Potassium oxalate monohydrate

The platinum printer’s developer contains no developing agent. Drop an exposed sheet into it and the image, in the Photographic Materials Group’s words, “comes up instantly” — but nothing in the tray is reducing anything. Potassium oxalate is a solvent — its job is to dissolve a compound that light has already made and that cannot move.

In photography

The platinotype developer. Ware records that Willis recommended neutral potassium oxalate at 120 to 130 grains per fluid ounce, which is 27 to 30 per cent w/v, and that later practice was little different, recommending a saturated solution of about 32 per cent w/v. Before 1892 the bath was used hot to speed the chemical reduction. Ware’s own work uses a standard 28 per cent w/v in potassium oxalate monohydrate at room temperature.

⚗The chemistry

Light acting on ferric oxalate leaves ferrous oxalate, which is insoluble. It sits where it was made, and the platinum salt sits where it was coated, and neither can reach the other: as Ware puts it, in the dry sensitiser the ions are immobile and cannot encounter one another. Potassium oxalate supplies free oxalate ions, which take the iron(II) into solution as a soluble oxalato complex. Now it can move, and now it can hand two electrons to one platinum(II). The potassium ions take no part at all, which is why Ware omits them from the balanced equation.

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

Ware's development equation: the oxalate dissolves the iron(II), which reduces the platinum

Why it must be neutral, or barely acid. The texts Ware quotes, citing Crawford, say that if much more than a trace of acid is present it inhibits the formation of platinum. The reason for wanting any acid is the one the texts give: the developer should be neutral or just slightly acid, to counter the alkali it accumulates from the water and the paper as it is reused. Some recipes carry 0.1 per cent oxalic acid or less; Ware records that some modern practitioners of the traditional process add about 2 per cent excess oxalic acid, and offers as a presumption rather than a finding that this lowers the pH below 6 and inhibits the hydrolysis of iron(III), so avoiding the yellow-brown stain that hydrolysed iron leaves in the paper. The Platinotype Company’s proprietary “Special D Salts”, said to give cooler, bluish tones, analysed as 85 per cent potassium oxalate monohydrate, 12 per cent potassium dihydrogen phosphate and 3 per cent oxalic acid dihydrate, made up to a working strength of only 8.6 per cent w/v for black papers, or 10.3 per cent for the sepia Japine papers used at 70 to 80 °C.

What more or less of it does. More oxalate dissolves more of the iron(II) photoproduct and develops faster and further; a hot bath does the same. Ware’s tests on palladiotype show the cost of overdoing it: the oxalate developer can reduce palladium(II) to some extent by itself, causing a brownish-grey fogging of the highlights by palladium metal, quite distinct from the yellow stain of iron, and it also produces a warmer colour and a higher speed than the trisodium citrate developer Willis specified for palladium. That is why the two traditions of palladiotype processing — the British citrate developer and the American habit of reusing old platinum oxalate baths — give different prints from the same sensitiser.

In the ferrous oxalate developer. Here the same solvent role builds the developer itself. Wall’s 1924 formulary explains the choice of cation in one sentence: potassium oxalate is used in preference to the ammonium and sodium oxalates “because it is more soluble. Consequently, it will dissolve more of the ferrous oxalate, which is the active agent in development.” His strongest version boils 500 g of potassium oxalate in a litre of water and dissolves 100 g of dry ferrous oxalate in it. In practice two stocks are kept — iron(II) sulfate with a little sulfuric acid, and potassium oxalate in hot water — and mixed one part of the iron to four of the oxalate, with the image appearing in 10 to 30 seconds and complete in 2 to 5 minutes.

In a sepia bleach, as an iron trap. Kodak’s 1928 primer gives the neatest single-sentence justification for an ingredient anywhere in the photographic literature. A trace of iron in the ferricyanide-bromide bleach — from a defective enamelled tray, for instance — produces blue spots of iron ferrocyanide on the print. “This tendency to form spots is reduced to a minimum by adding potassium oxalate to the bleach bath since the blue iron salt is soluble in the oxalate.” The primer’s own bleach, T-7a, accordingly carries 75 g of potassium ferricyanide, 75 g of potassium bromide and 195 g of potassium oxalate with 40 cm³ of 28 per cent acetic acid in 2 litres.

And elsewhere. Wall’s 1924 Belitzski reducer is built on potassium ferric oxalate, or on iron(III) chloride and neutral potassium oxalate mixed to make it. His chapter on the iron processes gives corrections on a blueprint as a solution of potassium oxalate at about 15 per cent, thickened with gum arabic if necessary — the oxalate dissolving Prussian blue where it is brushed on. Photographers’ Formulary’s kallitype kit supplies 60 g of it, and names it one of four chemicals in the kit needing special attention.

Properties

An odourless white solid that, in CAMEO’s phrase, “sinks in and mixes slowly with water” — the practical warning that a saturated developer is not made in a hurry. Wall’s 1912 dictionary gives the solubility as 1 part in 3 of water and records that it is insoluble in alcohol or ether; saturation is about 32 per cent w/v. CAMEO records that it loses its water at about 160 degrees — the entry does not say on which scale — and decomposes to the carbonate with no charring, and that the reaction is not hazardous.

Its solution is basic. CAMEO’s reactive-group classification is Salts, Basic: the solution neutralises acids with the generation of heat, though less than a strong base would. It is also classed as a weak reducing agent that can generate carbon dioxide. Both matter in a developer that is supposed to sit within a hair of neutral.

Neutral, not acid. Wall’s 1912 dictionary is careful to separate this salt from potassium binoxalate, the acid oxalate sold commercially as salt of sorrel, and gives a method of converting one to the other. A period formula that says “oxalate of potash” without qualification means the neutral salt; one that says salt of sorrel does not. He also gives a preparation for the neutral salt from 13 oz of potassium carbonate in 30 oz of water with about 9 oz of oxalic acid added gradually until the boiled solution is neutral to test paper, made up to 64 oz to give a solution 1 in 4.

Weights. The monohydrate is 184.23 to the mole and the anhydrous salt 166.22, so a formula written for the monohydrate needs 0.902 times the weight if only the anhydrous salt is to hand, on the course’s own arithmetic from the two molar masses. Because the solid effloresces in warm dry air, an old tub drifts toward the anhydrous figure on its own and every gram from it carries a little more oxalate than intended.

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, with serious eye irritation in 36.4 per cent and skin irritation in 23 per cent. CAMEO goes further than the classification, and the course follows CAMEO: 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. Ware’s own hazard note in the Platinomicon reads: “Harmful by ingestion and inhalation. Irritating to skin and eyes. If swallowed causes severe internal pain, followed by collapse.”

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” — the same reasoning that places oxalic acid at Level B. Level B’s criterion covering fine powders that must not be inhaled applies directly, since CAMEO describes systemic poisoning by the dust, and the concentrations actually handled are not small: a working developer is 28 to 32 per cent w/v, and 280 g of it goes into a litre. 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 the developer itself produces no vapour.

⚠Safety

Weigh over a tray without raising dust; gloves and goggles; eyewash within reach; hands washed before eating and nothing eaten at the bench, because ingestion is the route that has killed. The storage rule is as important as the personal one: 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 the oxalate developer and the silver nitrate live in different places and their wastes go into different bottles. Use distilled water: hard water throws calcium oxalate, which clouds the bath and, in a print, is as insoluble as anything on this page.

Waste

A platinum or palladium developer is used until it is spent rather than discarded after each print, so the volume is small but the concentration is high — 28 to 32 grams of this salt in every 100 millilitres, with dissolved iron and traces of the noble metal it did not deposit. The clearing baths carry most of the iron. Two rules govern. The first is a compatibility rule: oxalate waste and silver-bearing waste do not share a container, for the reason above. The second is that the platinum or palladium is 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 developer falls inside it — which decides nothing on its own, because what makes this a waste to collect is the oxalate and the metal, not the pH. 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 platinotype needed three things that were hard to get in 1873, and Ware names this salt as the third: after the platinum salt and the ferric oxalate came “the use of potassium oxalate as a developer”. Willis admitted that his attention was directed to it by “…a note by a French chemist…”, who has never been identified, and — the detail that dates the whole enterprise — he had to prepare the potassium oxalate himself, because at the time he could not obtain it in London.

What he had found was not a new developing agent but a new idea about what development could be. Every other developer in photography reduces something. This one dissolves something, and lets the reduction that light had already prepared finish itself. The Getty Conservation Institute’s atlas records the sequence in its list of the process’s steps: the exposure produces “a faint brownish image made from the photochemically generated ferrous oxalate”, and then “the exposed platinum paper is fully developed in a solution of potassium oxalate”, after which the image appears almost immediately.

The bath outlived the instructions written for it. On Paul L. Anderson’s evidence, which Ware quotes, many American workers developed their palladiotypes in their pre-existing, used platinotype developer — nearly saturated potassium oxalate, complete with its accumulated metal residues — rather than mixing the trisodium citrate bath the manufacturer specified, and cleared them in hydrochloric acid diluted 1 part in 60 rather than 1 in 200. Ware’s reading is that the two departures tended to be self-cancelling, the stronger acid etching away the palladium fog that the oxalate developer had produced. The prints came out well enough that the discrepancy went unexamined.