Alkalis, Buffers and pH

Under its formula for DK-50, Kodak Limited’s 1949 handbook prints a sentence that no other ingredient in a developer earns. Increasing or decreasing the quantity of the alkali makes it possible either to increase or decrease the contrast obtained in a given time of development, or to decrease or increase the time of development without affecting the contrast.

That is two levers from one substance, and it is why the alkali is the ingredient a formulator moves first. Everything else in a developer bottle is more or less fixed by its job: the agent has to be able to reduce silver, the preservative has to be there in quantity, the restrainer has to hold fog down. The alkali is the throttle. This page is about what is actually in your hand when you turn it.

The alkali does not develop anything

Part III established what pH is and the previous pages established what the developing agents are. Put the two together and the accelerator’s job is narrow and specific: it strips protons from the agent’s hydroxyl or amino groups, and the anion that results is the species that hands electrons to silver. Kodak’s 1928 primer states the dependence without 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.

Hydroquinone’s pKa of about 9.88 is why that dependence is so steep for one agent and so shallow for another, and the ionisation curve on the classical agents page is the picture of it: at a film developer’s pH only a few per cent of the hydroquinone is ionised, and every tenth of a unit you add is a large proportional change in that few per cent. Metol, whose amine is already neutral above about pH 8, barely notices.

The consequence is measurable in a manufacturer’s own time tables.

Activity against pH, from three ILFORD developers at their published pH

• DELTA 100 Professional at EI 100
• FP4 Plus at EI 125
• PAN F Plus at EI 50
• Kentmere PAN 100 at EI 100

Show the numbers behind this plot

Four rising sets of points, one per film, showing development rate against the published pH of three ILFORD powder developers. Rate is taken as the reciprocal of the development time to the same contrast, scaled so that ID-11 reads 1.00 for every film. PERCEPTOL, published at pH 7.68 to 7.82 and plotted at 7.75, gives 0.57 for DELTA 100 Professional, 0.71 for FP4 Plus, 0.46 for PAN F Plus and 0.72 for Kentmere PAN 100. ID-11, published at 8.60 to 8.70 and plotted at 8.65, is 1.00 for all four by construction. MICROPHEN, published at 8.67 to 8.93 and plotted at 8.80, gives 1.31 for DELTA 100, 1.06 for FP4 Plus, 1.44 for PAN F Plus and 1.06 for Kentmere PAN 100. Every film rises from left to right, so the developer held nearly a pH unit lower is roughly half to three quarters as fast, and the one held a little higher is faster again. The scatter between films at the same pH is large, about half as large as the whole effect, which the caption attributes to the three products differing in more than their alkali.

Series	Published pH of the fresh stock solution	Development rate relative to ID-11 at the same meter setting
DELTA 100 Professional at EI 100	7.75	0.57
DELTA 100 Professional at EI 100	8.65	1.00
DELTA 100 Professional at EI 100	8.80	1.31
FP4 Plus at EI 125	7.75	0.71
FP4 Plus at EI 125	8.65	1.00
FP4 Plus at EI 125	8.80	1.06
PAN F Plus at EI 50	7.75	0.46
PAN F Plus at EI 50	8.65	1.00
PAN F Plus at EI 50	8.80	1.44
Kentmere PAN 100 at EI 100	7.75	0.72
Kentmere PAN 100 at EI 100	8.65	1.00
Kentmere PAN 100 at EI 100	8.80	1.06

Measured, in the sense that every time and every pH is published by ILFORD on one sheet: the times target a Gbar of about 0.62, and each film here is listed at the same meter setting in all three developers, so these are times to the same contrast at the same speed. What it is not is a controlled experiment on pH. The three products differ in agent, preservative and everything else — MICROPHEN is a phenidone developer and ID-11 is not — and ILFORD publishes none of the formulas. Read the direction, which is unambiguous and holds for every film; do not read a slope off it.

Rule one of this course applies to that plot as much as to any formula: it shows a real published relationship and it does not isolate a variable. What it rules out is the idea that the pH difference between these products is a detail. Three quarters of a pH unit separates PERCEPTOL from ID-11, and PERCEPTOL takes roughly half again to twice as long to reach the same contrast.

Five alkalis, and what each one actually holds

An alkali in a developer is chosen for two numbers that are entirely independent: where it holds the pH, which is set by the pK_a of the conjugate pair it forms, and how much it holds, which is set by how many moles of that pair are in the litre. Part III proved that those two are separable; this page spends the rest of its length on what the separation costs and buys in a real formula.

Alkali	The pair it forms	pKa of that pair	Where you meet it
Borax	boric acid / borate, in equal amounts as supplied	9.27	D-76, 2.0 g/L
Sodium metaborate (Kodalk)	the same pair, but supplied as base only	9.27	DK-20 2 g/L, DK-50 10 g/L, DK-15 22.5 g/L
Sodium carbonate	hydrogencarbonate / carbonate	10.33	D-19b 48 g/L, D-72 67.5 g/L
Sodium hydroxide	none: it is free hydroxide	—	D-9 stock B, 52.5 g/L
Dibasic phosphate	dihydrogenphosphate / hydrogenphosphate, or hydrogenphosphate / phosphate	7.21 and 12.38	gold toning baths, not developers

The pK_a values are OpenStax’s Appendix H ionisation constants, converted: boric acid 5.4 × 10⁻¹⁰, carbonic acid 4.3 × 10⁻⁷ and 4.7 × 10⁻¹¹, phosphoric acid 7.5 × 10⁻³, 6.2 × 10⁻⁸ and 4.2 × 10⁻¹³.

Borax supplies both halves of its pair at once, which is what makes a plain borax solution a buffer with no second ingredient, and why 0.01 molal sodium tetraborate is a certified pH standard at 9.180 at 25 °C. Two grams per litre of the decahydrate, molar mass 381.4, is 5.24 × 10⁻³ mol/L; each formula unit yields two borate and two boric acid, so D-76 holds 0.0105 mol/L of each half and 0.021 mol/L of pair in total.

Metaborate supplies only the base half. That single difference is the whole of Kodalk. Ten grams per litre of the tetrahydrate, molar mass 137.86, is 0.0725 mol/L of borate with no boric acid beside it — seven times as much borate base as D-76 carries, and none of it pre-neutralised. The solution is therefore far more alkaline than a borax one and becomes a buffer as development generates the acid half in situ. To reach DK-50’s borate base with borax instead you would need 13.8 g/L, and you would drag in 0.0725 mol/L of boric acid with it, holding the pH down. That is what “intermediate in activity between sodium carbonate and borax” means arithmetically, and PubChem’s measured pH values for the tetrahydrate confirm the direction — 10.52 at 0.1 per cent w/v, 11.0 at 1 per cent, 11.4 at 4 per cent.

B(OH)₃ + 2 H₂O ⇌ B(OH)₄⁻ + H₃O⁺

Boric acid is a Lewis acid: it takes a hydroxide from water rather than giving a proton away, and borate is the conjugate base either way

Carbonate supplies the base half of a pair whose pK_a is a unit higher, and it is supplied in bulk: D-72’s 67.5 g/L of the anhydrous salt is 0.637 mol/L, thirty times D-76’s whole borate reserve. That is why a paper developer holds pH 10.3 to 10.6 through a tray session and a film developer does not hold 8.65 through a tank of film.

CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻

Why a carbonate solution is alkaline, and why it has a reserve: each carbonate that takes a proton leaves the rest of the carbonate untouched

Hydroxide supplies no pair at all. Kodak’s process tray developer D-9 is the extreme case in the corpus: stock B is 52.5 g of sodium hydroxide per litre, used one part to one part of a stock carrying 22.5 g/L each of hydroquinone, sodium bisulfite and potassium bromide. Even after the bisulfite has taken its share, the working bath holds of the order of half a mole of free hydroxide per litre. A borax developer at pH 8.65 holds 10⁻⁵·³⁵ = 4.5 × 10⁻⁶ mol/L. The two differ by five orders of magnitude. D-9 develops in about three minutes at 18 °C, and it carries about 11 g/L of potassium bromide — nearly three times D-19b’s dose and more than twenty times DK-50’s — for one reason. Without it, a bath that alkaline would develop the whole frame.

Phosphate is the instructive absence. No developer in this course’s corpus uses it; Kodak’s 1928 primer lists dibasic sodium phosphate with sodium acetate and borax as weak alkalis for gold toning. The reason is visible in the constants. A solution of the dibasic salt sits at about the mean of pK_a2 and pK_a3, ½(7.21 + 12.38) = pH 9.8 — squarely in the developer band. But 9.8 is also the point furthest from either pK_a, which is where a polyprotic buffer is at its weakest. Phosphate offers the right pH with the worst possible capacity at that pH, and a developer needs the opposite.

Where the reserve actually is, and the number this course inherits

Part III worked D-76 honestly and this page does not reopen it. Established there: the borate pair is 0.021 mol/L and the sulfite pair 0.79 mol/L, and ILFORD publishes ID-11 stock at pH 8.60 to 8.70. Inferred there, and still not confirmed by any photographic-chemistry source this course has read: that the borax sets the pH region while the sulfite carries most of the acid-absorbing reserve. That inference stands as an inference on this page too. It follows from two published concentrations and two published pK_a values; it has not been tested.

What can be added here is the shape of the same argument in the other formulas, because the proportions differ enormously.

Developer	Alkali reserve, mol/L	Sulfite, mol/L	Sulfite as a multiple of the alkali
D-76	borate pair 0.021	0.79	38×
DK-50	borate 0.072	0.24	3.3×
D-19b	carbonate 0.453	0.57	1.3×
D-72	carbonate 0.637	0.36	0.6×
D-23	none added	0.79	the sulfite is the alkali

Read down the last column and D-76 is revealed as the odd one out. In a paper developer the carbonate plainly is the buffer; in D-19b the two are comparable; in D-76 the sulfite outweighs the borax nearly forty to one. Whatever Part III’s inference is finally worth, it is a claim about this formula and its relatives and not about developers in general.

🧮Henderson and Hasselbalch, used on something you will actually mix

The relation is

pH = pK_a + log₁₀([A⁻] ÷ [HA])

Henderson and Hasselbalch

where [HA] is the acid member of the pair, [A⁻] its conjugate base, and pK_a minus the logarithm of the acid’s ionisation constant. In one sentence: the pH sits at the pK_a and moves by the logarithm of how lopsided the mixture is.

Worked, on a borax bath. Borax gives equal halves, so the ratio is 1, its logarithm is 0, and the pH is the pK_a: 9.27 by Appendix H, against 9.18 measured for the 0.01 molal standard at 25 °C. The nine-hundredths of a unit between those is a reminder that a computed pH is not a measured one.

Worked, on the acid load. Suppose the film puts 0.005 mol/L of acid into a litre of D-76. Of the 0.0105 mol/L of borate present, 0.005 becomes boric acid, so the ratio falls from 10.5 : 10.5 to 5.5 : 15.5 and the pH falls to 9.27 + log(5.5/15.5) = 8.82. Half a unit, from a load a carbonate developer would not notice — and this is exactly why the same arithmetic on the sulfite pair matters, since 0.005 mol/L against 0.79 mol/L of sulfite is almost nothing.

Two honest limits, both inherited from Part III. The relation assumes the amounts that ionise are small compared with the amounts you weighed, which fails as a pair runs out. And at the ionic strength of a 0.8 mol/L sulfite bath the ideal-solution assumption behind it is doing more work than the chemistry, so these are directions and magnitudes, not second decimals.

The pH of real developers, and what the numbers say

Only manufacturers measure the products they will not describe, so this table is short and every row is somebody’s published measurement of their own solution.

Solution	Published pH	What it implies
ILFORD PERCEPTOL stock	7.68–7.82	below hydroquinone’s useful range; an extra-fine-grain developer bought with time
KODAK XTOL working solution	8.20 ± 0.05	an ascorbate developer, and Kodak states it contains no hydroquinone
ILFORD ID-11 stock	8.60–8.70	the D-76 family’s working region
ILFORD MICROPHEN stock	8.67–8.93	a shade higher, and ILFORD attributes MICROPHEN’s fine grain to its low alkalinity
ILFORD BROMOPHEN stock	10.30–10.50	a paper developer, sitting on the carbonate pKa
ILFORD MULTIGRADE at 1+9	10.45–10.55	the same region, from a liquid concentrate
ILFORD PQ UNIVERSAL at 1+9	10.48–10.58	the same again

Two things fall out of that column. The first is that the film developers are packed into one pH unit and the paper developers into another, with nothing published in between; a full unit and a half separates the two families. The second is that every paper developer here sits within a quarter of a unit of 10.33, the second pK_a of carbonic acid, which is the pH at which a carbonate buffer is at its absolute strongest. That is not a coincidence and it is not chosen for the pH — it is what you get when you put a lot of carbonate in a litre and let it find its own level.

✕The figure this course cannot give you

There is no published pH here for D-76, D-23, D-19b, DK-50 or Rodinal. Kodak’s data sheets give times, capacities and keeping and no pH at all; the current Adox Rodinal datasheet, as the alternative agents page records, carries no extractable text in the copy this course mirrored. ID-11’s 8.60 to 8.70 is used throughout this part as the best available proxy for the D-76 family, and it is a proxy: ILFORD does not publish ID-11’s formula either, so nobody outside Mobberley can say how close the two are.

One formula is missing for a different reason. Kodak published a buffered variant of D-76, D-76d, in which some of the borax is replaced by a borax-and-boric-acid pair so that the pH holds better in use. It is named in the research plan for this page and it is not in this course’s corpus: the J-1 publication that would carry it was mirrored, and the copy the course holds has no extractable text. The variant is therefore mentioned and not printed, because Rule 6 does not permit reconstructing a formula from what its name implies.

The lab at the end of this part mixes D-76 and D-23 and measures both, which is how the course intends to close the gap on the pH figures — with its own measurement, labelled as its own.

Why a carbonate developer blisters a film and a metaborate one does not

Kodak led its Kodalk announcement not with the pH but with a fault, and the fault is the clearest example in this part of a chain running from a molecule to a ruined negative.

Carbon dioxide inside the gelatin, and the alkali that cannot make any

1. 1Swollen gelatin, carried into an acid bath — weakest exactly when the gas arrives: warm, wet and softened by the alkali it has just come from
2. 2Carbonate meets acid — CO3^2- + 2 H+ gives water and carbon dioxide — a gas, generated inside the layer
3. 3The blister — a bubble that cannot escape sideways lifts the emulsion off the base; on drying it leaves a permanent lens-shaped mark
4. 4Borate meets acid — B(OH)4- + H+ gives boric acid, which stays dissolved; there is nothing to lift the layer

Drawn to show the mechanism, not measured: the layer thicknesses are readable rather than to scale, and the bubble is drawn far larger than a real one. The chemistry and the consequence are Kodak Limited's 1949 statement that films developed in a Kodalk developer will not blister in an acid fixing bath even at high temperatures, because the alkali does not evolve carbon dioxide on acidifying.

CO₃²⁻ + 2 H⁺ → H₂O + CO₂

The blistering reaction, run inside a swollen gelatin layer instead of in a beaker

Kodak’s second claim for the same substance is quieter and worth the same attention: less tendency to precipitate aluminium sulfite from fixing baths containing alum. Carried-over alkali is what pushes a hardening fixer out of its acid window, and a developer with less carbonate alkalinity in it carries less over. The boric acid page treats the same problem from the fixer’s side; both are answers to the same collision.

What moves the pH in use, and how the manufacturers answer it

Three loads act on a developer’s buffer, and Part III drew them: the acid released by development itself, which scales with the film put through; carryover of acid on wet film from a stop bath; and carbon dioxide from the air, which is an acid on any carbonate bath and is why a half-empty bottle is worse than a full one for reasons beyond oxidation.

None of the manufacturers publishes a drift figure. All of them publish a replenisher, and the replenishers say the same thing four times over.

Developer	Its alkali	The replenisher’s alkali
D-76	borax 2.0 g/L	D-76R, borax 20.0 g/L
DK-20	Kodalk 2.0 g/L	DK-20R, Kodalk 20.0 g/L
D-23	none added	D-25R, Kodalk 20.0 g/L
D-19b	carbonate 48 g/L	D-19bR, carbonate 48 g/L plus 7.5 g/L of sodium hydroxide

The first two are tenfold. The third is the striking one: D-23 has no alkali in it beyond the sulfite, and its replenisher introduces one that was never in the developer. The fourth adds free hydroxide to a bath that already had half a mole of carbonate per litre. Four manufacturers’ answers to “what does a working tank run out of first”, and all four say the alkalinity.

✕Bicarbonate instead of carbonate

It is an easy substitution to make: the tubs look alike, the names differ by three letters, and one of them is in every kitchen. Kodak’s 1928 primer disposes of it in a clause — bicarbonate is practically useless as an alkali in photography — and adds that photographic carbonate is deliberately calcined until practically none of the bicarbonate survives, because leaving some behind is the manufacturing fault that matters.

The arithmetic says why. A 0.1 molar bicarbonate solution measures pH 8.3; a carbonate bath sits near 10.3. Two units is a hundredfold difference in hydroxide, and it lands in exactly the wrong place on hydroquinone’s ionisation curve: at 10.3 about two thirds of the hydroquinone is ionised, at 8.3 about two per cent. The negatives or prints come out thin and flat, no amount of extra time retrieves them, and nothing in the appearance of the bath announces the error. Weigh from a labelled tub, and if a tub is unlabelled it is not a photographic chemical.

Measuring the pH of a developer, as opposed to measuring a pH

The Part III laboratory session owns the instrument: the glass electrode, the constant K that gathers the reference and junction potentials and drifts from day to day, two-point calibration, and the difference between the slope correction that automatic temperature compensation performs and the sample’s own chemistry, which it cannot touch. Four things are specific to a developer.

You are measuring near or above the electrode’s honest limit. The response equation is quoted as valid over roughly pH 0.5 to 9, and alkaline error grows above about pH 9. A film developer at 8.65 is at the edge of that; a paper developer at 10.5 is well past it. Bracket the sample: calibrate at 7 and 10 for a paper developer, at 4 and 7 for a stop bath, and never extrapolate.

The sample’s own pH moves with temperature more than you expect. The borate standard’s certified value falls from 9.464 at 0 °C to 9.011 at 50 °C, about ten times as much movement as the phthalate standard shows over the same range — and borate is exactly the chemistry a film developer’s alkali uses. Record the temperature with every reading or the reading means nothing.

The ionic strength is not the buffer sachet’s. A litre of D-76 carries 0.79 mol/L of sulfite. The junction potential in a solution like that is not the junction potential in a dilute standard, and it is inside K. This is one more reason to treat a developer pH as a repeatable number of your own rather than as an absolute.

Compare like with like, and ILFORD says so. Its own sheet publishes the pH ranges above and then adds that the figures were obtained under controlled laboratory conditions, may differ from measurements users make in their own working areas, and that users should make their own control measurements from their own accurately mixed fresh solutions for later comparison. That is the correct use of a developer pH: a fingerprint of a batch you mixed, logged on the day, to be compared against the same measurement on the same meter later. It is not a purity test.

The borates, and the constraint that comes with them

Two of the four alkalis on this page are borates, and borate is the one class here with a classification serious enough to change how it is bought and handled.

The aggregated ECHA notifications on PubChem classify borax Danger on H360, may damage fertility or the unborn child, in 93.2 per cent of the 2,865 company reports that classify it, with H319 in 12.6 per cent. Sodium metaborate tetrahydrate carries H319 in 82.6 per cent of its reports, H360 in 45 per cent and the weaker H361 in 26.2 per cent — from only 149 reports across 7 notifications, so those percentages are thin and the notifiers disagree with each other about whether the reproductive hazard is established or suspected. HSE’s EH40 sets workplace exposure limits for the dust: 5 mg/m³ for the decahydrate, 1 mg/m³ for the anhydrous and pentahydrate forms.

The borax page sets out why that makes it Level B in this course’s rubric despite being unremarkable in every other respect, and the control is dust discipline: weigh without raising dust, over a tray, nitrile gloves and eye protection throughout, hands washed before eating, food kept out of the darkroom. A photographer who is pregnant, breastfeeding or trying to conceive should read that page before opening the tub.

!Purchasing: what the course can and cannot tell you

The research plan for this page asked for the sale restrictions that follow from the borate classification in the EU and the UK. No source in this course’s corpus establishes them. The classification is verified, from PubChem’s aggregation of the ECHA notifications and from EH40; the downstream question of what a retailer may sell to a private buyer, in which quantity and with what labelling, is a matter of regulation this course has not read and will not summarise from memory.

What is safe to say is what follows from the classification alone, and it is enough to act on. A reproductive-toxicity classification is the kind that drives consumer-supply rules, availability varies by market and changes, and if borax or metaborate is hard to obtain where you are, that is the likely reason rather than a supplier’s whim. Do not substitute another alkali gram for gram to get round it. The metaborate page gives the one substitution the chemistry supports, between the hydrates and the anhydrous salt, and the factors are large: 10 g of the metaborate tetrahydrate is 4.77 g of the anhydrous salt, and 2 g of borax decahydrate is 1.06 g of anhydrous borax. Weigh the wrong hydrate and the alkalinity is out by nearly a factor of two, which by DK-50’s own footnote is a change in both contrast and time.

Recap

• The alkali is the throttle, and Kodak states the two levers explicitly. More of it raises the contrast reached in a fixed time, or shortens the time needed for a fixed contrast.
• It works by ionising the agent, which is why the effect is steep for hydroquinone at pK_a 9.88 and shallow for metol, whose amine is done by pH 8.
• Two independent numbers describe any alkali: the pK_a of the pair it forms, which sets where it holds, and the moles of that pair per litre, which set how much it holds. Borax 9.27, carbonate 10.33, hydroxide no pair at all, phosphate the right pH at its weakest capacity.
• Metaborate is borate without its acid half, which is why 10 g/L of Kodalk is a stronger alkali than 2 g/L of borax by far more than the weights suggest, and why it makes no carbon dioxide to blister a film in an acid fixer.
• Published developer pH values fall in two clusters — film developers between 7.7 and 8.9, paper developers between 10.3 and 10.6, the latter sitting on the carbonate pK_a.
• The replenishers are the manufacturers’ statement of what runs out, and in four formulas out of four it is the alkalinity.
• A developer pH reading is a fingerprint of your batch, taken at a recorded temperature on a two-point calibration that brackets the sample, and compared with your own earlier readings.

Check your understanding

Question 1. D-76 carries 2.0 g/L of borax (M = 381.4) and DK-50 carries 10 g/L of sodium metaborate tetrahydrate (M = 137.86). How much more borate base does DK-50 hold, and why is the difference larger than the weights suggest?

• Five times, in proportion to the weights
• About seven times, because borax supplies only half its borate as base and the metaborate supplies all of it
• About fourteen times, because metaborate has twice as many borons
• The same, because both are borates

Show the answer and why

Answer: About seven times, because borax supplies only half its borate as base and the metaborate supplies all of it

Borax dissolves to give equal amounts of boric acid and borate, so 5.24 × 10⁻³ mol/L of borax gives 0.0105 mol/L of borate base. Sodium metaborate gives 10 ÷ 137.86 = 0.0725 mol/L, all of it base. That is 6.9 times as much, from five times the weight, and the extra factor is the missing acid half. It also explains the two behaviours Kodak claims for Kodalk in one breath: more alkalinity per gram than borax, and still a buffer rather than free hydroxide, because the acid half is generated in situ as development releases acid.

Question 2. A student mixes a paper developer and reaches for sodium bicarbonate because the carbonate tub is empty. What happens, and what does the negative or print look like?

• Nothing much: bicarbonate and carbonate are the same buffer pair
• The bath sits near pH 8.3 instead of 10.3, so almost none of the hydroquinone is ionised; prints come out thin and flat and extending the time does not fix them
• The bath becomes more alkaline and the prints fog
• The developer will not dissolve

Show the answer and why

Answer: The bath sits near pH 8.3 instead of 10.3, so almost none of the hydroquinone is ionised; prints come out thin and flat and extending the time does not fix them

CAMEO measures a 0.1 molar bicarbonate solution at pH 8.3; a carbonate bath holds near 10.3. That is a hundredfold difference in hydroxide, and it lands on the steep part of hydroquinone’s ionisation curve — roughly two thirds ionised at 10.3, about two per cent at 8.3. Kodak’s 1928 primer states the conclusion flatly: bicarbonate is practically useless as a photographic alkali, and photographic carbonate is calcined until practically none of it remains. The failure is silent, because the bath looks and smells identical, which is the argument for weighing only from labelled tubs.

Question 3. You measure a fresh paper developer at pH 10.42 against a published range of 10.45 to 10.55, using a meter calibrated at pH 7 alone. Which reading of that result is sound?

• The developer is under-alkaline and needs more carbonate
• The result is uninterpretable as an absolute: a one-point calibration corrects only the offset and assumes the ideal slope, and at pH 10.4 the electrode is also in its alkaline-error region
• The meter is faulty
• The developer has already been used

Show the answer and why

Answer: The result is uninterpretable as an absolute: a one-point calibration corrects only the offset and assumes the ideal slope, and at pH 10.4 the electrode is also in its alkaline-error region

Part III’s laboratory session shows what a one-point calibration misses: it finds the offset and assumes 59.16 mV per pH unit, so an ageing electrode running at 52 mV per unit reads about 0.36 low at pH 10. Add alkaline error, which the same source puts above about pH 9, and a three-hundredths discrepancy carries no information at all. Calibrate at 7 and 10 to bracket the sample, record the temperature, and then treat the number as a repeatable fingerprint of your own batch rather than as a verdict on the manufacturer’s.

Question 4. Kodak Limited says a film developed in a Kodalk developer will not blister in an acid fixing bath even at high temperatures. What is the mechanism, and why does warmth make the carbonate case worse?

• Metaborate hardens the gelatin, and warm gelatin is harder
• Acid meeting carbonate inside the swollen layer makes carbon dioxide gas, which lifts the emulsion; borate meeting acid makes dissolved boric acid instead, and warmth softens the gelatin and lowers the gas solubility at once
• Metaborate has a lower pH, so less acid is neutralised
• The acid fixer cannot penetrate a metaborate-developed film

Show the answer and why

Answer: Acid meeting carbonate inside the swollen layer makes carbon dioxide gas, which lifts the emulsion; borate meeting acid makes dissolved boric acid instead, and warmth softens the gelatin and lowers the gas solubility at once

The reaction is CO₃²⁻ + 2 H⁺ → H₂O + CO₂, and the thing that makes it a fault rather than a fizz is where it happens: inside a warm, swollen, alkali-softened gelatin layer, where a bubble cannot escape sideways and lifts a dome of emulsion off the base. Borate’s equivalent, B(OH)₄⁻ + H⁺ → B(OH)₃ + H₂O, produces a dissolved molecule. Warmth attacks from both sides, softening the layer and reducing the solubility of the gas in it, which is why Kodak’s tropical formula DK-15 is a metaborate developer.

Question 5. Kodak’s D-76 takes 2 g/L of borax and its replenisher D-76R takes 20 g/L; D-23 has no added alkali at all and its replenisher D-25R takes 20 g/L of Kodalk. What are the manufacturers telling you?

• That a replenisher is simply a stronger version of the developer
• That what a working tank runs out of first is its alkalinity — so much so that a formula with no added alkali still needs one put back
• That borax and Kodalk are interchangeable
• That the developing agent is consumed fastest

Show the answer and why

Answer: That what a working tank runs out of first is its alkalinity — so much so that a formula with no added alkali still needs one put back

A replenisher is not the developer topped up; it is a formula designed to put back what the bath spends, at the rate it spends it. Compare the columns and the sulfite is unchanged, the agents rise by half, and the alkali rises tenfold — and D-25R goes further, introducing an alkali D-23 never contained, because the mild alkalinity of the sulfite alone is what the working bath is losing. This is also why replenishment is not the same as dilution: adding fresh working-strength developer restores volume and dilutes the bromide, but leaves the replenished bath a weaker version of a used one.

Question 6. Why is a dibasic phosphate solution, which sits near pH 9.8, a poor choice of alkali for a film developer even though 9.8 is in the right region?

• Phosphate precipitates silver
• Because pH 9.8 is the midpoint between phosphoric acid’s second and third pKa values, which is exactly where a polyprotic buffer has its least capacity
• Phosphate is not soluble enough
• Phosphate reacts with sulfite

Show the answer and why

Answer: Because pH 9.8 is the midpoint between phosphoric acid’s second and third pKa values, which is exactly where a polyprotic buffer has its least capacity

An amphiprotic salt sits at about the mean of the pKa values either side of it: ½(7.21 + 12.38) = 9.8 from OpenStax’s Appendix H constants. A buffer pair is strongest at its own pKa and useful for roughly a unit either side, so the midpoint between two of them is the worst place to be — the right pH with the least reserve, which is the opposite of what a developer needs. Kodak’s 1928 primer accordingly lists dibasic sodium phosphate among the weak alkalis for gold toning, where only a mild alkalinity is wanted and nothing generates acid, and not among developer alkalis at all.

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