Ammonium thiosulfate

Buy a bottle of fixer today and this is almost certainly what is in it. ILFORD states it without hedging on the technical sheet for RAPID FIXER: the fixing agent is ammonium thiosulfate, and the product contains no sodium thiosulfate at all. The ion doing the work is the same one Herschel described in 1819. The cation is not, and the difference shows up on the clock.

In photography

It fixes by the same reaction as hypo. Thiosulfate takes the silver ion out of the solubility equilibrium of the halide crystal and holds it as a soluble complex; the halide goes with it, and the emulsion clears. Nothing about that argument changes when the counter-ion is ammonium rather than sodium.

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Fixing: the ion is the same one hypo supplies

What changes is the speed, and the honest answer is partly a gap. The observation is old and well recorded. Wall’s 1924 formulary reports that adding 2.5 to 5 per cent of ammonium chloride to a 20 per cent hypo bath increases the rapidity of fixing, that 10 per cent ammonia water does the same job, and — the detail that identifies the culprit — that with a 40 per cent hypo bath the addition has no effect at all. The acceleration therefore belongs to the ammonium ion, and it appears only where thiosulfate is not already in large excess.

⚗What the course can and cannot state

Two things are sourced. First, the ammonium ion accelerates fixing, on Wall’s evidence above, which he reports as an effect and not as a mechanism — he even notes the period belief that ammonium hyposulphite is formed as a supposition. Second, ammonium thiosulfate is far more soluble than the sodium salt: CAMEO gives 64% at 20 °C, against the 1 part in 2 of cold water Wall’s 1912 dictionary gives for hypo crystals, so a stronger bath is available before the solid runs out of solvent.

What the course could not verify from any open source it holds is the mechanism itself — whether the ammonium ion changes the rate at which the silver–thiosulfate complex forms, changes which complex forms, or acts on diffusion into the gelatin. The standard references on fixer chemistry are not open access and this course has not read them. Until it has, the mechanism is marked unverified rather than guessed at.

What it means at the tray. ILFORD’s sheet gives working figures for a 1+4 dilution: general purpose film 2 to 5 minutes, fibre-base paper 1 minute, resin-coated paper 30 seconds. Working-strength pH is 5.0 to 5.5 and specific gravity 1.070 to 1.080 at 1+4; capacity is 24 films of 135-36 per litre. The bath is non-hardening, and ILFORD is explicit that RAPID FIXER must not be used with a fixer hardener.

Two rules that decide print permanence. The first is the clearing-time test: fix film for twice the time the emulsion takes to clear, and discard the bath when the clearing time in used fixer exceeds twice the clearing time in fresh. The second is the silver limit, and it is much tighter for paper than for film. ILFORD allows 8 to 10 g/L of silver in a film bath, but for fibre-base prints wanting commercial permanence keeps it below 2 g/L, and for maximum stability in long-term storage below 0.5 g/L — about ten 8 × 10 inch prints per litre. Above that, compounds may remain in the paper base after washing and contribute to staining over time. ILFORD’s answer is the same two-bath technique the nineteenth century arrived at: half the time in the first bath, half in the second, the second promoted to first when the first is exhausted.

Properties

A white crystalline solid with an odour of ammonia, hygroscopic, very soluble in water and insoluble in alcohol and ether. Relative molecular mass 148.21, against 158.11 for anhydrous sodium thiosulfate and 248.19 for the hypo crystals — the lightest of the three per mole of thiosulfate, because ammonium is lighter than sodium and there is no water of crystallisation. In practice you will never weigh it: CAMEO’s second entry describes what is actually shipped, an ammonium thiosulfate solution of 60 per cent or less, an odourless colourless liquid.

Its pH behaviour is the property to respect. The working bath is mildly acid at pH 5.0 to 5.5. Push it the other way and the ammonium ion gives up its proton as ammonia gas: Kodak’s J-215, writing about silver recovery, says that although a higher pH generally helps electrolysis, the pH of an ammonium-based fixer should not be raised above 7.8 to 8.0 because of the ammonia evolved. Push it down instead, with acid, and you get the thiosulfate decomposition Kodak’s 1928 primer describes — free thiosulfuric acid falling apart into sulfurous acid and sulfur, the bath going milky. There is a narrow window and the manufacturer sets it: ILFORD’s instruction for a bath whose pH has drifted high is a few drops of 50 per cent acetic acid, added gradually with stirring, and not past the stated limits.

Handling

The aggregated ECHA notifications held by PubChem carry no classification: 168 of 173 reports, 97.1%, say ammonium thiosulfate does not meet GHS hazard criteria, and only 2.9% of companies filed any GHS information at all. Five reports carrying hazard codes is not a consensus. CAMEO’s datasheet fills some of the gap without being a classification: dust may irritate the respiratory system, ingestion could be harmful, eye and skin contact may cause irritation, and the primary hazard is the threat posed to the environment. Neither EH40 nor the NIOSH Pocket Guide lists the substance, and EH40 states that absence from its list does not indicate that a substance is without risk.

Why Level A. Against the course rubric, a fixing bath meets the Level A criteria: no agreed classification exceeds the “at most irritant or corrosive” ceiling at the concentrations handled, nothing is heated, and the waste is silver-bearing fixer collectable for recovery. Two Level B controls are added by preference: splash goggles for the undiluted concentrate, because 60 per cent is a concentrated reagent whatever its classification, and the discipline of never letting anything alkaline near it.

⚠Safety

Gloves, eye protection and an apron for mixing, as ILFORD’s own sheet recommends; goggles for the concentrate; ventilate. Two mixtures to avoid absolutely. Acid frees sulfur and, on Princeton’s guidance, sulfur dioxide, for which EH40 sets 0.5 ppm over eight hours — so never pour fixer into a vessel that has held a stop bath. Oxidisers are worse: CAMEO records that mixtures with sodium chlorate can react exothermically and then decompose explosively, and that chlorates, nitrates and nitrites release ammonia and hydrogen sulfide. Nothing in a darkroom requires either combination.

Waste

The stream is spent fixer holding silver as the thiosulfate complex. Recovery first: ILFORD states that any silver recovery method can be used with RAPID FIXER, and recommends the electrolytic method for maximum efficiency because the treated fixer can then be recycled or reused. Kodak’s J-215 describes the same cell, and the metallic replacement cartridge in which iron reduces the silver complex and passes into solution in its place at pH 5.5 to 6.5.

After desilvering, the residue is an oxygen-demanding solution carrying ammonium, and CAMEO puts the environmental threat at the head of this substance’s hazards. Kodak’s J-300 guidance for amateurs states that septic systems are not designed to treat photographic processing solutions and lists household hazardous waste collection among the routes for used fixer; ILFORD tells domestic users in the UK to bottle wastes separately, label them and take them to a household waste and recycling centre. Check your local regulations; they govern.

History

The ammonium salt is in the record from photography’s first months. Herschel made hyposulphite of ammonia in 1819 and described both its double salts with silver — one very readily soluble, one “extremely insoluble in water” — the same soluble-and-insoluble pair that would later force two fixing baths. In the note read to the Royal Society on 14 March 1839 he recommends it by name: a photograph fixed with pure water alone comes out brick-red, and “the black colour may be restored by washing it over with a weak solution of hyposulphite of ammonia”.

For the next century it stayed a modifier rather than a fixer. Wall’s 1924 formulary heads its paragraph “Rapid ammonia fixing bath” and describes photographers adding ammonium chloride or ammonia to an ordinary hypo bath on the supposition that ammonium hyposulphite is formed. The Image Permanence Institute’s account of negative processes marks the change: in the cellulose acetate era, ammonium thiosulfate — rapid fix — came into use as a fixer alongside sodium thiosulfate.

ILFORD’s two liquid fixers make that succession explicit: both the RAPID FIXER and the HYPAM sheet state that the fixing agent is ammonium thiosulfate and that the product contains no sodium thiosulfate. On many darkroom shelves the word “hypo” now survives chiefly in the names of the washing aids sold to get it out again.