Ammonium Thiosulfate

Ammonium thiosulfate ((NH₄)₂S₂O₃; CAS 7783-18-8), also known as ammonium hyposulfite in older literature and marketed as rapid fixer, is the faster-acting successor to sodium thiosulfate and the active ingredient in essentially every modern commercial photographic fixer — Kodak Rapid Fixer, Ilford Rapid Fixer, Fuji Hunt, and their generic equivalents are all ammonium thiosulfate formulations with minor variations in preservative and buffer.^[1] The underlying silver-halide dissolution chemistry is identical to sodium thiosulfate — the difference is that the ammonium cation accelerates complex-formation kinetics by roughly a factor of two, cutting working fix times from ~10 minutes to 3–5 minutes for film and proportionally for paper. The thiosulfate anion also washes out of fibre-based paper and emulsion more readily as an ammonium-counter-ion salt, improving archival permanence relative to sodium fixers.

Properties at a glance

Property	Value
Common name	Ammonium thiosulfate
Historical name	Ammonium hyposulfite (or hyposulfite of ammonia)
Photographic name	Rapid fixer (active ingredient)
Chemical formula	(NH₄)₂S₂O₃
Plain ASCII formula	(NH4)2S2O3
Molar mass	148.21 g/mol
CAS Registry Number	7783-18-8
EC Number	232-002-3
Density (60% aqueous solution)	~1.30 g/mL
Solubility in water (20°C)	640 g/L (very high)
Solubility in ethanol	Insoluble
pH of 10% aqueous solution	~7.0 (near-neutral)
Commercial supply form	56–60% aqueous concentrate (rarely sold as solid)
Dry-form stability	Hygroscopic / deliquescent; decomposes on standing

The dry crystalline form is essentially never available commercially because it decomposes too readily — every photographic supplier and every chemical house ships the 56–60% aqueous concentrate. The molar mass figure refers to the salt itself for stoichiometry purposes; the concentrate's "active strength" is whatever the bottle label specifies (commonly cited as "60% by weight" with sulfite + EDTA stabilizer additives).

Photographic mechanism

The fixing chemistry is the same complex-formation reaction as with sodium thiosulfate: residual AgCl/AgBr/AgI in the emulsion is converted to the soluble bis(thiosulfato)argentate(I) complex and washes away.^[2] Two kinetic effects make the ammonium form faster:

1. NH₄⁺ assists complex formation — the ammonium ion's smaller hydration sphere compared to Na⁺ produces less ion-pairing with thiosulfate and a more active fixing species. Observed rate increase is roughly 2× at standard working concentrations.
2. Washing kinetics — the ammonium thiosulfate anion-counter-ion pair diffuses out of gelatin and cellulose-fibre matrices faster than the sodium salt. This is a real archival-quality advantage, not just a convenience claim.

Sodium vs ammonium thiosulfate

The two thiosulfate fixers are interchangeable in chemistry but differ enough in kinetics, supply form, and cost that the choice matters in workflow planning:

	Ammonium thiosulfate	Sodium thiosulfate
Form	56–60% aqueous concentrate	Anhydrous powder or pentahydrate crystals
Working concentration	~10% (1:4 from 60% concentrate)	~25–30% (typical hypo bath strength)
Film fix time	3–5 min	8–10 min
Paper fix time	1–2 min	5–10 min
Wash time after fix (fibre)	15–25 min with wash-aid	25–45 min with wash-aid
Stock-solution stability	12–18 months sealed	Indefinite as solid; weeks–months as solution
Cost per litre working solution	Higher (commercial concentrate)	Lower (mix from bulk powder)
Best for	Time-pressed workflow, modern enthusiast use	Recipe-driven workflow, archival fine-art, mixing from raw chemicals

For most contemporary darkroom workers, ammonium thiosulfate (commercial rapid fixer) is the default — the time savings and the wash-time reduction make the supply-form tradeoff worthwhile. Sodium thiosulfate retains its place for working from formula-published recipes (F-5, F-6) and for workers who prefer to mix from bulk reagents.

Common photographic uses

• Film rapid fix: working dilution typically 1:4 from commercial concentrate (~10% working), 2–4 minute film fix time. The standard film fixer in every amateur darkroom since the 1970s.^[3]
• Paper rapid fix: working dilution 1:9 from concentrate (~5% working), 1–2 minute paper fix. Two-bath fixing is still recommended for archival work — the first bath does most of the work, the second catches what the first missed.
• Ferricyanide bleach step in toners: some modern toner formulas use ammonium thiosulfate instead of sodium thiosulfate for faster bleach / redeposit kinetics.
• Farmer's reducer Stock B: substitutes 1:1 for sodium thiosulfate as the silver-solvent component, with roughly 2× faster action. Workers who want faster tray-time use rapid fixer here; workers who want easier end-point control stick with the sodium form.
• Washing-aid component: ammonium-thiosulfate-based washing aid solutions accelerate fixer removal from fibre-based paper more strongly than the sodium-sulfite-based hypoclear systems.

Practical notes

Ammonium thiosulfate is almost never sold as dry crystals. It is unstable in dry form — hygroscopic to the point of deliquescence, and decomposes on standing to ammonia, sulfur, and sulfur dioxide. The commercial product is a concentrated aqueous solution (typically 56–60%) sold in glass or heavy-gauge plastic bottles, shelf-stable for a year or more. Working dilutions are mixed on demand from the concentrate.

Shelf life: concentrated stock lasts roughly 12–18 months in the original tightly-sealed bottle; working dilutions should be mixed fresh (ideally the same day; a week maximum in a covered tank). Exhausted working fixer shows the same yellow tinge, ammonia-plus-sulfur smell, and silver thiosulfate cloudiness as exhausted sodium thiosulfate — replace when any of these appear.

Incompatibilities: acid contact liberates ammonia gas and sulfur dioxide simultaneously (an unpleasant combination but not acutely dangerous at darkroom scale); strong bases liberate ammonia gas visibly. Keep the concentrate away from both.

Disposal

Same rules as sodium thiosulfate: dilute unused solution is environmentally benign, but used fixer carries dissolved silver as the thiosulfate complex and should be collected for silver recovery or treated as heavy-metal waste. The ammonium form does not change the silver-waste profile; both are identical in their dissolved-metal load.

Related compounds

• Sodium thiosulfate — the original, slower-acting fixer (anhydrous reference form). Ammonium thiosulfate substitutes for sodium thiosulfate in essentially every silver-halide fix formula, with the working-strength concentration typically halved to reflect the faster kinetics.
• Sodium thiosulfate pentahydrate — the hydrated crystalline form of sodium thiosulfate; common in classical recipes.
• Sodium sulfite — preservative companion in commercial rapid-fixer concentrates; protects thiosulfate from oxidation during shelf storage.