Experiment: Measuring Stop Bath Capacity

Every capacity figure in the last two lessons was somebody else’s. This session produces one of your own, from two baths, a meter and a litre of developer — and it does it by deliberately making the two baths disagree about how acidic they are while agreeing about how much acid they contain.

Purpose

To follow the pH of two working stop baths as measured volumes of D-76 are added to them, locate the point at which each is exhausted, convert that into a capacity in films or prints per litre using a carryover volume you measure yourself, and compare the result with what the manufacturers publish.

The hypothesis. Capacity follows acid content, not pH. The two baths are made to contain the same number of titratable protons per litre — 0.300 mol — but very different numbers of acid molecules: 0.300 mol/L of acetic acid, which has one ionisable proton, against 0.100 mol/L of citric acid, which has three. Because citric acid’s first pK_a is 2.87 against acetic acid’s 4.76, the citric bath will start about 0.7 pH units lower — it will look considerably stronger. The prediction is that this appearance buys it nothing: both baths will reach any given exhaustion pH within roughly ten per cent of the same volume of added developer.

The control. A third vessel holding one litre of the water you would use for a rinse, receiving the same 25 mL additions for the first four rounds. It has no acid reserve at all, so it shows what the pH does when nothing is holding it, gives the meter a fixed point to be checked against later in the session, and measures the water-rinse alternative from the previous lesson at the same time.

The one variable that changes between the two acid vessels is which acid is dissolved in them. Volume, temperature, titrant, stirring, addition schedule, electrode and operator are all held.

=What falsifies the hypothesis, and what each outcome means

If the two curves reach pH 5.2 within about 25 mL of each other, the hypothesis stands: matching on protons matched the capacity, and the 0.7-unit head start of the citric bath was worth nothing.

If the citric bath exhausts substantially earlier — say at two thirds of the acetic bath’s volume — then its third proton, whose pK_a of 5.68 sits above the endpoint, was not available in the working range. That is not a failed experiment. It is a measurement of how much of a polyprotic acid a stop bath actually gets to use, which is a number no manufacturer publishes.

If the citric bath exhausts substantially later, look for a mixing error first: 19.2 g of the anhydrous acid and 21.0 g of the monohydrate are not the same amount of acid, and the label is the only thing that tells you which you have.

Learning objectives

Titrate a bath against a real working solution rather than a standard; read an exhaustion curve and say where on it a bath stops being useful; convert a curve into a capacity using a carryover you measured; explain why a bath halfway up its pH range is not half spent; and state, with numbers, why an indicator dye is a warning device rather than a gauge.

Prerequisites

Why stopping works and stop bath formulations compared, which supply the arithmetic this session measures. From Part III, the pH laboratory — you must be able to calibrate a meter and know what its slope means before any number here is worth writing down — and buffers and buffer capacity. From Part II, concentration and dilution and measurement and uncertainty. A litre of D-76 stock from Part VIII’s mixing lab.

Safety classification

Level A, on the course rubric: dilute solutions in gram and tens-of-gram quantities, no concentrates opened, no dust beyond weighing about twenty grams of a solid, no heat, no mains equipment beyond a meter, and no silver anywhere in the session. Two criteria were weighed and did not raise it.

The acetic acid. Glacial acetic acid is H314 and H226 and would be Level B to handle. This session does not handle it: the bath is made from a pre-diluted product whose strength is stated on the label and in its own safety data sheet, exactly as the environmental-health guidance for photographic chemicals recommends, and the working bath is about 1.8 per cent w/v.

The sulfur dioxide. Adding a sulfite-rich developer to an acid is the acid-with-sulfite reaction on the incompatibilities page, and it does release sulfur dioxide. What keeps it Level A is scale and rate, quantified under Hazards below, plus covered vessels and the ventilation Level A already assumes: a printing session puts more developer through a stop bath than this experiment does, and the only difference is that here it arrives in sixteen deliberate steps.

What is not a hazard here, and why. There is no silver-bearing waste, because no film is fixed and no emulsion enters any bath — this is one of very few practical pages in the course whose waste contains no silver at all, and the silver-waste routine does not apply. There is no dust hazard from the citric acid worth a respirator: the safety card records a dust explosion as possible for the powder dispersed in air, but that is a bulk-handling statement, and twenty grams tipped gently from a scoop into a tared beaker over a tray raises nothing. And there is no risk of a runaway neutralisation: both acids are weak, both baths are dilute, the enthalpy involved is small enough that the thermometer will not show it, and the pH change per addition is a fraction of a unit. Those same substances behave very differently outside this page — glacial acetic acid is corrosive and flammable, citric acid dust in an industrial silo is an explosion hazard, and the same sulfite meeting a strong acid in a closed waste bottle is a serious matter — and it is the concentration, the quantity and the open, ventilated vessel that make the difference, not the identity of the chemicals.

Hazards

Sulfur dioxide, generated deliberately. HSE’s EH40 gives sulfur dioxide a long-term limit of 0.5 ppm (1.3 mg/m³) and a fifteen-minute limit of 1 ppm (2.7 mg/m³). The quantity in play is knowable. Each 25 mL addition of D-76 stock brings 19.8 mmol of sulfite into the bath. How much of that sits as dissolved sulfur dioxide rather than as hydrogensulfite is set by the pH and by sulfurous acid’s first pK_a of 1.80:

f = 1 ÷ (1 + 10^(pH − 1.80))

Fraction of the sulfur(IV) present as dissolved sulfur dioxide

Immediately after the first addition the acetic bath is near pH 3.6, which puts f at about 1.6 per cent; the citric bath is near pH 2.4, which puts it at about 19 per cent, twelve times more. Both fall steeply as the baths climb. So the worst moment of the whole session is the first addition to the citric bath, and the controls are pointed at it: add slowly down the wall of the vessel, with the bath already being stirred, and replace the cover before you pick up the meter. Environmental-health guidance for darkrooms makes the same point from the practical end, recommending a water rinse between developer and stop precisely to reduce sulfur dioxide formation.

Acetic acid vapour, if you use the acetic product. EH40 gives 10 ppm over eight hours and 20 ppm over fifteen minutes, and the International Chemical Safety Card notes that harmful air contamination can be reached rather quickly by evaporation at 20 °C. The working bath is dilute and covered; the concentrate bottle is opened once, over a tray, and closed.

Citric acid: H319 in 84.7 per cent of 4,373 ECHA reports and H335 in 23 per cent, with 359 of them finding it meets no criterion. Eye protection while weighing and while pouring.

The developer: metol is a skin sensitiser under a harmonised CLP entry and hydroquinone causes serious eye damage and is toxic to aquatic life. A litre of it is open on the bench for two hours. Gloves throughout, and no bare-handed recovery of a dropped cylinder.

Glass and a wet bench. The electrode is glass, it is expensive, and it breaks. It is the most likely casualty of the session.

Required PPE

• Single-use nitrile gloves, 0.2 mm, per HSE’s COSHH essentials sheet P1 for manual film development, changed if contaminated. The gloves page governs the choice.
• Eye protection throughout, not only while weighing: this is sixteen pourings into vessels of acid at chest height.
• An apron or overall, and dedicated labelled vessels that never see food use.
• An eyewash within reach, per the first-aid page, before the first bottle is opened.

Ventilation

Ventilation is a control here rather than a formality, because the session generates a gas on purpose. 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 minimum for this bench: a window open and a door ajar so air crosses the room, vessels covered between additions, and the draught carrying away from you. If you can smell sulfur dioxide — sharp, like a struck match, quite unlike the vinegar of an acetic bath — cover everything, leave for a few minutes and improve the airflow. Smell is not a control, but it is a usable alarm well below the limit.

Alternative route

The quarter-scale route. A reader whose workspace has poor ventilation, or who has less developer to spend, can run the session at 250 mL of bath and 6 mL additions. The ratio is identical, so the curve has the same shape and the same axis in millilitres per litre, and the gas is a quarter as much. What is lost is precision: 6 mL in a syringe carries more relative uncertainty than 25 mL in a cylinder.

Without a pH meter, the session works as a comparison rather than a measurement: narrow-range pH papers covering 2 to 7, read against their chart in daylight, resolve the two curves well enough to show that they cross. Record it as papers, not as pH, and write no decimal places the method cannot support.

Materials

Item	Quantity	Note
Distilled or deionised water	about 3 L	For the baths, the rinse bottle and the standards
pH 7.00 and pH 4.01 buffer standards	3 sachets or 150 mL	Two to calibrate, one unopened as a check
Lint-free tissue	a few sheets	Blotting the electrode, never wiping it
A scrap strip of 135 film on its spiral, or one sheet of the paper you print on	1	Sacrificial, for the carryover weighing: it is wetted with developer and thrown away, not processed
Squared paper, or a spreadsheet	1	The curve is plotted during the session, not afterwards

Chemicals

Chemical	Quantity	Form
Acetic acid, pre-diluted	18.0 g of acetic acid	Solution of stated strength: 64 g of a 28 % w/w product, or 22.5 g of an 80 % one
Citric acid, anhydrous	19.2 g	Solid; 21.0 g if your jar is the monohydrate
D-76 stock, from Part VIII’s lab	1.0 L, of which about 980 mL is spent	Solution: metol 2 g, sodium sulfite 100 g, hydroquinone 5 g, borax 2 g per litre
Commercial indicator stop bath concentrate	10 mL	Optional fourth track; makes 200 mL at 1+19
Sodium bicarbonate	about 50 g	For neutralising the spent acid streams before they are bottled

Equipment

A pH meter reading to 0.01 with its electrode in good order and its storage solution. Three 1.5 L vessels that can be covered — tall beakers with watch glasses, or wide-mouth jars with lids — plus a 250 mL clear glass jar for the indicator track. A 25 mL measuring cylinder or a 50 mL syringe. A balance reading to 0.1 g, checked per the balance SOP. A thermometer to 0.1 °C. A glass stirring rod for each acid vessel, never shared. A wash bottle of distilled water, a waste beaker, a timer.

Estimated cost

£, and it is the cheapest experiment in the course: about twenty grams of a solid acid, a similar mass of a dilute one, and a litre of developer that costs pennies in raw chemicals. The buffer standards are the largest single item and the meter is assumed from Part III. The planner carries what numbers the course has.

Estimated consumables cost

The cheapest experiment in the course: about twenty grams of a solid acid, a similar mass of a dilute one, and a litre of developer that costs pennies in raw chemicals. The buffer standards are the largest single item and the meter is assumed from Part III.

Consumed	This session	Sourced price	Cost this session
Citric acid, anhydrous	19.2 g, or 21.0 g as the monohydrate	£10.00 per 250 g of the monohydrate, checked 7 September 2026	£0.84 on the monohydrate route
Acetic acid, pre-diluted	18.0 g of acetic acid: 64 g of a 28 % product	£11.89 per 1 L of the 80 per cent acid, which is neither the strength nor the unit this row is written in	—
D-76 stock, from Part VIII	1.0 L, of which about 980 mL is spent	Costed in Part VIII’s mixing lab	—
Commercial indicator stop bath concentrate	10 mL, optional fourth track	£10.66–£12.18 per 500 ml of citric acid concentrate, diluted 1+19	£0.21–£0.24
Sodium bicarbonate	about 50 g, neutralising the spent acid streams	Supermarket item; the planner prices none	—
pH 7.00 and pH 4.01 buffer standards	3 sachets or 150 mL	None. A named price gap: a pH meter, buffer standards at 4.01 and 7.00, and narrow-range indicator papers	—
Distilled or deionised water	about 3 L	None. distilled-water carries a cost band and no dated figure	—
A scrap film strip or one sheet of paper	1, sacrificial, for the carryover weighing	A by-product, thrown away rather than processed	—
Lint-free tissue	a few sheets	None. glassware carries a cost band and no dated figure	—

The priced rows come to £1.05 to £1.08 for one run of this session, at the retail ranges read on 5 and 7 September 2026 and recorded in the laboratory planner. That is a floor, not a total: 4 of the 9 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 developer is the largest quantity on the page and the buffer standards are the largest purchase, and neither appears in the priced subtotal: the first because Part VIII paid for it, the second because it sits behind a named price gap. This is a session where the sourced figure is almost meaningless and the list is what matters.

Waste streams

1. Two spent acid baths, about 1.4 L each, containing acetate or citrate, hydrogensulfite, borate, metol and hydroquinone, at a final pH near 6.3. One labelled container, per the general waste SOP.
2. The spent control, about 1.1 L of dilute developer. The same container.
3. Electrode rinsings and buffer standards, a few hundred millilitres, dilute. The same container.
4. No silver-bearing waste. Nothing in this session touches an emulsion.

Streams 1 to 3 may be combined; the mixture is nearly neutral by construction, which is the whole point of the experiment. Nothing here goes anywhere near spent fixer.

Preparation

1. Open the laboratory per the opening SOP, and check the balance per the balance SOP.

2. Calibrate the meter per the calibration SOP, at 7.00 and 4.01 rather than 7 and 10: every sample here lives between pH 1.9 and 7, and you bracket the samples, not the round numbers. Record the slope and verify against the unopened third standard.

📓One reading you are entitled to distrust

The citric bath’s starting pH will be near 1.96, below the pH 4.01 standard and so outside the calibrated bracket. Record it, and record beside it that it is an extrapolation; if you have a pH 1.68 standard, calibrate at three points and say so. That is the difference between a number and a number you can defend, which is the subject of Part III’s laboratory.

3. Mix bath A, acetic. Weigh the pre-diluted acetic product to give 18.0 g of acetic acid — the mass, not the volume, so that no density is needed — into a tared beaker, transfer quantitatively into a 1 L vessel, rinse the beaker into it, and make up to 1.000 L. Follow the mixing SOP for the transfer. Label it per the labelling SOP.

4. Mix bath C, citric. Weigh 19.2 g of anhydrous citric acid (or 21.0 g of the monohydrate; read the jar) and dissolve to 1.000 L. It dissolves readily — the safety card gives 59 g per 100 mL at 20 °C — so no warming is needed.

5. Fill the control, bath W, with 1.000 L of the water you would actually use for a rinse, and stand all three in the same place so they reach the same temperature. Record it.

6. Measure your own carryover. This is what converts the curve into a capacity. Weigh a dry strip of film on its spiral, or a dry sheet of your paper, to 0.1 g. Immerse it in the developer, lift it out, drain it for the exact count you use in practice, and weigh it again. The gain in grams is the carryover in millilitres to better than five per cent, a working developer’s density being within a few per cent of water’s. Do it three times: the scatter is your uncertainty and it is worth as much as the mean.

7. Optional fourth track. Mix 200 mL of a commercial indicator stop bath at its stated dilution in the clear glass jar, and stand it where the light falling on it will not change during the session.

Procedure

The two acid baths are titrated in the same round, alternately, with one electrode. That is deliberate: it holds the meter, the operator and the clock identical between them, so a difference between the curves cannot be a difference between sessions.

1. Zero reading. Rinse, blot and read bath A. Rinse, blot, read bath C. Rinse, blot, read bath W. Record all three with the temperature. Photograph or note the colour of the indicator jar.
2. Add. Measure 25.0 mL of D-76 stock and add it to bath A slowly, down the inside wall, with the rod already stirring. Repeat for bath C. If you are running the indicator track, add 5.0 mL to its 200 mL, which is the same one-part-in-forty ratio.
3. Stir and settle. Stir each for 15 seconds, then cover. Wait 30 seconds.
4. Read. Rinse the electrode into the waste beaker, blot it on lint-free tissue — never wipe — stand it in bath A with the bulb fully covered and clear of the bottom, and wait until the reading stops moving rather than slows. Record. Rinse, blot, read bath C. Record. Note the indicator jar’s colour against a white card.
5. Repeat steps 2 to 4 for sixteen rounds — 400 mL into each acid bath — or until both have passed pH 6.0, whichever comes first. Sixteen rounds is set by the litre of developer you have and it is enough: it carries both baths past the pH at which an indicator dye first moves, past Kodak’s litmus endpoint, and past the steepest part of the acetic curve. Reaching the dye’s full turn near pH 6.8 would need a second litre and tells you nothing the shape has not already said.
6. The control gets four rounds only. Add 25 mL to bath W and read it in each of the first four rounds, then stop adding to it and record that you stopped. One litre of water has no reserve, so it will have said everything it has to say by round two; feeding it for the whole session would spend 300 mL of developer to watch a flat line.
7. Plot as you go. Put each point on squared paper as you take it. A curve drawn during the session catches a mistake while it can still be repeated; one drawn afterwards catches it when it cannot.
8. Final readings. With the last round done, read all three vessels once more after five minutes standing, to see whether anything is still drifting.
9. Close down per the closing SOP, returning the electrode to its storage solution.

✦The clock, and where to spend your time if it beats you

Two hours divides roughly as 35 minutes to open the lab, check the balance and calibrate the meter; 8 minutes for the carryover weighings; 12 minutes to mix three baths; 55 minutes for sixteen rounds at about three and a half minutes each; and 15 minutes to bottle the waste and close down. It is tight, and the way to make it fit is to have every vessel labelled, filled and standing at temperature before the first addition.

If the clock beats you: the information in a titration is in the bends, not in the flats. Between about pH 4.5 and 5.0 the acetic bath is at its flattest and you can widen to 50 mL steps without losing shape. Do not widen anywhere above pH 5.5 in either bath — that is where the curve turns, and it is the part you came for.

Developer budget. Sixteen rounds spend 400 mL into each acid bath, 100 mL into the control and 80 mL into the indicator jar: 980 mL of the litre, with no spare. Mix a second litre before you start if you want to carry the curves past pH 6.5.

Expected observations

The first addition is the biggest. Both baths jump about a whole pH unit on the first 25 mL and then settle into a crawl. That is a weak acid discovering that it has acquired a conjugate base: a fresh bath is a poor buffer and becomes a good one as it is used.

The curves cross. The predicted shapes are below. The citric bath starts 0.7 units lower, climbs more steeply through its first two ionisations, and meets the acetic curve at about pH 5.0 and 245 mL. Above that the two run together within a tenth of a unit. The plot runs to 500 mL because that is where the prediction becomes interesting again; a sixteen-round session stops at 400, which is past the crossing and past the steepest part of the acetic curve.

Predicted pH against added D-76 stock, for two baths matched in titratable protons

• Acetic acid, 0.300 mol/L (one proton)
• Citric acid, 0.100 mol/L (three protons)

Show the numbers behind this plot

Two rising curves against a horizontal axis of added developer from zero to 500 millilitres per litre of bath, and a vertical axis of pH from 1.5 to 7.5. The acetic acid bath at 0.300 molar begins at pH 2.63, rises steeply to 3.61 on the first 25 millilitres, then flattens into a long shallow climb through 4.30 at 100 millilitres, 4.80 at 200 and 5.04 at 250, before steepening again to 5.73 at 350, 6.22 at 400 and 6.55 at 450. The citric acid bath at 0.100 molar begins much lower at pH 1.96, climbs more evenly through 2.77 at 50 millilitres, 3.34 at 100, 4.48 at 200 and 5.01 at 250, then flattens relative to the acetic curve, reaching 5.98 at 350, 6.34 at 400 and 6.60 at 450. The two curves cross at about 245 millilitres and pH 5.0 and stay within a tenth of a unit of each other from there on. Two horizontal reference lines are marked: one at pH 5.2, where bromocresol purple is still yellow and the dye first begins to move, crossed at about 280 millilitres by the acetic bath and 267 by the citric; and one at pH 6.8, where that dye is fully purple, crossed at about 518 and 510 millilitres. The teaching point is that the bath which started a full pH unit more acidic reaches every one of those lines at essentially the same volume of developer.

Series	D-76 stock added per litre of bath (mL)	pH
Acetic acid, 0.300 mol/L (one proton)	0.00	2.63
Acetic acid, 0.300 mol/L (one proton)	25.00	3.61
Acetic acid, 0.300 mol/L (one proton)	50.00	3.93
Acetic acid, 0.300 mol/L (one proton)	75.00	4.14
Acetic acid, 0.300 mol/L (one proton)	100.00	4.30
Acetic acid, 0.300 mol/L (one proton)	125.00	4.44
Acetic acid, 0.300 mol/L (one proton)	150.00	4.57
Acetic acid, 0.300 mol/L (one proton)	175.00	4.68
Acetic acid, 0.300 mol/L (one proton)	200.00	4.80
Acetic acid, 0.300 mol/L (one proton)	225.00	4.92
Acetic acid, 0.300 mol/L (one proton)	250.00	5.04
Acetic acid, 0.300 mol/L (one proton)	275.00	5.17
Acetic acid, 0.300 mol/L (one proton)	300.00	5.32
Acetic acid, 0.300 mol/L (one proton)	325.00	5.50
Acetic acid, 0.300 mol/L (one proton)	350.00	5.73
Acetic acid, 0.300 mol/L (one proton)	375.00	5.99
Acetic acid, 0.300 mol/L (one proton)	400.00	6.22
Acetic acid, 0.300 mol/L (one proton)	425.00	6.41
Acetic acid, 0.300 mol/L (one proton)	450.00	6.55
Acetic acid, 0.300 mol/L (one proton)	475.00	6.66
Acetic acid, 0.300 mol/L (one proton)	500.00	6.74
Citric acid, 0.100 mol/L (three protons)	0.00	1.96
Citric acid, 0.100 mol/L (three protons)	25.00	2.44
Citric acid, 0.100 mol/L (three protons)	50.00	2.77
Citric acid, 0.100 mol/L (three protons)	75.00	3.05
Citric acid, 0.100 mol/L (three protons)	100.00	3.34
Citric acid, 0.100 mol/L (three protons)	125.00	3.64
Citric acid, 0.100 mol/L (three protons)	150.00	3.94
Citric acid, 0.100 mol/L (three protons)	175.00	4.22
Citric acid, 0.100 mol/L (three protons)	200.00	4.48
Citric acid, 0.100 mol/L (three protons)	225.00	4.74
Citric acid, 0.100 mol/L (three protons)	250.00	5.01
Citric acid, 0.100 mol/L (three protons)	275.00	5.28
Citric acid, 0.100 mol/L (three protons)	300.00	5.53
Citric acid, 0.100 mol/L (three protons)	325.00	5.76
Citric acid, 0.100 mol/L (three protons)	350.00	5.98
Citric acid, 0.100 mol/L (three protons)	375.00	6.17
Citric acid, 0.100 mol/L (three protons)	400.00	6.34
Citric acid, 0.100 mol/L (three protons)	425.00	6.48
Citric acid, 0.100 mol/L (three protons)	450.00	6.60
Citric acid, 0.100 mol/L (three protons)	475.00	6.69
Citric acid, 0.100 mol/L (three protons)	500.00	6.77

Computed, not measured. The curves come from the charge balance of each bath against D-76's published sulfite and borax content, using pKa values of 4.76 for acetic acid, 2.87, 4.35 and 5.68 for citric acid, 1.80 and 7.19 for the sulfurous system and 9.27 for boric acid, and assuming ideal behaviour. Your measured curves should have this shape and may sit a few tenths away from these numbers; where they differ is the interesting part of the report. 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.

The control does almost nothing after the first addition. One litre of water has essentially no buffer reserve, so 25 mL of a strongly buffered developer takes it straight to near the developer’s own pH and it barely moves again. Expect a reading somewhere in the region of 8.5 to 9.5.

!A model failure you should see, and should not paper over

The ideal-solution calculation behind the plotted curves puts pure D-76 stock at about pH 9.5. ILFORD publishes ID-11 stock — the same formula — at 8.60 to 8.70. That is nearly a whole unit of disagreement, and it is not a rounding error.

At least three things are unmodelled and all push the same way: at an ionic strength near 0.8 mol/L the activity coefficients are well away from one, so the concentration-based Henderson-Hasselbalch relation over-predicts; dissolved carbon dioxide from the air is an acid the model does not have; and the developing agents themselves are weak acids the model ignores.

So your control vessel is a measurement of a real solution, and the number in the plot is not. Record what you get, compare it with ILFORD’s published range and with the model, and say in your report which of the three you would bet on. That comparison is worth as much as either titration curve.

The indicator jar turns late, and slowly. Expect it to look unchanged for the first third of the session and to show a first hint of colour somewhere after pH 5. Note the round at which you first doubt the colour and the round at which you would actually change the bath in a real session: those two numbers are the useful ones.

What is happening chemically

Three reactions, in order of how much of the acid each consumes.

SO₃²⁻ + CH₃COOH → HSO₃⁻ + CH₃COO⁻

The sulfite, which is 98.7 per cent of D-76's acid demand

Every 25 mL of D-76 stock brings 19.8 mmol of sulfite, each of which takes one proton on its way to hydrogensulfite. That is what is actually being titrated. The borax contributes 0.26 mmol per addition and the developing agents essentially nothing at these pH values.

B(OH)₄⁻ + CH₃COOH → B(OH)₃ + CH₃COO⁻ + H₂O

The borate, which is the remaining 1.3 per cent

And, in the acid bath itself, the reaction that makes the curve a curve:

CH₃COOH ⇌ CH₃COO⁻ + H⁺

The acetate accumulating is what turns a weak acid into a buffer

The shape follows from those. At the start the vessel holds acetic acid with almost no acetate, so the Henderson-Hasselbalch logarithm is a large negative number, the pH sits well below the pK_a, and the first acetate produced moves it a long way. As acetate accumulates the ratio approaches one and the pH parks near 4.76, changing only logarithmically. Near the end the acid runs low, the ratio runs away, and the pH climbs steeply again.

The citric bath does the same thing three times over, at 2.87, 4.35 and 5.68. Because those constants are separated by only about 1.4 units, the three buffer regions overlap and the curve has no distinguishable steps — it looks like one broad, gentle climb rather than three plateaux, which is exactly what a polyprotic acid with closely spaced constants should look like and is worth pointing out in the report.

⚗Why the two baths end up in the same place

At pH 5.2, acetic acid is 1 ÷ (1 + 10^4.76−5.2) = 74 per cent ionised, so 0.300 mol/L has given up 0.223 mol of protons. At the same pH, the citric system averages 2.13 protons released per molecule, so 0.100 mol/L has given up 0.213 mol. The two numbers agree to within five per cent, and that agreement — not the starting pH, which differs by 0.7 units — is what the capacities follow.

This is the whole hypothesis in two lines of arithmetic, and it is why the experiment is worth doing: you can predict the answer, and then find out whether a real bath, a real developer and a real electrode agree.

Data to record

For every round: the round number, the cumulative volume of D-76 added, the pH of bath A, the pH of bath C, the temperature, and the indicator jar’s colour in words. For the session: the meter’s slope and check-standard reading before and after; the acetic product and its stated strength; whether the citric acid was anhydrous or the monohydrate; the age of the D-76; each bath’s weighed acid mass; and the three carryover weighings with their mean and spread.

📓The two numbers everything else hangs on

The carryover, in millilitres per film or per sheet, with the drain count you used. Without it the curve is a chemistry result and not a darkroom one.

The volume at which you would actually change the bath, decided by you at the bench and written down before you do any arithmetic. That is your operational definition of exhaustion, and every capacity you calculate afterwards is relative to it. Manufacturers have one too; they just do not publish it, which is why their capacity figures are not comparable with each other.

Analysis

1. Plot both titration curves on one pair of axes, pH against cumulative millilitres. Mark the crossing point if there is one.

2. Find the exhaustion point, three ways, and notice that they differ.

• The dye’s way. Read off the volume at which each curve passes pH 5.2, where a bromocresol purple indicator would begin to move. Its full turn at pH 6.8 is beyond a sixteen-round session — the prediction puts it near 510 mL — so extrapolate it from your own curve’s tail if you want it, and label it as an extrapolation.
• Kodak’s way. Read off the volume at pH 5, which is roughly where blue litmus stops turning red and therefore where the 1928 primer’s “before the bath becomes alkaline” test would call it.
• The steepest-slope way. Compute the change in pH per 25 mL for each interval and find the largest. That is the nearest thing to a mathematical endpoint, and on these curves it lands well above the other two.

3. Convert to a capacity. With V_exh the exhaustion volume in millilitres per litre of bath, and C the carryover you measured in millilitres per film or per sheet:

N = V(exh) ÷ C

Capacity of a stop bath

N is the number of films or sheets one litre of that bath will take. Do it for all three endpoints and report the range, not a single figure.

4. Compare with the published figures. ILFORD gives ILFOSTOP 15 films of 135-36, 60 resin-coated sheets of 20.3 × 25.4 cm or 30 fibre-base sheets per litre, unreplenished; Kodak’s SB-1 works out at 19.8 sheets of 8 × 10 inches per litre in both the 1928 primer and the 1949 handbook. Your bath is neither product, so compare magnitudes and ratios, not absolute figures. Ask three questions:

• Does your N land in the same decade as theirs, or is it out by a factor of five? A factor of five means a mistake in the carryover, not in the chemistry.
• Which endpoint definition brings your number closest to theirs? That is an inference about where the manufacturer drew its own line.
• ILFOSTOP’s fibre-base figure is exactly half its resin-coated one. Does your measured carryover for fibre paper come out at about twice your figure for resin-coated, if you measured both?

5. Test the hypothesis explicitly. State the two exhaustion volumes, the difference between them as a percentage of the acetic bath’s, and the difference in starting pH; then answer in one sentence whether capacity followed acid content or pH. If the citric bath came in early, estimate how many of its three protons were available by dividing its exhaustion volume by the acetic bath’s and multiplying by three.

6. Say what the uncertainty is. Sixteen additions of 25 mL from a cylinder good to ±0.5 mL accumulate to about ±2 mL in 400 if the errors are random — half a per cent, and negligible. The electrode’s ±0.02 is not negligible where the curve is flat, because a hundredth of a unit there is several millilitres of developer. Say which of your numbers is limited by which.

Troubleshooting

The pH will not settle. Give it longer: a dilute, poorly buffered solution equilibrates slowly at a glass membrane, and the control vessel is the worst offender. If a reading in a buffered bath still wanders after two minutes, suspect the junction — rinse thoroughly and check against the standard you kept back.

Both curves are far lower than predicted. Most likely more acid than you meant. Check whether your acetic product’s per cent is by mass or by volume, and whether your citric acid is the monohydrate sold as anhydrous.

Both curves are far higher. Either less acid than intended, or an aged D-76: sulfite is consumed by aerial oxidation, and an old stock demands less acid per millilitre, as Part VIII’s oxidation experiment measures directly.

A sharp smell after an addition. That is sulfur dioxide, and it means the addition went in too fast or into a bath that was not stirring, so a pocket of local low pH formed. Cover the vessels, improve the airflow, and add more slowly down the wall.

The indicator jar never changes colour. Check the light you are judging under. A safelight or a warm domestic lamp disguises a yellow-to-purple change badly; judge it against a white card in daylight and, if you cannot, record “could not judge” rather than a guess.

Clean-up

Rinse the electrode, blot it, and return it to its storage solution — not to distilled water, which damages it. Rinse the vessels, rods and cylinder into the waste container, then wash them. Wipe the bench and check under it: the commonest spill in this session is the waste beaker.

Storage

Nothing here is kept as a working solution: both acid baths have been deliberately spent and are waste. Unused stop bath, if you mix extra, keeps as the manufacturer states for a comparable product: ILFORD gives its concentrate five years in a full airtight bottle, twelve months in a half-full one, and the working strength seven working days. Store the citric acid jar dry and closed, away from strong oxidants, strong bases, metal nitrates and metals, and not in a metal container — the safety card records that it attacks metals. Keep the buffer standards capped and dated; an open sachet is not a standard.

Disposal considerations

The chemistry of what you have made is simple and it is the reason this experiment is cheap to finish. Streams 1 and 2 combined are a nearly neutral mixture of a spent weak acid and a spent developer: acetate or citrate, hydrogensulfite, borate, and the developing agents with their oxidation products. The acid has already done the neutralising that Kodak’s environmental guidance describes for a spent stop bath, where sodium bicarbonate is added slowly — slowly because the mixture foams as carbon dioxide comes off — in a ventilated place, with gloves, goggles and an apron. If a stream is still below about pH 5 when you bottle it, that is the procedure to use on it; add the bicarbonate a spoonful at a time and stop when the fizzing stops.

What neutralising does not do is remove anything. The metol and the hydroquinone are still there, and hydroquinone is notified as toxic to aquatic life. Bottle the combined stream, label it per the labelling SOP, and route it as general chemical waste per that SOP. Never combine it with spent fixer: acid on thiosulfate is the reaction the incompatibilities page calls the likeliest accident in a home darkroom.

Check your local regulations; they govern, they differ between authorities within one country, and they change. The disposal page sets out why that is a real limit on what this course can tell you rather than a formality.

Questions

1. Your two baths were matched on titratable protons and differed by 0.7 pH units at the start. Which quantity predicted the capacity, and what does that say about judging a bath by a pH reading?
2. The steepest part of your acetic curve is well above the pH at which an indicator dye would have turned. Which of the two is the better definition of exhaustion for a working darkroom, and why?
3. Take your measured carryover and ILFORD’s published 15 films per litre. What exhaustion volume would ILFOSTOP need to have to reach that figure with your carryover? Is it plausible against your curves?
4. The control vessel read close to the developer’s own pH after one addition. Explain, in terms of buffer capacity rather than pH, why a water rinse still does useful work in a process.
5. Your measured fibre-base carryover was not twice the resin-coated one. Give two explanations that do not require ILFORD to be wrong.

Further experiments

The paper developer run. Substitute a working-strength carbonate paper developer for the D-76. Its acid demand is 0.54 mol/L against D-76’s 0.80, and 78 per cent of it is carbonate rather than sulfite, so the same bath should take about half as many millilitres again — and you should be able to see carbon dioxide coming off, which the sulfite titration does not produce.

The blister question, which is still open. The first lesson records that this course found no measurement of how often a modern film blisters in an over-strong stop bath. You now have the arithmetic to design one: a carbonate developer, three stop baths at one, three and ten times normal strength, one film emulsion, and the strips examined dry by reflected light for the crater-like depressions Kodak’s 1928 primer describes. Run it with Part IX’s discipline about controls and identifiable strips and it would settle a piece of folklore with evidence.

The ILFOSTOP PRO question. ILFORD publishes two citric stop baths at the same dilution with capacities in the ratio 22 to 15. Titrate both against the same D-76 and see whether the ratio of their exhaustion volumes matches the ratio of their published capacities — a published capacity tested with nothing but a meter and a cylinder.

Check your understanding

Question 1. Bath A is 0.300 mol/L acetic acid and bath C is 0.100 mol/L citric acid. Why are these described as matched, when one is three times the concentration of the other?

• Because they have the same pH
• Because acetic acid has one ionisable proton and citric acid has three, so both baths hold 0.300 mol of titratable protons per litre
• Because they cost the same per litre
• Because both are weak acids and weak acids behave identically

Show the answer and why

Answer: Because acetic acid has one ionisable proton and citric acid has three, so both baths hold 0.300 mol of titratable protons per litre

Capacity is a count of protons available to neutralise the alkali arriving, not a count of molecules and not a pH. 0.300 mol/L × 1 proton and 0.100 mol/L × 3 protons are the same 0.300 mol of proton reserve. The pH is deliberately not matched — the citric bath starts about 0.7 units lower because its first pKa is 2.87 against acetic acid's 4.76 — and that mismatch is what makes the experiment a test rather than a demonstration: if pH predicted capacity, the citric bath would win, and it does not.

Question 2. Each 25 mL addition of D-76 stock brings 19.8 mmol of sulfite into the bath. Immediately after the first addition the acetic bath is near pH 3.6 and the citric bath near pH 2.4. Which vessel is the sulfur dioxide hazard, and by roughly what factor?

• The acetic bath, because acetic acid is volatile
• The citric bath, by about twelve times, because the fraction of sulfur(IV) present as dissolved sulfur dioxide is 1 ÷ (1 + 10^(pH − 1.80)) and that gives 19 per cent at pH 2.4 against 1.6 per cent at pH 3.6
• Both equally, since both received the same sulfite
• Neither, because the quantity is too small to matter

Show the answer and why

Answer: The citric bath, by about twelve times, because the fraction of sulfur(IV) present as dissolved sulfur dioxide is 1 ÷ (1 + 10^(pH − 1.80)) and that gives 19 per cent at pH 2.4 against 1.6 per cent at pH 3.6

Sulfurous acid's first pKa is 1.80, so the split between hydrogensulfite and dissolved sulfur dioxide is decided entirely by the pH the sulfite meets. Twelve times more of it is in the volatile form in the vessel that started lower, and the first addition is the worst moment in the session because that is when the pH is lowest and the sulfite is newest. The controls follow from the arithmetic rather than from a general caution: add slowly down the wall into a bath already stirring, cover between additions, and keep the through draught the ventilation standard specifies.

Question 3. You find the volume at which your acetic bath passes pH 5.2, pH 5.0 and its steepest slope, and get three different answers spanning nearly 200 mL. What is the right thing to report?

• The steepest-slope figure, since it is the only mathematically defined endpoint
• All three, with the endpoint definition attached to each, because "capacity" is meaningless until someone says where the line is drawn
• The average of the three
• The pH 5.2 figure, because that is where the dye turns

Show the answer and why

Answer: All three, with the endpoint definition attached to each, because "capacity" is meaningless until someone says where the line is drawn

This is the central methodological point of the session. The three numbers do not disagree about the chemistry; they disagree about the question. Kodak's 1928 capacity was measured to a litmus turn near pH 5, ILFORD's to whatever internal criterion it uses, and the steepest slope is a property of the curve with no photographic meaning at all. Averaging them mixes three definitions into one meaningless number. Reporting all three with their definitions is what makes your figure comparable with somebody else's — and it is why two published capacities for similar baths can differ by a factor of two without either being wrong.

Question 4. Your control vessel of plain water reads 8.9 after the first 25 mL of D-76 and 9.0 after the eighth. The ideal calculation says 9.5 and ILFORD publishes ID-11 stock at 8.60 to 8.70. What is the most defensible reading of that?

• The meter is faulty and the session should be repeated
• The measurement sits between the two, which is what you would expect: the model ignores activity coefficients at high ionic strength, dissolved carbon dioxide and the agents' own acidity, while the published figure is for a differently mixed batch of a nominally identical formula
• ILFORD's published figure must be wrong, since the calculation is from their own formula
• The water was contaminated

Show the answer and why

Answer: The measurement sits between the two, which is what you would expect: the model ignores activity coefficients at high ionic strength, dissolved carbon dioxide and the agents' own acidity, while the published figure is for a differently mixed batch of a nominally identical formula

A model that assumes ideal behaviour at an ionic strength near 0.8 mol/L will over-predict pH, and it has no term for the carbon dioxide the solution absorbs while it stands open or for the weak acidity of metol and hydroquinone. That is why the plotted curves on this page are labelled computed rather than measured. ILFORD's own sheet anticipates the other half of the answer by telling users to make their own control measurements from their own accurately mixed fresh solutions. A measurement landing between an idealised calculation and a manufacturer's range is the ordinary, healthy outcome, and saying so is better science than adjusting anything.

Question 5. You measure a carryover of 18 mL per 135-36 film and find your bath exhausts at 270 mL of D-76 per litre. ILFORD publishes 15 films per litre for ILFOSTOP. What have you learned?

• Nothing, because your bath is not ILFOSTOP
• That 270 ÷ 18 = 15 films per litre, so a bath of quite different composition reaches the same capacity — which supports the idea that the published figure is a statement about moles of acid rather than about the product
• That ILFOSTOP contains 0.300 mol/L of citric acid
• That your carryover measurement must be wrong, since the numbers agree too well

Show the answer and why

Answer: That 270 ÷ 18 = 15 films per litre, so a bath of quite different composition reaches the same capacity — which supports the idea that the published figure is a statement about moles of acid rather than about the product

The arithmetic is N = V(exh) ÷ C, and it lands on the published figure. What that supports is the general claim, not an identification: it says a capacity is set by the acid reserve divided by the alkali each piece of work brings in, and that two baths with the same reserve will have similar capacities whatever acid supplies it. Option 3 goes too far — the calculation constrains ILFOSTOP's proton reserve, not its concentration, because you do not know how many protons per molecule its acid delivers in the working range. Knowing exactly how much a coincidence like this licenses you to conclude is the difference between a result and an anecdote.

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