Sodium thiosulfate (anhydrous)

Development makes the picture; this ion makes it survive. Every crystal light did not reach is still in the emulsion when development stops, still sensitive, and only a short list of substances will take it out. Thiosulfate has topped that list since 1839. This page is the grade with no water in it.

In photography

It dissolves what water cannot. Kodak’s 1928 primer puts it plainly: only a few substances will dissolve silver bromide, and the one universally used is sodium thiosulfate. Not because thiosulfate is a better solvent in any ordinary sense, but because it removes the silver ion from an equilibrium and the halide follows.

⚗The chemistry

Silver bromide has a solubility product, and the LibreTexts general chemistry text gives it as K_sp = 5.35 × 10⁻¹³ at 25 °C, which puts Ag⁺ and Br⁻ at 7.31 × 10⁻⁷ mol/L in pure water. Thiosulfate binds silver as a complex ion with a formation constant of K_f = 2.9 × 10¹³:

Ag⁺ + 2 S₂O₃²⁻ ⇌ [Ag(S₂O₃)₂]³⁻

Add the two equilibria and their constants multiply: K = K_sp × K_f = 15. A number above one means the reaction runs, and the same source puts the resulting increase in the solubility of silver bromide at about 3 × 10¹³. The complex is soluble where the halide is not, and the whole of fixing is that multiplication.

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

Fixing: silver halide into the soluble thiosulfate complex

Why a tired bath is worse than useless. More than one silver–thiosulfate compound exists, and they do not behave alike. Kodak’s 1924 primer — a passage the 1928 revision dropped — states that two compound sodium silver thiosulfates exist, one almost insoluble in water and the other very soluble, and that only the soluble one forms while the bath retains appreciable fixing power. Reilly puts the modern condition on it: thiosulfate must be present in excess, or insoluble complexes form that cannot be washed out, one of them soluble only in fresh thiosulfate. Hence two baths. The 1924 primer adds the sting — the insoluble compound is invisible, so a negative moved to the wash as soon as it looks clear may be carrying it, and will keep it.

How much, and how long. Wall’s 1924 formulary gives 40 to 45 per cent as the strongest hypo bath that should be used and the most rapid in action, and warns that a bath grows saturated with silver salts as it works, so fixing slows and the chance of insoluble, transparent silver salts rises. Over-fixing is a fault of its own: Reilly records that fixation is complete fairly quickly, and that prolonged immersion drives thiosulfate into the paper fibres where it is almost impossible to remove.

Other jobs. Thiosulfate also tones and reduces. Ware sets out the hypo–alum sepia bath, in which alum’s acidity converts thiosulfate to colloidal sulfur (H⁺ + S₂O₃²⁻ → S + HSO₃⁻) that then sulfides the image; and Kodak’s 1928 primer explains Farmer’s reducer, in which ferricyanide oxidises the image silver and the hypo dissolves the product. Both live on the pentahydrate page, because both formulas are written in crystals.

Properties

A hygroscopic white solid; the ILO-WHO Chemical Safety Card calls it colourless crystals, and it reaches commerce as large crystals, dry powder and wet solid alike. The relative molecular mass PubChem computes is 158.11.

Solubility, with a discrepancy the course has not resolved. HSDB gives 50 g in 100 cm³ of water, rising to 231 g at 100 °C; the ILO-WHO card on the same PubChem record gives 20.9 g per 100 mL at 20 °C. Both cannot describe the same solid at the same temperature, and the record aggregates data filed against the anhydrous salt and its hydrates together. Take the ordering, not the number: Kodak’s 1928 primer says hypo dissolves in less than its own volume of water.

🧮Anhydrous against crystals

The pentahydrate has a relative molecular mass of 248.19 against 158.11 for the anhydrous salt, so 90.08 of every 248.19 — 36.3% — of the crystals is water of crystallisation.

• 1 g of anhydrous salt supplies the thiosulfate of 248.19/158.11 = 1.57 g of crystals.
• 1 g of crystals supplies the thiosulfate of 158.11/248.19 = 0.637 g of anhydrous salt.

So Kodak’s F-1 fixing bath, 480 g of hypo crystals made up to 2 litres, is 240 g/L of crystals and about 153 g/L of the anhydrous salt. Nearly every published fixer formula means the crystals, and weighing anhydrous salt against a crystal figure makes the bath more than half again as strong as it should be.

Stability. Dry, it keeps. In solution it does not: Reilly states that dissolved sodium thiosulfate partially decomposes to sodium sulfite and elemental sulfur, and mixes a print fixer shortly before use. Acid accelerates that decomposition, which is why an acid fixing bath needs sulfite to hold the equilibrium back — the argument is on the sodium sulfite page.

Handling

The aggregated ECHA notifications held by PubChem carry no classification: 2,637 of 2,963 reports, 89.0%, say sodium thiosulfate does not meet GHS hazard criteria, and the six notifications that do give codes cover too few reports for any code to be printed. Neither EH40 nor the NIOSH Pocket Guide lists it, and EH40 states that absence from its list does not indicate that a substance is without risk.

What the bath can do is another matter. Princeton University’s environmental health and safety guidance records that the powder is not significantly toxic by skin contact, that swallowing it has a purgative effect, and that on heating or long standing in solution it can decompose to release sulfur dioxide, to which asthmatics are particularly sensitive — and that darkroom ventilation exists partly to control that gas. EH40 sets sulfur dioxide at 0.5 ppm over eight hours, 1 ppm over fifteen minutes.

Why Level A. Against the course rubric, this meets the Level A criteria and no more: no agreed classification exceeds the rubric’s “at most irritant or corrosive” ceiling, nothing is heated above 50 °C in ordinary fixing, and the waste is silver-bearing fixer — exactly the stream Level A anticipates and expects to be collected. Splash goggles rather than glasses for the dry salt and concentrates is a Level B control taken by preference, and one Level A control does real work here: dedicated, labelled vessels, because the reaction to avoid is with acid.

⚠Safety

Gloves and eye protection for solutions, goggles for the powder, ventilation throughout. Never acidify a thiosulfate solution, and never put fixer into a vessel that has held an acid stop bath: the 1928 primer shows the solution going milky as sulfur precipitates, and Princeton’s guidance names sulfur dioxide as what follows. That same primer offers tasting a print as a test for residual hypo, and Herschel in 1819 described the silver complex as sweeter than honey. Nothing in this darkroom goes in anyone’s mouth.

Waste

The stream is spent fixer, carrying dissolved silver. Kodak’s J-300 guidance for amateurs states that the silver in photographic effluent is present as silver thiosulfate and is removed during secondary treatment at a municipal plant, that recovering it before discharge is nevertheless good practice, and that septic systems are not designed to treat photographic processing solutions at all.

Recovery works on the complex, not on the thiosulfate. Kodak’s J-215 gives three routes: an electrolytic cell, where silver plates onto the cathode while sulfite and thiosulfate are oxidised at the anode; a metallic replacement cartridge, where iron reduces the silver complex and passes into solution in its place, run best at pH 5.5 to 6.5; and chemical precipitation. The desilvered solution is still oxygen-demanding — Kodak’s J-52 gives 350 mg/L as the usual regulated mean limit for five-day biochemical oxygen demand, and a pH window of 5.6 to 9.4.

ILFORD’s guidance for domestic users in the UK 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. Keep fixer waste out of any container that has held an acid.

History

Photography’s fixer was found twenty years before photography needed it. In 1819 John Herschel published on the hyposulphites in the first volume of the Edinburgh Philosophical Journal and recorded, as one of their most singular characters, “the property their solutions possess of dissolving muriate of silver, and retaining it in considerable quantity in permanent solution”. He went further than the sentence usually quoted: he made the double salts, and noted that adding more silver chloride to a saturated solution turns it into a powder “of very difficult solubility in water”. The two-bath rule is implicit in his paper, a century before Kodak printed it.

Hearing in January 1839 what Talbot and Daguerre had done, Herschel remembered his own chemistry and tested it. His note to the Royal Society of 14 March states the requirement before naming the reagent: a fixing liquid must dissolve and wash off the unchanged chloride while leaving the reduced silver untouched, and “these conditions are best fulfilled by the liquid hyposulphites”.

The name that came with it is a trap. “Hyposulphite of soda” stuck to the photographic salt and is still printed on old formulas and old labels, and Kodak’s 1928 primer had to say in the same breath that chemists use the name hyposulphite for a different substance altogether. If a bottle says hyposulphite, check the formula on it.

Ware’s account of what thiosulfate then did to nineteenth-century prints is the other half of the story. Residual fixer left in the paper slowly converts image silver to silver sulfide, and for the nanoparticle silver of a salted-paper print that drops the optical density by a factor of about thirty. The reagent that made photographs permanent is also, badly washed, what fades them.