Chemical incompatibilities

A grid of crosses tells a reader that two bottles must not meet and gives them no way to recognise the next pair the grid does not list. This page gives the reaction instead. Read it once and the pattern behind two-thirds of the entries becomes obvious: an acid liberates a volatile weak acid from its salt, and photography’s shelves are full of the salts of volatile weak acids.

The storage lesson sets out the six separation groups a home laboratory sorts into and why sorting by group rather than alphabetically is what prevents these accidents. This page is the reference the labs cite for the pairs themselves.

The matrix, with the product in each cell

Read across: what the row meets, what the column is, what forms. A blank is not a permission — it is a pair the course has no sourced reaction for.

	Acids	Alkalis and ammonia	Oxidisers	Silver salts
Sulfites, bisulfites, metabisulfites	SO₂	—	heat, and products that may be flammable	—
Thiosulfates	SO₂ + colloidal sulfur	—	tetrathionate, which attacks an image	insoluble silver salts, then complexes
Sulfides	H₂S	—	violent reaction	the sulfide toning reaction itself
Cyanides and ferricyanides	HCN	—	fusion may explode	—
Hypochlorite bleach	Cl₂	—	—	—
Silver nitrate	—	Ag₃N, with ammonia	—	—
Alcohols	—	—	fire; silver fulminate with silver nitrate	silver fulminate
Oxalic acid and oxalates	—	salt formation	violent reaction	silver oxalate, explosive
Aluminium, tin, zinc	H₂	H₂	—	—
Organic material: paper, cloth, wood	charring, with concentrated acid	—	fire	fire as it dries

Each of those cells is unpacked below, with its evidence.

The acid-and-its-salt group

Acid with sulfite, bisulfite or metabisulfite: sulfur dioxide

Acidify a sulfite and you re-form sulfurous acid, which is unstable and gives up sulfur dioxide.

Na₂SO₃ + 2 HCl → 2 NaCl + H₂O + SO₂

A strong acid meeting a sulfite

CAMEO records that acids, hot water and steam produce corrosive material from sodium sulfite, and that acids liberate gaseous sulfur dioxide from sodium bisulfite and from the metabisulfites. Kodak’s industrial version of the rule is a waste rule: acid cleaning solution is collected separately to prevent the emission of sulfur dioxide when it mixes with developer and other processing solutions.

Why it matters at darkroom scale. EH40 gives sulfur dioxide a long-term limit of 0.5 ppm and a fifteen-minute limit of 1 ppm, among the tightest gas limits in the course. A spilled acid finding a tub of sulfite is a room full of an irritant gas; the same mixture made inside a closed waste bottle is worse, because it is made under a cap you will later unscrew.

Acid with thiosulfate: sulfur dioxide and a cloud of sulfur

Na₂S₂O₃ + 2 HCl → 2 NaCl + H₂O + SO₂ + S

A strong acid decomposing hypo

Kodak’s 1928 primer records the observation directly: a few drops of a strong acid in a weak hypo solution decompose it, and the solution turns milky as sulfur precipitates, because the acid frees thiosulfuric acid which breaks down to sulfurous acid and sulfur. That milkiness is the diagnostic sign, and it is why a spent fixer that has met a stop bath both smells and clouds.

This is the single most likely incompatibility accident in a home darkroom, because the two liquids are adjacent in the tray sequence and adjacent again in the waste corner.

Acid with sulfide: hydrogen sulfide

The Environment Agency’s waste guidance writes the reaction out, because it is the basis on which a waste is assigned the hazard statement for liberating an acute toxic gas on contact with acid.

Na₂S + 2 H⁺ → H₂S + 2 Na⁺

Any acid meeting a sulfide

CAMEO states the incompatibility plainly for sodium sulfide, and Kodak’s toning sheet gives the photographic form of the rule: sulfide toners must not be discarded with stop baths or fixing baths, because the combination generates hydrogen sulfide. Princeton’s guidance requires a print to be rinsed thoroughly after an acid bleach before it enters a sulfide or thiourea toner, which is the same rule applied to the acid a print carries on its surface. EH40 gives hydrogen sulfide 5 ppm long-term and 10 ppm over fifteen minutes.

Sulfide has a second, non-toxicological incompatibility worth stating in the same place: Kodak’s primer records that a very small quantity of hydrogen sulfide converts enough silver halide to sulfide to produce severe fog, and that no photographic material should be stored in a room where sulfides are kept or sulfide toning is done. The gas that hurts you also ruins the paper.

Acid with cyanide: hydrogen cyanide

KCN + HCl → HCN + KCl

Any acid meeting a cyanide

CAMEO records that potassium cyanide reacts with acids to generate poisonous hydrogen cyanide, and that in acidic water dangerous amounts form at once. EH40 gives hydrogen cyanide 0.9 ppm long-term and 4.5 ppm over fifteen minutes, with the Sk notation for skin absorption. Cyanide fixing is Level D in this course and no reader will hold the salt; the pair is on this page because the second half of it is not.

The ferricyanide that is not a cyanide, and the ways it can become one. Potassium ferricyanide is the reducer and the cyanotype reagent, and its cyanide is locked to iron in a complex. The international chemical safety card records that it reacts with acids to generate a toxic hazard and that the solid decomposes on heating to gases including hydrogen cyanide. Princeton’s guidance is more specific and more useful: potassium ferricyanide will release hydrogen cyanide if heated, if hot acid is added, or on exposure to strong ultraviolet such as a carbon arc, and it records that cases of cyanide poisoning have occurred through treating Farmer’s reducer with acid. The course writes no equation for that decomposition, because it has not read a balanced one in a source that meets its standard.

Acid with hypochlorite bleach: chlorine

NaOCl + 2 HCl → NaCl + H₂O + Cl₂

Household bleach meeting an acid

Princeton’s guidance states it directly: hypochlorite bleaches can release highly toxic chlorine gas when acid is added or when heated. This is a domestic hazard rather than a photographic one, and it belongs here because a darkroom is often a kitchen and the bleach is under the sink. Kodak gives the plumbing version, which is the one people actually meet: never pour photographic processing chemicals into a drain where chlorine-containing cleaning agents are present unless the drain has been thoroughly rinsed, and run plenty of water before and after. A stop bath poured onto bleach standing in a trap is the same reaction in the one place you cannot ventilate.

⚗The pattern, stated once

Sulfite, thiosulfate, sulfide, cyanide and hypochlorite are all salts of weak or unstable acids whose free form is a gas or decomposes to one. Add a stronger acid and you displace it. That single sentence predicts five of the entries above, and it will predict the next one you meet — which is why the course teaches it rather than listing the pairs and stopping.

Silver, which is in two groups at once

Silver nitrate with ammonia: silver nitride

This is the one genuinely explosive possibility in an ordinary photographic laboratory, and it is worth stating precisely rather than dramatically.

Add ammonia to a silver nitrate solution and you first precipitate silver oxide, which redissolves in excess ammonia to give the colourless diammine silver complex. Mike Ware’s cautionary note is that solutions of diammine silver can, with time, deposit a black solid precipitate of silver nitride — “fulminating silver” — which is a highly sensitive contact explosive, detonating at a touch and sometimes even when wet, and that storing such solutions is not recommended. CAMEO records a case in which 28 per cent ammonium hydroxide and silver nitrate solution treated with sodium hydroxide gave a black precipitate that exploded on stirring, and NIOSH’s silver entry lists ammonia second among its incompatibilities, after acetylene.

The course writes no equation for the formation, because it has not read a balanced one it can verify. The rules that follow need none. Ammonia solution is never stored next to silver nitrate. No ammoniacal silver solution is ever stored at all, or allowed to dry out. Where a later part uses an ammoniacal silver step, it is made immediately before use and quenched immediately after, and the page that does so says so.

Silver nitrate with alcohols: silver fulminate

CAMEO records that silver nitrate with alcohols may produce silver fulminate, which can explode when disturbed, and that mixtures with alkyl esters may explode through the formation of alkyl nitrates. The international chemical safety card for ethanol comes at the same pair from the other side, naming silver nitrate among the strong oxidants with which ethanol reacts violently.

What that forbids, and what it does not. It forbids mixing concentrated silver nitrate solution with alcohol, and it forbids storing the two together. It is not a statement about the small volumes of ethanol that several of this course’s emulsion formulas add to a made emulsion, where the silver has already been precipitated as halide and the alcohol is a few per cent of a gelatin-thickened batch. That distinction is the course’s reading of the sourced incompatibility rather than something the sources state, and it is flagged as such: what the sources support without interpretation is the storage rule and the prohibition on mixing the concentrates.

Silver compounds with oxalic acid and oxalates: silver oxalate

NIOSH’s pocket guide lists silver compounds outright among oxalic acid’s incompatibilities, and the international chemical safety card explains why: with certain silver compounds it forms explosive silver oxalate. NIOSH’s own silver entry returns the compliment, listing oxalic acid.

This is not a theoretical pairing in this course. The iron and iron-silver processes use ferric oxalate and potassium oxalate; the printing-out processes and the emulsion laboratory use silver nitrate. Oxalate waste and silver-bearing waste are never combined, and neither are the solutions on the bench.

Silver nitrate with organic material

Silver nitrate carries H272, may intensify fire; oxidizer, and CAMEO records that it accelerates the burning of combustible materials and blackens on contact with organic material as readily as in light. An oxidiser stored above a shelf of paper, cloth or a wooden bench is a fire question as well as a stain question, and a sensitised sheet drying in the dark is loaded paper.

The rest, each with its reason

Oxidisers with reducing and developing agents

Within the darkroom’s own inventory the relevant pairing is oxidiser against developing agent. NIOSH lists strong oxidisers and alkalis as hydroquinone’s incompatibilities; the acetic acid card names chromic acid, sodium peroxide and nitric acid. Persulfates, permanganates and — in the historical parts — the dichromates arrive as bleaches and reducers, and they belong on a shelf of their own from the day they arrive.

Two of those pairs are used deliberately and safely only because of how they are handled. The acid permanganate bleach exists because permanganate and sulfuric acid are kept as separate dilute stocks and combined immediately before use; Farmer’s reducer exists because ferricyanide and thiosulfate are mixed immediately before use and discarded after, since the mixture decomposes rapidly. A deliberate incompatibility is still an incompatibility, and it is never stored.

Concentrated alkali with aluminium, tin and zinc: hydrogen

NIOSH lists metals such as aluminium, tin and zinc among sodium hydroxide’s incompatibilities, and the international chemical safety card records that the caustic corrodes them with evolution of hydrogen, a combustible and explosive gas. Nothing in this course asks you to put alkali in a metal vessel; the pair is here because aluminium is everywhere in a kitchen — foil, a baking tray pressed into service as a drip tray, a pan used as a water bath. Use plastic.

Solid alkali with water, and concentrated acid with water

Both are heat, and the rule that follows is the same in both directions: the substance goes into the water, never the water into the substance.

CAMEO states that dissolving sodium hydroxide can liberate enough heat to cause steaming and spattering and to ignite adjacent combustible material; HSDB gives potassium hydroxide a heat of solution of 53.51 kJ/mol. For concentrated sulfuric acid the international chemical safety card records violent reaction with water generating heat and a fire or explosion hazard, and the card for nitric acid prints the instruction in capitals: never pour water into this substance.

Formaldehyde with hydrochloric acid: bis(chloromethyl) ether

Both NIOSH and CAMEO record that formaldehyde reacts with hydrogen chloride to form bis(chloromethyl) ether, which CAMEO describes as highly toxic. Formaldehyde is excluded from this course as a hardener on the emulsion literature’s own grounds before the hazard record is reached, but the pair belongs here because formalin is still sold and still met.

Chromium(VI) with anything combustible

NIOSH’s incompatibility list for chromates is paper, wood, sulfur, aluminium and plastics, and CAMEO records that ammonium dichromate detonates on heat or mechanical shock and, confined, turns its own decomposition into a burst container. Chromium(VI) is never used at any level in this course; see the chromium policy for the rule and for the chromium(III) salt that is a different substance.

Selenium and thiourea toners with acid

CAMEO’s reactivity profile for sodium selenite is a single sentence — incompatible with strong acids — and the safety card records that strong acids generate a toxic hazard from it. Thiourea’s profile records violent decomposition with hydrogen peroxide and with nitric acid, and thermal decomposition to very toxic oxides of nitrogen and sulfur. Princeton’s rule covers both in practice: a print is rinsed thoroughly after an acid bleach before it enters a toner.

Mercury(II) chloride with gelatin, alkalis and sulfites

CAMEO lists gelatin and albumin among mercury(II) chloride’s incompatibilities, which is the same fact the historical darkroom recorded from the other end — the mercury salt is tenaciously held by the gelatin of a bleached plate and needs an acid wash to remove it. Alkalis, ammonia, carbonates, sulfites, sulfides and bromides are all on CAMEO’s list, and several of them were the second bath of the historical intensifier, which is to say that the process was an intended incompatibility. Mercury is Level D; the entry is here for completeness of the matrix.

Using this on your own shelf

Three habits do most of the work, and none of them requires remembering the table.

Sort by group, not alphabetically. Acids away from everything on this page. Silver nitrate on a shelf of its own, because it belongs to two groups and takes the stricter rule of both. Ammonia with the alkalis and never beside the silver.

Give the waste containers the same separation as the shelf. Waste is where people relax, and the acid container is the one that gets used as a general receptacle at the end of a tiring session. Every pair above that produces a gas produces it just as readily inside a capped bottle.

When you meet a pair that is not here, ask the two questions that generate most of it. Is one of them a salt of a weak or unstable acid, and is the other an acid? Is one an oxidiser and the other something that burns? If either answer is yes, look it up before you pour.

⚠What this page does not cover

It lists the pairs the course has read in a tier 1 or tier 2 source. It is not a complete incompatibility list for any substance, and a blank cell in the matrix means the course found no sourced reaction, not that the pair is fine. The safety data sheet for the product in your hand, section 10, is the authority for that product; each chemical page in the encyclopaedia carries its own sourced incompatibility list, which is longer and more specific than anything a shared page can be.