Potassium persulfate

Potassium persulfate has been sold to photographers since before 1912 under a trade name, as a convenience: a spoonful in the wash water to get rid of the last of the hypo. It is also a respiratory sensitiser with one of the lowest exposure limits on any substance in this encyclopaedia, and this course gives no home procedure for it.

In photography

Anthion, the hypo eliminator. Wall’s 1912 dictionary defines it in a sentence: “Anthion. Persulphate of potassium K₂S₂O₈, a highly oxidising salt obtained by the electrolysis of potassium sulphate, is sold as a hyposulphite eliminator under the name of anthion.” His 1924 formulary repeats the recipe among a list of hypo eliminators — “potassium or ammonium persulphate in 1 per cent solution, made alkaline with ammonia, may also be used. The former has been sold as Thioxydant, Anthion, etc.” — alongside potassium permanganate, potassium percarbonate, sodium perborate and hydrogen peroxide. The idea is straightforward: an oxidiser destroys residual thiosulfate so that a short wash does the work of a long one.

Modern practice does not do this. A washing aid such as a sulfite-based hypo clearing agent works by ion exchange rather than by oxidation, does not attack the image, and carries no sensitisation statement. The course teaches that route.

Kodak’s Gold Toner T-21. The persulfate’s one surviving appearance in a manufacturer’s formula is the stock solution of Kodak’s Nelson gold toner, in publication G-23. Stock Solution A is 960 g of sodium thiosulfate crystals dissolved in 4 litres of water at 50 °C, into which 120 g of potassium persulfate is added with vigorous stirring — and then the instruction that tells you what is happening: “If the mixture does not turn milky, increase the temperature.” A solution of 5 g of silver nitrate and 5 g of sodium chloride in 64 mL is added after cooling to 27 °C; Stock B is 1 g of gold chloride in 250 mL; the working bath is all of A plus 125 mL of B, used at 43 °C.

⚗The chemistry

The milkiness is the point of the operation and Kodak does not explain it. What can be said from the sources read is that potassium persulfate is a strong oxidant — Chemical Safety Card 1133 says so plainly — and that thiosulfate is a reducing agent, so the two react. What the course has not found in any source is an equation for the reaction, a statement of what the milky precipitate is, or an account of why the toner needs the products of that reaction rather than plain hypo. The obvious guess is colloidal sulfur; a guess is not a citation, and this page does not print one.

The same silence covers Anthion. Wall records that a persulphate destroys residual thiosulfate and does not say what it turns it into.

Which persulfate a formula means. The two salts are not interchangeable in the literature, and it is worth being precise about which is which. The reducers are ammonium: Kodak’s R-1 persulphate reducer is 60 g of ammonium persulphate per litre with 3 mL of concentrated sulfuric acid, and Wall’s reducer formulas, including Bennett’s stock solution and Puddy’s thiocyanate variant, all specify the ammonium salt. The hypo eliminator is either, and the potassium salt is the one that was sold under a name. The gold toner stock in G-23 is potassium. A formula that says only “persulphate” is ambiguous. The two are different salts with different relative molecular masses, so they are not interchangeable gram for gram, and this course has fetched a hazard record only for the potassium salt — the ammonium salt has no page here and its classification has not been read.

Properties

White crystals, hygroscopic, specific gravity about 2.48. Relative molecular mass 270.33.

It decomposes below 100 °C, which Chemical Safety Card 1133 and CAMEO both record, and which is remarkable for a solid salt: there is no melting point to quote because it never reaches one. That is why Kodak’s T-21 stock is warmed to 50 °C and no further, and why nothing containing it is boiled.

It is only sparingly soluble. 5.2 g per 100 mL at 20 °C, against 1.75 g at 0 °C — so a 5 per cent solution is near saturation at room temperature and a winter darkroom will not make one at all. Insoluble in alcohol. The card adds that the solution in water is a medium strong acid, which is not obvious from the formula and matters for anything it is mixed with.

Handling

The harmonised classification under Regulation (EC) No 1272/2008 gives Danger, three pictograms and seven statements: oxidiser, harmful if swallowed, skin irritation, allergic skin reaction, serious eye irritation, allergy or asthma symptoms or breathing difficulties if inhaled, and respiratory irritation. The 1,150 aggregated ECHA reports agree at 100 per cent on every statement but the skin sensitiser, which 94.8 per cent give.

Chemical Safety Card 1133 turns that into a description of what actually happens. Inhalation of the dust may cause asthma-like reactions. Repeated or prolonged contact may cause skin sensitisation and repeated inhalation may cause asthma. It gives an ACGIH TLV of 0.1 mg/m³ as an eight-hour average — and then, in its notes, the three sentences that decide this page: “The symptoms of asthma often do not become manifest until a few hours have passed and they are aggravated by physical effort. Rest and medical observation are therefore essential. Anyone who has shown symptoms of asthma due to this substance should avoid all further contact.” Followed by: “Do NOT take working clothes home.”

HSE’s EH40 sets no British workplace exposure limit for persulfates, and states that absence from its list does not indicate that a substance is safe; the NIOSH Pocket Guide has no entry either.

Why Level C. Against the course rubric, Level C’s first criterion names reagents classified as sensitisers at the concentrations used, where a fume cupboard or specialist disposal is the recognised control — and for a respiratory-sensitiser powder with a 0.1 mg/m³ limit, engineered extraction is the recognised control; Card 1133 asks for local exhaust in terms. The course has already reasoned this way once, when it excluded glutaraldehyde from Part V on its sensitiser status alone, and the logic does not change because this salt is cheaper and older. The oxidiser statement is a second, independent reason: a Level A darkroom has no place to store a class 5.1 solid away from combustibles.

⚠The one operation that fixes this at Level C

Weighing the powder. Everything else about persulfate could be managed at Level B — the working solutions are dilute, the tray is cold, the splash risk is ordinary. What cannot be managed in a home darkroom is opening a jar of a respiratory sensitiser and putting it on a balance, because the control that the safety card asks for is extraction and the consequence of getting it wrong is permanent. A person sensitised to persulfates reacts to amounts far below any threshold they can perceive, and the only treatment is never to meet it again.

This course therefore offers no home procedure using potassium persulfate. Where residual hypo is the problem, the answer is a proprietary washing aid and a longer wash; where a gold toner is wanted, GP-1 made with sodium thiocyanate does the job without a persulfate in it.

Waste

Because the course gives no procedure, the best waste advice is not to acquire the substance. Where it exists, it is an oxidiser: it must not go into a bottle with organic waste, must not be absorbed in sawdust or paper, and — Card 1133 is explicit — must not be allowed to enter the environment. Kodak’s J-52 pH window of 5.6 to 9.4 does not settle this one, because the hazard is the oxidising power rather than the pH. Take advice from your local authority under the disposal caveat before buying it, not after.

History

Anthion is a good example of a pattern this course meets repeatedly: a substance sold to amateurs for decades under a trade name, on a benefit that turned out to be small, with a hazard nobody had characterised. Wall’s dictionary entry is neutral and useful — here is the salt, here is how it is made, here is what it is sold for — and it is exactly the kind of entry that could not be written today, because the intervening century produced an occupational-medicine literature on persulfate asthma that did not exist in 1912.

The chemistry never changed. What changed is that the harm became visible, and a hypo eliminator turned out to be a poor trade for it.