Iron(III) chloride, anhydrous

Open the jar and the crystals are already wet at the edges. Anhydrous iron(III) chloride is one of the few darkroom chemicals that changes identity while you look at it: it pulls water out of the air, hydrates, and finally deliquesces to a brown syrup. It is kept anhydrous because two jobs need it that way — etching a gravure plate at a defined strength, and building an iron sensitiser from its parts. For everything a printer does in a tray, the hexahydrate is the practical salt.

In photography

Etching the gravure and half-tone plate. This is the use that specifies the anhydrous salt by name. Wall’s 1924 formulary gives half-tone on copper as “perchloride of iron solution 35° Bé. or a 35 per cent solution of anhydrous ferric chloride”, a first bite of thirty seconds and three and a half to four minutes in total after stopping out. For photogravure it gives a ladder of baths at 43, 40, 38, 36 and 33 degrees Baumé, corresponding to 46, 41, 39, 36 and 33 per cent, with 6 per cent of freshly precipitated iron(III) hydroxide added to each and allowed to settle. The rule that governs their use is the one a beginner gets backwards: “The weaker the solution the stronger the etching.” The course reproduces Wall’s rule and does not claim to have verified the explanation usually given for it, which concerns how fast each strength of bath penetrates the swollen gelatin resist rather than how fast it attacks bare copper.

Building an iron sensitiser from its parts. The second use is subtler and much more interesting. Wall’s 1924 chapter on the iron processes offers, among the alternative blueprint sensitisers, one that contains no ready-made iron salt at all: 180 g of tartaric acid dissolved in three-quarters of a litre of water, 68 g of anhydrous iron(III) chloride dissolved into it, and then ammonia — about 175 cm³ — added until the acid is just neutralised, with the warning that excess ammonia must be avoided. Filter, and add potassium ferricyanide. What has been made in the bottle is an ammonium iron(III) tartrate, one of the light-sensitive iron carboxylates, and the chloride was only the delivery vehicle for the iron.

⚗The chemistry

That trick is a century older than the modern version of it and identical in principle. Ware’s 2022 Simple Cyanotype avoids commercial ferric ammonium citrate altogether and “effectively makes it in situ” from 12 g of iron(III) nitrate nonahydrate, 13 g of citric acid monohydrate and ammonia solution, for exactly the reason Wall’s formula implies: an iron(III) salt of a strong mineral acid is a clean, well-defined source of Fe³⁺, and the carboxylate that makes it light-sensitive can be supplied separately and in a known ratio. Both routes depend on the same fact — that in water, iron(III) chloride and iron(III) nitrate are simply an acidic solution of the hydrated Fe³⁺ ion waiting for a ligand.

Why it behaves as an acid. Iron(III) is small and triply charged, so its aqua ion pulls electron density away from the water molecules bound to it and lets a proton go. CAMEO records the result as a number: a 0.1 mol/L solution has pH 2.0, and its reactive-group classification for iron(III) chloride is not “salt” but Acids, Strong Non-oxidizing. ChEBI describes it as a Lewis acid and an astringent. Two practical consequences follow. Adding alkali to the solution does not neutralise it quietly; it precipitates a bulky brown iron(III) hydroxide, which is why Wall’s formula insists that excess ammonia be avoided and why his etching instructions say that if the iron solution is very acid, a little ammonia should be added “until a slight precipitate is formed, then boil and filter”. And the solution attacks metal: CAMEO calls it highly corrosive to most metals, so it is mixed in glass or plastic and never in a metal jug.

Properties

Black-to-brown hygroscopic crystals, in the words of the ILO-WHO chemical safety card PubChem quotes; the Merck Index adds the detail that the solid is red by transmitted light and green by reflected light. CAMEO describes an orange to brown-black noncombustible solid and notes that it is very hygroscopic and readily forms the hexahydrate. That last property is the reason a bottle of “anhydrous” ferric chloride that has been opened repeatedly is no longer anhydrous, and a gravure bath made up from it by weight is no longer the strength on the label. Baumé degrees, which the etching literature uses, are a hydrometer reading of the finished solution rather than a weight of solid, which sidesteps the problem entirely — a rare instance of an obsolete unit being the more robust one.

Solubility, and a disagreement worth seeing. HSDB gives 74.4 g per 100 cm³ in cold water at 0 °C and 535.7 g per 100 cm³ at 100 °C. The ILO-WHO card gives 92 g per 100 mL at 20 °C and marks it “(reaction)”. Wall’s 1912 dictionary gives 160 per cent in cold water. CAMEO’s general description, on the other hand, calls the solid “slightly soluble in water” and quotes 5 to 10 mg per mL. Those cannot all be right, and the course reports the spread rather than picking the convenient figure; the weight of evidence, and ordinary experience of the salt, is with the high values.

Weights. The anhydrous salt is 162.20 to the mole and the hexahydrate 270.29, so a formula written for the hexahydrate needs 0.600 times the weight if only the anhydrous salt is available, and one written for the anhydrous salt needs 1.666 times the weight of the hexahydrate, on the course’s own arithmetic from the two molar masses. Nineteenth-century texts add a further trap: Wall’s 1912 dictionary writes the formula as Fe₂Cl₆ with a molecular weight of 325, the dimer that iron(III) chloride forms in the vapour and in non-polar solvents, so a period formula quoting “molecular proportions” may be counting in units of two irons.

Handling

The aggregated ECHA notifications held by PubChem classify anhydrous iron(III) chloride Danger, corrosive and irritant: harmful if swallowed in 96.6 per cent of the 2,849 reports carrying hazard codes, severe skin burns and eye damage in 75.4 per cent, harmful to aquatic life with long lasting effects in 72.4 per cent, with a minority giving the milder skin-irritation and the separate serious eye damage statements. CAMEO’s health hazard entry is consistent: inhaling the dust irritates the nose and throat, ingestion irritates the mouth and stomach, the dust irritates the eyes, and prolonged skin contact causes irritation and burns.

Why Level B. Against the course rubric this is a Level B substance on three of that level’s criteria at once. It is handled as a concentrated acid in everything but name — CAMEO groups it with strong non-oxidising acids and records pH 2.0 at 0.1 mol/L. It is a fine powder that must not be inhaled, with a NIOSH recommended limit of 1 mg/m³ for soluble iron salts as iron and the identical British long-term figure in HSE’s EH40. And its failure mode is Level B’s own definition: a splash or a burn rather than a spoiled print. It does not reach Level C, whose criterion applies where a fume cupboard or specialist disposal is the recognised control; the recognised controls here are goggles, gloves, an apron, dust suppression, eyewash and working at an open window. The one place it edges toward Level C is disposal, because an etching bath carrying dissolved copper is a metal waste rather than a photographic one.

⚠Safety

Goggles and gloves before the jar is opened, and eyewash within reach. Weigh quickly over a tray and reclose: the salt fumes as it takes up moisture. Add the solid to water, never water to the solid, and CAMEO’s spill instruction is the reverse of the usual one — pick up and remove spilled solid before adding water. Mix in glass or plastic, never in metal, because the solution is corrosive to aluminium and most metals and the contact can produce flammable hydrogen. Keep it away from alkali metals, with which CAMEO records a violent reaction, and out of a fire, where it gives hydrogen chloride and where heating dissociates it to iron(II) chloride and chlorine.

Waste

Two streams leave a bench that uses this salt, and they are not alike. A sensitiser made from it is dilute, and what remains after coating is an acidic iron solution carrying chloride; the classification’s H412 governs, so it does not go onto a garden or into a soakaway. A spent etching bath is a different thing: it holds the dissolved copper or zinc it removed from the plate, and it is a metal-bearing chemical waste for a licensed route rather than anything a household drain should see. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most frequently set, and any working iron(III) chloride solution sits far below it. Raising the pH precipitates iron(III) hydroxide as a brown sludge — the same reaction Wall exploits deliberately in his etching bath and Ware in his sensitiser preparations — which converts a corrosive liquid into a solid that must still be filtered off and disposed of rather than washed away. 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 for a metal-etching waste they are usually specific.

History

Wall’s 1912 dictionary gives the substance three sentences that between them cover a century of photographic use: prepared by passing chlorine over hot iron filings or by dissolving iron wire in hydrochloric acid and oxidising with nitric acid; occurring in “yellowish-red opaque masses, which are very deliquescent”; and “used for the reduction of negatives, for cyanotype paper, also for etching copper and zinc”. Only the last of those three is still current, and the reason the middle one survives at all belongs to the hexahydrate’s page.

The first of the three died on its merits. Wall’s own 1924 verdict on iron reducers is flat: “Ferric chloride and sulphate either alone or with citric acid have been recommended as reducers; but they are extremely liable to stain the gelatine by the deposition of basic iron salts and should not be used.” That is the recurring failure of iron chemistry in a gelatin layer — the metal hydrolyses at the pH of the emulsion, and the brown hydroxide it leaves behind is permanent. It is the same failure that makes clearing baths necessary in platinum printing, and the reason nearly every iron process ends with a bath whose only job is to get the iron back out.