Sodium bisulfite

Take sodium sulfite and put one of sulfurous acid’s two hydrogens back. What you get is the acid salt, NaHSO₃, and it behaves like a mixture of the sulfite and the acid because functionally that is what it is. Kodak’s 1928 primer writes the relation directly.

Na₂SO₃ + H₂SO₃ → 2 NaHSO₃

Bisulfite as sulfite plus sulfurous acid

In photography

It preserves in acid, where sulfite cannot. Sodium sulfite is alkaline, and Kodak’s 1928 primer observes that oxidation progresses less readily in acid solution than in alkaline. So a developing agent that must sit in a bottle for months — pyrogallol above all — keeps better with an acid sulfite. The primer’s recommendation is the two-solution developer: one bottle holding the developing agent and the bisulfite, another the carbonate and the bromide, mixed at the point of use. Kodak Limited’s 1949 handbook adds the rule for the one-solution case: where sodium bisulfite appears in a formula it goes in with the sulfite, not after the alkali.

In a one-solution developer it quietly rewrites the formula. Put bisulfite and carbonate in the same bottle and they react.

NaHSO₃ + Na₂CO₃ → Na₂SO₃ + NaHCO₃

Bisulfite converting to sulfite at the expense of the alkali

⚗The chemistry

Read that as a formula edit rather than as chemistry. Every gram of bisulfite destroys an equivalent quantity of carbonate, so the developer ends up with less free alkali than the ingredient list suggests. Kodak’s 1928 primer names the consequence: the bisulfite exerts an apparent restraining action, and the developer appears to keep longer because some carbonate has been used up. Why not add sulfite instead, and leave the alkali alone? The primer answers that too — experiments showed that the sodium bicarbonate formed acts as an antifogging agent. The by-product is doing work.

More bisulfite therefore means a slower, cleaner-working, longer-lived developer with less alkali in it; less means a faster, more energetic one that fogs more readily and dies sooner — a lever you can pull deliberately once you know it is there.

In the fixer it supplies both the sulfite and the acid. An acid fixing bath needs sulfite, to keep carried-over developer from staining and to keep the acid from breaking thiosulfate down into sulfur; and it needs acid, to neutralise the alkaline developer arriving with every film. Kodak’s 1928 primer records that bisulfite is used very largely as a fixing-bath preservative precisely because it supplies both at once, and is honest about the limit: acetic acid is better where it can be had, because bisulfite gives a poorer reserve of available acid — and that reserve is what keeps the alum hardener working as the bath ages.

Properties

NIOSH describes the solid as white crystals or powder with a slight odour of sulfur dioxide, and that faint smell is the substance announcing what it is: an acid sulfite in equilibrium with dissolved sulfur dioxide. Kodak’s 1928 primer says the pure salt has an acid reaction and often carries a slight excess of sulfur dioxide; CAMEO lists the commercial solution separately, a clear colourless to yellow liquid corrosive to aluminium. The molar mass is 104.06 — the 104 Wall’s 1912 dictionary gives — and NIOSH puts the solubility at 29% and records that it decomposes rather than melting.

What is actually in the tub is the awkward part. Kodak’s 1928 primer states that ordinary commercial bisulfite consists chiefly of metabisulfite, converted into bisulfite when it dissolves, and PubChem’s synonym list for this compound includes “Sodium Bisulfite (mixture of NaHSO₃ and Na₂S₂O₅)”. The dry salt sold as sodium bisulfite is therefore usually sodium metabisulfite: one reagent in solution, two substances on the balance.

Ageing is a slow leak rather than a sudden failure. NIOSH’s note is that the salt is slowly oxidised to the sulfate on exposure to air, and sulfate is neither a preservative nor an acid: an old container has quietly weakened in both of the roles it was bought for.

Handling

The aggregated ECHA notifications on PubChem give sodium bisulfite the signal word Warning, the exclamation-mark pictogram and one hazard statement — H302, harmful if swallowed, from 99.9% of the classifying reports, with only one of 1,446 reports saying it meets no GHS criterion. That near-unanimity is unusual.

It is also incomplete, because the interesting hazard is the gas rather than the salt. CAMEO records that sodium bisulfite emits highly toxic gaseous sulfur dioxide when heated to decomposition or on contact with mineral acids, and calls the powder irritating to eyes, nose and throat. NIOSH sets a recommended exposure limit of 5 mg/m³ as a time-weighted average over a workday of up to ten hours, and EH40 the same figure over eight hours. For the sulfur dioxide the salt can release, EH40 is far tighter: 0.5 ppm over eight hours and 1 ppm over fifteen minutes.

Why Level A. Against the course rubric, this salt sits inside Level A: at the quantities used it is classified harmful if swallowed and irritant, which the first criterion admits; nothing is heated above 50 °C; and the waste is collectable spent fixer. HSE’s COSHH essentials sheet P1 supplies the numbers behind the controls — ventilation above five air changes an hour from a powered fan with a through draught, and single-use nitrile gloves 0.2 mm thick. The step that would leave Level A is deliberate acidification.

HSO₃⁻ + H⁺ → SO₂ + H₂O

What an acid does to bisulfite

That is a procedure generating a vapour, a Level B criterion. The faint sulfurous smell of a properly made acid fixing bath is the same reaction running very slowly, held in check because the bath is only weakly acid — which is why Kodak’s primer argues for a weak acid rather than a strong one.

⚠Safety

Weigh the powder over a tray and add it to water, not water to it. Gloves and eye protection for solutions; splash goggles for the dry salt and for the concentrated commercial liquid, which CAMEO calls a strong irritant to skin and tissue. Ventilate: the dust limit is 5 mg/m³ over eight hours, and the limit for the sulfur dioxide is a fortieth of that by mass. Keep it away from acids, bleaches, heat and bare metal. Read the data sheet for your own container — it may be a solution, and it is probably metabisulfite.

Waste

Sodium bisulfite rarely leaves a darkroom as itself. It goes out in a spent acid fixer, alongside thiosulfate and dissolved silver, or in a spent acid-preserved developer — and the silver decides the order, so the fixer is collected for recovery first.

What remains is acidic, reducing and oxygen-demanding, and two of the parameters Kodak’s J-52 publication lists as most often regulated apply directly: a pH window of 5.6 to 9.4, and a five-day biochemical oxygen demand limit of 350 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, which govern.

The mistake to avoid is the tidy-minded one: do not add acid to bisulfite waste, and do not pour bisulfite waste into a vessel holding acid. The product is sulfur dioxide, and it comes off faster than a bucket can disperse it.

History

Bisulfite’s photographic career began at the fixing bath rather than the developer, and it began suddenly. Eder records that in 1889 Alexander Lainer showed that sulfites could be mixed into a fixing bath as a clear solution, so that fixing and the clearing of fog happened together — which mattered because the developing agents of the late 1880s, eikonogen chief among them, fogged badly. Within months, Eder writes, M. Andresen published a fixing bath for eikonogen-developed plates of 250 g of hypo and 50 g of sodium bisulfite in 1,000 parts of water. Eder then claims a piece of it: the acid sulfite solution used generally afterwards for acidifying fixing baths he says he introduced in August 1889 — the author’s own account of his own contribution.

By the 1920s the choice between bisulfite and its potassium cousin was a question of accounting. Kodak’s 1928 primer notes that it was customary to substitute sodium bisulfite for potassium metabisulfite weight for weight, either being quite satisfactory, and that the sodium salt cost a third to a half as much — which is why so many formulas of the period offer the two on one line.