Sodium metabisulfite

Sodium bisulfite has a practical problem: as a dry salt it is awkward to make pure, and as a solution it keeps worse. The industry’s answer was to sell the anhydride instead — push sulfur dioxide into sodium sulfite and you get a stable white powder that turns back into bisulfite the instant it meets water.

In photography

Kodak’s 1928 primer gives the formation and the conversion in adjacent paragraphs, and it is the same reaction run backwards that makes the salt useful and makes it dangerous near acid.

Na₂SO₃ + SO₂ → Na₂S₂O₅

How the metabisulfite is made

⚗The chemistry

The primer then states that ordinary commercial bisulfite has been shown by analysis to consist chiefly of metabisulfite, converted into bisulfite when it dissolves in water. Written out, that sentence is one molecule of water splitting the disulfite ion in half. So sodium metabisulfite has no independent photographic chemistry at all: everything it does in a tray it does as bisulfite — preserving a developer in acid, supplying a fixing bath with sulfite and acid together, forming a near-neutral buffer when mixed with plain sulfite. Its whole contribution is that it survives the shelf and the postal system, which the acid it becomes does not.

Na₂S₂O₅ + H₂O → 2 NaHSO₃

Metabisulfite becoming bisulfite in the beaker

That resolves most of the confusion in old formulas. Kodak’s primer offered its own dry sodium bisulfite for formulas calling for either name; the modern reality is the mirror image, since a tub sold as sodium bisulfite is usually metabisulfite. Weigh what the label says.

🧮Do the maths

The weighing difference is real but small. One mole of metabisulfite, 190.11 g, takes up one mole of water, 18.02 g, and gives two moles of sodium bisulfite, 2 × 104.06 = 208.13 g. So a gram of the metabisulfite delivers about 1.095 g of bisulfite — a tenth more, gram for gram, than the salt it replaces. The 1920s substituted the two weight for weight without correcting for it, defensibly, when commercial “bisulfite” was largely metabisulfite anyway; it is less defensible with a modern analytical-grade bottle of each, and it matters most where the acid sulfite is also setting the pH.

Two current uses are worth naming, and both are clearing baths. ILFORD’s reversal processing sheet lists sodium or potassium metabisulphite among the chemicals required, gives the clearing solution as 25 g of either salt made up to a litre, and places it two minutes after the acidified permanganate bleach, noting that it removes the yellow staining bleaching causes. Mike Ware’s workshop notes for the print-out palladiotype give a 2.5% w/v bath — 25 g in a litre again — between a disodium EDTA bath and a tetrasodium EDTA one, named in his outline as “Reduce and Clear”, and not to be kept from one session to the next.

What more or less of it does belongs to the bisulfite page, because it is the bisulfite doing the work. Kodak Limited’s 1949 handbook keeps the mixing rule that applies either way: the acid sulfite goes into the water with the sulfite, before the alkali, never after it.

Properties

NIOSH describes it as white to yellowish crystals or powder with an odour of sulfur dioxide, and the smell is diagnostic: a metabisulfite that has stopped smelling has been oxidising in its tub. NIOSH gives the molar mass as 190.1 and records that it decomposes rather than melting; CAMEO puts the decomposition above about 150 °C, with toxic oxides of sulfur and sodium.

It is markedly more soluble than sodium sulfite: 66.7 g per 100 g of water at 25 °C from HSDB, 65 g per 100 mL at 20 °C from the ILO-WHO Chemical Safety Card. Concentrated stocks are therefore easy to make, which is why it is sold as a powder for making them. The solution is acidic, and CAMEO’s phrasing is blunt — it “acts as a corrosive acid when mixed with water”. The course has found no sourced pH figure for a working-strength solution.

Stability is the reason it exists and also its limit. NIOSH’s note is that it is slowly oxidised to the sulfate on exposure to air and moisture, and sulfate is neither a preservative nor an acid: age erodes both of the jobs the powder was bought to do, without much changing its appearance.

Handling

The aggregated ECHA notifications on PubChem give the signal word Danger, the corrosive and irritant pictograms, and two statements: serious eye damage from 99.5% of the classifying reports and harmful if swallowed from 98.9%. Only 11 of 3,060 reports say it meets no criterion. Against sodium sulfite, where a quarter of reports declined to classify at all, this is one of the clearest aggregated verdicts in the encyclopaedia — and it is about eyes.

NIOSH sets a recommended exposure limit of 5 mg/m³ as a time-weighted average over a workday of up to ten hours; EH40 gives the same figure over eight hours for the substance it lists as disodium disulphite. Neither covers the sulfur dioxide the salt gives off in acid, for which EH40 sets 0.5 ppm (1.3 mg/m³) over eight hours — forty times lower by mass than the dust limit.

Why Level A. Against the course rubric, this handling meets the Level A criteria: at the quantities involved — 25 g in a litre for either clearing bath above — the classification is corrosive to the eye and harmful if swallowed, which the first criterion admits explicitly; nothing is heated above 50 °C; and the waste is a collectable spent bath. One control is upgraded rather than left to judgement: splash goggles, not safety glasses, for powder and solution alike, because with an eye-damage classification carried by 99.5% of notifications, eyewear that leaves the sides open is not proportionate. The step that leaves Level A is putting metabisulfite and a strong acid together on purpose — a procedure generating a vapour, which is a Level B criterion.

⚠Safety

Weigh over a tray, face away from the tub, powder into water rather than water into powder. Goggles and nitrile gloves throughout — HSE’s COSHH essentials sheet P1 takes single-use nitrile 0.2 mm as splash protection, and asks for ventilation above five air changes an hour from a powered fan with a through draught. Keep the salt away from acids, bleaches and other oxidisers, from heat, and from bare metal, which CAMEO records can generate flammable hydrogen. Read the data sheet for the container you bought.

Waste

This salt is not usually discarded as itself: it reaches the waste bottle in a spent clearing bath, or in a spent acid fixer together with thiosulfate and silver. Silver sets the order, so the fixer is collected for recovery first.

Two properties govern what is left. It is acidic, and Kodak’s J-52 publication gives the pH window sewer codes most often set as 5.6 to 9.4 — a metabisulfite-rich waste sits at or below the bottom of that range and wants diluting. And it is reducing, so it takes dissolved oxygen directly as well as feeding the biological demand the five-day BOD test measures; J-52 gives 350 mg/L as the usual regulated limit against a typical photographic effluent of 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, which decide the matter. And do not try to neutralise metabisulfite waste with acid: acid is what turns it into gas.

History

Sulfite reached the fixing bath in 1889, when, as Eder records, Alexander Lainer showed in Vienna that sulfites could be mixed into a fixing bath as a clear solution; Eder adds that the acid sulfite solution used generally afterwards for acidifying fixing baths was one he introduced in August of the same year. Both were liquids, and liquids of this kind lose strength on the shelf and cannot be posted in a paper packet. The metabisulfite answers that logistical problem rather than a photographic one, and it is why a modern developer arrives as powder in a foil sachet.

The same logic made the salt a staple far outside photography. It is E223 in the European food additive list, and PubChem’s synonym list includes “Campden Tablets”, the form in which winemakers buy it — useful to know, because home-brewing suppliers sell it in quantities and at prices a photographic supplier will not match. Grade, purity and tabletting aids are then the supplier’s business, and that product’s own data sheet describes what is in the packet.