Sodium sulfite

Three jobs, one ion. Sodium sulfite is in nearly every black-and-white developer and most fixers, and it does something different in each.

In photography

It takes the oxygen. A developing agent gives up electrons easily, and the air is full of something that will take them. Kodak’s 1928 primer puts it plainly: sodium sulfite has a very strong affinity for oxygen, is easily oxidised to sodium sulfate, and so protects the developer from the oxygen of the air.

2 Na₂SO₃ + O₂ → 2 Na₂SO₄

Sulfite taking the oxygen the developer would otherwise take

The primer writes it with atomic oxygen. Two working rules follow. A sulfite stock should be as strong as possible — the primer says 20% of the desiccated salt — because at that strength it oxidises slowly and keeps, while weak solution takes up air readily and is soon useless. And Kodak Limited’s 1949 handbook warns that adding the alkali before the sulfite lets coloured oxidation products form, which is why the preservative is dissolved first.

It traps oxidised developer before it can stain. This is the job that decides what colour the negative is.

⚗The chemistry

Oxidise pyrogallol in plain solution and the product is yellow; develop in it and that product settles into the film beside the silver, so the image is part silver and part stain. Kodak’s 1928 primer gives the graded experiment: no sulfite, a very yellow negative; sulfite, a much less yellow one, because the sulfite is oxidised instead; a great deal of sulfite, an image almost as blue as one developed in Elon, Kodak’s purified metol. The mechanism is in the same book — with sulfite present, the oxidation products of hydroquinone are its mono- and disodium sulfonates, and those are colourless. The sulfite adds itself to the oxidised developer and takes it out of the staining business.

The sting is stated there too: because the sulfonates are colourless, a clear old Elon-hydroquinone developer is no evidence that its power is unimpaired. A brown developer is certainly spent; a colourless one is undiagnosed.

At high concentration it dissolves silver. Graininess is largely clumping — crystals lying close together developing as one once any of them is attacked. Kodak’s primer explains D-76 in those terms: it carries 400 g of sulfite in its four-litre recipe, 100 g per litre, and sulfite is a solvent for silver bromide and iodide, so development dissolves a little of each grain and greatly reduces clumping. Add carbonate and development speeds up and the grain coarsens again. The primer does not name the silver–sulfite complex, and this course does not guess at it.

In the fixer it does two things at once. Developer carried in on the film oxidises, browns the bath and stains prints, and sulfite is added against that. But the acid the bath needs, to neutralise the developer arriving with every film, turns thiosulfate into free thiosulfuric acid, which is unstable.

H₂S₂O₃ → H₂SO₃ + S

Why an acid fixer turns milky: thiosulfuric acid falls apart

The change runs both ways, so enough sulfite works against the acid and keeps the sulfur from precipitating: sulfite is what makes an acid fixer possible. It has a fourth job outside the silver processes — in Mike Ware’s platino-palladiotype sequence, a sulfite bath between two EDTA baths reduces residual iron(III) to iron(II) for the final bath to complex.

Properties

The salt sold for photography is the anhydrous one: a white crystalline powder, odourless and, CAMEO notes, slow to dissolve. Published solubilities differ, from CAMEO’s “soluble in 3.2 parts” to 22 g per 100 mL at 20 °C on the ILO-WHO Chemical Safety Card. In solution it is alkaline, for the reason Kodak’s primer gives in a clause: soda is a strong base and sulfurous acid a weak one. The course has found no sourced pH figure for a working-strength solution.

Hydrates matter here. A heptahydrate, Na₂SO₃·7H₂O, crystallises from cooled solution; Wall’s 1912 dictionary gives its relative molecular mass as 252 against 126 for the anhydrous salt, and Kodak’s primer states that when pure it holds 50% of dry sulfite — so one part of the anhydrous salt equals two parts of the crystals, which is Wall’s own rule. The primer distinguishes two dry grades, “desiccated” at about 92% sulfite and “anhydrous” at up to 96.5%; the 1949 Kodak handbook prefers the anhydrous for its stability, ease of solution and smaller bulk.

Left in air, sulfite oxidises to sulfate, and sulfate is not a preservative: the 1928 primer advises suspicion of any sulfite that has effloresced badly.

Handling

The aggregated ECHA notifications on PubChem classify sodium sulfite Danger, corrosive and irritant: severe skin burns and eye damage from 51.8% of the reports that classify it, serious eye irritation from 21%, skin irritation from 19.3%, harmful if swallowed from 18.3%. Nearly a quarter of all reports — 618 of 2,482 — say it meets no GHS criterion. That spread means suppliers of different grades reached different conclusions, not that the hazard is negotiable. CAMEO adds that solutions irritate the stomach by liberating sulfurous acid.

Why Level A. Against the course rubric, this handling meets the Level A criteria and no more: the classification tops out at corrosive and irritant at the concentrations actually handled, nothing is heated above 50 °C, and the waste is spent developer and silver-bearing fixer, both collectable. HSE’s COSHH essentials sheet P1 supplies two of the numbers — ventilation above five air changes an hour from a powered fan with a through draught, and single-use nitrile gloves 0.2 mm thick. One Level B control is added by preference, splash goggles rather than glasses for the dry salt and for concentrates, because more than half the classifying notifiers call the solid capable of causing serious eye damage. Neither NIOSH nor EH40 sets an exposure limit for sodium sulfite, and EH40 states that absence from its list does not indicate safety.

⚠Safety

Weigh over a tray, container away from your face, powder into water rather than water into powder. Gloves and eye protection throughout; goggles for powder and concentrates; ventilate. CAMEO records sodium sulfite as a reducing agent reacting with oxidisers such as peroxides, epoxides and oxoacids, and as producing corrosive material with hot water, steam or acids. Keep it from acids, bleaches and peroxides, and never put it in a vessel that has held an acid stop bath: tipped into anything strongly acidic it gives sulfur dioxide, for which EH40 sets 0.5 ppm over eight hours.

Waste

Sulfite leaves a darkroom in spent developer and in spent fixer, and the silver sets the order: fixer goes for recovery first. Sulfite is part of how that recovery works. Kodak’s J-215 publication states that in an electrolytic cell silver deposits on the cathode while sulfite and thiosulfate are oxidised at the anode, and that the sulfite must be held at 8 to 10 g per litre when silver is at 1 to 5 g per litre, or silver sulfide forms and the plating falls off.

Downstream, the property that makes sulfite useful makes it an oxygen-demanding waste. Kodak’s J-52 publication explains that the five-day biochemical oxygen demand measures how much dissolved oxygen micro-organisms take from the water, gives 350 mg/L as the usual regulated mean limit and a pH window of 5.6 to 9.4, and puts typical photographic effluent at 100 to 1,000 mg/L.

ILFORD’s guidance for domestic users in the UK is to bottle wastes separately, label them and take them to a household waste and recycling centre’s chemical cupboard; failing that, small amounts flushed down the drain with plenty of water, unmixed, and never into a septic tank. Check your local regulations; they govern. And do not acidify sulfite waste in the belief that you are neutralising it.

History

Eder’s history credits Herbert Bowyer Berkeley (1851–1891), chemist to the Platinotype Company, with radically improving the pyrogallol developer in 1882 by adding sodium sulfite, which stopped the solution browning and the negatives staining. Eder calls it an enormous advance, and one that did not stay with pyro: within a short time it was applied to hydroquinone, pyrocatechin and metol. Seven years later, Eder records, Alexander Lainer in Vienna found in 1889 that sulfites could be mixed to a clear solution with fixing baths, so that fixing and the removal of fog happened in one operation — the ancestor of every acid sulfite fixer since.