Potassium metabisulfite

Every job on this page could be done by sodium metabisulfite at lower cost, and photographers have known it since the 1920s. The potassium salt survives in formulas anyway, because it got there first and a formula written in 1890 tends to stay written.

In photography

In water it does what its sodium twin does: the disulfite ion takes up water and splits into two hydrogen sulfite ions, so the working reagent is potassium bisulfite.

K₂S₂O₅ + H₂O → 2 KHSO₃

The disulfite ion splitting in water

As a preservative it was the first metabisulfite in the darkroom. Wall’s 1912 dictionary records it as used to preserve pyrogallol, and adds the caveat that matters: on keeping, the solution becomes very acid and needs larger quantities of alkali. Kodak’s 1928 primer calls it often used, good crystals, convenient — and very costly compared with sodium bisulfite.

As an acidifier Wall’s entry gives its second use in a sentence: it may be added to a fixing bath to make the bath acid and prevent it staining.

As a clearing bath it is current practice, not history. ILFORD’s reversal processing sheet lists “sodium or potassium metabisulphite” among the chemicals required and gives the clearing solution as 25 g of either salt made up to a litre, two minutes after the acidified permanganate bleach, noting that it removes the yellow staining bleaching causes. The sheet does not explain the mechanism and this course will not invent one; what is sourced is that the bath before it is an oxidising bleach and that CAMEO calls metabisulfite a reducing agent.

🧮Do the maths

The two metabisulfites are interchangeable in solution and are not interchangeable on the balance. Take the molar masses from PubChem — K₂S₂O₅ is 222.33, Na₂S₂O₅ is 190.11 — and each gives two bisulfite ions.

• 1 g of K₂S₂O₅ = 1/222.33 = 0.004498 mol, giving 0.008996 mol of bisulfite.
• 1 g of Na₂S₂O₅ = 1/190.11 = 0.005260 mol, giving 0.010520 mol of bisulfite.

Gram for gram the sodium salt delivers about 17% more bisulfite, because potassium is the heavier passenger and you are paying to carry it. Kodak’s 1928 primer nevertheless records that it was customary to substitute sodium bisulfite weight for weight, either being quite satisfactory, and that the choice “simmers down to a matter of dollars and cents” — the same sum gives 1 g of NaHSO₃ as 0.009610 mol of bisulfite, about 7% more than a gram of the potassium salt. In a clearing bath the difference is irrelevant; in a developer where the acid sulfite also sets the alkalinity, it is not.

What more or less does. More in a developer means more acid and less free alkali — a slower, longer-lived, less fog-prone solution, with Wall’s warning that an old stock has drifted further acid than the formula intended. More in a fixing bath means a larger acid reserve against the alkaline developer carried in on the film.

Properties

CAMEO describes a white granular or powdery solid with a sharp odour of sulfur dioxide, density 2.3 g/cm³, decomposing between 150 and 190 °C. The smell is its own diagnostic: a tub that no longer announces itself has been oxidising to the sulfate.

The molar mass is 222.33, which is the 222 Wall’s 1912 dictionary gives. HSDB and the Chemical Safety Card both put the solubility at 49.5 g per 100 g of water at 25 °C, and Wall’s estimate of one part in three is in the same region. It is insoluble in ethanol, which is why Wall’s preparation route works: saturate potassium carbonate solution with sulfurous acid gas, then throw the metabisulfite out with alcohol. It is markedly less soluble than sodium metabisulfite, 49.5 g against 66.7 g, which matters when making a concentrated stock.

In CAMEO’s words, aqueous solutions are acidic and corrosive; the course has found no sourced pH figure for a working-strength one. The reactivity to remember is in the same profile: it generates gaseous sulfur dioxide in contact with acids, and reacts with oxidising agents to give heat and products that may be flammable — with strong oxidisers, explosively.

Handling

The aggregated ECHA notifications on PubChem carry Danger with the corrosive and irritant pictograms and three statements: skin irritation from 90% of the classifying reports, respiratory irritation from 84%, serious eye damage from 13.7%. Forty-three of 1,552 reports say it meets no criterion. Read against sodium metabisulfite, where eye damage carries 99.5% and no respiratory statement appears, the pattern is odd, because the chemistry in the beaker is the same. These are notifications by suppliers of different grades, not a laboratory comparison, so the course takes the union of the two: assume it can damage an eye and that the dust irritates the airway.

Exposure limits are an admitted gap. Neither the NIOSH Pocket Guide nor EH40 lists potassium metabisulfite; both give its sodium counterpart 5 mg/m³, NIOSH as a time-weighted average over a workday of up to ten hours and EH40 over eight, and treating this salt as comparable is an inference this course makes explicitly rather than a limit anyone has set. What is set is the limit for the gas: sulfur dioxide, 0.5 ppm over eight hours and 1 ppm over fifteen minutes.

Why Level A. Against the course rubric, this handling meets the Level A criteria: the classification is irritant and corrosive to the eye at the quantities used, 25 g in a litre for ILFORD’s clearing bath and grams to tens of grams elsewhere; nothing is heated above 50 °C; and the waste is a collectable spent bath. Two controls are upgraded. Splash goggles rather than safety glasses, because an eye-damage classification is on the record even at 13.7%. And a mask rated for fine particulates if weighing raises visible dust — a Level B control, brought in because 84% of the classifying notifications give “may cause respiratory irritation”, and because a sharp sulfur-dioxide odour tells you the dry salt is releasing gas before it touches water. Weighing over a tray and keeping the tub closed between scoops usually make the mask unnecessary.

⚠Safety

Goggles and nitrile gloves — 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. Ventilate before you open the tub, not after. Keep the salt from every acid, from bleaches and other oxidisers, from heat and from bare metal. If the sulfurous smell over the tray becomes noticeable rather than faint, stop and increase the ventilation.

Waste

A spent clearing bath is acidic and reducing, and in ILFORD’s reversal sequence it carries manganese from the permanganate bleach before it. Spent acid fixer carries thiosulfate and silver, and the silver decides the order: it goes for recovery first.

The residue behaves like the rest of the family. Kodak’s J-52 publication gives the pH window sewer codes most often set as 5.6 to 9.4, so a concentrated metabisulfite waste sits at or below the bottom of that range and wants diluting; and being reducing it takes dissolved oxygen from the water it reaches, on top of the biological demand the five-day BOD test measures, for which J-52 gives 350 mg/L as the usual limit against a typical 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 govern. The mistake this salt invites is mixing it with the bleach it follows: a permanganate bleach is acidified, and metabisulfite meeting acid gives sulfur dioxide.

History

Eder’s history dates sodium sulfite’s entry into the developer to 1882, when H. B. Berkeley added it to pyrogallol. Four years later, in 1886, the same history records that Mawson and Swan introduced potassium metabisulphite for preserving pyrogallol solutions, and for hydroquinone developers. The acid preservative was therefore not a later refinement of the alkaline one; it followed within four years, and it arrived as the potassium salt.

Why potassium first is not something this course can source. Wall’s 1912 dictionary had already recorded the working nuisance — a stored solution goes on becoming more acid and demands more alkali — a good reason to prefer a dry salt weighed fresh over an aged stock bottle, whichever metal is attached to it. What kept the name alive is that formulas outlive their reasons: ILFORD’s 2019 reversal sheet still offers the two salts as equals, and in the tray they are. The balance is where they stop being interchangeable.