Sodium thiocyanate

Gold does not deposit on a print by itself. It has to arrive as a soluble complex, be reduced at the image silver and stay in solution everywhere else, and the ligand that does that in most of the published gold toners is thiocyanate. This is the salt those formulas mean.

In photography

Kodak’s gold protective solution. GP-1 is the simplest gold toner in the published literature and one of the most useful, because it is not really a toner: Kodak lists it as providing print protection against atmospheric contaminants, with only a very slight change of image colour. The formula is 750 mL of water at 20 °C, 10 mL of a 1 per cent gold chloride stock, and 15.2 mL of sodium thiocyanate (liquid), made to a litre. The mixing order is not optional — the gold stock goes into the 750 mL of water, the thiocyanate is dissolved separately in 125 mL, and the thiocyanate solution is then added slowly to the gold with rapid stirring. A washed print is treated for 10 minutes at 20 °C, or until a very slight change to a bluish black is visible.

The course cannot convert that 15.2 mL to a weight, and says so rather than guessing. G-23 gives the thiocyanate as a liquid and a volume, and does not state the strength of the liquid anywhere the course has read. Anyone working from a solid salt needs the concentration of the commercial solution Kodak assumed, which is not in the publication.

Reilly’s toner for albumen and salted paper. The conservation literature gives a formula in weights: sodium thiocyanate 15 to 20 g, 1 per cent gold chloride 60 to 80 mL, water to 1 litre. Reilly sets that beside a rule for gold content — 0.1 to 0.2 g of gold chloride per litre for matte salted papers, 0.4 to 0.5 g for glossy albumen — and the printed thiocyanate formula carries 0.6 to 0.8 g, above both. The course notes the gap and does not reconcile it: Reilly presents the formulas and the strength rule as separate guidance, and the reasonable reading is that the gold is adjusted to the paper, but he does not say so and the course will not put the sentence in his mouth.

⚗The chemistry

Thiocyanate does two jobs in a gold bath and they pull in different directions.

Against silver it is a solvent. OpenStax’s Appendix K gives

Ag⁺ + 4 SCN⁻ ⇌ [Ag(SCN)₄]³⁻

Formation constant 1.2 × 10¹⁰

which is about a thousand times the diamminesilver ion’s 1.7 × 10⁷ and a hundred thousand times the dichloroargentate ion’s 1.8 × 10⁵. That is strong enough to dissolve finely divided silver, and it is why a thiocyanate gold toner both plates gold onto the image and etches the image it is plating.

Against gold it is a stabiliser: it holds gold in solution as a thiocyanate complex so that the bath does not simply precipitate metallic gold in the tray. Reilly’s practical test for the state of a gold bath — that an acidic, inactive bath keeps a yellow colour and a working bath is colourless — is a statement about which gold species is present, and the course has found no source that assigns the colours to named complexes, so it reports the observation and not a mechanism.

As a silver solvent on its own. Wall’s 1912 dictionary states flatly that the alkali thiocyanates “are used as solvents of the silver haloid salts, also in compounding toning solutions”. The same property makes the salt a workable fixer and a developer additive; for the developer side, where the potassium salt is what Kodak specified, see potassium thiocyanate.

Properties

Colourless hygroscopic crystals or a white powder, deliquescent. Relative molecular mass 81.07. Chemical Safety Card 0675 gives a melting point near 300 °C, decomposition at 368 °C, a density of 1.7 g/cm³ and a solubility of 139 g per 100 mL of water at 21 °C — a figure worth having, because the potassium salt’s card gives none, so the two salts cannot be compared from the sources read.

CAMEO carries a second datasheet for the commercial solution at 56 per cent or less, an odourless clear to pale yellow liquid. That is a different article from the crystals, and it is what Kodak’s GP-1 is written around.

Handling

The 799 aggregated ECHA notifications classify it Danger: harmful swallowed, harmful in contact with skin and harmful if inhaled, at 100, 98.1 and 98.1 per cent of classifying reports. On the eye the notifiers split — 36.5 per cent give serious eye damage against 15.3 per cent for irritation — and this page takes the harsher reading. Chemical Safety Card 0675 adds what the codes omit: effects on the central nervous system, excitement and convulsions acutely, and on repeated exposure effects on the thyroid resulting in hypothyroidism. That last is why the card asks for it to be stored away from food. 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 classification is harmful rather than toxic, which would sit inside Level A. Two things take it past. The eye statement puts an open tray of it under Level B’s criterion of a procedure whose failure mode is a splash of something corrosive; and the incompatibility is not a concentration question at all — a thiocyanate and an acid in one room is a hydrogen cyanide problem, and Level B’s eyewash and spill kit are the answer to it. It does not reach Level C: nothing in the record carries a carcinogenicity, mutagenicity, reproductive or sensitisation statement, and the recognised control is separation rather than a fume cupboard.

⚠The one operation that takes this out of Level B

Bringing an acid near it. Card 0675 records a violent reaction with acids and the old EC entry carries R32, contact with acids liberates very toxic gas. In a darkroom that acid is the stop bath, the clearing bath or the acid hardening fixer, and the toning sequence puts all of them within arm’s reach. Give the thiocyanate its own cylinder and its own waste bottle; wash a print between the toner and anything acid; and never pour a spent toner into a bottle whose label you have not read. Wall said it in 1912 of the double cyanides as a class — they “should be regarded with extreme caution, as simple cyanides may be very readily produced from them under unexpected conditions.”

Waste

A spent gold toner carries three things worth keeping out of the drain: dissolved silver etched off the print, unspent gold, and the thiocyanate itself, which is classified harmful to aquatic life with long lasting effects. Recover the metals first. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most often set, and a thiocyanate toner sits inside it, which again makes the pH the wrong question. Bottle it, label it, keep it away from every acid waste, and follow the disposal caveat and your local authority; ILFORD’s route for domestic users in the United Kingdom is a household waste and recycling centre’s chemical cupboard.

History

Every nineteenth- and early twentieth-century formula that calls for sulphocyanide is calling for this salt or its potassium twin. Wall’s dictionary explains the double name — whether CNS is treated as a halogen-like radical or as a cyanate with sulfur for oxygen — and records that photographic books of the period preferred “sulphocyanide”, which is why the word survives on old labels and in reprinted formulas long after chemistry settled on thiocyanate. The substance did not change; only the reading of its constitution did.