Sodium thiosulfate pentahydrate

Say “hypo” and this is what you mean. The chemistry belongs to the thiosulfate ion and is set out on the anhydrous page. What belongs here is the crystal: the form every published formula is weighed in, and a third water by mass.

In photography

Fixing, in the units the formulas use. Kodak’s 1928 acid hardening bath F-1 is 480 g of hypo made up to 2 litres; its deep-tank and motion-picture baths F-14 and F-2 are 960 g in 4 litres — the same 240 g per litre of crystals. Wall’s 1924 plain bath is 400 g per litre, and he calls 40 to 45 per cent the strongest and most rapid worth using. Every one of those numbers is crystals. Weigh the anhydrous grade against them and the bath is more than half again as strong as intended.

Alkaline for printing-out papers, acid for developed ones. Wall’s alkaline bath — 125 g of hypo, 7 g of dry sodium carbonate and a little common salt per litre — is the one he gives for silver printing-out images, and Reilly explains why. His print fixer is 150 g of the pentahydrate with 2 g of sodium carbonate per litre, and the slight alkalinity does two jobs: it stops acid carried in from decomposing the thiosulfate and liberating sulfur, and it keeps the bath from attacking the very finely divided silver of a print-out image, which an acid fixer bleaches out of the highlights and mid-tones. On capacity he is blunt: published estimates allow 150 prints of 8 × 10 inches per litre of a 15 per cent bath, but for optimum permanence he would fix ten to fifteen.

Toning. Hypo is not only a solvent for silver halide; acidified, it is a source of sulfur, and that makes it a toner in its own right.

⚗The chemistry

Alum in water is weakly acid — Ware writes the hydrolysis as [Al(H₂O)₆]³⁺ → [Al(H₂O)₅(OH)]²⁺ + H⁺ — and the liberated acid attacks thiosulfate:

H⁺ + S₂O₃²⁻ → S + HSO₃⁻

Kodak’s 1928 primer describes a hypo-alum bath held at the point where it is about to precipitate sulfur as effectively carrying free sulfur in solution, and that reactive sulfur converts the image directly to silver sulfide with no bleaching step. Kodak’s G-23 gives it as Hypo Alum Sepia Toner T-1a: 480 g of hypo crystals, 120 g of potassium alum and a little silver nitrate and sodium chloride, made to 4 litres and worked at 49 °C for 12 to 15 minutes. The silver is there for the reason the primer gives — a fresh bath eats the highlights until it has some silver in it, so a used hypo-alum bath works better than a new one.

The primer sets the limits too: the bath works best at 49 to 52 °C, and above about 54 °C the print blisters and the image bleaches. Kodak’s Gold Toner T-21 puts the same salt to a different use, 960 g of hypo crystals per 4 litres with potassium persulfate, before the gold chloride goes in.

AgBr + 2 S₂O₃²⁻ → [Ag(S₂O₃)₂]³⁻ + Br⁻

The reaction the crystals are bought for

Properties

Colourless, odourless crystals that deliquesce slightly in moist air; Wall’s 1912 dictionary calls them large watery crystals that should be free from acid and from any yellow tinge. Kodak’s 1924 primer records that what contaminates the commercial article is generally dirt, straw or wood dust from careless handling, though calcium thiosulfate, which decomposes far more readily than the sodium salt, does turn up. Kodak’s answer was Eastman Tested Hypo, granular crystals sold to dissolve easily.

Solubility and the cold beaker. Wall gives 1 part in 2 of cold water, 1 in 1 of boiling, insoluble in alcohol. Dissolving it takes heat out of the solution: Reilly instructs that a fixing bath be made with water slightly warmer than the working temperature, “since some heat is always consumed in the formation of the solution”, and warns that too great a difference between a cold fixer and the other baths can blister albumen paper. Wall’s trick is to tie the crystals in a cloth and suspend the bag in hot water, which also saves filtering.

🧮Crystals against anhydrous

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

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

So Kodak’s F-1 bath, 240 g/L of crystals, is about 153 g/L as the anhydrous salt. When a formula says only “hypo”, it means these crystals, and the course writes the hydrate into the name for exactly that reason.

Stability. Dry and closed, it keeps. In solution it does not: Reilly states that dissolved sodium thiosulfate decomposes in part to sodium sulfite and elemental sulfur, and mixes a print fixer shortly before use. Kodak’s 1924 primer gives the manufacture in one line — sodium sulfite boiled with sulfur, Na₂SO₃ + S → Na₂S₂O₃ — which is that equilibrium read backwards, and why a sulfite-rich fixer resists going milky.

Handling

The aggregated ECHA notifications held by PubChem carry no classification: 277 of 281 reports, 98.6%, say the pentahydrate does not meet GHS hazard criteria, and PubChem notes that only 1.4% of companies supplied any GHS information. 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.

The hazards worth controlling are the solution’s. Princeton’s environmental health and safety guidance records that the powder is not significantly toxic by skin contact, that swallowing it is purgative, and that on heating or long standing in solution it can decompose to release sulfur dioxide, to which asthmatics are particularly sensitive; EH40 sets sulfur dioxide at 0.5 ppm over eight hours.

Why Level A. Against the course rubric, fixing meets the Level A criteria and no more: no agreed classification exceeds the rubric’s “at most irritant or corrosive” ceiling, and the waste is silver-bearing fixer, exactly the stream Level A anticipates. Hypo-alum toning is the exception and sits at Level B: the rubric puts any solution held above 50 °C there, and this one is worked at 49 to 52 °C in an open tray. Splash goggles for the crystals and for concentrates are a Level B control taken by preference throughout.

⚠Safety

Gloves and eye protection for solutions, goggles for the crystals, ventilation throughout. Never acidify a hypo solution and never mix it in a vessel that has held an acid stop bath: the bath goes milky as sulfur precipitates, and Princeton’s guidance names sulfur dioxide as what follows. Kodak’s 1928 primer also warns that adding hardener to hypo that has not fully dissolved is likely to precipitate sulfur — dissolve first, add the acid hardener second, and add it slowly to a cool solution while stirring.

Waste

Spent fixer carrying dissolved silver as the thiosulfate complex, and — if you tone — a spent hypo-alum bath carrying aluminium and the silver deliberately added to it. Recovery comes first: Kodak’s J-215 describes the electrolytic cell, where silver plates onto the cathode while thiosulfate and sulfite are oxidised at the anode, and the metallic replacement cartridge, where iron reduces the silver complex and passes into solution in its place at pH 5.5 to 6.5.

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

⁇Who found it, and when

Reilly writes that Herschel “discovered the substance sodium thiosulfate” in 1819–1820 and noticed it was a solvent for silver chloride. Mike Ware states instead that the compound was first prepared by Chaussier in 1799, and credits Herschel with establishing the chemistry of the hyposulphites and reporting the solvent property in 1819. The course follows Ware, because Herschel’s own paper reads throughout as a correction and extension of earlier workers on these salts — Thomson and Berthollet among them — rather than as the announcement of a new substance. What neither disputes is the date that matters to photography: 29 January 1839, when Herschel recalled the property and tested it as a fixer.

✕The wrong Herschel

Wall’s 1912 dictionary states, under Sodium Hyposulphite, that the importance of the salt as a solvent for unacted-upon silver was “discovered by Sir William Herschel”. It was John Frederick William Herschel, William’s son, and both Reilly and Ware name him. A period reference book is evidence for period practice, not an authority on its own attributions.

Wall’s entry is worth reading anyway, because it prints both silver salts as equations and draws the conclusion: the first, AgNaS₂O₃, is almost insoluble in water and soluble in hyposulphite of soda, “therefore excess of hypo should always be used for fixing: the incomplete elimination of this insoluble salt is one of the chief causes of fading in prints”. From 1912, that is still the argument for two fresh baths.

Reilly adds the human part. Herschel told Talbot within days; Talbot, who had been stabilising with strong salt solution rather than fixing, at first disdained hypo and adopted it only later, as did Daguerre. Reilly also names the cost — the colour and density shift every printing-out paper suffers in the bath as silver chloride is dissolved away and the image silver packs into aggregates. What made the picture permanent made it weaker and yellower, and Ware records the working answer of about 1855: print such papers much darker than you want them, to survive the fixer.