Iron(II) sulfate heptahydrate

Pour it over a collodion plate lifted from the silver bath and the image arrives in ten seconds, out of a solution that contains no silver of its own. That is the peculiarity of the iron developer: the silver it lays down is already on the plate, dissolved in the film, and the iron’s only job is to hand it an electron at the right places.

In photography

In wet collodion it is the developing agent. A plate taken from the silver nitrate bath is still wet with that bath, so it carries free silver ions in the collodion as well as the silver iodide and bromide of the sensitive layer. Iron(II) reduces the free silver, and the metal deposits where the light has already made a nucleus.

⚗The chemistry

Iron(II) gives up exactly one electron to become iron(III), and reducing Ag⁺ to an atom of silver takes exactly one, so the stoichiometry is one to one. Hardwich’s 1864 manual states the principle before writing any equation — developers “have a tendency, when mixed with soluble Salts of Silver, to set Silver free” — and notes that salts of iron do it by uniting with the acid and the oxygen of the silver salt. Because the silver comes out of solution rather than out of the grain, this is physical development: the image is plated onto the exposed specks, which is why it looks grey and metallic, why it lies on the surface, and why the plate must not be allowed to dry or to be rinsed before the developer is poured.

Ag⁺ + Fe²⁺ → Ag + Fe³⁺

Physical development: silver from solution, one electron from iron(II)

What more or less of it does. Towler’s 1864 formulas set the two ends. For ambrotypes and melainotypes he gives three drachms of the crystals with four ounces of rain water, three drachms of acetic acid and two of alcohol; for negatives, two drachms of the crystals in thirty-two of water with three drachms each of acetic acid and alcohol. The negative gets less iron because it gets more exposure. Towler is explicit that the acid, not the iron, is the throttle: in warm weather or after a long exposure more acid is needed “to keep the reducing agent in check”, and for instantaneous work the iron may be used with no acid at all. He adds the observation that decides the whole balance — diminishing the iron and increasing the acid “are correlative expressions”. Modern practice keeps the same lever: Bostick and Sullivan supply a concentrated ferrous sulfate developer with a restrainer added and instruct that it be diluted 1+3 with water for tintypes and ambrotypes, developed for twenty to thirty seconds, and 1+5 for glass negatives, developed for two minutes after three stops of extra exposure.

As the source of ferrous oxalate. The second use makes a different developer entirely. Wall’s 1924 formulary precipitates ferrous oxalate by mixing 278 parts of ferrous sulfate in 500 parts of water with 184 parts of neutral potassium oxalate in 500 parts of water, then washing and drying the yellow solid: “The result should be 180 parts of ferrous oxalate.” In practice the precipitate is never isolated. Two stock solutions are kept instead — Wall’s are 330 g of ferrous sulfate per litre with 1 cm³ of sulfuric acid, and potassium oxalate in hot water — and mixed one part of the iron to four of the oxalate. The order matters absolutely. Wall’s 1912 dictionary states it in italics: the solution of iron must be added to the oxalate and not the reverse, or a thick yellow precipitate of ferrous oxalate forms, because there is then no excess of oxalate to dissolve it.

A double salt that was tried instead. Wall’s 1912 dictionary records ferrous ammonium sulfate, Fe(NH₄)₂(SO₄)₂·6H₂O, as a proposed substitute, more stable both as a salt and in solution, but with a much feebler action, so that more of it is needed for the same effect; it was frequently used for developing wet collodion plates.

Properties

Blue-green crystals, odourless, efflorescent in dry air. Wall’s 1912 dictionary gives the solubility as 1 g in 1.5 cm³ of water and records that the salt is insoluble in alcohol and ether. Kodak’s 1928 primer, whose solubility table is stated in ounces of chemical per 100 fluid ounces of saturated solution, gives ferrous sulphate as 29 at 40 °F (4.4 °C) and 41 at 70 °F (21.1 °C) — an increase of about two-fifths across the range a darkroom works in, which is why a stock solution made warm can throw crystals when the room cools.

The hydrate ladder. PubChem’s ChEBI description gives the dehydration steps: the heptahydrate loses water to the tetrahydrate at 57 °C and to the monohydrate at 65 °C. CAMEO’s datasheet gives a melting point of 64 °C and the loss of all seven waters at 90 °C. Nothing in a photographic procedure should approach those temperatures, but a jar on a sunny windowsill can, and the salt that comes out of it is no longer 278.02 to the mole. The conversion in the other direction is the one worth memorising: 278.02 against 151.91, so a formula written for the crystals needs 0.546 times the weight if only the anhydrous salt is to hand, on the course’s own arithmetic from the two molar masses. Get it backwards and the developer carries 83 per cent more iron than intended.

Oxidation is the shelf-life problem. Hardwich records that the crystals gradually absorb oxygen and go rusty at the surface, and that a solution, colourless at first, turns red and deposits a brown powder — a basic iron(III) sulfate — which sulfuric or acetic acid prevents because the brown powder dissolves in acid. Archer’s 1854 manual gives the same rule for the collodion developer: a minute portion of sulfuric acid prevents the decomposition. Wall’s 1924 instruction to keep the stock in small bottles filled to the cork follows from the same chemistry.

Handling

The aggregated ECHA notifications held by PubChem classify the heptahydrate Warning, irritant: harmful if swallowed in more than 99.9 per cent of the 2,617 reports carrying hazard codes, skin irritation and serious eye irritation in 99.8 per cent each. CAMEO’s health hazard entry describes what swallowing it does — abdominal pain, retching, diarrhoea, dehydration, shock, pallor, cyanosis, a rapid or weak pulse. That is a description of iron poisoning, and the quantities on a darkroom shelf are far larger than the quantities in a medicine cabinet.

Why Level A. Against the course rubric this meets the Level A criteria and no more: the classification tops out at irritant and harmful if swallowed, which is Level A’s stated ceiling; nothing is heated above 50 °C; and the waste is spent developer and rinse water, both collectable. NIOSH sets a recommended limit of 1 mg/m³ for soluble iron salts measured as iron, and HSE’s EH40 gives the same British long-term figure with 2 mg/m³ over fifteen minutes, so pulverising the crystals — which Towler’s formulas ask for — is done without raising dust. Two Level B controls are added by preference: splash goggles rather than glasses for the solid, and strict hand washing, because ingestion is the route that harms. The wet collodion procedure this developer belongs to is classified separately and higher, on account of its silver nitrate, its ether-and-alcohol collodion and its cadmium bromide; none of that severity comes from the iron.

⚠Safety

Weigh and grind over a tray with the container away from the face; powder into water, never water into powder. Gloves and eye protection for every solution, goggles for the solid. CAMEO records iron(II) sulfate as a weak inorganic reducing agent that reacts with oxidising agents to generate heat and products that may be flammable, combustible or otherwise reactive, so it is stored away from bleaches, peroxides, persulfates and chlorates. Keep it away from silver solutions except when the reduction is the one you want, and acidify stock solutions: the brown deposit that forms otherwise is basic iron(III) sulfate, and it stains paper and gelatin.

Waste

A spent iron developer is a dilute solution of iron(II) and iron(III) sulfate with acetic or sulfuric acid and, from a collodion plate, dissolved silver carried off the film. The silver decides the route: it is recovered from the first baths before anything else is considered, and the course treats every silver-bearing bath as a recovery stream. What remains is iron and acid. CAMEO’s datasheet says plainly that the primary hazard of ferrous sulfate is the threat to the environment, and Japan’s NITE classification of this hydrate carries H402, harmful to aquatic life, so a spent bath does not go onto a garden, into a soakaway or into a watercourse. Kodak’s J-300 guidance for amateur photographers sends developers, stop baths, fixers after silver recovery and wash water to the sewer where a household has one, and is explicit that septic systems are not designed to treat photographic processing solutions. ILFORD tells domestic users in the United Kingdom 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

Hardwich’s 1864 manual describes the salt as “an abundant substance” used “for a variety of purposes in the arts”, prepared on so large a scale that commercial material had to be recrystallised before it was pure enough for photography. Its photographic use begins in the 1840s. Hunt’s Researches on Light reproduces a contemporary account of a collodion process that credits “the photographic property of iodide, which was discovered by Dr. Woods of Parsonstown, in Ireland, and that of sulphate of iron, for which science is indebted to the researches of Mr. Robert Hunt” — Hunt’s own claim, reported at second hand in his own book, which is how a good deal of the period’s attribution works.

Archer’s 1854 manual of the collodion process fixes its reputation in a phrase: “a cheap and energetic developing agent, particularly when combined with gallic or pyro-gallic acid.” Ten years later Towler is teaching the acid-against-iron balance as a craft skill. By 1924 Wall records that the ferrous oxalate developer built on the same salt “has been completely abandoned”, because plates develop slower in it than in the organic developers and because calcium oxalate and basic iron salts deposit in the gelatin — but he keeps the entry, because ferrous oxalate gives an image of pure silver with no selective absorption, which made it the standard for photochemical measurement long after it stopped being used for pictures. The iron developer itself survived exactly where physical development is the point: on the wet plate, where the free silver is already in the film and there is no gelatin to stain.