Ammonium thiocyanate

If you buy a gold toning kit for salted paper, albumen, Van Dyke, kallitype or printing-out paper, this is the bottle marked A. Bostick & Sullivan’s kit is 500 mL of a 2 per cent ammonium thiocyanate solution and 500 mL of a 0.2 per cent gold chloride solution, and the toning bath is 50 mL of each in a litre of water — so the working bath, 1.1 litres in all, is about 0.09 per cent thiocyanate and 0.009 per cent gold chloride.

In photography

It is the ligand in a printing-out gold toner. The kit instructions record the visible sign that the chemistry has worked: on first mixing, the gold “will momentarily turn a brown or orange color in the toning bath but should quickly redissolve into solution”. That colour change is gold being complexed. The bath then cools a printed-out silver image from red-brown towards blue, maroon or purple, and the kit gives the reason a printer cares: gold plating enhances permanence and prevents loss of values through the fixing stage.

Wall’s formulary gives the same bath in weights. His normal sulphocyanide gold bath is 0.25 g of gold chloride and 3.9 g of “ammonium or potassium sulphocyanide” in a litre of boiling water, the gold dissolved in half the water and added to the thiocyanate dissolved in the other half. A red-tone variant adds 0.10 to 0.15 g of potassium iodide to 5 g of the ammonium salt and 0.25 g of gold. Valenta’s combined toning and fixing bath puts 25 g of ammonium sulphocyanide with 200 g of hypo, 10 g of lead nitrate and 20 g of alum in 500 ccm, heated to 50 °C for 10 minutes and filtered. And Puddy’s reducer, which acts on the shadows before the highlights, is 50 g of ammonium persulphate with 25 g of ammonium sulphocyanide in a litre — a rare appearance of the thiocyanate outside toning.

⚗The chemistry

Thiocyanate is a strong ligand for silver. OpenStax’s Appendix K gives

Ag⁺ + 4 SCN⁻ ⇌ [Ag(SCN)₄]³⁻

Formation constant 1.2 × 10¹⁰

That is what makes it a silver solvent, and it is also why the National Bureau of Standards found it cannot do the one thing ammonia does. Carroll, Hubbard and Kretschman’s study of hypersensitization turns on the charge of the complex: ammonia forms a positively charged silver complex which breaks down on drying and leaves an excess of silver in the emulsion, whereas “if the complex ion containing the silver is negatively charged, as in cyanide, thiosulphate, sulphite or thiocyanate solutions, its distribution will follow the same ratio as the bromide ion and there will be no excess of silver left in the emulsion and no hypersensitization.”

So a thiocyanate bath dissolves silver halide but does not ripen or hypersensitize a plate. That is a sourced negative result, and it is the reason this page’s photographic account is about toning.

A note on substituting one thiocyanate for another. Wall prints a single weight, 3.9 g, for “ammonium or potassium sulphocyanide”. The two salts have relative molecular masses of 76.12 and 97.18, so 3.9 g of the ammonium salt is 0.051 mol of thiocyanate against 0.040 mol for the same weight of the potassium salt — the ammonium salt delivers 28 per cent more. Wall does not say whether he means the substitution to be made by weight or by mole, and the course cannot tell from the text. If you are following a printed formula, follow it as printed and note which salt you used.

Properties

Colourless crystals, very deliquescent. Relative molecular mass 76.12 — the lightest of the three alkali thiocyanates the course uses, so a gram of it carries the most thiocyanate. Freely soluble in ethanol, soluble in methanol and acetone, practically insoluble in chloroform and ethyl acetate.

On solubility in water there is no number, and this page will not invent one. HSDB says “very soluble” and stops; there is no ILO-WHO Chemical Safety Card for the ammonium salt, where the sodium salt has one giving 139 g per 100 mL at 21 °C. The two are different salts and the figure does not transfer. At the concentrations photography uses — a 2 per cent stock at most — nothing turns on it.

Handling

The 347 aggregated ECHA notifications classify it Danger and harmful by all three routes: 99.7 per cent give harmful if swallowed, 99.1 per cent harmful if inhaled and 98.8 per cent harmful in contact with skin. Serious eye damage is a minority view at 38.6 per cent, and this page takes it. CAMEO’s datasheet says something that has to be quoted with its counterweight: the substance “contains no cyanide and therefore has low acute toxicity”, but adds in the same breath that “prolonged exposure may produce skin eruptions and sometimes mild or severe disturbances of the nervous system”. Both halves are true. Thiocyanate is not cyanide; it is also not harmless, and an acid will convert some of it into the thing it is not. Neither EH40 nor the NIOSH Pocket Guide sets an exposure limit, and EH40 states that absence from its list does not indicate that a substance is safe.

Why Level B. Against the course rubric the acute classification is harmful rather than toxic, and at the 0.1 per cent of a working toning bath that would sit comfortably inside Level A. What takes it past is the 2 per cent stock and the solid: an eye-damage statement in an open tray meets Level B’s criterion of a procedure whose failure mode is a corrosive splash, and the acid incompatibility is not a concentration question at all. It does not reach Level C — no carcinogenicity, mutagenicity, reproductive or sensitisation statement appears anywhere in the record, and the recognised control is separation from acids rather than engineered extraction. A printer who buys the kit and never opens a jar of solid is handling a 2 per cent solution and can work at Level A controls plus goggles, which is the argument for buying it made up.

⚠The one operation that takes this out of Level B

Putting it in the same sequence as an acid without a wash between. A printing-out workflow runs first wash, gold toner, fixer, wash — and if the fixer is an acid hardening fixer, or the paper has been through a citric acid clearing bath, an acid and a thiocyanate are in adjacent trays. The ILO-WHO card for the sodium salt records a violent reaction with acids and the risk phrase “contact with acids liberates very toxic gas”; the course applies that here by inference from the shared anion, because no card exists for the ammonium salt. Wash between baths, use a plain rather than an acid fixer where the process allows it, dedicate the cylinder, and never combine the wastes.

Waste

Spent toner and the wash after it, carrying dissolved silver and unspent gold; recover both before anything else. The H412 classification is a reason to bottle rather than pour even where the pH would allow it, and Kodak’s J-52 pH window of 5.6 to 9.4 is satisfied by a thiocyanate toner, which is exactly why pH is the wrong test here. ILFORD’s guidance to domestic users in the United Kingdom is to bottle wastes separately, label them and take them to a household waste and recycling centre; the disposal caveat governs and local regulation decides. One absolute: never the same bottle as an acid waste.

History

Wall’s 1912 dictionary gives the salt its formula as NH₄CNS = 76, records that it is “used in toning gelatino-chloride printing-out papers”, and then delivers a verdict that has held for a century: it “has also been recommended as a fixing agent instead of hypo, but from its comparatively high price, without any increased advantages, it is hardly likely to come into general use”. A thiocyanate fixer works. It has never been worth the money.

What did come into general use was the toning bath, and it is still sold that way. The kit a printer buys today for salted paper or Van Dyke is the same two-solution chemistry Wall printed — a thiocyanate and a gold chloride, mixed cold, the gold flashing orange and redissolving — at about a third of his gold concentration — 0.09 g of gold chloride per litre against his 0.25 g.