Experiment: The Activity Series - Alkali and Restrainer

Part VIII taught two levers with one name. The alkali raises the rate at which the agent gives up its electrons; the restrainer occupies the sites on the crystal where it would give them up. Both are called activity, both move contrast, both move fog, and they move them in opposite directions. A series that runs one without the other tells you half a story.

Seven strips, from one exposure batch, in one session, run both.

Purpose

To measure how far a metol-sulfite developer’s behaviour moves when its pH is raised by two different alkalis, and how far a metol-hydroquinone developer’s behaviour moves when a restrainer is added at four levels — and then to read the two sets against each other.

Hypothesis, arm A. Raising the pH of a metol-sulfite base raises the rate of development, so at a fixed time the threshold step moves further up the wedge, the scale length shortens, and chemical fog rises — rising fastest at the top of the ladder, where Kodak’s 1928 primer says too much alkali gives fog. And the magnitude is genuinely open, because Part VIII establishes that metol’s amine is already neutral above about pH 8 and so its ionisation barely changes across this range. If the effect is large anyway, the ionisation account does not explain it on its own. If it is small, that is a sharp contrast with a hydroquinone developer, whose pK_a of 9.88 puts it right in the steep part of its curve.

Hypothesis, arm B. Adding potassium bromide to D-76 lowers base plus fog and moves the threshold step down the wedge — and does the first sooner than the second, so that at the lowest addition fog has already fallen while the image is still developing strongly. That is Hurter and Driffield’s Experiment 15 read as a prediction: their unexposed density reached its floor by 8 parts per thousand while the highest exposure was still building density.

The controls. Three, doing three different jobs, and the design turns on keeping them apart.

• A1, the metol-sulfite base with nothing added, is arm A’s control: it differs from A2 and A3 in the alkali and in nothing else.
• B0, D-76 with no bromide, is arm B’s control, on the same terms.
• One shared reference strip, from the same exposure batch, developed at the standard condition set by the test-negative lab — D-76 1+1, 11 minutes, 20 °C. It is not a control for either arm. Its job is to tie this session to that one and to every other session in the part, so that drift in the batch, the bottle or the reader is measured rather than assumed.

The one variable that changes. In arm A it is the added alkali, with the agent, the sulfite, the film, the exposure, the time, the temperature, the agitation and the volume identical between the three cells. In arm B it is the bromide concentration, on the same terms. Nothing else moves within an arm, and the honest limit of the design is stated below: the two arms use different bases, so they can be read against each other for direction and shape but not crossed as a two-factor grid.

!Two things this page predicts and does not know

No source in this course’s corpus gives a pH for D-23, for D-76, or for either with an alkali added. Part VIII says so plainly and does not fill the gap; nor does this page. Every pH below is computed from published dissociation constants and published formula quantities, presented as the prediction your meter will test. If your reading disagrees with the arithmetic, the arithmetic is what was wrong — it assumes ideal behaviour in a solution at 0.8 mol/L, where that assumption is poor.

Nor has this experiment been piloted by the course. The bromide levels are taken from Kodak’s own formulas and the alkali levels from published Kodak quantities and the arithmetic, so no number here is invented — but whether a modern film shows a readable difference between 0.5 and 1 g/L within the resolution of a by-eye step reading is exactly the question, and if the answer turns out to be no, that is a result and the page tells you what to run next.

Learning objectives

By the end of this session you will be able to:

• Prepare a three-step alkali ladder from one developer base, and justify each quantity from a published formula or from buffer arithmetic rather than from habit.
• Predict the pH of each cell from the Henderson–Hasselbalch relation and say why the steps are uneven.
• Measure the pH of a strongly alkaline, sulfite-rich developer and state the two ways that measurement can be wrong.
• Prepare a concentration series from a percentage stock, holding the added volume constant across every cell so that dilution is not confounded with the variable.
• Develop seven cells and a reference at equal time in staggered vessels, with one written agitation script.
• Read threshold step, scale length and base plus fog on each strip, by eye and by scanner, and say which differences exceed your rig’s resolution limit.
• Explain the results in terms of ionisation, buffer capacity, adsorption and self-restraint, and name the one thing this design cannot separate.
• Archive every strip so that Part XXVII can measure it without asking you anything.

Prerequisites

• Lab: preparing standard test negatives, which produced the exposure batch, the standard condition and your resolution limit.
• Experimental design for the darkroom, for the readings, the blinding and the notebook fields.
• Alkalis, buffers and pH, which owns the alkali and is not re-derived here.
• Restrainers and antifoggants, which owns the restrainer and Hurter and Driffield’s data.
• Measuring pH and buffers and buffer capacity.
• Concentration and dilution, for the percentage stock arithmetic.
• The SOPs for opening the laboratory, calibrating a pH meter, checking the balance and labelling a container.

Safety classification

Level A: a standard home darkroom — with a weighing step worked at Level B controls, and an optional extension that is Level B throughout.

• The developers in use. Alkaline solutions of an agent, a preservative and an alkali, handled by the hundred millilitres in open cylinders. With gloves and eye protection that is Level A, and it is the same handling as Part VIII’s mixing lab.
• The weighing step, which is not. Two of the three solids are Level B substances in this course’s encyclopaedia: borax on a reproductive-toxicity classification carried by 93.2 per cent of the reports that classify it, and benzotriazole in the optional extension. Four grams of borax and two of carbonate are weighed with the Level B controls Part VIII sets — a tray, still air, one jar open at a time, gloves, eye protection, a particulate mask, and no film open in the room. Borax’s classification is may damage fertility or the unborn child; a reader who is pregnant, breastfeeding or trying to conceive should read the borax page before opening the tub and should take the alternative route below.
• The carbonate cell. Sodium carbonate at 10 g/L is a mild alkali by the standards of this course and a serious eye irritant by the standards of the classification, which is why eye protection is on from the moment the jar opens rather than from the moment something splashes.
• No hydroxide. This experiment deliberately stops at carbonate. A caustic ladder would reach a higher pH and would leave Level A, and the question it answers is not worth the change of level here.
• Waste. Alkaline developer in one labelled container; fixer and its first rinse, which are silver-bearing, in another.

What is not a hazard here, and why. There is no sulfur dioxide risk, although 0.8 mol/L of sulfite passes through your hands in every cell. Sulfite releases the gas on meeting an acid, and this session contains no acid at any point: the stop is a plain water rinse, chosen partly for that reason, and the fixer is met only after the rinse and goes to a separate container. The risk is real one bench along, in the waste bottle, which is why the two wastes are never combined.

Equally, there is no dust hazard once the three solids are in solution — the whole inhalation route exists for about four minutes at the balance and is controlled there — and nothing is heated: the warmest thing in the room is a water bath at 20 °C.

Hazards

Metol: skin sensitisation. H317, may cause an allergic skin reaction, in every ECHA notification that classifies it, and it is present in all seven cells. Sensitisation is cumulative and does not reverse. Gloves throughout, and no finger in a cylinder.

Hydroquinone: serious eye damage and more. Present in the four arm-B cells and the reference, as part of D-76. Eye protection is not optional in this session.

Borax: reproductive toxicity, and dust. H360 at 93.2 per cent of reports, with a workplace exposure limit for the dust set by HSE’s EH40 — 5 mg/m³ for the decahydrate. The control is dust discipline at the balance.

Sodium carbonate: eye irritation, and a divergence worth knowing. H319 in 99.8 per cent of ECHA reports, but Japan’s NITE-CMC and Safe Work Australia both go to H318, serious eye damage. Where authorities disagree the course takes the union, so the control is the one appropriate to H318.

Potassium bromide: eye irritation. H319 in 91.4 per cent of the reports that classify it, with H315 and H335 as minority views. It is the mildest thing you will weigh today and it still goes on the tray.

Benzotriazole, in the optional extension only. H302, H319, and from a quarter to two fifths of notifiers H332 and an aquatic hazard statement. It is classified Level B as a substance in this course and the extension is worked at Level B controls.

Aquatic toxicity of the whole waste stream. Metol and hydroquinone are notified as very toxic to aquatic life with long-lasting effects, and this session produces about 650 ml of alkaline developer waste that still contains nearly everything that was weighed into it.

Required PPE

• Single-use nitrile gloves, 0.2 mm, per HSE’s COSHH essentials sheet P1, changed when contaminated.
• Eye protection from the first jar opening to the last container being capped, with the carbonate divergence above as the reason it is not relaxed after the weighing.
• A particulate mask for the weighing step, which is what allows a Level B weighing inside a Level A session. It does not replace the tray or the still air.
• An apron or overall kept for laboratory work.
• Clean dry hands, or lint-free gloves, for the film itself, which is a control on the archive and not on you.
• Dedicated labelled utensils, and a separate set for fixer.

Ventilation

Nothing in this session evaporates in a way that puts a substance into the air, so ventilation is not being asked to remove a vapour. It is doing two other jobs. The first is dilution during the four minutes when three jars of powder are open, which is a general-ventilation duty: HSE’s COSHH essentials sheet P1 sets more than five air changes an hour with a through draught for manual film development, and that is the standard here. The second is to carry away the ammonia smell of a rapid fixer over a five-hour session, which is an amenity problem rather than a control failure and will still drive you out of a sealed room.

A darkroom has to be light-tight and ventilated at once, and Part II’s laboratory layout owns how.

Materials

Item	Quantity	Note
Test strips from the Part IX exposure batch	8, plus 2 for the pilot	Datum-notched, unexposed identity nips still to be applied
Negative sleeves and card label slips	8	One slip per strip, eight fields each
Opaque tin	1	To hold the unused batch while the darkroom light is off
Random reading cards	8	Shuffled, for blind reading

Chemicals

Chemical	Quantity	Form
Metol-sulfite base (Kodak D-23: metol 7.5 g/L, sodium sulfite anhydrous 100 g/L), from Part VIII’s lab	600 ml	Solution, split into three 200 ml portions
D-76 stock (metol, hydroquinone, sodium sulfite, borax), from the same lab	450 ml	Solution: 400 ml at stock for arm B, 40 ml diluted 1+1 for the reference
Borax	4.00 g	Solid, weighed at Level B controls
Sodium carbonate, anhydrous	2.00 g	Solid
Potassium bromide	10.00 g	Solid, for 100 ml of a 10 % w/v stock; about 0.35 g is actually consumed
Benzotriazole	1.00 g, optional	Solid, for 100 ml of a 1 % w/v stock — extension only
Rapid fixer (ammonium thiosulfate type)	500 ml at 1+4	Fixed for twice the clearing time
Wetting agent	500 ml at 1+200	Final rinse
Water, at 20 °C	about 4 L	Rinse and wash

Equipment

Eight 100 ml measuring cylinders in a rack, standing in a tray deep enough to serve as a water bath. A balance reading to 0.01 g, checked against a reference. A pH meter with pH 7 and pH 10 buffers, or the best pH indicator strips you have. A thermometer to 0.1 °C. A 1 ml syringe and a 5 ml syringe for the bromide stock. Three 250 ml beakers for the arm-A cells. Three shallow trays — one for the water rinse, one for the fixer and one for the wash — each long enough to lay a 135 mm strip flat, since the rinse and the fixer are not variables and every strip goes through the same ones. Tongs, one pair per solution, and a separate set for fixer. A drying line with pegs, a lightbox and a flatbed scanner.

Estimated cost

££, and almost all of it is film and the chemicals mixed in Part VIII. The solids consumed on this page amount to about 4 g of borax, 2 g of carbonate and a third of a gram of bromide; the bromide stock is made at 10 g because 0.35 g cannot be weighed usefully on a 0.01 g balance in one go, and the surplus keeps. The planner carries the numbers.

Estimated consumables cost

Almost all of the band above is film and the D-76 and D-23 stocks mixed in Part VIII. The solids this page consumes on its own account are about four grams of borax, two of carbonate and a third of a gram of bromide — and the bromide is weighed at 10 g because 0.35 g cannot be weighed usefully in one go, so the surplus keeps.

Consumed	This session	Sourced price	Cost this session
Metol-sulfite base (D-23), from Part VIII	600 mL of stock	Costed in Part VIII’s mixing lab	—
D-76 stock, from Part VIII	450 mL	Costed in Part VIII’s mixing lab	—
Borax	4.00 g	£9.98 per 200 g, decahydrate	£0.20
Sodium carbonate, anhydrous	2.00 g	£7.20 per 500 g, anhydrous	£0.03
Potassium bromide	10.00 g weighed, about 0.35 g consumed	£23.00 per 250 g (£0.09 a g)	£0.03
Benzotriazole	1.00 g, optional extension only	None. The planner carries no item for this substance at all	—
Test strips from the Part IX exposure batch	8, plus 2 for the pilot	Costed in preparing-standard-test-negatives	—
Rapid fixer concentrate	100 mL, to make 500 mL at 1+4	£21.05–£25.98 per 1 L of ammonium thiosulfate concentrate, diluted 1+4 for film	£2.10–£2.60
Wetting agent	2.5 mL, for 500 mL at 1+200	£28.70 per 1 L of concentrate, diluted 1+200	£0.07
Negative sleeves and card label slips	8 pockets, 8 slips	None. A named price gap: sleeving that passes the Photographic Activity Test	—

The priced rows come to £2.44 to £2.93 for one run of this session, at the retail ranges read on 5 September 2026 and recorded in the laboratory planner. That is a floor, not a total: 2 of the 10 rows carry no dated price, so they are counted as nothing here and are certainly not free. A priced entry is a dated range to plan against, never a quotation.

The bromide row prices what is consumed, not the 10 g weighed out, because the stock is kept and reused across Part IX; that is the opposite convention from the silver rows in Part V, where the jar is bought whole and the surplus is the point. Both conventions are stated where they are used so that the calculator does not have to guess.

Waste streams

1. Alkaline developer, about 650 ml, carrying nearly all of the metol and hydroquinone it was mixed from, plus borax, carbonate and bromide. One labelled container, routed per the general waste SOP.
2. Spent fixer and the first rinse after it, silver-bearing, to the silver stream.
3. Rinse and wash water. The rinse that follows the developer carries developer and goes with stream 1; the wash water after fixing goes with stream 2 for its first change and to the general stream thereafter.

Never combine streams 1 and 2. Stream 1 holds 0.8 mol/L of sulfite and stream 2 is acidic.

Alternative route

The session needs darkness only for one operation: taking eight strips out of the tin and giving each its identity nips. That is about fifteen minutes with the light off.

The orthochromatic route. If the batch was made on ILFORD ORTHO Plus, that quarter of an hour happens under a deep red safelight instead — ILFORD’s 906 filter with a 15 W bulb at not less than 1.2 m — and the rest of the session runs in room light with the strips in closed vessels. Whichever film the batch is on, it stays on it.

The route without borax. A reader avoiding borax on the reproductive classification can run arm A as a two-cell ladder — the base alone against the base with 10 g/L of sodium carbonate — and read arm B unchanged, since D-76’s own borax is already in the bottle mixed in Part VIII and is not weighed here. That loses the middle rung and the buffer-capacity argument that goes with it, and the page would rather say so than pretend the substitution is free. The alkalis lesson carries the chemistry the missing rung would have shown.

Preparation

This is about 75 minutes of work and it is done in a separate sitting, the evening before. The 300 minutes on this page is the Procedure.

Working out the ladder before you weigh anything

The base is Kodak’s D-23: 7.5 g of metol and 100 g of anhydrous sodium sulfite in a litre. Two numbers come out of that.

The sulfite is 100 g of Na₂SO₃ at 126.04 g/mol, which is 0.793 mol/L. Metol is a sulfate salt, (C₇H₉NO)₂·H₂SO₄ at 344.39 g/mol, so 7.5 g is 0.0218 mol/L — and each of those releases two protons, so the base already contains about 0.044 mol/L of bisulfite made at the moment of mixing, when the metol’s sulfuric acid met the sulfite.

HSO₃⁻ + OH⁻ → SO₃²⁻ + H₂O

The couple that sets the base's pH: bisulfite and sulfite, with pKa2 of sulfurous acid at 7.19

Put those two numbers into Henderson–Hasselbalch with pK_a2 = 7.19:

pH = 7.19 + log(0.7498 / 0.0436) = 7.19 + 1.24 = 8.43

Predicted pH of the metol-sulfite base

Now add borax. Each mole of Na₂B₄O₇·10H₂O gives two moles of borate and two of boric acid, and borate is the stronger base of the two couples, so it takes protons from the bisulfite:

B(OH)₄⁻ + HSO₃⁻ → B(OH)₃ + SO₃²⁻ + H₂O

Borate stripping a proton from bisulfite: the equilibrium lies to the right by about two orders of magnitude

And here is why the D-76 quantity of borax will not do. Two grams per litre of borax is 5.24 × 10⁻³ mol/L, giving 0.0105 mol/L of borate — under a quarter of the bisulfite already present. Conserve the protons across both couples and the pH moves from 8.43 to 8.51. Less than a tenth of a unit, which is inside the specification of a teaching electrode and which no darkroom meter will resolve against its own drift. That is the buffer capacity of a hundred grams of sulfite; it is the whole reason D-23 needs no alkali at all; and it is why this ladder uses 20 g/L, the quantity Kodak puts in the D-76 replenisher — ten times the working formula’s. At 0.105 mol/L of borate the same calculation converges at pH 9.00.

The carbonate rung uses the same argument in reverse.

CO₃²⁻ + HSO₃⁻ → HCO₃⁻ + SO₃²⁻

Carbonate stripping a proton from bisulfite: pKa2 of carbonic acid is 10.33, over three units above the sulfite couple

Ten grams per litre of anhydrous sodium carbonate is 0.0943 mol/L. It takes the 0.0436 mol/L of bisulfite outright, leaving about 0.051 mol/L still as carbonate against 0.043 mol/L now as bicarbonate — a ratio of about 1.19, and a pH of 10.40, the same band ILFORD publish for their paper developers. Ten grams is between a fifth and a seventh of what Kodak’s D-19b and D-72 carry, so it is well inside the range manufacturers use and nowhere near a change of safety level.

🧮The ladder is 8.4, 9.0, 10.4 — and the steps are not equal

0.57 of a unit and then 1.41. That is not the tidy one-unit ladder an experimental design would like, and the reason is worth more than the tidiness would have been: you cannot get equal steps out of three buffers whose pK_a values are 7.19, 9.27 and 10.33 while keeping the sulfite at the concentration that makes the base D-23 in the first place. The sulfite resists the first step and the carbonate takes over for the second.

Every number here assumes ideal behaviour in a solution at 0.8 mol/L, where that assumption is poor. Read them for direction and magnitude, write them in the notebook as predictions, and let the meter decide.

The rest of the preparation

1. Open the laboratory to the SOP, and check the balance and thermometer.
2. Weigh and dissolve, at Level B controls: 4.00 g of borax into 200 ml of the base (cell A2) and 2.00 g of anhydrous sodium carbonate into another 200 ml (cell A3). The third 200 ml is A1 and gets nothing. Stir until fully dissolved, cap, and label each per the container SOP with its cell, its addition in g/L and the date.
3. Make 100 ml of a 10 % w/v potassium bromide stock: 10.00 g made up to 100 ml. Label it.
4. Calibrate the pH meter at pH 7 and pH 10 per the SOP, bracketing the samples rather than sitting to one side of them.
5. Measure and record the pH of A1, A2 and A3 at 20 °C, rinsing the electrode between and reading only when the display has settled. Record the number you got, not the number above.
6. Run the two pilot strips. One in A1 and one in D-76 stock, both for 10 minutes at 20 °C. Both should give a readable threshold and a usable run of separable steps. If the A1 strip is nearly blank, raise the session’s equal time to 12 minutes and note the change; if the D-76 strip is at maximum density from step 1 to step 12, drop it to 8.

✕Two ways the pH reading is wrong, and neither is the meter's fault

Alkaline error. A general-purpose glass electrode is specified over roughly pH 0.5 to 9, and its error grows above that as sodium ions begin to be read as hydrogen ions. The carbonate cell at about 10.4 sits well past it, in a solution carrying nearly 0.2 mol/L of sodium. Use a low-sodium-error electrode if you have one, calibrate at 7 and 10 rather than 4 and 7, and write down that the carbonate reading is the least trustworthy of the three.

Ordinary accuracy. A sealed teaching electrode is specified at ±0.2 pH units. The gap between A1 and A2 is predicted at 0.57 units, so the instrument’s own uncertainty is a third of the effect it is being asked to measure. That is not a reason to skip it — a reading with a stated uncertainty is a datum, and an assumption is not — but it is a reason to record the uncertainty next to the number.

Procedure

The session is one exposure batch, seven cells, one reference strip, and one equal development time.

Stage 1 — bring everything to temperature and mix arm B

1. Stand the eight cylinders in the water bath and bring the whole bench to 20 °C, holding it within 0.3 °C, which is the limit Kodak’s process-control publication sets before temperature affects the result.

2. Re-measure the pH of A1, A2 and A3 on the day and record it again. A metol-sulfite bath that stood overnight has begun to oxidise, and the pH is where you find out.

3. Mix the four arm-B cells. Each is 100.0 ml of D-76 stock plus exactly 2.0 ml of added liquid, made up as follows:

   Cell	10 % KBr stock	Water	Potassium bromide in the cell
   B0	—	2.0 ml	0 g/L
   B1	0.5 ml	1.5 ml	0.49 g/L
   B2	1.0 ml	1.0 ml	0.98 g/L
   B3	2.0 ml	—	1.96 g/L

   📓Why every cell gets 2.0 ml even when three of them are water

   Add the bromide stock alone and B3 would be two per cent more dilute than B0 — so the cells would differ in bromide and in developer strength, and the experiment would have two variables in it. Two per cent is far below what a step reading can see, which is exactly why it would never have been noticed and exactly why it has to be removed rather than tolerated. Topping every cell to the same 2.0 ml costs nothing and makes the dilution identical to the precision of a syringe.

   The concentrations in the last column are the honest ones: 0.49 rather than 0.50, because the 0.05 g of bromide is in 102 ml and not 100. Kodak’s own film developers carry 0.5 g/L in DK-20 and DK-50 and 1.9 g/L in DK-15 and D-72, with none at all in D-76 and D-23, so this ladder spans the range manufacturers actually use. Those figures are per litre of the formula as mixed, which is a comparison of formulas rather than an exact equivalence of what the emulsion sees.

4. Mix 80 ml of D-76 1+1 for the shared reference strip, freshly, as Kodak’s own sheet directs for a 1+1 working solution.

5. Fill each cylinder with at least 80 ml of its solution — enough to cover a 135 mm strip with room to spare. Check the depth with a scrap and water first.

Stage 2 — allocate the strips, in darkness

6. With the light off, take eight strips from the batch tin and give each its identity nips along the top edge, working from the datum notch: one nip for A1, two for A2, three for A3, four for B0, five for B1. B2, B3 and the reference get a nip on the lower edge, one, two and three, so no strip is ambiguous by touch.
7. Close the tin, put it away, and only then turn the light on. Lay each strip beside its cylinder.

Stage 3 — develop, in two staggered batches

Equal time is the point of the whole design: activity has to show as a difference in density, not be compensated away by a difference in time. Every cell gets 10 minutes at 20 °C. That is 25 per cent longer than ILFORD’s published D-76 stock time for FP4 Plus at EI 125, so the restrained cells still develop something, and two thirds of the time Part VIII derives for D-23, so the carbonate cell is less likely to reach maximum density on every step. The reference strip is the exception and keeps the standard condition — D-76 1+1 for 11 minutes — because its job is to be comparable with the other sessions, not with the cells beside it.

The wet run: two batches of four, staggered by a minute

A1 — base, no alkali

develop 10 minrinsefix

A2 — borax 20 g/L

develop 10 minrinsefix

A3 — carbonate 10 g/L

develop 10 minrinsefix

Shared reference

develop 11 minrinsefix

B0 — no bromide

develop 10 minrinsefix

B1 — 0.49 g/L

develop 10 minrinsefix

B2 — 0.98 g/L

develop 10 minrinsefix

B3 — 1.96 g/L

develop 10 minrinsefix

0 min10203035

Four strips at a time, one minute apart, is the most a single pair of hands can agitate on a one-minute cycle. Eight at once would need eight lifts inside every minute and the script would break, which is the sort of thing a procedure has to be walked through rather than assumed.

8. Batch one: A1, A2, A3 and the reference, started one minute apart. Lower each strip in and move it gently up and down for the first 30 seconds; then lift, drain for two seconds and re-immerse once at the start of each subsequent minute. The same script that the control set was developed with, in the same words.

9. Lift each strip at its own ten-minute mark, rinse for one minute in two changes of plain water, and fix for twice the clearing time.

   ⚗Why a water rinse and not an acid stop, for this session only

   Carbonate carried into an acid bath makes carbon dioxide inside a warm swollen gelatin layer, and Part VIII draws what happens next. Cell A3 is the one at risk, and treating A3 differently from the other six would put a second variable into the arm.

   So every strip in this session gets the same plain water rinse. The cost is real and ILFORD name it: a water rinse may be substituted for a stop bath but increases the risk of processing marks and stains. That is the trade — a small uniform risk of marks on all seven, against a large risk of blisters on one and a confounded arm either way.

10. Batch two: B0, B1, B2, B3, the same way, started as soon as batch one is out of the developer.

11. Wash all eight together, 5 to 10 minutes in water within 5 °C of the developer, final-rinse in wetting agent at 1+200, and hang to dry in still, dust-free air. Do not squeegee.

Stage 4 — the optional extension

12. If you are running the benzotriazole arm, mix three more cells while batch two develops: 100.0 ml of D-76 stock plus 2.0 ml of added liquid, exactly as arm B, made up as follows. The lowest cell takes 0.2 ml of a 0.1 % w/v stock, itself made 1+9 from the 1 % — a serial dilution, because 0.02 ml of the 1 % stock is twenty microlitres and no darkroom syringe will deliver it. The middle cell takes 0.2 ml of the 1 % stock and the top cell 2.0 ml of it. That gives roughly 0.002, 0.02 and 0.2 g/L in the cell. Top every one to the same 2.0 ml with water and develop them as a third batch at the same 10 minutes.

⚠The extension is a range-finder, not a comparison, and the page says why

This course’s corpus contains no published grams-per-litre figure for benzotriazole in any developer, and Part VIII declines to invent one. Nor does this page. The 1 % stock is chosen so that small doses can be measured with a syringe, not because any source specifies it, and the three volumes span two orders of magnitude precisely because their purpose is to find the range rather than to compare with the bromide ladder. Start from the supplier’s own sheet if it gives a figure; if it does not, this wide bracket is what an honest first run looks like, and the result is a range for next time.

The course has no verified solubility figure for benzotriazole in water either. If the gram will not dissolve in 100 ml, make the stock weaker and scale the volumes rather than reaching for a solvent that would add another variable. Weigh it at Level B controls: gloves, tray, mask, and the spent cells into the developer waste, which is where its aquatic hazard statement belongs.

Expected observations

Arm A, as the pH rises. The strips should get visibly denser overall. Look at three things in this order: the masked patch, which should darken; the foot, which should reach further up the wedge; and the separation between the darkest steps, which should compress. The carbonate strip is the one to inspect for an even grey veil right out to the film edges — that is chemical fog and it is the thing this arm exists to find.

Arm A, as a possible surprise. If A1, A2 and A3 look nearly alike, that is a result and not a failure, and it is the one the hypothesis flagged as open. Metol’s amine is already neutral above about pH 8; a metol-only developer may simply not care about this ladder in the way a hydroquinone developer would.

Arm B, as the bromide rises. The masked patch should get cleaner, and it should reach its floor early — by B1 or B2 if Hurter and Driffield’s pattern holds. The foot should retreat down the wedge. Between B0 and B1 the middle of the strip may look slightly steeper rather than merely lighter, which is the shadows being held back further than the highlights.

The reference strip should look like the control set from the test-negative lab. If it does not, that is the first thing to write down and the last thing to explain away.

Both arms. Expect the differences to be smaller than you hope. A step or two, not a transformation.

What is happening chemically

Arm A: what the alkali actually does

Kodak’s 1928 primer states the dependence and not the mechanism: most developing agents cannot develop at all by themselves, they must be in an alkaline solution, and the energy depends upon the amount of alkali present. Part VIII supplies the mechanism — the alkali strips protons from the agent’s hydroxyl or amino groups, and the anion that results is what hands electrons to silver.

That account predicts a steep pH dependence for an agent whose pK_a sits in the working range, and a shallow one for an agent whose does not. Hydroquinone’s is 9.88, so at pH 8.65 only about six per cent of it is ionised and every tenth of a unit is a large proportional change in that six per cent. Metol’s amine is already neutral above about pH 8, so across a ladder from 8.4 to 10.4 its ionisation barely moves.

Which is why this arm is worth running rather than assuming. If the metol-sulfite base turns out to be strongly pH-dependent anyway, then something other than the agent’s ionisation is carrying the effect — the gelatin swells more at higher pH and lets developer in faster, the electron-transfer step itself may be pH-sensitive, and the fog reaction certainly is. If it turns out to be weakly dependent, you have a clean demonstration that the alkali’s grip on a developer runs through the agent’s pK_a and not through some general property of alkalinity. Either answer is worth a strip.

Buffer capacity is the other half. All three cells hold 0.79 mol/L of sulfite, which is an enormous reservoir sitting two to three units below the working pH, and the arm-A arithmetic above is really an account of how hard it is to move a solution that concentrated. It also means the three cells resist the acid that development itself releases very differently: A1 has only the sulfite couple far off its pK_a, A2 adds a borate couple sitting near its own, and A3 sits almost exactly on the carbonate pK_a, which is where a buffer is strongest. Over ten minutes and one small strip that difference is unlikely to show. Over a session of films it is the difference between a developer that holds and one that fades, which is what Part VIII’s alkalis lesson is about.

Where fog appears. Development is selective because the barrier to reducing silver at a bare crystal is higher than at a latent-image speck — not because it is infinite. Raise the driving force and the higher barrier starts to be cleared too. That is chemical fog, and it is why Kodak’s primer pairs “too much [alkali] gives chemical fog” with “too little is slow” in one sentence: they are the same knob.

Arm B: what the restrainer actually does

Bromide is the developer’s own exhaust, added in advance. Two mechanisms run at once and the experiment cannot separate them, which is worth knowing before you interpret it.

AgBr(s) ⇌ Ag⁺ + Br⁻

The common-ion effect: added bromide pushes this equilibrium left and lowers the silver ion concentration at the crystal surface

The common-ion effect lowers the concentration of free silver ions available to be reduced. Adsorption is the other: bromide ions occupy sites on the crystal surface where reduction would otherwise be initiated. Part VIII is explicit that the corpus does not settle which dominates or why the unexposed grain suffers most, and this page does not settle it either.

What the corpus does settle is the shape. Hurter and Driffield’s Experiment 15 ran a pyro-soda developer at 0, 2, 8, 32 and 128 parts of bromide per thousand and measured the whole family. Their unexposed density fell from 0.160 to 0.090 to 0.060 and had reached its floor by 8 parts per thousand, while the image at the highest exposure was still developing strongly at that concentration. Fog is suppressed sooner than image, and that asymmetry is the entire commercial justification for the ingredient.

They also found that nothing ever stops. Up to a 12 per cent solution, with ferrous oxalate and with alkaline pyrogallol, the reaction was never brought to a halt — “It is simply retarded, and, if sufficient time be allowed, the image will make its appearance in full force.” Your equal-time design is therefore measuring rate at a fixed time, which is not the same as measuring what the developer could eventually do, and the difference between those two statements is the whole of the contested question about whether bromide costs speed.

Self-restraint. Every strip develops in a bath that is accumulating bromide from the film itself, so even B0 restrains itself a little as the ten minutes pass. That is the reason the volume per cell is specified rather than left to the vessel: each strip is about a twelfth of the area of a 36-exposure roll and gets 80 ml, which is a generous margin against local exhaustion, and a starved cylinder would add a restrainer to the cell that was supposed to have none. It is also why every cell is one-shot.

Why the threshold moves before the slope does. The foot of the curve is made by the smallest latent-image specks, which are the ones a restrained developer gives up on first. Take those away and the curve loses its bottom before it loses its middle. That is the same fact seen from the other side as Hurter and Driffield’s observation about very slight deposits, and it is why the threshold step is the sensitive reading in this arm while the scale length is the sluggish one.

The organic antifoggant, if you ran the extension

A halide restrainer works through the equilibrium above. An organic antifoggant works by forming an extremely insoluble silver salt on the grain surface — so insoluble that analysts determine silver gravimetrically by precipitating it with benzotriazole. Part VIII owns the detail; the one thing to carry into this session is that the reaction needs the anion, and benzotriazole’s ring N–H has a pK_a of 8.64.

D-76 sits at about the same pH. So in this bath roughly half the benzotriazole is the active anion and half is not, which is precisely the steep part of its curve — and it means the extension’s cells are sensitive to any pH drift in a way the bromide cells are not. A halide restrainer does not care about pH. That difference, more than the potency, is what separates the two families.

Data to record

Session header, as the design lesson sets out, plus the batch block copied by name from the test-negative lab so that this session’s strips can be traced to that exposure run.

The pH table, with four columns: cell, predicted pH from the arithmetic, measured pH the evening before, measured pH on the day — and a fifth for the electrode’s stated accuracy, written next to every reading rather than once at the bottom.

The cell table, one row per strip: cell name, notch code, random reading number, developer and version, the addition and its actual concentration, volume, time, temperature at start, middle and end, agitation script, vessel.

The reading table, one row per strip: threshold step, scale length in steps, a written description of the masked patch against clear fixed base, matched densities if taken, scanner code values with the scan settings, and the date each reading was made.

The deviations. Every one. A cylinder knocked, a lift missed, a strip that touched its neighbour, a thermometer found reading low at the end.

Analysis

1. Read blind. Shuffle the eight cards, assign a random number to each strip, read all eight in one sitting in random order on the same lightbox, and open the key only when the reading table is full.
2. Apply your resolution limit first, before you look at any pattern. It came from the control set in the test-negative lab. Any difference between two cells smaller than that limit is not reported as a result — and if that rules out most of the arm, say so, because a null result honestly bounded is worth more than a trend read into noise.
3. Plot arm A: threshold step against measured pH, and scale length against measured pH. Use the pH you measured, never the pH you predicted.
4. Plot arm B: threshold step against bromide concentration, and scale length against it.
5. Rank base plus fog across all seven cells against the reference and against clear fixed base. You cannot put a number on it without a densitometer; you can rank it, describe it, and scan it, and the ranking is the arm’s most important single output.
6. Check the reference strip against the control set from the lab. If it has moved by more than the resolution limit, the whole session inherits that shift and every conclusion is about a difference between cells rather than about an absolute.
7. Cross the two arms. Look for a pair — one cell from each — with the same scale length. Then look at what else differs between them: the threshold step, and the fog.
8. Scan everything in one pass with the wedge included, and repeat steps 3 to 5 on the scanned numbers. Where the eye and the scanner disagree about the order of two cells, that pair is inside the resolution of both methods and neither of them has separated it.

Arm A: what to write down before you develop anything

• Predicted threshold step (higher = more sensitive)
• Predicted scale length, steps (shorter = more contrast)

Show the numbers behind this plot

Two predicted lines drawn against pH from 8.4 to 10.4. The upper line is the threshold step, rising from about 17 at pH 8.4 through 18 at pH 9.0 to about 19.5 at pH 10.4, which means the developer is finding density further and further down the exposure scale as the pH rises. The lower line is the scale length in steps, falling from about 12 at pH 8.4 through 11 at pH 9.0 to about 8.5 at pH 10.4, which means the tonal range is being compressed into fewer steps and the contrast is therefore rising. Both lines are drawn shapes and not measurements: they are the prediction the hypothesis makes, plotted so that the reader can lay their own data over the top and see immediately whether the direction was right and whether the size was anything like it. The plot deliberately carries no fog series, because fog is read as a rank and a description rather than in wedge steps and cannot honestly share this axis.

Series	Measured pH of the cell at 20 °C	Reading, in wedge steps
Predicted threshold step (higher = more sensitive)	8.43	17.00
Predicted threshold step (higher = more sensitive)	9.00	18.00
Predicted threshold step (higher = more sensitive)	10.40	19.50
Predicted scale length, steps (shorter = more contrast)	8.43	12.00
Predicted scale length, steps (shorter = more contrast)	9.00	11.00
Predicted scale length, steps (shorter = more contrast)	10.40	8.50

Drawn, not measured: these are the shapes the hypothesis predicts, printed so that your own three points can be laid over them. If your data run flat, the hypothesis was wrong about the magnitude and the page said in advance that it might be. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

Arm B: the same, for the restrainer

• Predicted threshold step (lower = more restrained)
• Predicted scale length, steps

Show the numbers behind this plot

Two predicted lines drawn against bromide concentration from zero to about two grams per litre. The upper line is the threshold step, falling from about 18 with no bromide to 17 at half a gram per litre, 16 at one gram and 14 at two grams, which means the developer needs progressively more exposure before it will put down any density at all. The lower line is the scale length in steps, which shortens slightly from about 11 at no bromide to about 10 at half a gram, reflecting the shadows being held back further than the highlights, and then lengthens again to 11 and 13 as the higher additions begin to suppress the image itself rather than only the fog. The non-monotonic shape of the lower line is the prediction most worth testing, because it is the one an intuition about restraint would not produce. As with the arm A figure, both lines are drawn shapes rather than measurements, and fog is deliberately absent because it is recorded as a rank rather than in steps.

Series	Potassium bromide in the working solution, g/L	Reading, in wedge steps
Predicted threshold step (lower = more restrained)	0.00	18.00
Predicted threshold step (lower = more restrained)	0.49	17.00
Predicted threshold step (lower = more restrained)	0.98	16.00
Predicted threshold step (lower = more restrained)	1.96	14.00
Predicted scale length, steps	0.00	11.00
Predicted scale length, steps	0.49	10.00
Predicted scale length, steps	0.98	11.00
Predicted scale length, steps	1.96	13.00

Drawn, not measured. The dip and recovery in the lower line is Hurter and Driffield's observation that a small bromide addition makes the curve steeper before a large one flattens it, translated into the only units this session can read. The curve is drawn to show the shape, not measured from a real material. Your own materials will differ, and measuring them is what the sensitometry part of the course is for.

Reading the two arms against each other

The activity map: two routes to the same contrast, and what still tells them apart

1. 1Shared control — the base developer, with neither alkali added nor bromide
2. 2Alkali arm — up and right — contrast bought, fog paid, threshold rises up the wedge
3. 3Restrainer arm — down and right, then down and left — fog floor bought, threshold falls, speed paid
4. 4The equal-contrast line — two cells, one contrast, two different fog floors and two different thresholds

This is what makes the two arms one experiment. Read either alone and contrast looks like the output of a single knob; read them together and it is obvious that the same contrast can be reached from opposite directions and that the difference lives somewhere else.

So which pair should match? On the predictions above, cell A2 (borax, about pH 9.0) and cell B1 (0.49 g/L of bromide) both land near a scale length of ten or eleven steps. If your data agree, you have two developers of the same contrast, and the two readings that separate them are the threshold step — higher for A2, lower for B1 — and the fog, higher for A2 and lower for B1. That is the answer to the question the whole page exists for: contrast on its own does not specify a developer, and a target written as contrast alone can be hit by more than one bath with different consequences for speed and for the shadows.

✕The thing this design cannot separate, said out loud

The two arms use different bases — a metol-sulfite bath in arm A and a metol-hydroquinone bath in arm B — so a cross-arm pair differs in the agent set as well as in the variable. The map above is therefore a reading of directions and shapes, and it is not a two-factor grid. Anyone who reports it as one has confounded pH with the presence of hydroquinone.

There is a second, smaller confound inside arm A itself, and it is unavoidable: you cannot change a pH without adding something. Twenty grams of borax and ten of carbonate also change the ionic strength of the cell and put a different anion in the bath. “pH” in this arm really means “pH, and whatever the alkali brought with it”.

Both are fixable, and the fixes are in Further experiments. Naming them is not a disclaimer; it is the result of walking your own design through the design lesson’s checklist, which is the exercise that page sets.

What a photographer does with this

The formulas in Part VIII stop being a list once these strips are dry. Bromide is scarcest exactly where speed matters most — none at all in D-76 and D-23, both fine-grain film developers — and heaviest where the alkali is most violent, at about 11 g/L in D-9, a caustic hydroquinone bath developing in three minutes. The two arms of this experiment are the two halves of that sentence, and you have now watched both.

The working rules that follow are modest and they are yours rather than the course’s, because they rest on your strips.

• The alkali is the contrast control, and fog is its price. If you want more contrast from a developer you already have, more alkali is the fastest route and the first thing to inspect afterwards is the unexposed rebate.
• The restrainer is the fog brake, and speed is its price. It buys a clean floor under the shadows, which is what lets a print’s darkest tones separate instead of sitting on a veil, and it costs exposure at the toe.
• They are not interchangeable, even where they meet. Two cells of equal contrast reached by opposite routes give you different negatives, and which one you want depends on whether your subject is short of shadows or short of separation.
• Neither is free, and neither moves one thing. That is the lesson Part VIII’s ingredient map drew and this page measured.

Troubleshooting

What you see	Likely cause	What to do
A1, A2 and A3 look identical	Either the pH ladder did not move the developer, which is a result, or the alkali did not dissolve	Check the measured pH values first. If they climbed as predicted and the strips did not, record the null result — it is the hypothesis’s stated open question
The measured pH of A2 is barely above A1	The borax did not fully dissolve, or the balance is out	Borax dissolves slowly in cold solution. Warm gently, stir, re-measure. Then check the balance against a reference
The carbonate pH reads far below 10	Alkaline error on a general-purpose electrode, or a stale pH 10 buffer	Re-calibrate with fresh buffer at 7 and 10. If the reading stays low, record it with the electrode’s specification beside it rather than adjusting it
The carbonate strip has blisters or small round lifts	Carbonate met an acid bath somewhere	Confirm every bath after the developer is a plain water rinse. Part VIII draws the mechanism
All four arm-B strips look the same	The bromide levels are below what this film and this reading can resolve	This is the pilot question the page flagged. Re-run at 2, 4 and 8 g/L — 4 g/L is Kodak’s own D-19b level — and record that the lower ladder was under the resolution
B3 is nearly blank	Ten minutes was not enough for a bath at 2 g/L, which retards rather than stops	Genuine result. Note it, and if you want the compensated version, develop a spare strip at 20 minutes: Hurter and Driffield showed the image appears in full force given time
The reference strip does not match the lab’s control set	Batch drift, developer age, a thermometer offset, or a different agitation	Work through those four in that order. Until it is explained, every conclusion in the session is relative and none is absolute
Streaks running from the dense steps	Uneven agitation, or too little solution in the cylinder	Check the depth covers the strip. Consistent lift-and-drain to a count
Drying marks read as density	Squeegeed, or the wetting agent was wrong	Re-wash and re-dry. Never squeegee a strip that Part XXVII will measure

Clean-up

Pour each cylinder into the alkaline developer container as you finish, rinse it twice into the same container and only then into the sink. Fixer and its first rinse go to the silver container. Wash the developer utensils and the fixer utensils separately and keep them separate, because a developer contaminated with fixer is the failure Kodak’s troubleshooting publication names first. Wipe the balance and the tray. Throw the gloves away. Close the laboratory to the SOP.

Storage

Do not keep the seven cells. They are one-shot working solutions and three of them are experiments in what an alkali does to a metol bath, which includes what it does to its keeping. Discard them.

Keep the D-23 base and the D-76 stock, capped full, dated, with their version numbers legible; the solvent-series and agent-comparison experiments draw on both.

Keep the 10 % bromide stock. It is a stable inorganic salt solution and the later experiments will want it. Label it with its concentration, the date and the balance used.

Archive every strip. This is the deliverable, and it matters more than any number you write down today.

📓The archive, and what Part XXVII will need from it

Each strip is fixed for twice its clearing time, washed to the published standard, dried without being touched, and filed in a sleeve with a card slip carrying the eight fields the test-negative lab sets out — with this session’s specifics filled in: arm and cell, the alkali or bromide addition in g/L, the measured pH with its temperature, the developer’s version number, the equal time, the temperature range and the agitation script.

Two honest gaps. The course cannot yet cite a residual-thiosulfate limit for archival permanence, because Part XII owns washing and permanence and has not been written; until it is, “fix for twice the clearing time and wash to the maker’s published figure” is the standard this page can defend and it may not be enough for a decade. And the course has not verified that silver density on a properly fixed and washed strip is stable, over months, to better than the uncertainty budget Part XXVII will work to. The mitigation is structural rather than reassuring: the shared reference strip in every session, and a spare control strip archived alongside, so that any drift can be measured later instead of argued about.

Disposal considerations

Two containers, labelled with contents and date, never combined.

The alkaline developer waste, about 650 ml, carries nearly all the metol and hydroquinone that went into it, because seven small strips reduce very little silver. Both agents are notified as very toxic to aquatic life with long-lasting effects. It also carries borate and carbonate from the arm-A cells and bromide from arm B, and — if you ran the extension — benzotriazole, which a quarter to two fifths of notifiers classify with an aquatic hazard statement of its own.

The fixer and its first rinse are silver-bearing and go to the silver stream, where the silver can be recovered. Combining them with the developer gains nothing and spoils the recovery.

ILFORD’s advice to domestic users in the United Kingdom is to bottle each waste chemical separately, label it and take it to a Household Waste and Recycling Centre. That is one country’s answer. The disposal page sets out why the question is jurisdictional and why the course gives no universal instruction. Check your local regulations; they govern, and they differ between authorities within one country.

Questions

1. Your measured pH values come out at 8.6, 9.1 and 10.2 against predictions of 8.43, 9.00 and 10.40. Which of the three disagreements would you take most seriously, and what would you check first?
2. A2 and B1 both come back with a scale length of ten steps. Write the two sentences you would put in the report: one saying what they have in common, and one saying what still separates them.
3. Cell B3 shows a threshold four steps below B0. Using Hurter and Driffield’s conclusion that bromide retards rather than stops, design the smallest additional test that would tell you whether the film speed has actually changed. Name the control it needs.
4. Somebody proposes running the alkali ladder on D-76 instead of the metol-sulfite base, “because D-76 is the developer people actually use”. What would that change about the result, and why did this page not do it?
5. The carbonate cell’s strip is denser everywhere, including the masked patch. Which part of that is evidence about development and which is evidence about fog, and which single reading separates them?
6. Your resolution limit from the control set is one step. Between B1 and B2 the threshold differs by one step. May you report that as an effect of bromide? Give the reasoning, not just the answer.

Further experiments

Close the confound: one alkali, three levels. Run the arm-A ladder with borax alone at 2, 20 and 60 g/L. The anion, the ionic strength and the buffer identity then move together instead of being swapped, and the pH becomes very nearly the only thing that changed. It is the cleaner experiment, it was not the one the course ran, and the reason — that borax and carbonate are the two alkalis a reader will actually meet in published formulas — is a teaching decision rather than a scientific one.

Cross the arms properly. Put the bromide ladder into the borax cell of arm A rather than into D-76. Both arms then share one base and one agent, and a genuine two-factor grid becomes available: three pH levels by three bromide levels is nine strips, which is one long session and no new chemicals.

The compensated arm. Repeat B3 at 20 and 30 minutes as well as 10, and see whether the image comes up in full force as Hurter and Driffield say it does, and whether the threshold comes back with it. That single series is the direct test of the contested question Part VIII sets out — whether bromide costs speed or only time — on your film rather than on an 1890 plate.

The seasoning arm. Develop four strips one after another in the same 200 ml of D-76, then compare their thresholds against the fresh-bath control. You will be measuring the bromide the film itself released, which is the same variable arm B added from a bottle, arriving by the route it arrives by in real work.

Fog against time at fixed pH. Take the carbonate cell and develop strips at 5, 10, 15 and 20 minutes. Both image and fog rise with time, but if fog rises faster there is a development time beyond which the bath costs more than it gives, and that time is a number nobody has published for your film in your developer.

Check your understanding

Question 1. Why does this experiment use 20 g/L of borax rather than the 2 g/L that D-76 itself carries?

• Because 2 g/L would not dissolve in a sulfite solution
• Because the base already holds about 0.044 mol/L of bisulfite from the metol sulfate, and 2 g/L of borax supplies only 0.0105 mol/L of borate — enough to move the pH by about 0.05 of a unit, which no darkroom meter resolves
• Because Kodak specifies 20 g/L for all borax developers
• Because a higher borax level is needed to buffer against the acid released by development

Show the answer and why

Answer: Because the base already holds about 0.044 mol/L of bisulfite from the metol sulfate, and 2 g/L of borax supplies only 0.0105 mol/L of borate — enough to move the pH by about 0.05 of a unit, which no darkroom meter resolves

The quantity is set by what is already in the bath. A hundred grams per litre of sodium sulfite is 0.79 mol/L, and the metol arrives as a sulfate salt whose acid converts about 0.044 mol/L of that to bisulfite at mixing time — so the base is a buffer before anything is added to it. Two grams of borax offers a quarter of that pool in borate and shifts the ratio hardly at all. Twenty grams per litre is not an arbitrary jump either: it is the borax level in Kodak’s own D-76R replenisher, so the ladder uses a published quantity rather than a chosen one.

Question 2. Every arm-B cell receives exactly 2.0 ml of added liquid, and in B0 all of it is water. What error is that guarding against, and why would it otherwise go unnoticed?

• Evaporation from the open cylinder during development
• That adding the bromide stock alone would leave B3 two per cent more dilute than B0, so bromide and developer strength would vary together — and two per cent is far below what a step reading can see, so the confound would never announce itself
• That the bromide would not mix evenly without a carrier volume
• That the cells would be at different temperatures

Show the answer and why

Answer: That adding the bromide stock alone would leave B3 two per cent more dilute than B0, so bromide and developer strength would vary together — and two per cent is far below what a step reading can see, so the confound would never announce itself

It is the classic small confound: a second variable that moves in lockstep with the first and is too small to be detected in the output. Two per cent of developer strength is nowhere near a wedge step, so no strip would ever look wrong, and the arm would quietly be a test of bromide-plus-dilution rather than of bromide. The fix costs one extra syringe and removes the ambiguity entirely, which is a much better trade than deciding after the fact that the effect was probably negligible.

Question 3. Part VIII establishes that metol’s amine is already neutral above about pH 8. What does that do to arm A’s hypothesis?

• It refutes it, so arm A should not be run
• It makes the magnitude genuinely open: if the metol-sulfite base is strongly pH-dependent anyway, something other than the agent’s ionisation is carrying the effect; if it is weakly dependent, that is a clean demonstration that the alkali works through the agent’s pKa
• It means the pH must be measured more accurately
• It means carbonate should be replaced by hydroxide to see any effect

Show the answer and why

Answer: It makes the magnitude genuinely open: if the metol-sulfite base is strongly pH-dependent anyway, something other than the agent’s ionisation is carrying the effect; if it is weakly dependent, that is a clean demonstration that the alkali works through the agent’s pKa

A hypothesis whose answer is already known is not worth a strip, and this one turns out not to be known. The ionisation account predicts a steep pH dependence for hydroquinone, whose pKa of 9.88 puts it in the middle of the working range, and a shallow one for metol, whose amine is not. So the metol-only base is the sharp test of whether alkalinity acts on a developer through the agent’s ionisation or through something else — gelatin swelling, the electron-transfer step, the fog reaction — and both possible outcomes are informative. Running the same ladder on D-76 would answer a different and easier question.

Question 4. Hurter and Driffield found the unexposed density had reached its floor by 8 parts of bromide per thousand while the image was still developing strongly. What does that asymmetry justify?

• Using bromide as a contrast control
• The whole commercial case for the ingredient: fog is suppressed sooner than image, so there is a concentration range that buys a clean floor without paying for it in image density
• Developing for longer whenever bromide is used
• Replacing bromide with an organic antifoggant

Show the answer and why

Answer: The whole commercial case for the ingredient: fog is suppressed sooner than image, so there is a concentration range that buys a clean floor without paying for it in image density

If restraint fell on fog and image at the same rate, adding bromide would be equivalent to shortening the development and would be pointless. The asymmetry is what makes it an ingredient rather than a nuisance: across their series the unexposed density fell from 0.160 to 0.060 and stopped falling, while the highest exposure was still building density at the same concentration. The manufacturers’ formulas are that observation turned into practice — none at all in the fine-grain film developers where speed matters most, and about 11 g/L in a caustic hydroquinone bath where the alkali would otherwise develop the whole frame.

Question 5. The session runs a shared reference strip at D-76 1+1 for 11 minutes, which is neither arm’s condition. Why is that not a wasted strip?

• It is a spare in case a cell is spoiled
• It is the only thing tying this session to the test-negative lab and to the other sessions in the part: it measures batch drift, developer age, thermometer offset and reader drift, none of which an internal control can see
• It gives the reader a familiar negative to judge the others against
• It is required for the scanner to be calibrated

Show the answer and why

Answer: It is the only thing tying this session to the test-negative lab and to the other sessions in the part: it measures batch drift, developer age, thermometer offset and reader drift, none of which an internal control can see

An internal control answers "what did the alkali do", because it differs from its cells in exactly one thing. It cannot answer "is this session comparable with the last one", because everything in it changed together with everything else in the session. The reference strip is held at one fixed condition across the whole part precisely so that it can, and it is the reason the archive can be re-measured months later in Part XXVII against a known point rather than against a hope. It is also why the two arms cannot share a control: their bases differ, so neither arm’s zero cell is the other arm’s unchanged comparison.

Question 6. Cells A2 and B1 come back with the same scale length. What has the experiment demonstrated, and what has it not?

• That borax at 20 g/L and bromide at 0.49 g/L are interchangeable
• That the same contrast is reachable by opposite routes, with the threshold step and the fog floor still separating the two — but not that the two are equivalent, because the arms use different bases and so differ in the agent set as well
• That contrast is independent of pH
• That the resolution limit is too coarse for this experiment

Show the answer and why

Answer: That the same contrast is reachable by opposite routes, with the threshold step and the fog floor still separating the two — but not that the two are equivalent, because the arms use different bases and so differ in the agent set as well

The positive finding is the point of running both arms together: contrast on its own does not specify a developer, so a target written as contrast alone can be hit by more than one bath with different consequences for speed and for the shadows. The threshold step should be higher for the alkali route and lower for the restrained one, and the fog floor higher for the first and lower for the second, so two readings still separate them. The limit is equally important and belongs in the report: arm A is a metol-sulfite base and arm B is a metol-hydroquinone one, so a cross-arm pair differs in the agent set too and the map is a reading of directions rather than a two-factor grid.

Check my answersClear