Sulfuric acid

Potassium permanganate on its own barely touches a developed image. Kodak’s 1928 primer records that a plain 0.25 per cent solution has only a very weak action, which is exactly why it can be used to lift dichroic fog without attacking the picture underneath. Add sulfuric acid and the same permanganate becomes a bleach that will strip a negative to bare gelatin. The acid is not the oxidant; it is the condition the oxidant needs.

In photography

The reversal bleach. ILFORD’s reversal sheet, which turns a black-and-white negative film into a transparency, needs the first, negative image destroyed while the unexposed halide is left intact. Its bleach is two stock solutions: A, 2 g of potassium permanganate in 500 mL of water; B, 10 mL of concentrated sulfuric acid added to 490 mL of water — or, if only a 10 per cent acid can be had, 100 mL of that into 400 mL of water; the sheet gives that figure without a basis, and the course reproduces it as written. The stocks keep for a long time separately; equal parts are mixed fresh for each film and discarded after. Five minutes with continuous agitation removes the silver, and the film goes creamy-yellow. The sheet notes in passing that the bleach, being strongly acid, stops development almost instantly, and that the yellow stain it leaves is cleared afterwards in 25 g/L of sodium or potassium metabisulfite.

⚗The chemistry

Kodak’s 1928 primer gives the mechanism in one sentence: permanganates are very strong oxidising agents, and a permanganate solution containing sulfuric acid applied to a negative “will oxidize the silver to silver sulphate, which is sufficiently soluble in water to be dissolved”. Metallic silver is insoluble; silver sulfate is not, so oxidation is what makes the image removable at all. Without the acid the permanganate has only a weak action — which the primer turns into a technique, using about 0.25 per cent plain permanganate to remove dichroic fog, the finely divided silver being attacked while the image is not. The acid sets how far and how fast the oxidation goes, and that is the variable a reducer’s dilution is really controlling.

2 Ag⁺ + SO₄²⁻ → Ag₂SO₄

The product Kodak names: image silver, once the acid permanganate has oxidised it, leaves the gelatin as a sulfate soluble enough to wash out

The permanganate reducer. Kodak’s formula R-2 is the same chemistry aimed at a finished negative rather than a whole image. Stock solution A is 52.5 g of potassium permanganate in a litre; stock solution B is 32 mL of chemically pure sulfuric acid in a litre of cold water; for use, one part of A, two parts of B and sixty-four parts of water. When the negative has been reduced enough it goes into plain hypo or a fresh acid fixer to clear the yellow stain, and then is washed. The proportional reducer R-5 uses a much smaller dose — 0.3 g of permanganate with 16 mL of 10 per cent sulfuric acid, against a persulfate second solution.

Elsewhere. Kodak’s chrome alum fixing bath F-16 carries 8 mL of pure concentrated sulfuric acid with 60 g of potassium chrome alum in the litre of its B solution, poured into the hypo solution slowly while that is stirred rapidly. And the primer’s first chapter builds the alums themselves out of this acid: replace the hydrogen of sulfuric acid with potassium and you have potassium sulfate, replace it with aluminium and you have aluminium sulfate, and the two sulfates combine to give the alum that hardens gelatin.

Properties

An odourless, colourless, oily, hygroscopic liquid of relative density 1.8, freezing at 10 °C and decomposing at 340 °C to sulfur oxides. It chars wood, paper and cotton on contact. It is miscible with water “with the generation of much heat and with contraction in volume”, and CAMEO adds the qualification that decides how it is handled: the reaction with water is negligible unless the acid strength is above 80 to 90 per cent, above which the heat of hydrolysis is extreme and may cause severe burns. Laboratory concentrate is 96 to 98 per cent w/w, so it is the dangerous case, and a 10 per cent solution is not.

⚗Two protons, only one of them strong

OpenStax sets out the two ionisations. The first is complete — an ionisation constant above 10², which in water means every molecule gives up its first proton — while the second, from the hydrogensulfate ion, has K_a2 = 1.2 × 10⁻², which is weak enough to be an equilibrium. So a sulfuric solution is a strong acid with a weak acid dissolved in it, and the sulfate it finally leaves behind is the anion of the alums, of silver sulfate, and of the sodium sulfate in a gold toner. Almost everything sulfuric acid does in photography ends with a sulfate somewhere.

Handling

The aggregated ECHA notifications classify it Danger with the corrosion pictogram alone and a single statement: H314, severe skin burns and eye damage, in more than 99.9 per cent of 8,811 company reports. There is no acute toxicity entry and no carcinogenicity entry. The exposure limits tell a different story, because they are written against the mist and not the liquid: HSE’s EH40 sets 0.05 mg/m³ over eight hours for sulphuric acid mist, defined as the thoracic fraction; NIOSH sets 1 mg/m³ with an IDLH of 15 mg/m³; and Chemical Safety Card 0362 states that mists of this strong inorganic acid are carcinogenic to humans, giving a TLV of 0.2 mg/m³ in carcinogen category A2. CAMEO adds that contact with the eyes may result in total loss of vision.

Why Level B. The course rubric puts the handling of concentrated acids in Level B, together with procedures whose failure mode is a splash or a burn rather than a spoiled negative, and that is precisely this substance: a tray of working bleach, about one per cent v/v acid once the two stocks are mixed in equal parts, is an ordinary darkroom operation, and the bottle it was made from is not. The reason it does not go to Level C is that the classified carcinogen is the mist, and the control is not to make one — no heating, no spraying, no electrolysis, no compressed air. Any procedure that would generate an aerosol of this acid needs engineered extraction and belongs at Level C; pouring a measured 10 mL into 490 mL of cold water in a ventilated room does not generate one.

⚠Safety

The acid goes into the water. Never the water into the acid. Chemical Safety Card 0362 prints the instruction in capitals; ILFORD prints it in the health and safety notice at the front of the reversal sheet; Kodak printed it in 1928 next to formula R-2, with the consequence spelled out — the solution may boil and spatter acid on the hands or face, causing serious burns. Pour slowly, down the side of a vessel of cold water, stirring, in goggles and an apron, with eyewash and running water within reach; ILFORD directs that skin contact be flushed at once with water containing sodium bicarbonate. Keep it away from everything organic, from every metal, and from the fixer. The permanganate and the acid meet only as dilute stock solutions, mixed for the film in hand and thrown away after it.

Waste

A spent bleach is three problems at once: it is strongly acid, it carries manganese, and in a reversal process it carries the silver of the negative image it has just dissolved. The silver decides the route — Kodak’s recovery guidance treats a solution that has taken image silver into solution as a recovery stream rather than an effluent, and a bottle of spent bleach is worth keeping for that reason alone. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most frequently set, and a fresh bleach is far below it, so any neutralisation is slow, deliberate, ventilated and done in the same protection the mixing needed. Chemical Safety Card 0362 records the substance as harmful to aquatic organisms.

The one absolute is that it does not go into the fixer bottle. Kodak’s 1928 primer states that a few drops of a strong acid such as sulfuric or hydrochloric decompose thiosulfate, turning the solution milky as sulfur is thrown out; a litre of spent bleach into a jerrycan of spent fixer does that at scale. ILFORD’s guidance for domestic users 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 with plenty of water, unmixed, and never into a septic tank. Check your local regulations, which govern.

History

Oil of vitriol is one of the oldest reagents in the book, and photography’s debt to it begins before photography. Frederick Scott Archer’s account of gun-cotton — the pyroxylin dissolved in ether and alcohol to make collodion, and so the wet plate — has sulfuric acid in the very first step, not as a reagent but as a condition. Nitric acid alone will not nitrate cotton; the nitric acid must be, in Archer’s phrase, “in what is called the nascent state, free to combine”, and the sulfuric acid is what puts it there, holding the water that the nitration releases. His recipe is one measured ounce of nitric acid of specific gravity 1.45, one measured ounce of ordinary sulfuric acid and 80 grains of cotton, ten minutes in the acids and then a washing in which “water must not be spared”. That preparation is not one this course teaches, and the reasons are in the same paragraph: it must be done in the open air or where there is draught enough to carry off the nitric acid vapour.

The permanganate bleach has had a longer and quieter life. Kodak’s R-2 of 1928 and ILFORD’s reversal bleach of 2019 are the same two bottles — a permanganate and a dilute sulfuric acid, kept apart until the moment of use.