Palladium(II) chloride

Palladium chloride is the form in which palladium is cheapest, and it is not the form in which anybody prints. Every practical palladium sensitiser and every palladium toner uses a chloropalladate — the sodium, potassium or ammonium salt of the [PdCl₄]²⁻ anion — and the reason this dark brown powder still sits on the shelf is that you can make one of those from it in a beaker for less than the made-up salt costs. Ware gives both routes side by side and calls them quite equivalent; the choice, he says, depends on economy and availability.

In photography

It is a precursor, not a sensitiser in itself. Palladium salts are not light-sensitive at all — Photographers’ Formulary state it in the first line of their kit sheet — so palladium chloride’s job is to become the anion that the light-generated iron(II) will reduce. What makes it awkward is that it barely dissolves in plain water. What makes it useful is that it dissolves readily once chloride is present, because the chloride completes a square-planar complex and it is the complex that is soluble.

⚗The chemistry

Ware’s two preparations are the same reaction with different spectators. For the traditional platinum-palladium sensitiser, sodium chloride: 2.92 g of it, 0.05 mol, dissolved in 30 cc of water and heated to about 70 °C, then 4.43 g of powdered palladium chloride, 0.025 mol, added a little at a time with stirring until a dark orange-brown solution results — two chlorides for every palladium. For his print-out palladiotype, ammonium chloride instead: 1.8 g of it in 20 cc of water, heated to about 70 °C, and 3 g of well-powdered palladium chloride added a little at a time.

PdCl₂ + 2 NH₄Cl → (NH₄)₂[PdCl₄]

Ware's ammonium route to the print-out palladiotype sensitiser salt

Both take from ten minutes to an hour, both are made up to a stated final volume and filtered into a brown bottle, and Ware notes that the resulting solution is stable indefinitely. The instruction he attaches to the powdering step is the only bare hazard warning in his palladiotype notes: “Hazard! Wear a dust mask.”

The cation you choose is a choice about the process, not about the palladium. Dissolve the chloride in sodium chloride and you have the sodium tetrachloropalladate of a Photographers’ Formulary or Bostick & Sullivan kit, for the traditional develop-out palladiotype. Dissolve it in ammonium chloride and you have the ammonium salt Ware’s print-out process needs, where the ammonium ion and a controlled paper humidity are part of the working chemistry. The potassium salt is the one chemical catalogues stock. The anion is identical in all three.

What the metal does once it is reduced. Two iron(II) hand over one electron each to palladium(II), and palladium metal is deposited in the paper fibres. Ware’s redox table puts the palladium couple at +0.62 V against +0.02 V for the photochemically generated iron(II) oxalato-complex; his aquation constants explain why the reaction is faster than the platinum equivalent. The print is browner, lower in contrast and smoother than a platinum print, and contrast is restored deliberately with potassium chlorate in the sensitiser: Bostick & Sullivan state that palladium requires twice as much chlorate as platinum to reach the same contrast, and Photographers’ Formulary supply 0.26 g of it for palladium against 0.18 g for platinum in an otherwise identical 30 ml bottle of ferric oxalate.

A caution that belongs to the finished print. Photographers’ Formulary warn that palladium metal can be etched from a print by a concentrated acid, which is why their palladium clearing bath is much weaker than the platinum one. The image metal is noble, not invulnerable.

Properties

The solubility contradiction is the property worth learning. CAMEO says soluble; the Hazardous Substances Data Bank says soluble in water, ethanol and acetone; a supplier sheet says insoluble in water. All three appear in PubChem’s record, and HSDB supplies the reconciliation in its next line: readily soluble in hydrochloric acid and in solutions of alkali metal chlorides. That is not a quibble about a number, it is the whole practical chemistry of the compound — and it is why every preparation in the literature specifies a chloride solution, heat, powdering the solid, and up to an hour of stirring.

Deliquescent, and decomposing. CAMEO’s air and water reactions entry is two words, “Deliquescent. Water soluble”, and its physical properties give a melting point of 934 °F with decomposition. The decomposition is worth writing out, because it names the second product:

PdCl₂ → Pd + Cl₂

CAMEO: decomposition at high temperature to metallic palladium and chlorine

CAMEO also records that the compound is a weak oxidising agent and that it is reduced in solution by hydrogen or carbon monoxide to metallic palladium — the reaction on which its industrial life as a catalyst rests, and a reminder that a palladium solution left with a reductant will not stay a solution.

Molar mass and hydrates. PubChem gives 177.32 for PdCl₂ and Ware 177.31, and unlike the tetrachloropalladates there is no hydrate ambiguity to trip over in the photographic literature: both of Ware’s preparations weigh the anhydrous compound. Because the solid is deliquescent, however, an old bottle left loosely capped will weigh more than it should, and the error runs the same way as a hydrate error.

Handling

The aggregated ECHA notifications classify palladium dichloride Danger, with the corrosive, acute-toxic, irritant and environmental pictograms. This is the biggest aggregation in the family — 299 reports across 19 notifications — and the most divided one. The single statement with the clearest majority is an allergic skin reaction, at 77.6 per cent; corrosivity to metals, serious eye damage, harmful if swallowed and the aquatic statements each sit between 58 and 65 per cent; skin irritation, serious eye irritation and respiratory irritation each around a fifth; and 12.7 per cent classify it as toxic rather than harmful if swallowed. CAMEO’s health-hazard entry is narrower and older, recording corneal irritation and calling the compound an eye irritant. Ware’s summary adds “possible risk of irreversible effects” and “possible carcinogen”, phrases from the European risk phrases that preceded GHS; nothing in the current aggregation corresponds to them, and the course records the discrepancy without resolving it, because it has not found a current classification either way.

Why Level B. The acute-toxicity pictogram takes this past Level A of the course rubric on its own, so the question is B against C. Level C applies “where a fume cupboard or specialist disposal is the recognised control”, and no source this course holds names engineered extraction for palladium chloride; the control the sources do name is a dust mask at the weighing and powdering step, which is Level B’s own criterion on fine powders that must not be inhaled. The skin-sensitisation statement is Level B’s first criterion, and the corrosive eye-damage statement its fourth. The waste is collectable and worth collecting, which is the same reasoning that keeps silver nitrate at Level B despite an identical aquatic classification. And the statement that pushed the platinum salt to Level C is absent here: no block of this record gives allergy or asthma if inhaled, and EH40’s asthmagen notation and 0.002 mg/m³ limit apply to halogeno-platinum compounds, not to palladium. Neither EH40 nor the NIOSH pocket guide sets a limit for palladium compounds, and EH40 states that absence from its list does not indicate that a substance is without hazard.

⚠Safety

The dust is the hazard to design against. Weigh the powder over a tray in still air, wear a mask rated for fine particulates while doing it, and add it to the hot chloride solution in small portions so that nothing is thrown up by stirring — that is Ware’s own sequence, with his own warning attached. Gloves and splash goggles throughout, and gloves changed rather than rinsed: at 77.6 per cent this is the most consistently classified skin sensitiser in this family, 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, not metal. Do not heat the solid beyond what the preparation asks: CAMEO records decomposition to palladium and chlorine.

Waste

Very little palladium chloride is thrown away as such, because it is consumed in making a solution that is then consumed in making prints. The streams that matter are the ones the printing produces — sensitiser washed off, spent developer, clearing baths — and they carry palladium, iron and an organic acid anion. Nearly six in ten notifiers classify the compound as very toxic to aquatic life with long lasting effects, so those liquors are bottled and labelled rather than discharged, and kept apart from silver-bearing waste. Any solid residue in a beaker after a preparation is palladium, and is worth keeping for the same reason the liquors are: this is a precious-metal stream, and treating it as a disposal problem throws away the most expensive thing on the bench. 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 hazardous household waste service. Check your local regulations; they govern, and they differ.

History

Palladium was the last of the four noble metals to arrive and the last to be used for photography. Ware records that it was discovered in platinum ores by William Hyde Wollaston, the inventor of the camera lucida, and names it for Pallas Athena and for the asteroid Olbers had found in 1802. Ware’s narrative gives the discovery as 1803 in two places while his own chronology table gives 1802; the course reports both as he prints them rather than choosing between them.

The photographic proposals came early and went nowhere for sixty years. Ware lists tentative palladium printing processes suggested by Burnett in 1856, by Willis himself in 1878 and by Pizzighelli and Hübl in 1882, and gives the reason none of them took: palladium was scarcer than platinum at the time, and the image was more prone to reversal on heavy exposure. Palladium toning of silver prints was better established by the 1890s, particularly for photoceramics, where a palladium-toned image survived the kiln under a glaze — but the manuals dismissed it as impractical, and the figures Ware gives explain why. Palladium then cost six pence a grain, about £12 an ounce, against roughly £2 an ounce for platinum. The metal that is now the affordable one was six times the price of the metal it replaced.

What changed that was the First World War, the embargo on photographic platinum and Willis’s palladiotype papers. What changed it again was the collapse of commercial manufacture and the hand-coating revival that followed: the Getty Conservation Institute’s atlas records that the revival succeeded through kits and chemicals rather than through paper, and that the Ziatype of 1996 built a whole new palladium variant on double salts of the metal. All of them start, one way or another, from this brown powder.