Sulfite: Preservative, Buffer, Silver Solvent

D-76 carries 100 g of sodium sulfite against 5 g of hydroquinone, 2 g of metol and 2 g of borax. By mass the preservative outweighs the other three put together by eleven to one, and it is ninety-two per cent of the dry weight of the whole formula.

The easy answer is that sulfite is the preservative and a developer needs protecting from the air. That answer is true, incomplete, and — as the arithmetic below shows — cannot possibly account for a hundred grams. Sulfite is doing three separate jobs in that litre, and the whole difficulty of formulating with it is that you cannot change one without changing the other two.

Three jobs, one ion

1. 1Oxygen scavenger — oxidised to sulfate instead of the agent; sulfate is not a preservative and the sulfite is gone
2. 2Acceptor for the oxidised agent — either gives the agent back, or takes it away for good as a colourless sulfonate
3. 3Silver halide solvent — finer grain, a little less speed, and dichroic fog if it goes too far
4. 4Mild alkali and buffer — the reason D-23 needs no alkali at all, and the reason bisulfite is a lever

One ion, four consequences, no independent controls. The drawing is a map of the page rather than a mechanism.

Job one: taking the oxygen — and why that cannot be the reason for 100 g

Part III established the setting: oxygen sits at +1.229 V, above everything in the bottle, and a developing agent is by design a substance with a great affinity for it. Kodak’s primer states the remedy in one line — sodium sulfite has a very strong affinity for oxygen, is easily oxidised to sulfate, and so protects the developer from the oxygen of the air.

2 Na₂SO₃ + O₂ → 2 Na₂SO₄

Sacrificial oxidation: the primer writes it with atomic oxygen; this is the same statement balanced with the molecule

Sulfate does nothing at all. Every route by which sulfite protects a developer ends with sulfite becoming sulfate, and the primer draws the shelf-level conclusion: a tub that has effloresced badly is to be viewed with suspicion, since the sulfate is not a preservative.

🧮The molar arithmetic that makes the rest of this page necessary

Work D-76 out in moles rather than grams, using the molar masses from the encyclopaedia: sodium sulfite 126.05, hydroquinone 110.11, metol as the hemisulfate 344.39.

n = 100 g/L ÷ 126.05 g/mol = 0.793 mol/L

Sulfite in D-76

n = 5 g/L ÷ 110.11 g/mol = 0.0454 mol/L

Hydroquinone in D-76

n = 2 × (2 g/L ÷ 344.39 g/mol) = 0.0116 mol/L

Metol in D-76, counted as the developing base rather than the salt

Total developing agent: 0.057 mol/L. Sulfite: 0.79 mol/L.

The sulfite is present at about fourteen times the molar quantity of everything it is supposed to be protecting. Take the ratio against hydroquinone alone and it is seventeen to one.

That is the number this page exists to explain. Sacrificial protection is a stoichiometric job: a mole of sulfite intercepts a mole’s worth of oxidation, and there is no argument from oxygen chemistry that gets you to fourteen equivalents. Kodak’s own shelf lives make the same point from the other end — a full, tightly closed bottle of D-76 keeps six months and a half-filled one two, so the quantity of oxygen a bottle actually meets is set by the headspace and not by the sulfite. If a hundred grams were about oxygen, ten would do.

Job one and a half: what happens to the oxidised agent

Protection has a second half that a scavenger alone does not cover. The developer’s own product, quinone, is an oxidising agent in its own right, and Kodak’s primer gives it two possible ends in a sulfite solution — in two different chapters, and they are not the same end.

Route one: the agent comes back. “If we add sulphite to quinone, the quinone oxidizes the sulphite to sulphate and is itself reduced again to hydroquinone.”

C₆H₄O₂ + SO₃²⁻ + H₂O → C₆H₆O₂ + SO₄²⁻

Regeneration — the developing agent restored at the sulfite's expense

Route two: the agent is taken away for good. In the chapter on the useful life of developers the same book says something quite different: “In the presence of sodium sulphite, however, the oxidation products of hydroquinone consist of hydroquinone mono- and disodium sulphonates which are colorless.”

C₆H₄O₂ + HSO₃⁻ → C₆H₅O₅S⁻

Sulfonation — the course's own balancing of the product Kodak names, not a mechanism

⚗Reading two routes that the primer never reconciles

The two accounts predict different futures for the same bottle. On route one the sulfite is a regenerator and the developing agent is conserved until the sulfite runs out. On route two the sulfite is a terminator: sulfonation removes the quinone permanently, so the agent is consumed and the solution nevertheless stays clear, because the sulfonates are colourless.

Kodak states both and reconciles neither, and this course will not reconcile them either. What can be said is which is more likely to be operating where. Route one plainly happens: the 1924 primer records that adding a little bisulfite to a browned hydroquinone solution bleaches the colour out, which is a colour change you can perform on a bench. Route two is the primer’s own explanation of a practical observation, and it is the observation that matters most:

The fact that an old Elon-hydroquinone developer is colorless is, therefore, no indication that the original developing power is unimpaired.

A brown developer is certainly spent. A colourless one is undiagnosed. That asymmetry is the single most useful thing on this page, and it is why the course’s answer to “is this still good?” is a fogged-strip activity test rather than a look at the bottle.

Two things remain unsourced and are not asserted here. Whether hydroquinone monosulfonate is itself a developing agent — the photographic literature says it is, and no document this course holds says so — and which species actually adds to which, since the primer names a product and gives no mechanism. The equation above is the course’s own balancing of Kodak’s named product and is labelled as such. Part III recorded sulfonation as a route it could not source and deferred it to this part; it is now sourced, and the next question along is not.

The ten per cent rule, and what changes between 5 and 100 grams

Here is the sourced fact that reframes the whole quantity question, and it is not about the developing agent at all. From the primer’s chapter on keeping:

A plain solution of sodium sulphite oxidizes readily in contact with air at a concentration below 10%, but above this concentration it oxidizes very slowly.

Sulfite protects itself better when it is concentrated. The primer’s instruction follows directly: stock solutions containing sulfite, alone or with a developing agent, should be made at around 10 per cent w/v for maximum keeping. D-76’s 100 g per litre is exactly 10 per cent w/v. That is one reason for the number, and the third job below is the other; the oxygen argument is not among them.

The primer adds the exception that the previous page made a mixing rule of: “owing to the relative insolubility of Elon in a sodium sulphite solution, it is not possible to prepare such stock solutions with Elon.” There is no concentrated metol-and-sulfite stock, at any strength, ever.

Sulfite	Molarity	Keeping	Solvent action	What the negative shows
5 g/L (0.5% w/v)	0.040 mol/L	below the primer’s 10% threshold, so the sulfite itself oxidises readily	negligible	staining agents stain; grain as the emulsion made it; developer life short
25 g/L (2.5% w/v)	0.198 mol/L	still in the region the primer calls readily oxidised	slight	a working middle: preservative enough for a tray, well short of where grain changes
100 g/L (10% w/v)	0.793 mol/L	the primer’s own optimum	the fine-grain regime of D-76 and D-23	less clumping, finer grain, a little less effective speed
Saturated, about 220 g/L at 20 °C	1.75 mol/L	above the optimum, no further benefit sourced	past the useful range	dichroic fog becomes a real risk

The saturation figure is the ILO-WHO Chemical Safety Card’s 22 g per 100 mL at 20 °C, and it matters practically: a 10 per cent w/v solution is less than half-saturated, so it mixes without a struggle and will not crystallise in a cold darkroom, while a formula asking for much more is asking for trouble at 15 °C.

!Anhydrous, crystals, and the factor of two

Sulfite is sold in more than one state and the difference is not small. The heptahydrate, Na₂SO₃·7H₂O, crystallises from cooled solution, and Kodak’s primer states that when pure it holds 50 per cent of dry sulfite — so one part of the anhydrous salt equals two parts of the crystals. Kodak’s own formulas print both columns for exactly this reason: D-76 is 16 oz of the crystals or 8 oz of the anhydrous per 80 oz.

Weigh crystals against a recipe written for the anhydrous salt and you have halved the sulfite, which by the table above moves the developer out of the fine-grain regime, out of the primer’s keeping optimum, and towards staining. The water of crystallisation arithmetic is Part III’s; the consequence is this part’s. Kodak Limited’s 1949 handbook prefers the anhydrous salt on three grounds — greater stability, ease of solution, smaller bulk in storage — and a modern packet is almost always anhydrous, but the label is the authority, not the habit.

Job two: a mild alkali, and half of a buffer pair

Kodak’s primer explains the alkalinity of a sulfite solution in a clause: soda is a strong base and sulfurous acid a weak one. The sulfite ion therefore takes a proton from water.

SO₃²⁻ + H₂O ⇌ HSO₃⁻ + OH⁻

Why a sulfite solution is alkaline: the conjugate base of a weak acid hydrolyses

That is enough alkali to run a developer with no other alkali in it, and Kodak’s D-23 is the proof: 7.5 g of metol and 100 g of sodium sulfite in a litre, nothing else, developing in about 18 minutes at 18 °C. It is about as simple as a working developer gets, and it exists because job two is real.

Add the conjugate acid and you have a buffer. OpenStax’s Appendix H gives sulfurous acid Ka₂ = 6.4 × 10⁻⁸, so pKa₂ = 7.19, and a sulfite/bisulfite mixture buffers around there.

Where a sulfite/bisulfite mixture sits, and what D-25 does with it

• pH of the mixture

Show the numbers behind this plot

A curve falling from left to right, giving the pH of a sulfite and bisulfite mixture against the percentage of the total sulfur present as bisulfite. With five per cent as bisulfite the pH is 8.47; with ten per cent, 8.14; with twenty per cent, 7.79; with fifty per cent, 7.19, which is the second dissociation constant of sulfurous acid and the flattest part of the curve; with seventy per cent, 6.82; and with ninety per cent, 6.24. One point is marked: Kodak's developer D-25, which is D-23 plus fifteen grams of sodium bisulfite per litre, has fifteen point four per cent of its sulfur as bisulfite and therefore a computed pH of 7.93. A horizontal guide line is drawn at pH 7.75, labelled as the published pH of ILFORD PERCEPTOL stock, an extra fine grain developer; the caption is explicit that the coincidence of value is not a claim about that product's composition, which ILFORD does not publish. The teaching point is that adding a modest quantity of the acid salt to a sulfite developer moves it into a genuinely buffered region more than two units below a carbonate developer, and that this is what an extra fine grain formula does.

Series	Percentage of the sulfur present as bisulfite	pH of the mixture (Henderson-Hasselbalch, pKa₂ 7.19)
pH of the mixture	5.00	8.47
pH of the mixture	10.00	8.14
pH of the mixture	15.00	7.94
pH of the mixture	20.00	7.79
pH of the mixture	30.00	7.56
pH of the mixture	40.00	7.37
pH of the mixture	50.00	7.19
pH of the mixture	60.00	7.01
pH of the mixture	70.00	6.82
pH of the mixture	80.00	6.59
pH of the mixture	90.00	6.24

The curve is arithmetic: pH = 7.19 + log([sulfite]/[bisulfite]), with pKa₂ from OpenStax Appendix H and the composition from Kodak's published D-25. Two honest limits. The ideal-solution assumption behind Henderson-Hasselbalch is poor at the ionic strength of a 0.8 mol/L sulfite bath, so read the shape and the direction rather than the second decimal; and the horizontal guide is ILFORD's published measurement of its own product, placed here only to show where that pH falls on this scale, since ILFORD does not publish PERCEPTOL's composition and this course makes no claim about it.

Kodak’s D-25 is D-23 with 15 g of sodium bisulfite added, and its heading states the consequences without explaining them: “‘Elon’ extra fine-grain developer — a simple formula for minimum grain in films or plates. (This developer requires 50 to 100% increase in exposure.)” It develops in about 18 minutes at 25 °C, where D-23 takes about the same time at 18 °C.

Read those two formulas together and you have a controlled experiment Kodak ran for you. The sulfite is identical in both — 100 g/L. The only difference is 15 g of the acid salt, and what it buys is finer grain; what it costs is a stop of speed and seven degrees of temperature to keep the time the same. Since the sulfite has not moved, none of that is solvent action. It is pH, and pH alone, which is why alkalis-buffers-and-ph owns the general case and this page only points at it.

✕The pH the course will not give you

It would be natural to want the pH of D-23 next to D-25’s, and this page does not give it. The sodium sulfite page records that the course has found no sourced pH figure for a working-strength sulfite solution, and none has appeared since.

The calculation could be done — hydrolysis of a 0.79 mol/L weak base is a standard exercise — but at that concentration the ideal-solution assumptions behind it are doing more work than the chemistry, and a number produced that way would be presented as arithmetic and read as a measurement. The buffer-ratio calculation above survives the same objection much better, because it depends on the ratio of two similar ions rather than on their absolute activities. That distinction is worth carrying: some calculations degrade gracefully when the solution is concentrated, and some do not.

Job three: dissolving the silver

The third job is the one that surprises people, and Kodak explains D-76’s whole character with it. Graininess, the primer says, is largely clumping: several silver halide crystals lying close together may develop as one, since an unexposed crystal in contact with a developing one can be dragged into development with it. Then the mechanism:

In the special developer (Formula D-76) there is a high concentration of sulphite which is a solvent for silver bromide and iodide. As development progresses therefore, the sulphite actually dissolves a small quantity of each grain and thereby minimizes greatly the tendency for clump formation which would increase the graininess.

Solvent action is therefore not an accident of the preservative; it is what a hundred grams per litre is for, in a fine-grain developer, and it is the second reason for the number. The primer adds the control that makes the point testable: add carbonate to such a developer and development speeds up and the graininess is accentuated again. It gives no reason for that, and the course marks its own reading as a reading — a faster development leaves the solvent less time to work on the grains before they are gone.

⚗What the course will not name

Sulfite dissolving silver bromide is complex formation, and Part III puts it in the table of ligands beside thiosulfate. But there the honesty begins.

The 1928 primer does not name the silver–sulfite complex, and this course does not supply one. No formula, no coordination number, no formation constant — Part III states the same and gives none. Nor has the course found a quantitative measurement of the solubility of silver bromide in sulfite solutions of any strength, which is why the table above says “slight” and “the fine-grain regime” rather than milligrams per litre.

What is established is the chain of consequences, and it is enough to formulate with: high sulfite dissolves a little of each grain; less clumping follows; grain is finer; and the same indiscriminate attack removes some marginal latent-image clusters, which is where the speed cost comes from. solvent-action-and-grain follows the dissolved silver to where it is redeposited.

The overshoot has a name and an appearance. The primer’s account of dichroic fog is precise: negatives developed in a bath containing an excess of sulfite, or hypo, or ammonia may show a fog that is yellowish-green by reflected light and pink by transmitted light, caused when dissolved silver salts are reduced to metallic silver in a very fine state of subdivision — particularly in the shadows, where no bromide is liberated during development to hold the free silver down. Fine-grained emulsions are recorded as the most susceptible, which is exactly the wrong combination, since fine-grained films are what a solvent developer is usually pointed at.

Bisulfite, metabisulfite, and the two-part bottle

The preservative for something that has to sit on a shelf for a year is not sulfite. The primer is explicit: a developing agent stored for a considerable time keeps best with an acid sulfite, such as sodium bisulfite, rather than sodium sulfite, which is slightly alkaline. Bisulfite also “keeps satisfactorily in more dilute solutions and is a better preservative than sulfite in the absence of carbonate”, and the readily oxidisable agents — pyro, amidol — are customarily kept that way.

Hence the two-solution developer, which is a storage decision rather than a chemical one: one bottle holds the agent and the bisulfite, the other the carbonate and the bromide, and they meet at the point of use. Kodak Limited’s 1949 handbook gives the single-solution rule for the same ingredient — where sodium bisulfite appears in a formula, it is added with the sulfite, not after the alkali.

Two arithmetic traps sit in that paragraph.

The salt you buy is probably not the salt you are reading about. Kodak’s primer records that ordinary commercial bisulfite has been shown by analysis to consist chiefly of metabisulfite, converted to bisulfite when it dissolves. The modern position is the same, and PubChem’s entry says so plainly: the dry salt sold as sodium bisulfite is usually largely sodium metabisulfite.

Na₂S₂O₅ + H₂O → 2 NaHSO₃

Metabisulfite becoming bisulfite in the beaker

The masses do not match. One mole of metabisulfite, 190.11 g, gives two moles of bisulfite, 2 × 104.06 = 208.13 g, so a gram of the metabisulfite delivers about 1.095 g of bisulfite — a tenth more. Substituting weight for weight was defensible when commercial bisulfite was largely metabisulfite anyway; it is less defensible with two labelled analytical bottles, and it matters most where the acid sulfite is also setting the pH, which on this page is always.

And in a one-solution formula the bisulfite quietly rewrites the alkali.

NaHSO₃ + Na₂CO₃ → Na₂SO₃ + NaHCO₃

Bisulfite converting to sulfite at the carbonate's expense

Every gram of bisulfite destroys an equivalent quantity of carbonate, so the developer ends up with less free alkali than the ingredient list implies. The primer names the consequence — the bisulfite exerts an apparent restraining action and the developer appears to keep longer — and then adds the detail that makes it interesting rather than merely annoying: experiments showed that the sodium bicarbonate formed acts as an antifogging agent. The by-product is doing work.

The hazard that only appears when you mix wastes

Sodium sulfite’s classification aggregates 2,482 company reports across 19 ECHA notifications, and the result is Danger: H314, severe skin burns and eye damage, from 51.8 per cent of the reports that classify it, with H319, H315 and H302 behind it — and 618 reports, nearly a quarter, saying it meets no GHS criterion at all. That spread means different grades reached different conclusions, not that the hazard is negotiable. Neither NIOSH nor HSE’s EH40 sets an exposure limit for the salt itself.

The hazard that is specific to this substance is not on that list, because it belongs to a mixture you make by accident.

SO₃²⁻ + 2 H⁺ → SO₂ + H₂O

Sulfur dioxide from sulfite and any strong acid

CAMEO records sodium sulfite as producing corrosive material with acids, hot water or steam, and the product with acid is sulfur dioxide. Unlike the sulfite, SO₂ does have a workplace exposure limit: HSE’s EH40 sets 0.5 ppm over eight hours. A darkroom holds an acid stop bath and an acid fixer, and a sulfite-rich developer, and a single waste container is exactly the way to bring them together.

⚠Controls, and the two rules that are specific to sulfite

The general controls are the ones HSE’s COSHH essentials sheet P1 sets for manual film development: general ventilation above five air changes an hour with a through draught, single-use nitrile gloves at 0.2 mm, eye protection and an overall. Weigh the powder over a tray with the container away from your face, and add powder to water rather than water to powder. Splash goggles rather than spectacles are worth the upgrade for the dry salt and for concentrates, because more than half the classifying notifiers call the solid capable of causing serious eye damage.

Two rules belong to this substance in particular. Never put sulfite waste into a container that has held an acid stop bath or an acid fixer, and never acidify it in the belief that you are neutralising it: the product is a gas with an exposure limit of 0.5 ppm. And do not treat a caked or effloresced tub as still good, because it has been oxidising to the sulfate on the shelf and the sulfate does nothing.

Downstream, sulfite is an oxygen-demanding waste, which is a different question from toxicity and is the chemical waste page’s subject. Bottle the streams separately, label them, and check your local regulations; they govern.

Recap

• Sulfite in D-76 is 0.79 mol/L against 0.057 mol/L of total developing agent — about fourteen equivalents. Sacrificial oxidation is a stoichiometric job and cannot account for that, so the quantity has to be explained by something else.
• Two things explain it, and both are sourced. A plain sulfite solution oxidises readily below 10 per cent w/v and very slowly above it, so 100 g/L is where sulfite protects itself best; and at that concentration sulfite is a solvent for silver bromide, which is what makes D-76 a fine-grain developer.
• The oxidised agent has two possible fates and Kodak names both: reduction back to hydroquinone, or addition as a colourless sulfonate. Because the sulfonates are colourless, a clear old MQ developer proves nothing. Whether the monosulfonate develops is not something this course can source.
• Sulfite is a mild alkali on its own — D-23 has no other — and with bisulfite it buffers near pKa₂ 7.19. Kodak’s D-25 is D-23 plus 15 g of bisulfite, which computes to about pH 7.9, and which Kodak says needs 50 to 100 per cent more exposure and seven degrees more temperature for the same time. Same sulfite, different pH: that speed loss is not solvent action.
• The silver-sulfite complex is not named by Kodak’s primer, and this course does not name one, nor does it have a measured solubility of silver bromide in sulfite solution. What it has is the chain of consequences: less clumping, finer grain, a little less speed, and dichroic fog if pushed too far.
• Anhydrous is not crystals. One part of the anhydrous salt equals two parts of the heptahydrate, and getting that wrong halves the sulfite and moves the developer out of both the keeping optimum and the fine-grain regime.
• Bisulfite is the preservative for storage, better than sulfite in the absence of carbonate; the salt sold under that name is usually metabisulfite, which delivers about 1.095 g of bisulfite per gram; and in a one-solution formula it converts carbonate to bicarbonate, reducing the free alkali and leaving behind a by-product that acts as an antifoggant.
• Sulfite plus acid gives sulfur dioxide, for which EH40 sets 0.5 ppm over eight hours. That is a waste-container rule, not a bench rule.

Check your understanding

Question 1. D-76 holds 100 g/L of sodium sulfite (M = 126.05) and 5 g/L of hydroquinone (M = 110.11). What is the molar ratio, and what does it rule out?

• About 2 to 1, which is what sacrificial oxidation needs
• About 17 to 1, which is far more than any stoichiometric account of oxygen scavenging requires, so the quantity must be explained by something other than protecting the agent from air
• About 1 to 1
• About 140 to 1

Show the answer and why

Answer: About 17 to 1, which is far more than any stoichiometric account of oxygen scavenging requires, so the quantity must be explained by something other than protecting the agent from air

100 ÷ 126.05 = 0.793 mol/L of sulfite against 5 ÷ 110.11 = 0.0454 mol/L of hydroquinone, a ratio of 17.5 to 1; counting the metol as well, the sulfite is about fourteen times the total developing agent. A scavenger works mole for mole, so fourteen equivalents is not an oxygen argument. Kodak supplies the two reasons that do work: a plain sulfite solution oxidises readily below 10 per cent and very slowly above it, and 100 g/L is 10 per cent w/v; and at that concentration sulfite is a solvent for silver bromide, which is what makes D-76 fine-grained.

Question 2. A bottle of MQ developer mixed a year ago is water-clear. Which inference is sound?

• It is unimpaired, because oxidised developer is brown
• None about its power: with sulfite present the oxidation products of hydroquinone are colourless sulfonates, so clarity is consistent with a fully spent developer
• The sulfite has gone but the agent remains
• It has been contaminated with fixer

Show the answer and why

Answer: None about its power: with sulfite present the oxidation products of hydroquinone are colourless sulfonates, so clarity is consistent with a fully spent developer

Kodak’s primer states it directly and the asymmetry is what to remember: brown means certainly spent, because an unprotected oxidation product is coloured, but colourless means only that the sulfite was still intercepting the product while the agent was being consumed. The two routes the primer describes have different consequences here — regeneration returns the agent, sulfonation removes it — and the book does not say which dominates. The practical answer is not to look at the bottle at all but to develop a fogged strip against a known time, which is the test the experiment page in this part builds.

Question 3. Kodak D-25 is D-23 plus 15 g/L of sodium bisulfite, and its own heading says it needs 50 to 100 per cent more exposure. Since both formulas carry 100 g/L of sulfite, what causes the speed loss?

• Extra solvent action from the additional sulfur compound
• The fall in pH: the bisulfite converts the bath into a sulfite/bisulfite buffer near pH 7.9, and a less active developer at a lower pH reaches fewer marginal latent-image clusters
• The bisulfite reacting with the metol
• The higher development temperature

Show the answer and why

Answer: The fall in pH: the bisulfite converts the bath into a sulfite/bisulfite buffer near pH 7.9, and a less active developer at a lower pH reaches fewer marginal latent-image clusters

This is a controlled experiment Kodak published without labelling it as one: the sulfite is identical in the two formulas, so nothing about solvent action has changed, and the only variable is the acid salt. Henderson-Hasselbalch with the pKa₂ of sulfurous acid, 7.19, puts a mixture that is 15.4 per cent bisulfite at about pH 7.9. The other half of the evidence is in the development instructions — D-23 takes about 18 minutes at 18 °C and D-25 about 18 minutes at 25 °C, so seven degrees are being spent to hold the time. Read the numbers only for direction and magnitude: the ideal-solution assumption is poor at 0.8 mol/L.

Question 4. A formula calls for 100 g of sodium sulfite per litre and you weigh out 100 g of the heptahydrate crystals. What have you actually made?

• The intended developer
• A developer with about half the intended sulfite, so it is out of the primer’s keeping optimum, largely out of the fine-grain regime, and more prone to staining
• A developer with twice the intended sulfite
• A developer that will not dissolve

Show the answer and why

Answer: A developer with about half the intended sulfite, so it is out of the primer’s keeping optimum, largely out of the fine-grain regime, and more prone to staining

Kodak’s primer states that the heptahydrate, when pure, holds 50 per cent of dry sulfite, so one part of the anhydrous salt equals two parts of the crystals — which is why Kodak’s own formulas print 16 oz of crystals or 8 oz of anhydrous for the same litre. Halving the sulfite drops the bath from 10 per cent w/v to 5, below the concentration at which the primer says a sulfite solution stops oxidising readily, and takes most of the solvent action with it. Hydrates are the commonest silent arithmetic error in a formulary, which is why the course keeps anhydrous and hydrated apart in its terminology and asks which hydrate every time.

Question 5. Why does the course refuse to give a formula for the silver-sulfite complex that makes D-76 fine-grained?

• Because no such complex exists; the effect is physical
• Because Kodak’s 1928 primer, which supplies the effect and the explanation, does not name the complex, and no other source the course holds names it or gives a formation constant
• Because complexes are covered in Part III
• Because sulfite dissolves silver only in the presence of thiosulfate

Show the answer and why

Answer: Because Kodak’s 1928 primer, which supplies the effect and the explanation, does not name the complex, and no other source the course holds names it or gives a formation constant

The primer states that a high concentration of sulfite is a solvent for silver bromide and iodide, that it dissolves a small quantity of each grain during development, and that this reduces clumping — and it stops there. Part III’s complex-formation page takes the same position and gives no formation constant for the same reason. The course also has no quantitative measurement of silver bromide’s solubility in sulfite solutions, which is why the effect is described as a regime rather than in milligrams per litre. What is established is a usable chain of consequences: less clumping, finer grain, a small speed loss, and dichroic fog when it is pushed too far.

Question 6. Which single practice creates a sulfur dioxide hazard from ingredients that are otherwise handled at the bench without one?

• Mixing the sulfite into water that is too hot
• Pouring spent sulfite-rich developer into a container that has held acid stop bath or acid fixer, or acidifying sulfite waste deliberately
• Storing sulfite in a clear bottle
• Using the anhydrous salt instead of the crystals

Show the answer and why

Answer: Pouring spent sulfite-rich developer into a container that has held acid stop bath or acid fixer, or acidifying sulfite waste deliberately

Sulfite plus a strong acid gives sulfur dioxide, and CAMEO records sodium sulfite as producing corrosive material with acids. The sulfite itself has no workplace exposure limit in EH40 or the NIOSH guide; sulfur dioxide has one, at 0.5 ppm over eight hours. The hazard is therefore created by combining two streams that are individually manageable, which is why the course keeps waste streams separate and labelled and why the instinct to neutralise an alkaline waste with acid is exactly the wrong instinct here. Local regulation governs what may then be done with the labelled container.

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