Potassium oxalate, anhydrous

There are two white powders called potassium oxalate and a photographic recipe almost never says which one it means. This is the lighter of the two: 166.22 to the mole against 184.23 for the monohydrate, which is the form a supplier normally ships and every historical formula assumes. Weigh a platinum developer out with this salt against a recipe written for the crystals and the bath carries 10.8 per cent more oxalate than intended, on the course’s own arithmetic from the two molar masses.

In photography

The active part is the anion, and it is the same anion. Everything potassium oxalate does in photography, it does as the oxalate ion, C₂O₄²⁻; the potassium is a spectator and the water of crystallisation is packaging. The monohydrate page sets out the uses — the platinotype developer, the ferrous oxalate developer, the iron trap in a sepia bleach, the blueprint correcting solution — and each of them is a use of this ion. What the anhydrous form changes is the arithmetic, and in an alternative process the arithmetic is the formula.

⚗The chemistry

Why oxalate dissolves iron compounds that water will not touch is a matter of shape. Two carboxylate groups on adjacent carbons let the ion grip a metal through two oxygens at once — a bidentate ligand — and three of them close around a single iron(III) to give the octahedral trisoxalatoferrate(III) anion, [Fe(C₂O₄)₃]³⁻, whose structure Ware records as well determined both in the solid and in aqueous solution. The same grip works on iron(II). That is why a bath of this salt lifts insoluble ferrous oxalate into solution in a platinum developer, why 15 per cent of it brushed on a blueprint dissolves the Prussian blue for a correction, and why Kodak’s 1928 primer adds it to a sepia bleach so that stray iron cannot deposit as blue spots — “since the blue iron salt is soluble in the oxalate”.

2 FeC₂O₄ + 4 [C₂O₄]²⁻ + [PtCl₄]²⁻ → 2 [Fe(C₂O₄)₃]³⁻ + Pt + 4 Cl⁻

Ware's development equation: the oxalate ion does the dissolving, whichever salt supplied it

How to tell which salt a formula means. Usually by its number rather than its name. Wall’s 1924 preparation of ferrous oxalate calls for “Potassium oxalate, neutral 184 parts” against 278 parts of iron(II) sulfate — and 184 is the monohydrate’s formula weight, just as 278 is the heptahydrate’s, so both are identified by their weights and neither by its name. Where a formula gives a percentage rather than a molar proportion, as most platinum developers do, the difference is smaller but real: Ware’s standard 28 per cent w/v developer is specified explicitly in potassium oxalate monohydrate, and making it up with the anhydrous salt at the same weight gives a bath about a tenth stronger in oxalate.

Where this form genuinely turns up. Suppliers list both, and the anhydrous salt is sometimes the cheaper or the only one in stock. It is also what the monohydrate slowly becomes: the Merck Index records that the crystals effloresce in warm dry air, and CAMEO records that the monohydrate loses its water at about 160 degrees — the entry does not say on which scale — and then decomposes to the carbonate without charring. A tub that has stood open on a warm shelf for years has been quietly converting itself, and its grams are no longer the grams the recipe assumed.

The other confusion, which is worse. Wall’s 1912 dictionary is at pains to separate the neutral oxalate from potassium binoxalate, the acid oxalate sold as salt of sorrel, and gives a method of converting the one to the other by heating 200 parts of the acid salt in 1,000 parts of water with potassium carbonate or bicarbonate until faintly alkaline, then adding back a few grains of oxalic acid. Substituting salt of sorrel for the neutral salt in a platinum developer would not make the bath a tenth too strong; it would make it acid enough to inhibit the formation of platinum, which the texts Ware quotes say is exactly what more than a trace of acid does. When a period source says “oxalate of potash” without qualification, it means the neutral salt.

Properties

A colourless, odourless crystalline solid; commercial material is described as white crystals or chunks. Its aqueous solution is basic — CAMEO’s reactive-group classification for the monohydrate is Salts, Basic, neutralising acids with the generation of heat, though less than a strong base would — and it is also classed as a weak reducing agent that can generate carbon dioxide with oxidisers.

Solubility. The sources the course holds give figures for the monohydrate rather than for this form: Wall’s 1912 dictionary gives 1 part in 3 of water, insoluble in alcohol or ether, and platinum practice takes saturation as about 32 per cent w/v. Kodak’s 1928 primer lists potassium oxalate in its table of chemical solubilities, but the two temperature columns did not survive legibly in the copy the course holds, so its figures are not quoted here. The course has not found a separately sourced solubility for the anhydrous salt and does not assume that the monohydrate’s figure transfers unchanged; in practice the difference matters far less than the weighing does, because a saturated solution is defined by what dissolves rather than by what was weighed.

The conversion, both ways. 166.22 against 184.23. To follow a recipe written for the monohydrate using this salt, multiply the stated weight by 0.902; to go the other way, multiply by 1.108. Both figures are the course’s own arithmetic from the two molar masses. Ware gives the monohydrate’s formula weight as 184.24 and Wall as 184, and nothing in photographic practice turns on the second decimal place.

Handling

The aggregated ECHA notifications held by PubChem classify it Warning, irritant: harmful if swallowed and harmful in contact with skin, each given by every notifier who classifies it, and serious eye irritation by 60.7 per cent, from 300 reports across five notifications. CAMEO holds no separate datasheet for the anhydrous salt, so the course applies the monohydrate’s, which is considerably harsher than the notified classification: inhalation of the dust “can cause systemic poisoning”, and ingestion causes burning pain in the throat, oesophagus and stomach, whitening of the exposed mucous membrane, vomiting, severe purging, a weak pulse and possible cardiovascular collapse, with neuromuscular symptoms if death is delayed.

Why Level B. Against the course rubric, harmful in contact with skin is an acute dermal toxicity entry outside Level A’s ceiling of “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”, which is the same reasoning that places oxalic acid and the monohydrate at Level B. Level B’s criterion covering fine powders that must not be inhaled applies with particular force to this form, which carries no water of crystallisation to weigh the dust down, and CAMEO’s description of systemic poisoning by inhalation is precisely that criterion’s concern. It does not reach Level C, whose criterion applies where a fume cupboard or specialist disposal is the recognised control; gloves, goggles, dust suppression, eyewash and hygiene are the recognised controls, and an oxalate bath produces no vapour.

⚠Safety

Weigh over a tray without raising dust, container away from the face; gloves and goggles; eyewash within reach; wash hands before eating and eat nothing at the bench, because ingestion is the route that harms most. Keep every oxalate away from every silver salt: the NIOSH pocket guide lists silver compounds among oxalic acid’s incompatibilities and the ILO-WHO chemical safety card names explosive silver oxalate as the reason, so oxalate wastes and silver-bearing wastes go into different bottles. Make solutions with distilled water, because calcium oxalate is essentially insoluble and hard water will cloud the bath.

Waste

What leaves the darkroom is a spent platinum or palladium developer, the rinse from an oxalate-containing sepia bleach, or a discarded ferrous oxalate developer — in each case a solution of this ion with whatever metal it has been dissolving. The compatibility rule is the one that matters: oxalate waste and silver-bearing waste do not share a container, for the reason above. Platinum and palladium are worth recovering rather than discarding. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most frequently set, and a neutral oxalate solution falls inside it, which settles nothing on its own, because what makes this a waste to collect is the oxalate and the metals travelling with it. 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. Check your local regulations; they govern, and they differ.

History

The nineteenth century made this salt for itself. Wall’s 1912 dictionary gives the preparation as neutralising oxalic acid with potassium carbonate or caustic potash, and describes a convenient working method: dissolve 13 oz of potassium carbonate in 30 oz of water, add about 9 oz of oxalic acid gradually until the boiled solution is neutral to test paper, filter, and make up to 64 oz for a solution 1 in 4. Willis did the same when he was inventing the platinotype, Ware recording that he had to prepare the potassium oxalate himself because he could not obtain it in London.

The ambiguity between the hydrates is older than the ambiguity between the neutral and acid salts is resolved. Wall’s dictionary states the formula as K₂C₂O₄·H₂O and the weight as 184, which is the monohydrate; a great many later recipes simply write “potassium oxalate” and leave the reader to infer it. The course’s convention follows the practice of the sources: a photographic formula means the monohydrate unless it says otherwise, and where a formula gives a molar proportion the number itself identifies which salt the author had on the balance.