Sodium tetrachloropalladate(II)

Open a platinum-palladium kit and the bottle marked “Palladium #3” holds this compound. Bostick & Sullivan name it on their palladium toning sheet as sodium chloropalladite; Photographers’ Formulary name it in full — the kit’s 15 per cent palladium salt solution is prepared from sodium tetrachloropalladate, Na₂PdCl₄. It is the working form of palladium in modern alternative-process printing, and the reason is neither chemical nor photographic: the anion behaves the same whatever its cation, and the sodium salt is what the trade settled on.

In photography

It supplies the metal that becomes the image. Photographers’ Formulary put the mechanism in one sentence: palladium salts are not light-sensitive, so the paper is coated with a mixture of sodium tetrachloropalladate and light-sensitive ferric oxalate. Ultraviolet reduces the iron(III) to iron(II); an oxalate or citrate developer mobilises that iron(II) by dissolving the ferrous oxalate it made; the iron(II) then hands two electrons to palladium(II) and palladium metal is left in the paper fibres. Ware’s table of noble-metal potentials puts the palladium couple at +0.62 V against +0.02 V for the iron(II) oxalato-complex, so the reaction runs downhill; his aquation constants — 0.17 for palladium against 0.015 for platinum — explain why it runs downhill faster than the platinum equivalent, and why palladium prints are smoother.

⚗Two routes to the same bottle

Ware gives the standard 0.5 molar palladium solution in two forms and calls them quite equivalent. The first dissolves the solid salt: 8.7 g of the trihydrate, or 7.35 g of the anhydrous salt, made up to exactly 50 cc. The second makes the salt in the beaker, and is the cheaper route when palladium(II) chloride is the more available reagent — 2.92 g of sodium chloride dissolved in 30 cc of water, heated to about 70 °C, and 4.43 g of powdered palladium chloride added a little at a time with stirring until a dark orange-brown solution results, which may take from ten minutes to an hour.

PdCl₂ + 2 NaCl → Na₂[PdCl₄]

Ware's in-beaker route: palladium chloride plus brine gives the tetrachloropalladate

Look at the quantities and the equation is there: 0.05 mol of sodium chloride to 0.025 mol of palladium chloride, exactly two chlorides for one palladium. The sparingly soluble simple chloride dissolves because the added chloride completes the square-planar [PdCl₄]²⁻ ion, and it is the complex, not the simple salt, that goes into solution.

Contrast comes from an oxidant, not from the palladium. Photographers’ Formulary’s Sensitizer B is ferric oxalate with potassium chlorate added — 0.26 g into a 30 ml bottle for palladium against 0.18 g for platinum — and the sheet explains why: light makes iron(II), the iron(II) converts the palladium salt to free metal, and the chlorate reconverts iron(II) back to iron(III), acting as a restrainer, increasing contrast and holding the whites. The chlorate decomposes over weeks, so the sheet advises a test strip about once a week or before a printing session. Bostick & Sullivan’s kit carries the same idea as two ferric oxalate bottles, #1 plain and #2 chlorated, mixed in varying proportions.

As a toner it is the gentle one. Their palladium toner for printing-out paper, Van Dyke and kallitype is 10 g of citric acid in a litre of distilled water with seven to fifteen drops of the palladium solution — more drops for faster toning, fewer when there is inspecting to be done. It tones the shadows first, needs a five-minute rinse before fixing in 15 per cent sodium thiosulfate, and is refreshed with five to ten further drops after ten to twenty-five prints. Where a platinum toner must be acid enough to attack the image and yellows the highlights if pushed, this one is a warm-tone treatment run at 1 per cent citric acid.

The mixing ratio is a solubility artefact. Ware mixes ferric oxalate and palladium solutions in equal volumes, but ferric oxalate to platinum at 0.9 to 1, because the platinum solution can only be made up to about 0.45 M while the palladium reaches 0.5 M. It is a small detail with a satisfying cause: the same low solubility that makes platinum printers coat twice also changes the arithmetic on the mixing pipette.

Properties

The hydrate question is the practical one. Ware records that the solid is usually supplied hydrated with approximately three waters, but sometimes anhydrous, and gives 348.3 for the trihydrate against 294.19 for the anhydrous salt — figures that agree with PubChem’s computed 294.2 for the anhydrous formula unit. “Approximately” is doing real work in that sentence, and it is why he specifies weights in moles-worth for both forms rather than trusting a single gram figure. On the course’s own arithmetic from those two formula weights, a gram of the anhydrous salt carries about 18 per cent more palladium than a gram of the trihydrate — enough to change a print.

Solubility. PubChem’s record for this compound carries no solubility entry and no physical description, and the course invents neither. What the sources establish is behavioural: 7.35 g dissolves in well under 50 cc of water with stirring and gentle warming in Ware’s preparation, the photographic stock solutions run at 15 to 19 per cent w/v, and Ware’s own is 0.5 M in palladium. The solid is filtered through a Whatman Grade 1 paper into a brown bottle after dissolving, which is Ware’s standard practice for every one of these solutions.

It is the stable partner in the kit. Bostick & Sullivan state that the platinum and palladium solutions last a very long time and neither oxidise nor age, and Ware calls the palladium solution made by the palladium-chloride route stable indefinitely. Everything that perishes in a platinum-palladium kit is on the iron side.

Handling

The aggregated ECHA notifications classify this salt Danger, with the corrosive, irritant and environmental pictograms. Across 176 reports from 10 notifications, 98.3 per cent give harmful if swallowed and 96 per cent corrosive to metals; about three-quarters give an allergic skin reaction, serious eye damage and very high aquatic toxicity; a quarter give the milder eye statement instead. One report of the 176 states that it meets no GHS criteria at all. Ware’s own hazard note, abstracted from safety data sheets of the period, reads: harmful by ingestion, irritating to eyes, possible risk of irreversible effects, possible carcinogen — the last two phrases belong to the pre-GHS European risk phrases and have no counterpart in the current aggregation, which carries no carcinogenicity statement. The course records both and follows the classification for its level.

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. Two statements take this past that cap: an allergic skin reaction from three-quarters of the notifiers, and serious eye damage from the same proportion — which is Level B’s first criterion, sensitisers and corrosive reagents, and its fourth, a procedure whose failure mode is a splash rather than a spoiled print. It stops short of Level C for the reason the rubric itself gives: that criterion applies where a fume cupboard or specialist disposal is the recognised control, and no source this course holds names engineered extraction for a palladium salt, while the waste is collectable and worth collecting for recovery. Neither 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. Note in particular what is absent here that is present two pages over: no block of this record gives allergy or asthma if inhaled, and palladium carries no asthmagen notation in EH40, which is the whole of the difference between this page and the platinum salt’s.

⚠Safety

Gloves, splash goggles, glass rather than metal, and no airborne dust at any stage — not when weighing the solid, not when powdering palladium chloride into hot brine, and not when drying a coated sheet with warm air. Change gloves when they are contaminated: three-quarters of the notifiers call this a skin sensitiser, and HSE’s HSG262 makes the point that after sensitisation very small amounts provoke a reaction and the only remedy left is to prevent further exposure. Palladium sensitisation of the skin is documented in the classification whatever the reputation of the metal, and gloves that are reused wet are the commonest way a printer gets a repeated low dose.

Waste

Three streams, as with any siderotype: unexposed sensitiser washed out of the print, the spent developer, and the clearing baths that strip the residual iron. They carry palladium, iron and the organic acid anion. Bottle each separately, label them, and keep oxalate-bearing waste away from silver-bearing waste. The aquatic statements are carried by three-quarters of the notifiers and they settle the question of the drain on their own. Treat the collected liquor as a precious-metal stream: the AIC Photographic Materials Group’s entry records palladium among the elements detectable in these prints by X-ray fluorescence, and the Getty atlas records that residual iron survives even good clearing, which together say that a fair part of what you coat ends up in a tray rather than on the paper. Kodak’s environmental guidance gives the general principle for metal-bearing solutions, ILFORD’s guidance to domestic users is a labelled bottle and 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

This salt has no nineteenth-century history of its own, and that is the point worth making. Willis’s palladiotype papers of 1917 were factory-coated, and the sensitiser was his business rather than the photographer’s. What made the sodium salt the standard was the collapse of commercial manufacture — the Getty Conservation Institute’s atlas records that platinotype paper ceased to be made in the United States in the 1930s and in Great Britain in 1941, and that the revival which began in the late 1960s succeeded not by reintroducing coated paper but through the commercial production of chemicals and kits that let photographers coat their own. A hand-coating printer needs a stable, storable, accurately made-up metal solution, and this is it.

The atlas records two milestones of that revival. In 1986 Mike Ware and Pradip Malde introduced a modern version of Pizzighelli and Hübl’s 1887 print-out platinum process; in 1996 Richard Sullivan introduced the Ziatype in Santa Fe, a palladium variant using double salts of palladium with lithium, caesium and other metals. Both are palladium chemistry, and both changed the cation deliberately — which is the best evidence that the choice of sodium here is a matter of trade convention rather than of chemistry, and that anyone who changes it is doing so for a reason worth writing down.