Sodium cyanide

This is a study entry. Sodium cyanide is here because the cyanide era of photography is part of the subject and because a reader will meet the word on modern labels that have nothing to do with this salt. The page gives the chemistry, the hazard data and the historical role, and it gives no quantity, no bath, no time and no sequence, because the course classifies cyanide fixing at Level D and a Level D page is not a procedure.

In photography

The period literature says potassium, and this page says so. The nineteenth-century manuals name cyanide of potassium almost without exception; Hardwich’s 1864 account of the wet plate is written about that salt, and potassium cyanide carries the historical argument in this encyclopaedia. Sodium cyanide does the same chemistry with the same anion, and the course does not manufacture a history for it that the sources do not support.

What it does do is turn up in modern hazard documents. Florida Atlantic University’s health and safety guidance for photographic chemicals lists intensifiers containing “extremely hazardous components” and names five: mercuric chloride, mercuric iodide, potassium cyanide, sodium cyanide and uranium nitrate. That is the register a reader is most likely to meet the substance in — an institutional list of what a photographic laboratory must not casually keep.

⚗The same complex, by a shorter route

Fixing is complex formation. Two cyanide ions lift a silver ion out of the crystal lattice and hold it in solution as the dicyanoargentate ion:

AgBr + 2 CN⁻ → [Ag(CN)₂]⁻ + Br⁻

Cyanide dissolves silver bromide as a dicyanoargentate complex

Hardwich’s description of why the period preferred it is a description of this equilibrium: cyanide is a far more energetic solvent of the insoluble silver salts than hyposulphite of soda, and the double salt it forms is not decomposed by dilution with water, so less washing is needed than a thiosulfate fix demands. Both halves of that sentence are consequences of how stable the complex is — and so is the drawback, which is that a solvent this strong does not politely stop at the undeveloped halide. The sodium salt and the potassium salt give the identical anion in solution, so nothing in this equation distinguishes them.

The label most readers actually meet is not this substance. Photographers’ Formulary’s cyanotype kit sheet makes the distinction explicitly, because it has to: potassium ferricyanide contains cyanide groups, but they are chemically bound to the iron atom and not free to act as a poison, and the sheet states that potassium ferricyanide “is not the insidious poison that sodium cyanide is”. It then adds the condition under which that reassurance stops holding — strong acid, which releases hydrogen cyanide from the ferricyanide too, and which the cyanotype process does not use. A reader who has handled potassium ferricyanide in a Farmer’s reducer has not handled a cyanide in the sense this page means.

Properties

A white, hygroscopic, freely soluble solid, non-combustible in itself. The ILO-WHO card’s odour note is the one to remember: it has a characteristic odour but is odourless when dry, so smell is not a detector and never was.

The chemistry that governs everything else is that cyanide is the anion of a very weak acid, so its salts hydrolyse in water and the solution is alkaline:

CN⁻ + H₂O ⇌ HCN + OH⁻

Hydrolysis: a cyanide solution is always releasing a little hydrogen cyanide

That equilibrium sits well to the left, but it is never at zero, which is why a cyanide solution has a headspace even when nothing has been added to it. Add acid and the equilibrium is not nudged, it is demolished:

CN⁻ + H₃O⁺ → HCN + H₂O

Any acid drives the whole of the cyanide to hydrogen cyanide

Hydrogen cyanide boils at ordinary room temperature, so the product leaves the liquid as fast as it forms. NIOSH states the consequence in the fire section of its own entry, where a reader would least expect to find a toxicological warning: non-combustible solid, but contact with acids releases highly flammable hydrogen cyanide.

Handling

The aggregated ECHA notifications classify it Danger, and the three acute-toxicity statements arrive together and unanimously — fatal if swallowed, fatal in contact with skin, fatal if inhaled, each in 100 per cent of 433 reports. Skin absorption is not a footnote here; it is one of the three.

NIOSH sets a ceiling of 5 mg/m³ as CN over ten minutes, an immediately-dangerous-to-life level of 25 mg/m³, and four exposure routes including skin absorption. Its symptom list runs from lassitude, headache and confusion through nausea and vomiting to a breathing rate that first rises and then becomes slow and gasping, with the eyes, skin, cardiovascular system, central nervous system, thyroid and blood as target organs.

⚠Two limits for one substance, and the course quotes the tighter

HSE’s EH40 lists cyanides twice, and the two entries do not agree. The group entry — cyanides, except hydrogen cyanide, cyanogen and cyanogen chloride, measured as CN — sets a long-term limit of 5 mg/m³ with the Sk notation for skin absorption. The named entry for sodium cyanide, CAS 143-33-9, sets a long-term limit of 1 mg/m³ and a short-term limit of 5 mg/m³, also with Sk. Potassium cyanide is listed by name with the same pair of figures.

So the specific entries are five times tighter than the group entry that would otherwise cover them, inside one document, on one page. The course quotes the named entry, states the difference and does not attempt to reconcile them: which figure applies to a given task is a judgement for whoever is making the assessment, and a reader is better served by knowing that the regulator publishes both. For comparison, EH40 gives hydrogen cyanide itself 0.9 ppm long-term and 4.5 ppm over fifteen minutes, with Sk — and hydrogen cyanide is what the acid makes.

☠Why Level D

The classification rubric names cyanide fixing in its Level D criteria and the Level D policy argues it in full; this page cites both rather than restating them. What is specific to this entry is the geometry of a darkroom. The decisive hazard is not the salt’s own toxicity but its incompatibility, and a darkroom is a room that contains an acid stop bath by design. The failure mode is not a slow exposure to be managed down; it is one tray meeting another.

The second D criterion applies as well: a safer route exists and is taught instead. Thiosulfate does the same job by the same kind of chemistry, and the course teaches it throughout Part XI.

The one operation that would take this entry out of Level D is the removal of every acid from the room, which is not an operation a photographic darkroom can perform on itself.

Waste

There is no waste stream here, because there is no use. What is worth carrying away is why the ordinary photographic waste habit is exactly inverted for this substance. Spent fixer is usually treated by acidifying and precipitating; acidifying a cyanide bath converts it to gas. The Environment Agency’s WM3 puts the same fact in regulatory language, using sodium cyanide as its worked example for hazardous property HP 12, produces toxic gases in contact with water, air or acid, assessed against the 0.2 per cent concentration limit for EUH032 — and naming it again under HP 6, acute toxicity, and HP 14, ecotoxic. Check your local regulations; they govern.

History

The cyanide fixer belongs to the wet plate, and it went out with it. What did not go out is the word, and that is the reason this entry exists at the end of a batch of substances the course refuses. Three compounds in ordinary alternative-process practice have “cyan” in their names — potassium ferricyanide, potassium ferrocyanide and the Prussian blue that the cyanotype is made of — and none of them is this. The bound cyanide in an iron complex is not free cyanide, as the Formulary’s own kit sheet takes the trouble to say, and a reader who confuses the two will either be needlessly frightened of a cyanotype or dangerously relaxed about the real thing.

The course’s position is the one the nineteenth century arrived at slowly and for the wrong reasons: the chemistry mattered, the specific reagent did not, and a thiosulfate bath reaches the same equilibrium without putting a gas generator on the bench.