Potassium permanganate

Almost everything this crystal does in a darkroom turns on one switch: whether the solution is acid. A plain permanganate bath will lift a stain off a negative and barely touch the picture. Add sulfuric acid to the same permanganate and it will strip the negative to bare gelatin in five minutes. The oxidant does not change; the conditions do.

In photography

R-2, the reducer that does not raise contrast. Kodak Limited’s 1949 handbook heads it permanganate reducer for reducing density of negative materials without loss of contrast, and that sentence is the whole reason it exists beside Farmer’s reducer. The 1928 Eastman primer and the 1949 London handbook print it at the same quantities:

• Stock A — 52.5 g of potassium permanganate, water to make 1 litre.
• Stock B — 32 mL of concentrated sulfuric acid added very gradually to 1 litre of cold water, with constant stirring.
• For use, 1 part of A, 2 parts of B and 64 parts of water, which is a 1 in 67 dilution and puts the working bath at about 0.78 g/L of permanganate.
• After reduction, a 2 per cent sodium bisulphite bath to remove the stain, then a fresh acid fixing bath, then a thorough wash.

Two of the handbook’s own warnings belong with the formula. The negative must be washed thoroughly before the treatment or an iridescent, irremovable scum may appear on drying. And the stock solutions keep well but the mixture must be used immediately — the same rule that governs Farmer’s reducer, for a different reason.

⚗Why the acid changes everything

Permanganate is manganese in its highest oxidation state, +7, and how far it falls decides what the bath does. In strong acid it takes five electrons and ends as the colourless manganese(II) ion, which stays in solution.

5 Ag + MnO₄⁻ + 8 H⁺ → 5 Ag⁺ + Mn²⁺ + 4 H₂O

Acid permanganate: five silver atoms per permanganate ion

Kodak’s 1928 primer states the consequence in a sentence — the silver is oxidised to silver sulphate, “which is sufficiently soluble in water to be dissolved”. Metallic silver is insoluble; the silver(I) ion is not, and that is what makes the image removable at all.

Without enough free acid the permanganate stops three electrons in, at insoluble manganese dioxide, and the negative goes brown.

3 Ag + MnO₄⁻ + 4 H⁺ → 3 Ag⁺ + MnO₂ + 2 H₂O

Not enough acid: the brown stain is manganese dioxide

Which is why every permanganate formula in the literature ends with a bisulphite bath. Bisulphite reduces the dioxide the rest of the way to a soluble manganese(II) salt and the stain goes.

MnO₂ + HSO₃⁻ + H⁺ → Mn²⁺ + SO₄²⁻ + H₂O

Clearing: bisulphite takes the manganese the rest of the way down

The 1928 primer gives that last step in words and the course has written the equations from the oxidation states; the primer prints no equations of its own.

ILFORD’s reversal bleach, which is the same chemistry sold today. This is not a museum formula. ILFORD’s reversal-processing sheet, revised in June 2019, still specifies potassium permanganate by name: solution A is 2 g in 500 mL of water, solution B is 10 mL of concentrated sulfuric acid in 490 mL of water, the stocks keep for a long period separately, equal parts are mixed fresh for each film and discarded after. Five minutes with continuous agitation strips the first, negative image and leaves the film creamy-yellow; a metabisulphite clearing bath then takes off the yellow stain. The sheet notes in passing that the bleach, being strongly acid, stops development almost instantly.

Reversal is the one process where destroying a silver image completely is the object rather than an accident, and permanganate is what a manufacturer still reaches for to do it. The sulfuric acid page carries the same formula from the acid’s side.

🧮A reducer and a bleach, in the same two chemicals, at ten times the acid

Comparing the two formulas as stocks is misleading, because R-2 is diluted 1 in 67 at the tray and ILFORD’s is diluted 1 in 2. Compare the working baths instead.

• Kodak R-2, working: 52.5 g/L × 1⁄67 = 0.78 g/L of permanganate, and 32 mL/L × 2⁄67 = 0.96 mL of concentrated sulfuric acid per litre.
• ILFORD’s reversal bleach, working: 4 g/L × 1⁄2 = 2 g/L of permanganate, and 20 mL/L × 1⁄2 = 10 mL of concentrated sulfuric acid per litre.

The reversal bleach carries about two and a half times the oxidant and ten times the acid. That ratio is the difference between the two jobs written in numbers: a reducer takes some of the image away and has to be stoppable by eye, while a bleach takes all of it away in five minutes and is finished when the film goes creamy-yellow. The course has done this arithmetic from the two published formulas; neither manufacturer sets them side by side.

R-5, where the permanganate is the minority partner. Kodak’s proportional reducer works by argument rather than by a single reagent. No one substance reduces proportionally; permanganate is a slightly cutting reducer and ammonium persulphate is a flattening one, so a mixture can be made to sit between them. Stock A is only 0.3 g of permanganate with 16 mL of 10 per cent sulfuric acid per litre — a hundred and seventy-five times weaker than R-2’s stock — and stock B is 30 g of persulphate per litre, used one part of A to three of B. Wall’s 1924 handbook publishes the same pair independently at 0.25 g and 25 g per litre in the same 1 + 3 ratio, cleared in 1 per cent potassium metabisulphite.

Dichroic fog, where the weak action is the technique. The 1928 primer records that a plain permanganate solution at about 0.25 per cent has only a very weak action on a negative, and turns that into a use: dichroic fog is very finely divided silver, the fine silver is attacked, and the image silver is not appreciably touched. The dichroic fog entry records the treatment and prescribes none, and this page does not change that — what has changed with this entry is that the reagent can now be looked up, not that the course has adopted a procedure for it.

The pyro difference, which is the most useful thing on this page for anyone using a staining developer. On a negative developed in pyrogallol the image is partly silver and partly the oxidation-product stain. The 1928 primer records that ferricyanide removes the silver and leaves the stain, so the negative appears to grow yellower as it reduces, while permanganate does the opposite — it attacks the stain image in preference to the silver, and makes the negative less yellow. That is a real choice between two reducers on a real class of negative, and it is decided by which half of the image you want to keep.

The hypo test, and a trap in Kodak’s own numbering. Permanganate is at once decolourised by thiosulfate, so a violet solution that loses its colour has met residual hypo. Both handbooks print the test at the same strength: the 1928 primer gives 0.3 g of permanganate with 0.6 g of sodium hydroxide in 250 mL, and the 1949 handbook gives 1.2 g with 2.4 g in a litre, which is the same solution written per litre instead of per quarter. One millilitre of it goes into 250 mL of pure water and the washed material drips in.

The trap is the code. The 1928 primer heads the test HE-1; the 1949 handbook heads it HT-1a and uses HE-1 for a hypo eliminator, which is a different formula doing a different job. A reader moving between the two books can ask for one and get the other.

The two also disagree about what they saw. The 1928 primer reads the sequence as violet turning green with a little hypo and then to deep yellow; the 1949 handbook reads it as violet turning orange in about thirty seconds and then to yellow. The course records both and reconciles neither. The 1949 note is the more useful half in any case, and it is a real limitation: oxidisable organic matter in the water reacts with the permanganate in the same way hypo does, so the test only means anything against a comparison run on a sample of the same water.

Wall’s 1924 handbook gives the same idea as a bath rather than a test, adding 1 per cent permanganate to the wash water until the colour is no longer discharged. The course teaches the silver-nitrate HT-2 residual-hypo test instead, because it is a stain comparison against a printed patch rather than a colour judgement, and because it does not require this substance.

As a bleach with bromide. One further use, recorded in the 1928 primer and not developed here: add a bromide to the permanganate solution and silver bromide is formed instead of a soluble silver salt, giving the same rehalogenating action as a ferricyanide-bromide bleach. The primer gives no formula for it, and this page gives none either.

Properties

Dark purple crystals, odourless, density 2.7 g/cm³, relative molecular mass 158.03. It decomposes at 240 °C rather than melting cleanly, and the decomposition gives toxic gases and irritating fumes.

It is only moderately soluble, and the ILO-WHO card says so in that word: 6.4 g per 100 mL at 20 °C. HSDB agrees at 6.40 × 10⁴ mg/L at 20 °C and adds 25 g per 100 at 65 °C. Wall’s 1912 dictionary, a century earlier, gives 6.3 per cent in cold water. Three sources, one number.

The practical consequence is printed in the 1928 primer. R-2’s stock A wants 52.5 g in a litre, which is 5.25 g per 100 mL — 82 per cent of saturation at 20 °C, so it will dissolve, but grudgingly and slowly. The primer’s instruction follows from that: dissolve the crystals in a small volume of water at about 82 °C, shake or stir vigorously until they have gone, and then dilute to volume with cold water. What it is guarding against is not an impossible solution but a slow one, and an undissolved crystal is not a nuisance — Wall’s dictionary records that a single particle landing on a film produces a mark on the negative.

🕰A formula that halves

Wall’s 1912 dictionary writes potassium permanganate as K₂Mn₂O₈ = 316 and gives its preparation by fusing potash hydrate and chlorate with black oxide of manganese. The modern formula is KMnO₄ and the relative molecular mass is 158.03 — exactly half of Wall’s figure, because the period wrote the salt as a doubled formula.

Nothing about the substance changed. A reader taking a quantity out of a pre-war text does not need to convert anything, because the weights in those formulas were always weights of the article on the balance. But anyone doing molar arithmetic from a nineteenth-century formula will be out by a factor of two, and the dictionary entry is the reason.

The same entry says the salt is used “for intensifying negatives”, which sits oddly beside the permanganate reducer the dictionary prints thirty pages later and beside every other source read here. The course follows the reducer, and records the line as it stands rather than correcting a source it cannot check.

It stains. CAMEO’s health-hazard note puts it first — burns and stains the skin dark brown — and Wall’s dictionary records that a single undissolved particle landing on a film produces a mark on the negative. Brown fingers are the visible sign that a control has failed, and brown fingers on a darkroom bench are also how permanganate gets onto a print.

The acidified solution does not keep. Wall’s 1912 dictionary records that the acid diminishes the stability of the permanganate even in a very dilute solution, and offers potash alum in place of the acid to get a bath that keeps almost indefinitely. Kodak reaches the same practical rule from the other direction: keep the permanganate and the acid as two stocks that keep well, and mix them only when you are about to use them.

Handling

The harmonised classification under Regulation (EC) No 1272/2008 gives Danger with H272, H302, H361d — suspected of damaging the unborn child — H400 and H410. The aggregated ECHA notifications add H314, severe skin burns and eye damage, in 48.5 per cent of 472 reports, and the ILO-WHO card prints the same statement in its own classification under UN GHS criteria without any percentage beside it. The course carries H314. The full spread, including the 37 reports out of 472 that say the substance meets no criteria at all, is set out in the hazard note above and is not reconciled here.

Chemical Safety Card 0672 describes what the codes mean in practice, and the two sentences that matter most are about delay. Inhalation of the dust may cause lung oedema, “but only after initial corrosive effects on eyes and/or airways have become manifest”, and the effects may be delayed, so medical observation is indicated. The card’s standing instruction ends with a line it prints for very few substances: IN ALL CASES CONSULT A DOCTOR!

⚠The exposure limits are quoted as manganese, and they disagree by a factor of fifty

There is no workplace exposure limit anywhere for potassium permanganate under its own name. Every figure below is for manganese, applied to this compound as a manganese compound — which is an inference the course is making explicitly and not a reading anyone has published about this salt.

• HSE’s EH40, Table 1, Manganese and its inorganic compounds (as Mn): 0.2 mg/m³ inhalable fraction and 0.05 mg/m³ respirable fraction over eight hours, with no short-term limit and no Carc, Sen or Sk notation. This is the British figure and the one a COSHH assessment works to.
• The ILO-WHO card, quoting the ACGIH TLV as Mn: 0.1 mg/m³ inhalable and 0.02 mg/m³ respirable, with A4, not classifiable as a human carcinogen. It quotes an EU-OEL of 0.2 and 0.05, agreeing with EH40, and a German MAK of 0.2 and 0.02 with pregnancy risk group C.
• The NIOSH Pocket Guide, Manganese compounds and fume (as Mn): REL 1 mg/m³ as an eight-hour average with a short-term limit of 3 mg/m³, an OSHA ceiling of 5 mg/m³, and an IDLH of 500 mg/m³. NIOSH has no entry for potassium permanganate itself; its CAS index carries no 7722-64-7.

The spread between the ACGIH respirable figure and the NIOSH eight-hour figure is fifty-fold. The course quotes all three, names the spread and reconciles nothing, because the disagreement is between national authorities and not something a photography course can adjudicate. What follows practically is the same under any of them: the numbers are small, they are quoted as a fraction of an inhalable dust, and a purple powder that stains is a powder you can see travelling.

Why Level C. Three criteria of the rubric apply and any one of them would do. The first names reagents classified as reproductive toxins at the concentrations used, where a fume cupboard or specialist disposal is the recognised control — H361d is in the harmonised entry, which is the binding classification and not a minority notification, and Card 0672 asks for local exhaust for the dust. The fourth names any procedure whose waste cannot lawfully enter a domestic drain: the same card directs that this substance be stored in an area without drain or sewer access and names it a marine pollutant, which is about as direct a statement of that criterion as a safety card gets. And the corrosive statement makes the failure mode a burn rather than a spoiled negative, which is the Level B criterion exceeded rather than met.

Two things this level does not say. It does not say that a bath made up from a bought solution is Level C — the operation-sets-the-level ruling is explicit that a level belongs to a procedure, and the crystals on a balance are the procedure being classified here. And it does not say that permanganate is unavailable or exotic. It is sold as an antiseptic, a water-treatment chemical and a pond additive, and a reader will meet it under those names. A domestic article with a harmonised reproductive-toxicity statement and a fifty-fold disagreement about its exposure limit is exactly the sort of substance this encyclopaedia exists to slow a reader down over.

☠Concentrated sulfuric acid and permanganate crystals

CAMEO records that mixing potassium permanganate with concentrated sulfuric acid in a vessel containing moisture caused an explosion, through the formation of manganese heptoxide — a green oil that detonates. Grinding with antimony or arsenic ignites the metals; acidified solutions in contact with benzene, carbon disulfide, diethyl ether, ethanol, petroleum or organic matter can explode; glycerol may explode on contact; and permanganate transported through a polypropylene tube once ignited the tube.

Both published photographic formulas that pair permanganate with sulfuric acid — Kodak’s R-2 and ILFORD’s reversal bleach — do it as two separate dilute aqueous stocks mixed immediately before use, and both makers say so in print. That is not a stylistic choice about shelf life. It is the arrangement that keeps the two reagents from ever meeting except as dilute solutions, and it is why the formulas are printable at all. Nothing in this course brings the concentrated acid near the solid, in either order.

One more thing worth noticing about a modern sheet. ILFORD’s reversal-processing document carries a health and safety block at the top, and every word of it is about the sulfuric acid — highly corrosive, severe burns, may be fatal if swallowed, always pour the acid into the water. The permanganate is not mentioned in it at all. The sheet does the right thing by telling the reader to obtain and study the manufacturer’s safety data sheets for every substance, but a reader who reads only the warning box would come away thinking the acid was the whole of the hazard. It is not, and this page exists partly so that it can be looked up.

Waste

A spent permanganate bath is its own stream. The environmental half of this substance’s classification is the strongest part of its record — H400 and H410 in the harmonised entry, over 91 per cent agreement across the aggregated reports, marine pollutant on the safety card, and the storage instruction to keep it away from a drain or sewer. That instruction is about the unused solid; a used bath deserves no less.

A bath that has gone brown has thrown manganese dioxide, which is a solid and is collected rather than rinsed. A bath used as a reducer or a reversal bleach also carries the image silver it has just dissolved, so it is a silver-bearing stream as well, and under Kodak’s silver-recovery guidance a solution that has taken up image silver is a recovery stream rather than an effluent. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most often set, and an acid permanganate bleach sits well below it — but pH is not what decides this one, and neutralising it would not make it routable.

Bottle it separately, label it with the substance and the date, keep it away from anything combustible and from any organic waste, and take it to a licensed hazardous waste route; ILFORD’s guidance for domestic users in the United Kingdom points at a household waste and recycling centre. Check your local regulations under the disposal caveat; they govern, and they differ.

History

Permanganate reached photography as an established laboratory oxidant rather than as a photographic invention. Wall’s 1912 dictionary describes a manufacture that would be recognisable to a chemist of 1860 — fuse potash hydrate and chlorate with the black oxide of manganese, boil the product with water, purify and crystallise — and then lists the photographic uses almost as an afterthought.

Its career in the darkroom has been unusually stable. The reducer Wall prints in 1912, complete with the observation that the acidified bath decomposes spontaneously and the alum substitution that fixes it, is recognisably the bath Kodak printed as R-2 in 1928 and reprinted in London in 1949. The bleach ILFORD published in 2019 is the same two-stock arrangement made up rather stronger. The clearing bath has been bisulphite throughout. In more than a century the only substantial change is that the formula moved from the reduction of negatives, which few people now do, to reversal processing, which some still do.

What did change is the record around it. In 1912 the entry ran to a preparation, a solubility and three uses. Today the same crystal carries a harmonised classification with a reproductive-toxicity statement, an aquatic toxicity that puts it on the marine-pollutant list, an exposure limit that can only be quoted as manganese, and three national authorities who disagree about that limit by a factor of fifty. The chemistry is the same. What accumulated is the knowledge of what it costs.