Oxalic acid

Rhubarb leaves, sorrel and the crust of a wine cask all contain it, and so does every bottle of platinum sensitiser ever coated. Oxalic acid is the strongest of the three vegetable acids that make iron light-sensitive, and in a platinum or palladium print it is doing three jobs at once: keeping the iron salt in solution, supplying the anion that gives up the electrons, and holding the developer acid enough that the iron does not hydrolyse and stain the paper.

In photography

Excess oxalate in the sensitiser. Ware’s Platinomicon traces the practice to Willis’s own patents, in which he added enough oxalic acid to render his ferric oxalate “freely soluble”. Later workers used anything from none to 5 g, usually about 2 g, per 100 cc of a 25 per cent w/v ferric oxalate solution. Ware tested the range and adopted 2 g per 100 cc as standard — 2 per cent w/v of the dihydrate, 0.16 mol/L in excess oxalate — and his instructions for the standard solution are 12.5 g of ferric oxalate, 1.0 g of oxalic acid dihydrate and exactly 44 cc of distilled water, stirred in the dark at room temperature for about twenty hours and never heated.

What more or less of it does. The excess oxalate converts part of the neutral ferric oxalate into the trisoxalatoferrate(III) anion, and Ware calculates that the customary 2 per cent w/v converts about a tenth of the sensitiser. That matters because the two forms behave differently in light: the photoproduct of the neutral salt, ferrous oxalate, is insoluble and stays where it was made, so a dry sensitiser prints out hardly at all, whereas the anion’s photoproduct remains soluble and can migrate far enough to find the noble-metal salt before development. Ware’s tests over 0 to 5 per cent w/v found the excess acid increased print-out slightly, with little effect on the degree of reversal, and that the reversal effect has an optimum near 2 per cent w/v.

⚗The chemistry

The IUPAC compilation records pK_a1 1.252 at 25 °C in 0.1 mol/L sodium perchlorate, assessed as reliable, with pK_a2 3.81 in the same medium. That first figure is what separates oxalic acid from its relatives: citric acid’s first proton comes off at 2.87 and acetic acid’s at 4.76, so oxalic acid is about forty times stronger than citric and about three thousand times stronger than acetic in its first ionisation, on the course’s own arithmetic from those three constants. It is a plant acid that behaves nearly like a mineral one. The second property that matters is shape: two carboxylates on adjacent carbons make oxalate a bidentate ligand, and three of them close around one iron(III) to give the trisoxalatoferrate(III) anion. Ware writes the light reaction in dissected form as two iron(III) plus one oxalate giving two iron(II) and two molecules of carbon dioxide — the oxalate is oxidised away as gas, and the electrons it loses are what make the image possible.

2 [Fe(C₂O₄)₃]³⁻ → 2 [Fe(C₂O₄)₂]²⁻ + 2 CO₂ + C₂O₄²⁻

Ware's fuller equation: trisoxalatoferrate(III) to bisoxalatoferrate(II), with carbon dioxide evolved

A trace in the developer. The traditional platinotype developer is potassium oxalate, and the texts Ware quotes say it should be neutral or only just acid — much more than a trace of acid is said to inhibit the formation of platinum. Some recipes carry 0.1 per cent oxalic acid or less; Irving Penn’s was 30 per cent potassium oxalate with 0.5 per cent oxalic acid; the Platinotype Company’s proprietary “Special D Salts” analysed as 85 per cent potassium oxalate monohydrate, 12 per cent potassium dihydrogen phosphate and 3 per cent oxalic acid dihydrate, made up to only 8.6 per cent w/v for black papers. Where those developer figures appear here without w/v or w/w after them, it is because the source gives them bare, and the course reproduces them as written rather than assuming a basis. Modern practitioners, Ware records, sometimes add about 2 per cent excess oxalic acid to bring the pH below 6, which inhibits the hydrolysis of iron(III) and so avoids staining.

As a chrysotype developer. Among the alternative first baths for a gold print, Ware lists oxalic acid at 1 to 2 per cent w/v and singles it out: it “causes the most intense and striking red/blue colour splits and the longest tonal range”.

Where it is the wrong acid. Papers buffered with chalk destroy an iron sensitiser, and the obvious remedy is an acid pre-bath. Ware is emphatic that oxalic acid is not it — calcium oxalate is as insoluble as the calcium carbonate it is meant to remove, so the popular use of oxalic acid for this purpose is, in his words, chemically inappropriate. Dilute hydrochloric or sulphamic acid is used instead.

In the older literature. Wall’s 1924 collection prepares ferrous oxalate developer by precipitating it from ferrous sulfate with a soluble oxalate, noting that 126 parts of oxalic acid may be used instead of the potassium salt, and that the precipitate dissolves in excess neutral potassium oxalate — which is the oxalate developer in another form. The same book gives a reducer of 75 g of dry sodium sulfite and 30 g of oxalic acid per litre, and records that adding 0.5 per cent oxalic acid to a cyanotype sensitiser gives brighter prints.

Properties

Hygroscopic colourless crystals or a white powder, odourless, specific gravity 1.90, subliming rather than melting cleanly. Chemical Safety Card 0529 gives 9 to 10 g per 100 mL of water at 20 °C and calls that moderate; NIOSH gives 14 per cent. It is the least soluble of the acids on these pages by a wide margin, which is why platinum sensitiser recipes give it in grams rather than tens of grams.

Which powder is in the jar matters more here than usual. The anhydrous acid has a molar mass of 90.03; the commercial dihydrate is 126.07. A formula written for the dihydrate — and Ware’s, and every alternative-process recipe the course has read, are — needs 1.400 times the weight if only the anhydrous acid is to hand, the course’s own arithmetic from the two molar masses. Get that backwards and a sensitiser carries two-fifths more acid than intended.

The card records that it decomposes on heating or on contact with flames to formic acid and carbon monoxide, and that the aqueous solution is a moderately strong acid.

Handling

The aggregated ECHA notifications classify it Danger: H302, harmful if swallowed, in 99.8 per cent of the 1,282 company reports carrying hazard codes, H312, harmful in contact with skin, in 99.7 per cent, and H318, serious eye damage, in 31.1 per cent. NIOSH sets a recommended limit of 1 mg/m³ over eight hours and 2 mg/m³ over fifteen minutes, with an IDLH of 500 mg/m³; HSE’s EH40 sets the identical British figures. The symptom list is where this substance stops resembling vinegar: irritation and eye burns, then local pain, cyanosis, shock, collapse, convulsions and kidney damage, with the kidneys named among the target organs. CAMEO records that ingestion of five grams has caused death, and Chemical Safety Card 0529 that exposure may affect the calcium balance and that repeated exposure can produce kidney stones, slow-healing ulcers and black finger nails.

Why Level B. The course rubric caps Level A at a classification “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”. Harmful in contact with skin is an acute toxicity entry outside that list, and so is serious eye damage; Level B’s criterion of fine powders that must not be inhaled is what the safety card describes. Level B’s criterion of a failure mode that is a splash or a burn rather than a spoiled print applies too, since the card calls the substance corrosive to the eyes, skin and respiratory tract even at the 1 to 2 per cent w/v of a working sensitiser or developer. It does not reach Level C, whose criterion applies where a fume cupboard or specialist disposal is the recognised control; the recognised controls here are dust suppression, gloves, goggles, eyewash and hygiene. The honest caveat is that Japan’s NITE classification adds suspected reproductive toxicity and organ damage on repeated exposure, which the European aggregation does not carry — if that classification were the one the course followed, the substance would sit closer to the Level B and C boundary.

⚠Safety

Weigh it over a tray without raising dust; goggles, nitrile gloves, eyewash within reach, hands washed before eating and nothing eaten in the darkroom, because ingestion is the route that has killed. Do not warm it: the card records decomposition to formic acid and carbon monoxide. And keep it away from silver — NIOSH lists silver compounds among its incompatibilities and the card names explosive silver oxalate as the reason, which in a darkroom means that the oxalic acid bottle, the oxalate developer and the silver nitrate live in different places and their wastes go into different bottles.

Waste

Spent oxalate developer is reused until it is spent rather than discarded after each print, so the volumes are small; the clearing baths carry the iron stripped off the print and traces of platinum or palladium. The governing rule for this substance is a compatibility rule rather than a concentration one. Oxalate waste and silver-bearing waste do not share a container, for the reason above. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most frequently set, and a working oxalate bath sits below it. Japan’s NITE classification records the substance as harmful to aquatic life with long-lasting effects, so a neutralised bath still does not go on a garden. ILFORD’s guidance for domestic users is to bottle wastes separately, label them and take them to a household waste and recycling centre’s chemical cupboard; failing that, small amounts flushed with plenty of water, unmixed, and never into a septic tank. Check your local regulations, which govern.

History

Oxalic acid entered photography through Willis’s platinotype patents of the 1870s, and the reason was practical rather than theoretical: ferric oxalate is awkward to dissolve, and a little free acid makes it behave. What the nineteenth century did not know is why the addition also changed the print. Ware’s own contribution is to have measured it — to have shown that the customary 2 per cent w/v converts about a tenth of the sensitiser to the ferrioxalate anion, and that this is what produces the slight print-out by which a platinum printer judges exposure. The recipe survived a century as a rule of thumb before anyone tested the range it sat in.