Gold(III) chloride

Read any toning formula published between 1850 and 2006 and it will call for gold chloride. Kodak’s G-23 sheet asks for one gram of it in Gold Toner T-21 and for a 1 per cent stock solution in GP-1 and T-26; Wall’s 1924 formulary measures it out in grains; Reilly’s alkaline baths take 40 ml of 1 per cent solution per litre. None of them means the substance on this page. Reilly says so outright: the gold chloride sold to photographers is always an acidic substance, technically chlorauric acid, and true gold chloride — made by passing chlorine gas over gold leaf at elevated temperature — is unstable and not usually commercially available. This page exists so that the distinction is written down somewhere, because it is the kind of gap that swallows a gram of gold.

In photography

It is the name, not the reagent. Every practical role belongs to chloroauric acid: toning printed-out silver on salted paper, albumen and printing-out paper; protecting a silver gelatin print; making a blue tone with thiourea; and, in Ware’s New Chrysotype, printing in gold outright. Ware’s own list of alternative names for the acid trihydrate ends with the note that the earlier literature, “which was ambiguous and inaccurate”, called it gold chloride, and his list even includes “gold(III) chloride hydrate” — because that is what some suppliers print on the label. The two substances are not interchangeable, and the difference is quantitative.

⚗What the substitution costs

Ware’s table of gold compounds gives the gold content of each by mass. Gold(III) chloride, Au₂Cl₆, relative molecular mass 606.66, is 64.93 per cent gold. Hydrogen tetrachloroaurate trihydrate, H[AuCl₄]·3H₂O, relative molecular mass 393.83, is 50.01 per cent gold; the sodium salt dihydrate is 49.51 per cent, which is why Ware says the two can be used interchangeably to within one per cent while the true chloride cannot. Weigh out a gram of gold(III) chloride where the formula meant a gram of the acid and you have put in about thirty per cent more gold than the recipe intended — an expensive error in the direction that also over-tones the print, since Reilly warns that too strong a bath deposits gold superficially and the layer is then largely removed in the fixer.

Ware’s assay formula makes the same point in the direction of use: for a 0.35 molar gold stock solution, V = 0.1451 × A × W cubic centimetres, where A is the assayed percentage of gold in the compound and W the weight taken. Change the compound and A changes, and the volume of water with it.

Where the gold ends up is the same either way. Both compounds carry gold in the +3 oxidation state, and the toning chemistry set out on the chloroauric acid page — three silver atoms lost for each gold atom deposited from gold(III), one for one once the bath has been brought to gold(I) — depends on the oxidation state and the ligand, not on which solid was weighed. What the course has not verified is a primary source for the speciation of dissolved gold(III) chloride in a photographic bath, and so this page writes no equation for it. The practical instruction stands without one: use the reagent the formula was written for, and if the label says gold(III) chloride hydrate, read the assay rather than the name.

Gold in an emulsion is a different use again, and a different salt. Part IV’s account of gold sensitisation rests on Ilford’s 1946 patent by Waller, Collins and Dodd, which adds small quantities of a soluble gold salt before or during digestion, at a pH not greater than about 8, for a considerable increase in sensitivity. The patent lists the salts it means — potassium aurate, potassium chloraurite, sodium chloroaurate, potassium auricyanide, potassium aurithiocyanate and their relatives, along with complex organic gold salts such as pyridine trichlorogold — and its worked examples use 1:10,000 solutions of sodium chloroaurate and of potassium chloroaurite. Simple gold(III) chloride is not among them. The quantities are so small, the patent notes, that they make no appreciable difference to the tone of the developed image, and too much gold loses the speed gain rather than increasing it.

Properties

The formula unit is a convenient fiction. PubChem’s record gives AuCl₃ with a molecular weight of 303.32 and the IUPAC name trichlorogold, and ChEBI classes it as a gold coordination entity. Ware’s table lists the same substance as Au₂Cl₆ at 606.66 — the dimer, which is what the solid actually is. The two descriptions agree on composition and disagree on molecularity, and a recipe in grams is unaffected either way, but anyone converting to moles needs to know which unit is meant.

Colour and solubility. Haz-Map’s compilation gives Hawley’s description as a yellow to red solid, soluble in water, and a supplier’s sheet as a red odourless crystalline solid soluble to 680 g/L at 20 °C. That figure is the only quantitative solubility the record carries, it comes from a safety data sheet rather than from a determination the course can trace, and it is reported here as such. Note the contrast with the acid, whose defining physical property is deliquescence: no source in this course’s corpus describes gold(III) chloride as deliquescent, and the course does not assume it.

Oxidising, and gold-bearing. Ware’s general warning about the gold salts — corrosive and oxidising, attacking most metals and organic matter, staining skin a spectacular purple — applies here. Gold(III) is the most readily reduced of the noble-metal states the alternative processes use; Ware’s table of redox potentials gives the tetrachloroaurate couple at +1.00 V, above silver at +0.80 V and well above platinum and palladium, which is why gold plates onto image silver spontaneously and why a gold solution left with anything organic will not stay a gold solution.

Handling

The aggregated ECHA notifications classify gold trichloride Danger, with the corrosive and irritant pictograms. The aggregation is thin — 51 reports across 7 notifications — and it is split. Just under nine in ten notifiers give skin irritation, serious eye irritation and respiratory irritation; the remaining one in eight give severe skin burns and eye damage. Japan’s NITE-CMC classifies the same substance separately and lands with the smaller group, adding an allergic skin reaction. Where a classification is divided, the course follows the more severe reading and says that it has done so.

Why Level B. Against the course rubric, Level A is capped at substances classified at most as irritant, harmful if swallowed, or corrosive at the concentrations actually handled — and on the majority reading of the notifications this substance would sit inside that cap. Two things take it past. The first is the minority-plus-NITE-CMC classification as corrosive, with an allergic skin reaction, which brings it under Level B’s first criterion covering corrosive reagents and sensitisers. The second is the respiratory irritation statement carried by 88.2 per cent of the notifiers, which puts a fine solid weighed on an open bench under the same criterion’s clause on powders that must not be inhaled. It does not reach Level C: that criterion is qualified by the words “where a fume cupboard or specialist disposal is the recognised control”, and the classification carries no acute toxicity, no carcinogenicity and no respiratory sensitisation. Neither HSE’s EH40 nor the NIOSH pocket guide sets an exposure limit for gold compounds, and EH40 states that absence from its list does not indicate that a substance is without hazard.

⚠Safety

Gloves, splash goggles, and no dust: weigh over a tray, in still air, and close the bottle between operations. Change gloves when contaminated rather than rinsing them, because NITE-CMC’s reading adds skin sensitisation and HSE’s HSG262 makes the point that once a person is sensitised, very small amounts of the allergen provoke a reaction and the only remedy left is to prevent further exposure. Use glass and inert plastic; Ware records that these gold compounds attack most metals. Keep it away from the fixer, from reducing agents and from organic matter, all of which will convert it to metallic gold in the bottle.

Waste

The stream is the same as for any gold salt: a solution containing gold, whatever metal it took out of a print, and whatever ligand held it. Collect it. The aggregated classification here carries no aquatic hazard statement, and it would be a mistake to read that as a licence — the same anion under its other name is classified as toxic to aquatic life with long lasting effects by a fifth of its notifiers, and the difference lies in who filed what, not in the chemistry. Bottle gold waste apart from thiosulfate waste, label it with its contents and date, and take it to the route your authority provides; ILFORD directs domestic users to a household waste and recycling centre, and GOV.UK’s page on hazardous waste from households names the local service in the United Kingdom. Gold is worth recovering rather than discarding. Check your local regulations; they govern, and they differ.

History

Gold(III) chloride’s photographic history is mostly a history of a name. When Fizeau gilded daguerreotypes in 1840 and when Le Gray’s sel d’or passed to salted paper after 1850, the “chloride of gold” in the recipes was made, as it still is, by dissolving gold metal in a mixture of nitric and hydrochloric acids and evaporating — a route that yields the acid, not the neutral chloride. Reilly records that nineteenth-century photographers commonly made their own by dissolving gold coins in the mixed acids and then evaporating to dryness or neutralising the excess acid with chalk, and adds that because of the fumes and the concentrated acids this is not a procedure for a home laboratory. That neutralisation with chalk is worth noticing: it is the same operation that later became a deliberate step in alkaline toning, where Reilly describes calcium carbonate used to take the free acid out of the gold stock before the toner is compounded.

The compound also has a walk-on part in the platinotype’s beginnings. The Getty Conservation Institute’s atlas records that William Willis’s first patent of 1873, “Improvements in Photo-chemical Printing”, described an early version of the platinum process and suggested that salts of iridium and gold could be used in an analogous way. They can, and Herschel had already shown it in 1842 with the chrysotype; but the analogous gold process took another 145 years to become controllable, and it took a chemist working on the ligand rather than on the gold.