Potassium tetrachloropalladate(II)

The photographically interesting part of this compound is the anion. Potassium, sodium and ammonium tetrachloropalladate all dissolve to give the same square-planar [PdCl₄]²⁻ ion, and it is that ion that light-generated iron(II) reduces to the palladium metal of a print. Which salt you buy decides what you weigh and what else is in the solution; it does not decide the chemistry. This page is therefore as much about the ion as about the potassium salt, and about why palladium turned out to be the better metal for the job that platinum was invented for.

In photography

Palladium prints by the same route as platinum, and prints better. The paper carries ferric oxalate and a tetrachloropalladate; ultraviolet reduces iron(III) to iron(II) and takes the oxalate off as carbon dioxide; an oxalate or citrate developer dissolves the insoluble ferrous oxalate so the iron(II) can move; the mobile iron(II) reduces palladium(II) to metal. Photographers’ Formulary states the division of labour in its first line — palladium salts are not light-sensitive, so the paper is coated with a mixture of sodium tetrachloropalladate and light-sensitive ferric oxalate. Two iron(II) are needed for each palladium atom, because palladium(II) takes two electrons.

⚗The chemistry

Ware writes the reduction for platinum and notes that similar equations can be written for the other metals. For palladium it reads:

2 [Fe(C₂O₄)₂]²⁻ + [PdCl₄]²⁻ → 2 [Fe(C₂O₄)₂]⁻ + Pd + 4 Cl⁻

Iron(II) oxalate complex reduces palladium(II) to the metal

The reason palladium is the easier metal lies one step earlier. Ware’s account has the electron transfer proceed not through the [MCl₄]²⁻ ion but through its aquated form, in which water has displaced one chloride:

H₂O + [PdCl₄]²⁻ ⇌ [PdCl₃(H₂O)]⁻ + Cl⁻

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

At 20 °C the equilibrium constant is 0.17 for palladium against 0.015 for platinum, so at sensitiser strength more than half the palladium is aquated and less than a fifth of the platinum. Palladium’s solution chemistry is more labile as well, so the reduction is faster. Together these give what Ware summarises as the practical difference: palladium is faster to print and achieves a finer, smoother quality than platinum under a wider range of conditions and paper choices.

What that buys and costs on the print. Ware records that palladium gives a lower-contrast, browner, very smooth image — and records that adding mercury(II) salts to a platinum sensitiser produces an image resembling palladium’s, lower in contrast, brown and very smooth, which is how the sepia platinotype was made before palladium was used. Contrast has to be put back deliberately, and the standard method is an oxidant in the sensitiser: Photographers’ Formulary supplies 0.26 g of potassium chlorate in the 30 ml ferric oxalate bottle for palladium against 0.18 g for platinum, and explains the mechanism plainly: light makes iron(II), iron(II) converts the palladium salt to free metal, and the chlorate reconverts iron(II) to iron(III), so the chlorate acts as a restrainer, increases contrast and holds the whites. Bostick & Sullivan put the comparison in words instead of grams, stating that palladium requires twice as much chlorate as platinum to achieve contrast.

The salt is also mild enough to tone with. Where a platinum toner needs an acid bath and still attacks the silver image, a palladium toner is gentle: Bostick & Sullivan’s kit for printing-out paper, Van Dyke and kallitype is nothing but 1 per cent citric acid with seven to fifteen drops of palladium solution per litre, worked by inspection with continuous agitation and refreshed after ten to twenty-five prints. Their solution is the sodium salt; the anion is the same one.

Where the cation does matter. Two places. The first is the mass on the balance: at 326.4 against 294.2 for the anhydrous sodium salt, a gram of the potassium salt is about a tenth less palladium. The second is the rest of the chemistry. Ware’s print-out palladiotype deliberately uses the ammonium salt, or palladium(II) chloride dissolved in ammonium chloride, because that process turns on ammonium ions and controlled humidity; and on the platinum side the ammonium cation is forbidden where hexachloroplatinate(IV) is present, because ammonium hexachloroplatinate is barely soluble and crystallises out. A tetrachloropalladate solution is never only a source of the anion.

Properties

Form and keeping. Alfa Aesar’s description, as Haz-Map compiles it, is a red-brown crystalline powder that is hygroscopic — so it gains water on an open bench, and a weighed gram becomes an uncertain gram. PubChem’s record carries no solubility figure for this salt and the course supplies none; what the sources establish is that the working solutions of the photographic kits are 15 per cent for the sodium salt and 19 per cent w/v for Ware’s ammonium salt, which are dissolved without difficulty, and Ware’s standard palladium solution is 0.5 M in the metal. The corresponding platinum solution can only reach about 0.45 M, which is why Ware sets the sensitiser mixing ratio for platinum at 0.9 parts ferric oxalate to 1 of platinum and for palladium at 1 to 1.

Redox position. Ware’s table of noble-metal potentials gives E([PdCl₄]²⁻/Pd, 4Cl⁻) = +0.62 V, against +0.73 V for platinum, +0.80 V for silver and +1.00 V for gold, while the photochemically generated iron(II) oxalato-complex sits at +0.02 V. Every one of those reductions is thermodynamically allowed; what separates them in practice is how fast the complex will accept electrons, which is the aquation story above. Ware also records the negative result that makes the point: ferrioxalate sensitiser produced no images at all with Pt(NH₃)₄²⁺, Pt(NO₂)₄²⁻, Pd(NH₃)₄²⁺, RuCl₆²⁻, IrCl₆³⁻ or RhCl₆³⁻, because those complexes are kinetically inert.

Stability in the bottle. Unlike the iron salt it is mixed with, the palladium solution does not perish. Bostick & Sullivan state that the platinum and palladium solutions last a very long time and neither oxidise nor age — the shelf life of a palladium kit is the shelf life of its ferric oxalate.

Handling

The aggregated ECHA notifications classify this salt Danger, with the corrosive, acute-toxic, irritant and environmental pictograms. Across 152 reports from 9 notifications the notifiers divide about two to one: roughly seven in ten give toxic if swallowed, an allergic skin reaction, serious eye damage and very high aquatic toxicity, while roughly three in ten give the milder eye statement, respiratory irritation and skin irritation. Ware’s own summary for the palladium salts, abstracted from safety data sheets, reads: harmful by ingestion, irritating to eyes, possible risk of irreversible effects, possible carcinogen — a phrasing from the pre-GHS European risk phrases that the current aggregation does not carry. Neither HSE’s EH40 nor the NIOSH pocket guide sets an exposure limit for palladium compounds, and EH40 states that absence from its list does not indicate that a substance is without hazard.

Why Level B and not Level C. The acute-toxicity pictogram puts this beyond Level A of the course rubric on its own, so the real question is B against C, and it turns on the rubric’s own qualifier: Level C applies “where a fume cupboard or specialist disposal is the recognised control”. No source this course holds names engineered extraction as the control for a palladium salt. What the sources do name is dust control at the moment of weighing — Ware’s “Hazard! Wear a dust mask” when powdering palladium chloride into hot solution, and Bostick & Sullivan’s warning about hair dryers — which is Level B’s own criterion on fine powders that must not be inhaled, alongside its criterion on sensitisers. The aquatic classification would meet Level C’s waste criterion if the waste had nowhere to go, but a palladium liquor is collectable and worth collecting, which is the same reasoning that keeps silver nitrate at Level B. The decisive contrast is with the platinum salt: no block of this record, and no block of the record for any palladium salt in this encyclopaedia, carries a respiratory sensitisation statement, and no exposure limit or asthmagen notation exists for palladium in EH40.

⚠Safety

Gloves, splash goggles, no dust and no metal. Change gloves when contaminated rather than rinsing them: about seven in ten notifiers classify this salt as a skin sensitiser, 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. Ware, writing about platinum asthma, says that a photographer who develops that allergy could still use palladium, “which is non-allergenic”. Read that sentence for what it is — a statement about respiratory sensitisation to platinum, the endpoint he is discussing — and not as a clean bill of health: the classification for every palladium salt in this family carries skin sensitisation from a clear majority of its notifiers, and the course treats palladium as a sensitiser on the skin while agreeing with Ware that the asthma hazard is platinum’s.

Waste

Palladium leaves the darkroom in three streams: unexposed sensitiser washed out of the print, the spent developer, and the clearing baths. Together they carry palladium, iron and oxalate or citrate. Bottle them, label them and keep them out of the drain — the aquatic statements are not marginal here, and they are the majority reading. Keep oxalate-bearing waste physically apart from silver-bearing waste, because the incompatibility of oxalates with silver compounds is a storage rule this course applies everywhere. Then treat the bottles as a recovery stream rather than a disposal problem: palladium is a precious metal, and the AIC Photographic Materials Group’s entry records that palladium is one of the elements detectable in these prints by X-ray fluorescence, which is a reminder that only part of what you coat stays on the paper. Kodak’s environmental guidance gives the general principle, ILFORD’s guidance to domestic users is a labelled bottle and a household waste and recycling centre, and GOV.UK names the local service. Check your local regulations; they govern, and they differ.

History

Palladium entered photography as a substitute and stayed as a preference. The Getty Conservation Institute’s atlas records that Willis’s second patent, in 1878, already suggested palladium as an image-forming metal, and that platinum was declared a strategic metal by the British government during the First World War and forbidden in photography — which is what forced the change.

⁇1916 or 1917?

The Getty atlas dates the introduction of the palladiotype process to 1916. Ware dates it to 1917 and gives the evidence: an unannounced demonstration of a “brand new printing paper” by W. H. Smith, technical manager of the Platinotype Company, to the Croydon Camera Club in January 1917, and the public demonstration of the revealed “Palladiotype” to the Camera Club of London on 8 March 1917, with Willis & Clements advertising “Palladiotype Sepia” in the United States that year. This course follows Ware, because his date is tied to named meetings and dated advertisements while the atlas gives the year alone; a reader who needs the date for a catalogue entry should cite both.

Willis treated it as a different process rather than a cheaper platinum. Ware notes that he recommended trisodium citrate at 20 per cent with 2 per cent citric acid as the palladiotype developer, and a much weaker version of the same as the clearing bath, rather than the potassium oxalate and hydrochloric acid of the platinotype — and Ware’s reading is that Willis, a shrewd and painstaking chemist, would not have changed the chemistry if the old developer had served. Photographers’ Formulary’s modern kit sheet preserves the same distinction in the same terms, warning that palladium metal is etched from a print by acid strong enough to be routine for platinum.

The two metals then quietly mixed. The Getty atlas records that low concentrations of palladium turn up in some early platinotypes without any deliberate addition, that most platinotypes made before the First World War are simple platinotypes, and that only after 1917 did photographers begin to use palladium and then platinum-palladium mixtures on paper they coated themselves. That is where the practice still is: the “Pt/Pd print” of a modern portfolio is a mixture, and the ratio is a control the printer chooses.