Potassium tetrachloroplatinate(II)

Platinum allergy was first observed in 1911, and Ware records where: as an occupational disease of the photographic factory workers who handled platinotype paper. The chemistry on this page made the most admired print in photography and made asthma in the people who coated it, and both facts belong to the same salt. Everything below follows from that pairing.

In photography

It is the platinum of the platinotype. Ware states it flatly: the platinum salt universally employed by Willis and all his successors from 1873 until 1986 was potassium tetrachloroplatinate(II), also called potassium chloroplatinite. Paper is coated with a mixture of this salt and light-sensitive ferric oxalate, dried in the dark and contact-printed under ultraviolet. Light reduces iron(III) to iron(II) and takes the oxalate away as carbon dioxide; the iron(II) then reduces platinum(II) to platinum metal, and that metal is the image. The salt itself is not light-sensitive. Photographers’ Formulary’s kit sheet says as much in its opening line, and Bostick & Sullivan’s says that ferric oxalate is the only light-sensitive compound in the kit.

⚗The chemistry

Ware gives the platinum couple as E([PtCl₄]²⁻/Pt, 4Cl⁻) = +0.73 V, and the photochemically made iron(II) oxalato-complex at +0.02 V, so the reduction is thermodynamically downhill. Two iron(II) are needed for each platinum atom, because platinum(II) takes two electrons and each iron supplies one. But thermodynamics is not the whole story, and the interesting part is kinetic.

There is no easy route for putting electrons onto the [PtCl₄]²⁻ ion itself. Ware’s account has the reaction proceed through the aquated anion instead, in which one chloride has been replaced by water:

H₂O + [PtCl₄]²⁻ ⇌ [PtCl₃(H₂O)]⁻ + Cl⁻

Aquation of the tetrachloroplatinate(II) anion (Ware §11.7)

2 [Fe(C₂O₄)₂(OH₂)₂]²⁻ + [PtCl₃(H₂O)]⁻ → 2 [Fe(C₂O₄)₂(OH₂)₂]⁻ + Pt + 3 Cl⁻ + H₂O

Ware's overall stoichiometry for the iron(II)-platinum(II) reduction (§11.8)

The equilibrium constant for the first reaction is 0.015 for platinum against 0.17 for palladium at 20 °C, so at sensitiser strength fewer than a fifth of the platinum ions are aquated while more than half the palladium ions are. That single number explains most of the practical difference between the two metals — why palladium prints faster and more smoothly, and why platinum needs help.

It also explains a piece of darkroom folklore. Establishing the aquation equilibrium is slow for platinum: Ware cites a measured half-time of 2.4 hours, so ten half-lives is about a day. That is why a freshly made tetrachloroplatinate solution is left to mature for twenty-four hours before first use, and Ware suggests it may also be why the published solubility figures for the salt disagree with one another.

Contrast used to be bought with an oxidant. Pizzighelli and Hübl’s 1882 formulae adjusted contrast by adding potassium chlorate to the sensitiser; the chlorate re-oxidises some of the iron(II) photoproduct so that it is unavailable to reduce platinum, truncating the exposure scale. Ware records that this costs image quality and grain, and that the alternative nineteenth-century oxidant, sodium hexachloroplatinate(IV), is still sold at a high price for the same purpose. He argues that with a correctly calibrated negative none of them is necessary.

As a toner it behaves in the opposite direction from gold. Reilly’s account is precise: gold toning goes best in an alkaline bath, platinum toning needs a neutral or acid one. Platinic chloride toners have so little energy that they are useless on albumen; the platinous salt — “chiefly potassium chloroplatinite” — is a very active toner in acid, and gives brown and brownish-black tones where gold gives purples and blue-blacks. Reilly’s working formula is 50 ml of a 1 per cent solution of the salt with 4 g of citric acid in 750 ml of water, and he warns that over-toning yellows the highlights.

Properties

Nobody has pinned down its solubility, and it matters. Ware devotes a paragraph to the point: Cassell’s Cyclopedia has one part of the salt in six of water as saturated, about 15.6 per cent w/v; Paul Anderson and others prescribe one in five, about 18.5 per cent; and the figure in the CRC Handbook of Chemistry and Physics, repeated in Wikipedia, is in Ware’s judgement “certainly erroneous”. His own standard solution — 5 g in exactly 25 cc of distilled water, warmed to 40–50 °C and stirred slowly for as much as several hours, then filtered into a brown bottle — comes out at roughly 18.5 per cent w/v or 0.45 M in platinum, and he calls the preparation tentative. PubChem’s record carries no solubility entry at all, so this page reports the disagreement rather than a number.

The consequence of that ceiling is double-coating. Ware notes that the apparent solubility is not ideally high enough for the best results, which is why some printers coat the paper twice to reach a higher maximum density. It also means the working solution sits close to saturation: Photographers’ Formulary’s 20 per cent solution is described as near saturation, and the fine red precipitate commonly seen in the bottle is potassium hexachloroplatinate(IV), K₂PtCl₆, which is much less soluble and must not be transferred into the sensitiser.

Because the platinum solution is not light-sensitive, it keeps. Bostick & Sullivan state that the platinum and palladium solutions last a very long time and neither oxidise nor age — the perishable component of a platinum kit is the ferric oxalate, not the metal.

Handling

The aggregated ECHA notifications classify this salt Danger, with five pictograms: corrosive, acute toxic, irritant, health hazard and environmental. Across 212 reports from 6 notifications, 99.5 per cent give toxic if swallowed, skin irritation, an allergic skin reaction, serious eye damage and — the statement that decides this page — may cause allergy or asthma symptoms or breathing difficulties if inhaled. Ware’s own hazard note reads: toxic by ingestion, irritating to eyes, can cause dermatitis and skin sensitisation, exposure can cause asthma, shortness of breath and cyanosis, allergenic, evidence of mutagenic effects.

Two occupational-hygiene authorities put numbers on it. HSE’s EH40 lists halogeno-platinum compounds (as Pt) — its paragraphs 25 and 26 define these as coordination compounds in which the platinum atom is directly bonded to halide ions, which is exactly what the tetrachloroplatinate anion is — with a long-term workplace exposure limit of 0.002 mg/m³ and the notation Sen, meaning capable of causing occupational asthma. NIOSH’s pocket guide entry for platinum soluble salts gives the same 0.002 mg/m³ as both its recommended limit and the OSHA permissible limit, an immediately dangerous concentration of 4 mg/m³ as platinum, and the instructions to prevent skin contact, prevent eye contact, wash when contaminated, remove clothing when wet or contaminated and change daily.

Why Level C. Two of the rubric’s Level C criteria are met, not one. The first names reagents classified as acutely toxic or as sensitisers at the concentrations used, where a fume cupboard or specialist disposal is the recognised control: this salt is both, and for a respiratory sensitiser engineered control genuinely is the recognised measure. HSE’s paragraphs 53 to 56 are explicit that exposure to an asthmagen should be prevented wherever reasonably practicable, that once the airways are hyper-responsive further exposure to tiny quantities may provoke symptoms, that it is impossible to identify in advance who will become hyper-responsive, and that health surveillance is appropriate for anyone exposed or liable to be exposed. A glove and a good intention are not that. The fourth criterion — waste that cannot lawfully enter a domestic drain — is met by the aquatic classification. Level B’s criteria would cover the corrosive and skin-sensitising statements on their own; they do not cover an asthmagen with a workplace exposure limit two thousandths of a milligram per cubic metre.

That is a classification of the substance, which is what someone buying a bottle needs. It is not by itself a verdict on every procedure: the course’s own printing lab in Part XXV is a palladium print, and Ware’s advice to anyone who develops the allergy is the same — platinum printing is not for you, but palladium remains open.

⚠Safety

The hazard that matters here is inhalation, and the control is to make sure there is nothing to inhale. Do not dry a coated sheet with a hair dryer: Bostick & Sullivan warn that it blows particles of the coating into the air, and Ware calls dust inhalation the worst hazard and advises against storing large amounts of dried sensitised paper. Never touch the surface of sensitised paper, and never put an ungloved finger into the processing solutions. Change gloves when contaminated rather than rinsing them — HSE’s HSG262 makes the point that after sensitisation very small amounts of an allergen provoke a reaction and the only remedy is to prevent further exposure. Glass only, never metal, or the platinum plates out of solution onto the container. One reassurance is well founded and Ware states it: platinum metal itself is not implicated in the allergy, so a fully processed platinotype is not a source of exposure.

Waste

The spent sensitiser, the oxalate developer and the acid clearing baths between them carry most of the platinum that did not become an image, along with the iron. Just under half the notifiers classify the salt as very toxic to aquatic life with long lasting effects, so the collection rule is not a preference. Bottle each stream, label it, and keep oxalate-bearing waste apart from silver-bearing waste. Treat the result as a precious-metal liquor: the Getty Conservation Institute’s atlas records that enough iron survives even the best clearing to be detected in a finished platinotype by X-ray fluorescence, which is a reminder of how much of what you coated ends up in the trays rather than in the print, and the AIC Photographic Materials Group’s entry records platinum, palladium, iron and sometimes silver, mercury, lead and uranium as the elements detectable in such prints. Kodak’s environmental guidance for amateur photographers gives the general principle for metal-bearing solutions, ILFORD’s guidance to domestic users is to bottle wastes separately and take them to a household waste and recycling centre, and GOV.UK names the local hazardous household waste service. Check your local regulations; they govern, and they differ.

History

Willis began in 1872 where everyone before him had begun, with platinic chloride — platinum(IV) — and by his own later account “all my early experiments were naturally made with platinic chloride”. They failed, and Ware explains why: platinum(IV) is not reduced to the metal readily enough, and the surviving specimen in the National Media Museum has deteriorated because platinic chloride is strongly acidic and oxidising and the paper was never washed. In 1873 Willis turned instead to the little-known platinous salts, obtained by partial reduction, and recalled that “after a troublesome operation, I made some potassic chloro-platinite”. Heinrich Gustav Magnus had first reported a preparation of platinous chloride in 1828, but his method was difficult and uncertain and the compound had remained a rarity. Willis’s finding a use for it, Ware suggests, is what prompted preparative chemists to look for easier syntheses; the chemist Chapman Jones later noted that platinum toning of silver prints did not succeed until Willis and the Platinotype Company made potassium chloroplatinite available.

Reilly’s parallel history of platinum toning fits into the same gap. M. De Caranza proposed toning with acidified platinic chloride in Istanbul in 1856 and was ignored, for the same reason Willis’s first experiments failed. Potassium chloroplatinite remained obscure until 1879, when Willis began to market the platinotype; in 1886 J. Reynolds found it to be a very energetic toner of silver prints, and in 1889 Stieglitz published a formula of nitric acid, potassium chloroplatinite and water. Platinum toning then had its own thirty-year vogue on matte papers, and the olive-black of a 1900s studio portrait is usually gold and platinum together.

The commercial arc closes quickly. The Getty atlas records that the first platinotype papers went on sale in 1880, that the Platinotype Company listed fifteen kinds in 1911, that the British government declared platinum a strategic metal during the First World War and forbade its use in photography, and that commercial manufacture ended in the United States in the 1930s and in Great Britain in 1941. What revived it was not a paper but a kit: chemicals sold for hand-coating, which is how almost every platinum print since has been made — and which is why the hazard that used to belong to a factory now belongs to whoever opens the bottle.