Potassium hydroxide (caustic potash)

Two strong alkalis sit on the darkroom shelf and only one ion in either of them does any work. Kodak’s 1928 primer disposes of the difference in two sentences: caustic potash is very similar to caustic soda and is prepared in the same way, and fifty-six parts of caustic potash are chemically equivalent to forty parts of caustic soda. That ratio, 1.40, is the whole of the arithmetic, and knowing why it holds is worth more than memorising it.

In photography

The hydroxide does the developing; the metal is a spectator. Both salts dissociate completely in water, and what a developing agent responds to is the hydroxide ion concentration. So the two are interchangeable at equal molar amounts, and because a mole of potassium hydroxide weighs 56.1 g against 40.0 g for the sodium salt, you weigh 1.4 times as much to get the same alkalinity. Kodak’s figure of 56 to 40 is exactly that ratio of molar masses.

⚗The chemistry

Developing agents work as anions. Hydroquinone, for instance, only becomes a strong reducing agent once its hydroxyl groups have lost protons, and the concentration of that active form is set by the pH. A strong base supplies hydroxide in full and at once, so the agent is driven into its active form and development is fast — and the pH then falls steadily as development itself releases acid, because a strong base is not a buffer and holds no reserve. Swap the two at equal moles and nothing about that changes. Swap them gram for gram and it does: a gram of sodium hydroxide carries 40% more hydroxide than a gram of the potassium salt, so substituting one way over-alkalises the bath by 40% and the other way leaves it nearly 30% short.

It belongs to the hydroquinone-caustic developers. The 1928 primer explains why: developing agents of low reduction potential need the strongest alkali, and hydroquinone’s is the lowest of the family. Wall’s 1924 collection gives the pattern in practice — a two-solution hydroquinone developer for black-and-white line work with hydroquinone, potassium metabisulfite and potassium bromide in part A and 17 g of caustic potash per litre in part B, mixed in equal parts; and Lainer’s rapid soft-working developer with 50 g per litre in its alkali stock, mixed ten parts A to one part B and finished in about a minute. Lainer’s one-solution version of the same idea carries 165 g of caustic potash per litre before its fourfold dilution, which is as concentrated an alkali as a photographic formula gets. Wall’s 1912 dictionary describes the material as it was sold: white sticks, poisonous, burning to the skin, used in alkaline developers, extremely deliquescent.

It also cleans. Wall’s photomechanical section soaks old collotype plates in a 5% caustic potash solution for four days before the old film is scraped off — the alkali attacking hardened gelatin exactly as it softens an emulsion left too long in an over-alkaline bath.

Why choose it over caustic soda? The course could not verify a photographic reason from its sources. The 1928 primer offers only the equivalence and the phrase “very similar”, and PubChem’s use list places both in photographic processing without distinguishing them. What is documented is a practical difference: the potassium salt is the more deliquescent and the harder to keep at a known strength. Where a formula names one, weigh that one, or convert by molar mass and record that you did.

Properties

A white hygroscopic solid, sold as lumps, rods, flakes, sticks or pellets and sometimes as a solution of stated strength; melting point 380 °C, density 2.04 g/cm³. Solubility rises steeply with temperature — HSDB gives 97 g per 100 g of water at 0 °C, 112 g at 20 °C and 178 g at 100 °C.

Dissolution is strongly exothermic. HSDB puts the heat of solution at 53.51 kJ/mol in water and adds that much heat is generated when it dissolves in water or alcohol, or when the solution is treated with acid. Fifty grams — the alkali stock of Lainer’s developer — is close to 0.9 mol, so nearly 48 kJ goes into the water as the solid disappears: enough to lift a litre by something over 10 °C, and far more than that locally where the pellets sit. CAMEO flags the substance water-reactive for exactly this reason, and Kodak’s 1928 primer directs that both caustic alkalis be dissolved in cold water, because a solution that gets too hot is apt to boil and spatter on the hands or face.

Deliquescence is the other half of the problem. HSDB records that it readily absorbs moisture and carbon dioxide and deliquesces, forming a syrup. Both changes cost strength: water dilutes it, and carbon dioxide converts it to potassium carbonate, a far weaker alkali. A jar that rattled when new and pours as a slush a year later is no longer the reagent the formula assumed.

Handling

The aggregated ECHA notifications classify it Danger, and one statement separates it from its sodium counterpart: H302, harmful if swallowed, appears in 96.1% of the 4,709 reports, alongside H314 in 99.4%. Chemical Safety Card 0357 records serious local effects by all routes of exposure and calls the substance very corrosive to the eyes, skin and respiratory tract, with dermatitis from repeated skin contact. HSE’s EH40 sets a short-term exposure limit of 2 mg/m³ and no long-term one; NIOSH sets the same figure as a ceiling.

Why Level B. The course rubric puts the handling of a concentrated alkali at Level B, and the second criterion applies with equal force: the failure mode here is a splash, a burn or a spill of something corrosive rather than a spoiled negative, and the splash can be hot because the solution heats itself as it forms. It stays below Level C because no fume cupboard is the recognised control — the controls are splash protection, cold water and eyewash — and because the classification carries no carcinogen, mutagen, reproductive-toxin or sensitiser entry.

⚠Safety

Chemical Safety Card 0357 gives the rule in capitals: NEVER pour water into this substance; when dissolving or diluting always add it slowly to the water. Cold water, the solid in small portions, stirring throughout, and an open vessel with room for the heat to escape. Goggles, nitrile gloves and an apron before the lid comes off; eyewash within reach. Keep it from every acid in the room, and from aluminium and zinc vessels, which the caustic solution attacks with evolution of hydrogen.

KOH → K⁺ + OH⁻

Complete dissociation: the same hydroxide, 56 g of it where soda needs 40

Waste

Spent caustic developer leaves strongly alkaline. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most frequently set, along with a five-day biochemical oxygen demand of 350 mg/L and a silver limit of 1.2 mg/L; a caustic bath is far outside the pH window, so dilution or neutralisation is the first question a disposal route must answer, and any bath that has fixed film goes for silver recovery first.

Neutralising it is a real reaction with a real exotherm — HSDB notes that treating the solution with acid generates much heat — so it is done in an open vessel with room to rise, never in a sealed bottle, and never by tipping caustic and acid wastes into one container to save space. 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.

History

Potash is the older of the two alkalis by centuries — the word records the ash it was leached from — but Kodak’s 1928 primer describes both caustic alkalis as products of the same modern industry: electrolysis, or causticising the carbonate with lime. What changed over the following century was not the ion but the formula around it. The hydroquinone-caustic developers Wall collected in 1924 are line and copy developers, made to give a fully black or fully clear result in a minute or two; as practice moved towards continuous tone, fine grain and long tank life, the alkali of choice moved down the scale — first to the carbonates, then to borax and the metaborates, whose entire virtue is that they refuse to hand over their alkali all at once.