Ammonium iron(III) oxalate

This is what ferric oxalate would be if chemistry had been kind. It is crystalline, its structure is known, it dissolves readily to a concentrated solution that keeps for years, it costs little, and it is available everywhere. It is also four to eight times more light-sensitive than the citrate of the classic cyanotype. Much of the modernisation of the iron-based processes over the last forty years — Ware’s print-out palladiotype, his New Cyanotype — comes down to putting this salt where something less definite used to be.

In photography

The anion is the whole story. Dissolved, the substance provides the trisoxalatoferrate(III) ion, [Fe(C₂O₄)₃]³⁻ — one iron(III) held octahedrally by three bidentate oxalates, a monomer rather than the polymer that ferric oxalate is. Ware records that this anion’s photochemistry has been by far the most extensively investigated of all the iron(III) carboxylates, and that it provides the basis of a well-known chemical actinometer, a light meter made of chemistry.

⚗The chemistry

The measured quantum yield per mole of iron(II) formed in aqueous solution is about 1.2 between 250 and 420 nm, falling slightly to 0.9 at 500 nm and very sharply thereafter — so more than one iron(II) per photon absorbed, because the radical formed in the primary step goes on to reduce a second, unexcited complex. Ware gives the Hatchard and Parker mechanism: light transfers an electron from one oxalate ligand to the iron(III), leaving an iron(II) complex carrying an oxalate radical anion; that radical dissociates and reacts with another iron(III) complex, transferring its second electron and leaving as two molecules of carbon dioxide. The theoretical ceiling is a quantum yield of 2, and the reverse dark reaction is what keeps it below.

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

Ware's stoichiometry for the photolysis of trisoxalatoferrate(III)

Why that matters more than the speed. Look at what is on the right-hand side. The iron(II) photoproduct is [Fe(C₂O₄)₂]²⁻, a soluble complex ion — not the insoluble ferrous oxalate that dry ferric oxalate gives. Ware’s explanation of the practical consequence is the pivot of modern siderotype: if the sensitised paper holds enough water, which any cellulose paper does at a relative humidity of 70 to 80 per cent, where it carries about 8 per cent of water by weight, the ions can migrate and reduce the platinum(II) to metal during the exposure. The image is formed by light rather than developed afterwards, and only clearing baths are needed.

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

Ware's in-situ reduction: two iron(II) for one atom of platinum

Ware lists what follows from that. Print-out is self-masking, because the darkening shadows inhibit their own further darkening, so a longer density range in the negative can be accommodated simply by exposing longer; prints can be judged by inspection without test strips; and the process appears to be immune to the “plague of black spots” that sometimes afflicts the ferric oxalate development route.

New Cyanotype, 1995. Ware’s replacement for the classic sensitiser uses 30 g of ammonium iron(III) oxalate trihydrate, 10 g of potassium ferricyanide and 0.1 g of ammonium dichromate made up to 100 cc — but it cannot simply be mixed, because potassium iron(III) oxalate is only sparingly soluble and crystallises out, giving what Ware calls a gritty sensitiser and, in the paper, “quite pretty, but totally unwanted fern-like patterns”. The preparation therefore turns the problem into the method: a hot, concentrated potassium ferricyanide solution is added to an excess of very concentrated ammonium iron(III) oxalate, the mixture is left in the dark to cool and crystallise for one to two hours, and about 15 g of solid potassium iron(III) oxalate is filtered off and discarded. What is left is a single-bottle sensitiser with most of the potassium removed, a shelf life of at least four to five years, and an image of what Ware calls an “ammonium blue” of very good colour, “more resistant to peptization and alkalies” than the potassium form.

What more or less of it does. Ware notes that the sensitiser may be made more dilute, up to 200 cc instead of 100, which prints faster but yields a less intense blue. Against the classic citrate formula the gains he lists are systematic: four to eight times the light sensitivity, so two or three stops less exposure; no mould on the stock solution; better penetration of the paper fibres, so less image washes away; an exposure scale of about 2.2 against about 0.9 for classic cyanotype, which is the difference between a process that can print a normal negative and one that needs a very flat one; and, because the oxalate can be mixed in chemically equivalent amounts with the ferricyanide, no excess iron(II) to bleed into the highlights.

In the trade, a century earlier. None of this was new in kind. Wall’s 1912 dictionary already lists the salt as “used in platinotype and blue-printing processes”, and his 1924 formulary gives blueprint sensitisers containing sodium or ammonium ferric oxalate, noting flatly that they give “greater speed than the above, but the paper does not keep so well”. Ware’s transcription of Kwech’s commercial formulae shows the same trade-off industrialised: the fastest blueprint papers, exposing in fifteen seconds of bright sunlight against four minutes for the “regular” formula, carry ammonium ferric oxalate and a very low ferricyanide concentration.

Properties

Bright green crystals, water-soluble, light-sensitive, not deliquescent. Wall’s 1912 dictionary gives the solubility as 90 parts in 100 parts of cold water and 126 parts in 100 of boiling water; Ware gives a saturated aqueous solution as approximately 1.4 molar, stable for years in the dark. CAMEO’s reactivity profile classes it as an acidic salt giving a solution below pH 7 and as a weak reducing agent that may release carbon dioxide with oxidising agents.

Which weight the recipe means. The commercial reagent is the trihydrate, and Ware gives it as (NH₄)₃Fe(C₂O₄)₃·3H₂O with a formula weight of 428.07 and CAS 13268-42-3. PubChem’s computed molecular weight of 374.02 is for the anhydrous formula unit, CAS 14221-47-7. A formula in grams means the trihydrate unless it says otherwise, and weighing the anhydrous salt against a recipe written for the trihydrate would put about 14 per cent too much iron in the bottle, on the course’s own arithmetic from the two weights.

Storage behaviour is the practical difference from its rivals. Ferric oxalate’s solution drifts in six to nine months; ferric ammonium citrate’s solution grows mould within a fortnight and the solid compacts into an intractable mass. This salt does neither. It is the only one of the three whose bottle can be trusted a year later.

Handling

The aggregated ECHA notifications held by PubChem classify it Warning, irritant, with H302 and H312 — harmful if swallowed and harmful in contact with skin — each at 100 per cent, from 2,015 reports across just three notifications. Two other sources describe it more severely. CAMEO’s health hazard entry records that ingestion causes burning pain in the throat and stomach, that the mucous membranes turn white, and that vomiting, a weak pulse, cardiovascular collapse and death may follow; that contact with the dust irritates eyes and skin and may cause severe skin burns. Ware’s own hazard note calls it corrosive and “very destructive of mucous membranes”, and adds that its toxicology is not fully investigated.

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 and ferric oxalate at Level B. Level B’s criterion covering fine powders that must not be inhaled applies to weighing 30 g of it, and the concentrations actually handled are high: the New Cyanotype stock is 30 per cent w/v. CAMEO’s description of what swallowing it does is the reason the course does not read the notified classification as the last word. It does not reach Level C, whose criterion applies where a fume cupboard or specialist disposal is the recognised control — with one exception: a sensitiser containing ammonium dichromate is a Level C matter on account of the chromium(VI), not the iron, and the rubric names chromium(VI) compounds in any quantity.

⚠Safety

Gloves and goggles, dust kept down when weighing, eyewash within reach, hands washed before eating and nothing eaten at the bench. Keep it 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 bottle and the silver nitrate live in different places and their wastes go into different containers. Where the New Cyanotype preparation is followed, note that it requires solutions at 70 °C and 50 °C, which is a Level B operation in itself, and that the dichromate it uses as a preservative is a chromium(VI) compound with its own, much stricter, handling and disposal requirements.

Waste

What leaves a bench using this salt is the unexposed sensitiser washed out of the print, the clearing baths, and — where New Cyanotype is made up from scratch — about 15 g of solid potassium iron(III) oxalate filtered off during the preparation. All of it is oxalate waste and is bottled apart from anything silver-bearing, for the reason above. CAMEO states that the primary hazard of ferric ammonium oxalate is the threat to the environment and that immediate steps should be taken to limit its spread, so nothing goes onto a garden, into a soakaway or into a watercourse. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most frequently set, and this is an acidic salt whose solutions sit below it. Where dichromate has been used as a preservative, the chromium(VI) governs the whole stream and it becomes a waste for a licensed contractor rather than anything a household can deal with. 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

Herschel had the choice in 1842 and did not take it. Ware notes that the oxalate was “shunned, or overlooked” by him — whichever it was — and that his siderotypes ran instead on ammonium iron(III) citrate, which sufficed for gold, silver and mercury but could not reduce platinum. The oxalate’s first documented print-out application came later: Ware credits Giuseppe Pizzighelli in 1887 with observing print-out using sodium iron(III) oxalate, the sensitiser whose behaviour the Platinomicon reconstructs.

By 1912 Wall could list the salt as a standard photographic chemical of the platinotype and blueprint trades, and the commercial blueprint industry used it to make its fastest papers. Wall’s own caveat about those papers — that they “do not keep so well” — is, on the course’s reading, why the citrate remained the amateur’s salt even while the trade used the oxalate; and when the drawing office stopped making blueprints, the fast papers went with it.

Its modern return is Ware’s work. The Platinomicon records thirty years of preferring the well-characterised ammonium salt to the ill-defined ferric oxalate, on the grounds that it is a highly crystalline pure substance of known structure, universally available at low cost, and dissolves readily to a stable concentrated solution — with the single precaution of keeping potassium ions out of the sensitiser. New Cyanotype followed in 1995, and applied the same substitution to Herschel’s process a hundred and fifty-three years after he declined to make it.