Ammonium iron(III) citrate

This page has no formula and no molar mass, and that is not an omission. Ammonium iron(III) citrate is not a compound. It is a family of iron-citrate-ammonium complexes of variable composition, sold under one name, whose iron content ranges from 14 to 28 per cent by weight and whose colour runs from light green to dark brown as that proportion rises. Chemists call such a material ill-characterised, which Ware describes as a condemnatory term reflecting the variability of its properties. Nearly every cyanotype made in the process’s first century and a half was made with it.

In photography

The classic cyanotype sensitiser. Herschel’s process of 1842 is two solutions mixed just before coating. Ware’s version of the classic formula is 20 g of the solid ammonium iron(III) citrate made up to 100 cc for stock A, and 10 g of potassium ferricyanide made up to 100 cc for stock B, mixed in equal volumes, with two to five drops of 20 per cent Tween 20 per 10 cc to help the solution into the paper fibres. Exposure to sunlight or a UV lamp runs 20 to 30 minutes; the image prints out, first darkening to blue and then reversing to a pale blue-grey, and is correctly exposed when it looks “blocked up”. Twenty minutes face down in running water follows, after which the shadows darken and the colour intensifies over several hours as atmospheric oxygen turns Prussian white into Prussian blue.

How much of it, and of what. Ware compared about sixty published recipes by converting them all to per cent w/v in the final mixed sensitiser. The recipes using the brown salt cluster around 10 per cent citrate with 8 per cent ferricyanide; those using the green salt around 13 per cent citrate with 6 per cent ferricyanide. Ware quotes Christina Z. Anderson’s 2020 summary of the work behind her 2019 book Cyanotype: while writing it she came to prefer a 10/10 classic mix, 10 per cent of each solution combined in equal volumes, which she describes as having a long exposure scale, little grain and a more turquoise blue, having previously used the 20/8 proportions the books recommend. Ware’s own conclusion is the one that matters most to a beginner: “the quality of the ferric ammonium citrate available to any particular practitioner is of far greater importance to a good outcome than their actual choice of concentration ratios.”

In the silver processes. The same salt sensitises the Van Dyke brown print and the argyrotype. Photographers’ Formulary’s Van Dyke kit is ferric ammonium citrate (green) 9.0 g, tartaric acid 1.5 g and silver nitrate 3.8 g, made up as three solutions and combined in order under subdued light. Ware’s argyrotype replaces the silver nitrate with silver sulfamate made in situ, on the grounds that nitrate is an oxidising anion which tends to dissolve the colloidal image silver during wet processing, and uses 22 g of the green citrate per 100 cc.

⚗The chemistry

Why the citrate can print in silver and gold but not in platinum begins with one number. Ware gives the redox potential of the iron(III)/iron(II) citrate couple as +0.372 V, against +0.02 V for the oxalate couple, and reads it as showing the citrate photoproduct to be the weaker reducing agent of the two. He then states the outcome: it “does not reduce platinum(II) or palladium(II), although it will reduce gold(III) and silver(I), under the printing conditions”, the four metal couples being gold at +1.00 V, silver at +0.80 V, platinum at +0.73 V and palladium at +0.62 V. Those numbers on their own do not explain the outcome — palladium sits below the citrate couple’s own value and is still not reduced — and Ware notes elsewhere that a noble metal is liberated from its complex only “kinetic factors permitting”. The course reports the observed division and does not offer the potentials as a complete account of it. What is certain is the consequence: Herschel could print in gold, silver, mercury and Prussian blue with the citrate, and platinum printing had to wait for Willis and the more energetic ferric oxalate.

The photochemistry, as far as it is known. Ware records that the iron(III)/citrate system in solution has a photo-inactive monomer predominating between pH 0.5 and 1.5 and a photo-active dimer above pH 2; two dinuclear citratoferrate(III) complexes have been isolated and their structures determined by X-ray diffraction. The quantum yield at 365 nm reaches a maximum of about 0.45 at pH 4 and falls to 0.28 at 436 nm — roughly a third of the trisoxalatoferrate(III) figure, which is why the classic cyanotype is slow. The initial photochemical product has been identified as acetone dicarboxylic acid, CO(CH₂COOH)₂, which may lose two more molecules of carbon dioxide to give acetone.

No balanced equation is offered here, and the reason is worth stating. Ware writes the reaction as the dimeric anion [Fe₂(C₆H₄O₇)₂(OH₂)₂]²⁻ going, under light, to two iron(II) species plus acetone dicarboxylic acid and carbon dioxide — but he writes those iron(II) species simply as “Fe^II”, without ligands or charge, because the nature of the iron(II) photoproduct remains unknown. His equation is therefore a schematic of what becomes of the citrate, not a balanced account of what becomes of the iron, and the course reproduces it as such rather than closing the balance by invention.

Green against brown. Ware calls the preparation of a green form of the salt by Eduard Valenta in 1897 “the only significant chemical advance in the entire history of the cyanotype process”. His comparison table is worth having whole: the brown salt dates from about 1840, carries 19 to 28 per cent iron and is basic; the green dates from 1897, carries 14 to 18 per cent and is acidic. The green prints about a stop faster in the midtones, shows stronger reversal and a higher maximum density, gives a brighter blue and no distinctive edge-etch effect, and is more susceptible to light fading. Every cyanotype made before 1897 used a variety of the brown. If only the brown salt is available, Ware’s remedy is to add about one third of its weight of citric acid to bring the citrate-to-iron ratio closer to the green.

Properties

Why there is no formula. The pharmacopoeias listed the substance as Ferri et Ammoniae Citras and recognised the problem from the start; Ware quotes their verdict that the “…want of chemical compactness, the loose state in which the iron is combined, precludes their recognition as well-defined chemical compounds.” The reason lies in the preparation, which has not changed: iron(III) hydroxide is precipitated from iron(III) sulfate with ammonia, filtered, washed, dissolved in citric acid and neutralised with ammonia, then evaporated to a syrup and heat-dried to an amorphous solid. Iron(III) hydroxide is a highly polymeric, often colloidal substance, so the product inherits its indefiniteness: the proportions of ammonium, iron and citrate vary from batch to batch, and, as Ware puts it, no single chemical formula can be written for the actual molecular species present.

The analyses attempted show the scale of the problem rather than solving it. Ware tabulates Valenta’s formulations — a brown form written as 4Fe(Hcit)·3(NH₄)₃(Hcit)·3Fe(OH)₃, formula weight 2030 and 19.26 per cent iron, and a green form of 1956 at 14.27 per cent — beside L. P. Clerc’s quite different pair from 1937, at 615 and 733; in a later edition of his manual, in 1954, Clerc declined to give any formula at all. One rare investigation, at the Mallinckrodt Chemical Works in 1949, did obtain both forms as crystalline solids giving sharp powder X-ray lines, but the authors could draw no structural inferences from the data, not even unit cell parameters.

What the printer actually sees. Wall’s 1912 dictionary gives the solubility as 1 part in 0.5 part of water, which is to say extravagantly soluble, and describes small transparent reddish-brown scales. The material is highly deliquescent, so an opened tub takes up water and goes sticky, and on long storage compacts into a mass that has to be broken up. Its solution grows mould within a week or two.

!Important

Which salt is in the jar? Ware records that most present-day catalogues offer a product described as the “green” salt containing rather more than 14 per cent iron, commonly up to about 18, and that it “frequently does not appear green at all, but light brown”, while many products sold as the “brown” salt are stated to contain much more than 19 per cent, up to 28 in one instance. Names, colours and iron contents no longer line up. The label is not evidence; the iron content on the certificate of analysis is, and a printer who calibrates a digital negative against one batch should expect to recalibrate against the next. What the course cannot resolve is whether any current product corresponds to Valenta’s green salt as analysed in 1897.

Handling

The aggregated ECHA notifications held by PubChem classify it Warning, irritant: skin irritation and serious eye irritation, each in 89.8 per cent of the 1,415 reports that carry hazard codes, with 161 of the 1,576 reports — one in ten — saying it meets no GHS criterion. CAMEO’s health hazard entry is consistent and mild by the standards of this family: inhaling the dust irritates the nose and throat, ingestion irritates the mouth and stomach, the dust irritates the eyes and causes mild irritation of the skin on prolonged contact.

Why Level A. Against the course rubric this substance meets the Level A criteria and no more. Its classification tops out at irritant, which is exactly Level A’s ceiling; nothing about handling it is heated above 50 °C; and its own waste is a dilute iron and citrate solution. NIOSH’s recommended limit of 1 mg/m³ for soluble iron salts as iron, matched by HSE’s EH40, is a reason to keep the dust down when weighing, not a reason to move up a level. One Level B control is added by preference: splash goggles rather than glasses for the solid, because the eye statement is near-unanimous among notifiers who classify it at all.

The level of the sensitiser is not the level of this salt. A classic cyanotype sensitiser is this substance plus potassium ferricyanide, and the ferricyanide sets the handling and the waste rules — above all the rule that acid must never reach a ferricyanide solution. A Van Dyke sensitiser is this substance plus silver nitrate, which the rubric names explicitly under Level B. And Ware’s “improved” classic formula, which adds oxalic acid and ammonium dichromate, contains a chromium(VI) compound and is a Level C matter on that ground alone; Ware’s own comment is that if one is prepared to go to that trouble it would be more worthwhile to use his New Cyanotype method, and that the improved sensitiser should not be placed in the hands of children.

⚠Safety

Gloves and eye protection; goggles and a tray when weighing the solid; keep the dust down. Dedicate a separate labelled syringe or pipette to each stock solution and a third to the mixture, because one drop of ferricyanide in the citrate bottle, or the reverse, makes Prussian blue on the spot and the stain does not wash out. Remember that the mixed sensitiser and the wash water also carry potassium ferricyanide, whose own entry sets the rules for handling and disposing of them. And do not use a dichromate bath to force the reoxidation of Prussian white, which Ware states should under no circumstances be done although it has been recommended in the past; dilute hydrogen peroxide at 0.3 per cent for about half a minute does the same job.

Waste

The wash from a cyanotype is mostly unexposed sensitiser: iron, citrate and ammonium from this salt, and potassium ferricyanide from the other bottle. The ferricyanide governs that stream, and its own entry carries the rules for it. The citrate’s own contribution is an environmental one: CAMEO records that the primary hazard of ferric ammonium citrate is the threat to the environment and that immediate steps should be taken to limit its spread, so no sensitiser solution 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. Where the salt has been used in a Van Dyke or an argyrotype, the wash and the thiosulfate fixer carry silver and belong to the silver recovery stream. 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 substance reached photography from the chemist’s shop. Ware records that it originated in early pharmacy before the cyanotype existed, and that its preparation reflects the need for a product uncontaminated with anything noxious, because it was meant to be swallowed as an iron tonic. Herschel did not find it himself: Ware credits the young scientist Alfred Smee (1818-1877), whose chemical expertise supplied him with both information and samples of newly synthesised substances, the ammonio-citrate among them.

Valenta’s green salt of 1897 is the only chemical advance Ware is prepared to call significant, and after 1900 the majority of cyanotypes were probably made with it; its sources were at first a few German manufacturers, and it soon became available in the United States. The Getty Conservation Institute’s atlas records the commercial arc around it: the first commercial cyanotype paper, papier ferro-prussiate, from Marion et Cie in Paris in 1872; a reprographic era running from roughly 1870 to 1950; and replacement by the diazo processes.

Ware’s judgement on the chemistry is unsparing — “All this unreliable chemistry is bad science!” — and he has twice tried to end it. New Cyanotype, in 1995, replaced the citrate with ammonium iron(III) oxalate, a crystalline substance of known structure. Simple Cyanotype, in 2022, avoided commercial ferric ammonium citrate altogether by making an iron citrate in the bottle from iron(III) nitrate, citric acid and ammonia in a known ratio. Both are attempts to give the oldest and cheapest of the alternative processes a chemical it can be sure of. The classic formula survives anyway, because it is two bottles, a brush and the sun.