Iron(III) chloride hexahydrate

This is the salt a photographic formula means when it says “ferric chloride” with nothing else attached. It is yellow-brown rather than black, it dissolves quickly instead of grudgingly, and at 270.29 to the mole it carries six waters that the anhydrous salt does not. Almost everything a printer does with iron(III) chloride is done with this form.

In photography

Herschel’s blue developer, 1842. The very first cyanotype experiments used potassium ferricyanide by itself on paper, and were extremely slow. Among the entries in Herschel’s Memoranda that Ware transcribes in Cyanomicon is a remedy: “Very weak perchloride of Iron acidulated with S. Acid, develops a deep & most superb blue. But the ground also is somewhat blued.” That second sentence is the whole problem in one clause. Ware records that developing the image in a bath of iron(III) chloride or nitrate gives greater speed and higher densities but “invariably causes blue fog in the highlights”, and reads this as one of the reasons Herschel abandoned the ferricyanide-only method in favour of the far faster sensitiser built on the ammonium iron(III) citrate that Alfred Smee recommended to him.

⚗The chemistry

The fogging is not a defect of technique but a consequence of the chemistry. Ware’s account of the photolysis of the hexacyanoferrate(III) anion is that light aquates it, releasing hydrogen cyanide, and that the aquapentacyanoferrate(III) so formed undergoes a facile reduction and hydrolysis, so that after exposure “the ingredients are available to form Prussian blue”. Those ingredients are half-made everywhere on the sheet, not only where the light fell. Flooding the whole surface with iron(III) supplies the other half at once, so the unexposed ground has every opportunity to go blue as well. Ware’s composition table gives the two pigments as soluble Prussian blue, KFe[Fe(CN)₆], and insoluble Prussian blue, Fe₄[Fe(CN)₆]₃, the latter with its four-to-three ratio of iron(III) to iron(II).

4 Fe³⁺ + 3 [Fe(CN)₆]⁴⁻ → Fe₄[Fe(CN)₆]₃

Insoluble Prussian blue: the reaction a developing bath of iron(III) chloride cannot confine to the image

The positive-working blueprint. The most ingenious use of this salt turns on a property that has nothing to do with colour. Ware records Poitevin’s observation of 1863 that iron(III) salts harden gum and similar colloids and make them insoluble in water, whereas iron(II) salts do not. Henri Pellet built a process on it, patented in Britain in 1877 and in the United States in 1881: iron(III) chloride and gum arabic together in the coating, so that the layer is hardened all over before exposure. Where light falls, the iron is reduced to iron(II), the gum becomes soluble again, and on development in potassium ferrocyanide the Prussian white formed there can be wiped away — leaving the blue in the unexposed regions and so a positive from a positive, which is what a drawing office copying a line drawing actually wants.

Two clearer published versions survive. Pizzighelli and Itterheim’s of 1881 takes three stocks — gum arabic 20 g per 100 cc, ammonium iron(III) citrate 50 g per 100 cc, iron(III) chloride 50 g per 100 cc — mixed in the volume ratio 20 : 8 : 5, developed in potassium ferrocyanide at 20 g per 100 cc and cleared in dilute hydrochloric acid. The version usually attributed to Waterhouse, who credited it to Fisch, drops the citrate entirely: gum arabic 26 g, tartaric acid 27 g and iron(III) chloride 100 g per 100 cc, mixed 13 : 3 : 2, left in the dark for twenty-four hours and diluted to a specific gravity of 1.100.

In commercial blueprint paper. Ware’s transcription of Kwech’s commercial sensitiser formulae shows iron(III) chloride at 7 per cent appearing in the faster papers alongside ammonium iron(III) oxalate, with exposures to bright sunlight falling from four minutes for the “regular” formula to fifteen seconds for the fastest. He suggests a second reason for its presence: formulations for some commercial blueprint papers contain added iron(III) chloride, which may inhibit the formation of the easily peptised “soluble” Prussian blue that washes out of a classic cyanotype during processing. The course reports that as Ware states it — a plausible mechanism, not a demonstrated one.

As a bleach and a reducer. Wall’s 1924 formulary gives Abney’s remedy for dichroic fog: bleach the negative in 24 g of iron(III) chloride and 18 g of potassium bromide per litre, wash well, and redevelop. The pattern is the same one Kodak’s G-23 sepia bleach uses with ferricyanide in place of iron(III) — an oxidant to convert the image silver back to a silver ion, and a halide to decide what salt it becomes — and the course reads Wall’s bath that way, noting that Wall gives the formula and the instruction but not the mechanism. The same book’s Belitzski reducer offers a variant that makes its own reagent: instead of 50 g of potassium ferric oxalate, use 32.5 g of iron(III) chloride with 62.5 g of neutral potassium oxalate, “and the potassium chloride formed is negligible”. That reducer, Wall notes, attacks the shadows more than the highlights and therefore increases contrast.

And the uses Wall told photographers to abandon. Iron(III) chloride also appears in his ferrogallic or “ink” process, with iron(III) sulfate, tartaric acid and gum or gelatin, developed on gallic and oxalic acid to give black lines on white; and in the vanadium-iron toners for bromide prints, where “more ferric chloride gives bluer tones”. But his verdict on iron reducers for the negative itself is unambiguous: iron(III) chloride and sulfate, alone or with citric acid, “are extremely liable to stain the gelatine by the deposition of basic iron salts and should not be used.”

Properties

A brownish-yellow to orange solid with a mild odour of hydrogen chloride, highly hygroscopic and readily soluble; Wall’s 1912 dictionary calls it “yellowish-red opaque masses, which are very deliquescent” and gives the solubility as 160 per cent in cold water, with the salt also soluble in alcohol and ether. PubChem’s solubility entries for iron(III) chloride disagree sharply among themselves — HSDB gives 74.4 g per 100 cm³ at 0 °C, the ILO-WHO chemical safety card 92 g per 100 mL at 20 °C flagged “(reaction)”, and CAMEO’s own summary calls the salt only slightly soluble — and the course reports the spread rather than choosing.

It is an acidic solution, not a neutral one. CAMEO gives the number that settles the point: a 0.1 mol/L solution has pH 2.0, and CAMEO’s reactive-group classification for iron(III) chloride is Acids, Strong Non-oxidizing. The hydrated Fe³⁺ ion loses a proton readily, which is why a sensitiser made with it needs no added acid to stay clear, and why adding alkali to it throws down a brown iron(III) hydroxide instead of neutralising it quietly.

Weights and water. The hexahydrate is 270.29 to the mole against 162.20 for the anhydrous salt, so a formula written for the crystals needs 1.666 times the weight if only the anhydrous salt is to hand, and the reverse conversion is 0.600, on the course’s own arithmetic from the two molar masses. Because the salt is deliquescent, a jar that has been opened repeatedly weighs heavy in water and light in iron. For anything where the strength matters, make a solution once, label it with its date and its per cent w/v, and work from that.

Handling

The aggregated ECHA notifications held by PubChem classify the hexahydrate Danger: harmful if swallowed in 99.6 per cent of the 223 reports carrying hazard codes, serious eye damage in 72.8 per cent, skin irritation in 71.9 per cent, severe skin burns and eye damage in 27.2 per cent, harmful to aquatic life with long lasting effects in 20.5 per cent, and corrosive to metals in 12.5 per cent. CAMEO’s health hazard entry for ferric chloride adds the practical detail: the dust irritates nose, throat and eyes, ingestion irritates the mouth and stomach, and prolonged skin contact causes irritation and burns.

Why Level B. Against the course rubric the deciding criterion is that the substance is handled as a concentrated acid — CAMEO’s own grouping, and pH 2.0 at 0.1 mol/L — and that its failure mode is Level B’s own wording, a splash or a burn rather than a spoiled print. Serious eye damage is outside Level A’s ceiling of “at most irritant, harmful if swallowed, or corrosive at the concentrations actually handled”, and here the concentrations actually handled are not small: Waterhouse’s positive sensitiser is 100 g of the salt in 100 cc of water. The Level B criterion of a fine powder that must not be inhaled applies too, with a NIOSH recommended limit of 1 mg/m³ for soluble iron salts as iron and the same British long-term figure in HSE’s EH40. It does not reach Level C, whose criterion applies where a fume cupboard or specialist disposal is the recognised control; goggles, gloves, an apron, eyewash and an open window are the recognised controls here.

⚠Safety

Goggles and gloves before the jar is opened; eyewash within reach; an apron for the strong solutions. Solid into water, never water into solid, and mix in glass or plastic rather than metal, because CAMEO records the solution as highly corrosive to most metals and notes that the contact can produce flammable hydrogen. Keep it away from every alkaline bottle in the darkroom, which precipitates iron(III) hydroxide from it, and away from alkali metals, with which CAMEO records a violent reaction. Assume that any drop that lands on a print, a bench or a brush that has touched ferrocyanide will become Prussian blue and stay there.

Waste

What leaves the bench is an acidic solution of iron and chloride, sometimes thickened with gum, sometimes carrying the silver an iron reducer has stripped from a negative. Any silver-bearing bath goes to recovery first. The rest is governed by two facts: the classification’s H412, harmful to aquatic life with long lasting effects, and the pH. Kodak’s J-52 publication gives 5.6 to 9.4 as the window sewer codes most frequently set, and a working iron(III) chloride bath is far outside it. Raising the pH precipitates iron(III) hydroxide as a brown sludge — the same reaction that makes the salt useless once an alkali has reached it — which converts a corrosive liquid into a solid that still has to be filtered off and taken away rather than washed down. Nothing goes onto a garden, into a soakaway or into a watercourse. ILFORD’s guidance for domestic users in the United Kingdom 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

Iron(III) chloride is the iron salt that kept nearly getting there. It was in Herschel’s hands in 1842 and gave him a superb blue with a fogged ground. It was in Poitevin’s hands in 1863, where its hardening of gum became the basis of every positive-working blueprint. It was in Pellet’s 1877 patent, which Ware notes is written in such deliberately vague language that no working method can be extracted from it — Lietze, transcribing an abridged version, records that he “has tried Pellet’s process, several times … without succeeding in producing good prints. The background was always more or less colored.” By 1943 the process was deemed defunct.

What survives of all this is instructive rather than practical. The positive blueprint is the reason the phrase “blueprint” means a white-on-blue drawing to most people and a blue-on-white one to a photographer, and the two are chemically opposite processes. The colloid hardening Poitevin noticed reappears, with dichromate in place of iron, as the whole basis of gum, carbon and photogravure. And Wall’s flat refusal of iron reducers — “extremely liable to stain the gelatine by the deposition of basic iron salts” — states the constraint that governs every iron process to this day: iron(III) hydrolyses at the pH of a paper or an emulsion, and the brown it leaves behind does not wash out. That is why platinum, palladium, kallitype and Van Dyke printing all end not with a fixer but with a clearing bath whose only job is to get the iron out.