Sodium Thiosulfate Pentahydrate

Sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O; CAS 10102-17-7), commonly called "hypo crystals" or "hyposulphite of soda", is the hydrated crystalline form of sodium thiosulfate — the traditional fixer salt supplied as large clear colourless crystals before the anhydrous powder became the modern photographic standard.^[1] The molar mass is 248.18 g/mol (vs 158.11 g/mol for the anhydrous form); the pentahydrate is roughly half water by weight. Chemically identical to anhydrous sodium thiosulfate for photographic purposes — the difference is packaging, density, and weight-per-gram of active sulfur. See the primary sodium thiosulfate page for full photographic chemistry, common uses, and the fixer-bath mechanism.

Properties at a glance

Property	Value
Common name	Sodium thiosulfate pentahydrate
Photographic name	Hypo crystals
Historical name	Hyposulphite of soda
Chemical formula	Na₂S₂O₃·5H₂O
Plain ASCII formula	Na2S2O3.5H2O
Anhydrous form	Na₂S₂O₃
Molar mass (pentahydrate)	248.18 g/mol
Molar mass (anhydrous)	158.11 g/mol
Water of crystallization	5 H₂O (90.07 g/mol = 36.3% by weight)
CAS Registry Number	10102-17-7
EC Number	231-867-5
Density (20°C)	1.69 g/cm³
Melting point	48.3°C (incongruent — melts in own water of crystallization)
Boiling point (decomposes)	~100°C with loss of water
Solubility in water (20°C)	701 g/L (pentahydrate)
Solubility in water (50°C)	~1840 g/L
Solubility in ethanol	Insoluble
pH of 0.1 M aqueous solution	~7.0
Crystal form	Large prismatic monoclinic crystals; transparent and colourless
Crystal habit	Tabular, sometimes acicular when grown from concentrated solution

The pentahydrate is the form most commonly stocked by photographic suppliers and chemical houses for archival fixer use; the anhydrous form is favoured for industrial bleach-bath chemistry and rapid-fixer concentrates where weight-per-gram of active sulfur matters.

Pentahydrate vs anhydrous

Both forms dissolve to give chemically identical solutions of the thiosulfate ion (S₂O₃²⁻) plus sodium ions plus (in the pentahydrate's case) some additional water. The choice between them is practical, not chemical:

• Storage stability: the pentahydrate is hygroscopic-resistant under moderate humidity but effloresces in dry air — it slowly loses water of crystallization and crusts over. The anhydrous form is mildly hygroscopic and prefers a sealed container.
• Weight handling: anhydrous gives more active fixer per gram (158 g/mol vs 248 g/mol). At equivalent moles, the pentahydrate weighs 1.57× more.
• Dissolution behaviour: the pentahydrate dissolves rapidly in cold water with a strongly endothermic effect (the bath temperature drops 5–10°C). Anhydrous dissolves more slowly and with a much smaller temperature drop.
• Heated bath behaviour: the pentahydrate melts in its own water of crystallization at 48°C. If you heat a tin of pentahydrate above 50°C without adding water, the crystals turn to a syrupy slurry. Anhydrous powder remains free-flowing to over 200°C.
• Historical recipes: 1890s–1950s formulas that say "hyposulphite of soda" or "hypo" without qualifying the form intend the pentahydrate (the only form readily available at that time). Formulas from the 1970s onward usually specify anhydrous unless explicitly marked "pentahydrate" or "crystal".

Weight conversion

To substitute one form for the other in any recipe:

From	To	Multiplier
Pentahydrate weight	Anhydrous weight	× 0.637 (i.e., 158.11 / 248.18)
Anhydrous weight	Pentahydrate weight	× 1.570 (i.e., 248.18 / 158.11)

Worked example — Kodak F-5 fixer. The classical F-5 formula calls for 240 g sodium thiosulfate (form unspecified, but the formula dates from a period when anhydrous was the standard photographic stock). If your supplier sells only the pentahydrate, weigh out 240 × 1.570 = 377 g pentahydrate to give the same molar quantity of thiosulfate ion. Conversely, a historical formula calling for "8 oz hypo crystals" (~227 g pentahydrate) substitutes to 227 × 0.637 = 145 g anhydrous.

The conversion is simple, but two pitfalls catch authors:

• Don't compensate for the water of crystallization in total bath volume. The water released by 240 g of pentahydrate is only ~87 mL — small compared to a typical fixer bath of 1 L. Some references over-correct by reducing water; this is unnecessary precision.
• Don't confuse "hypo" weight with "hyposulphite of soda" weight. The two terms are interchangeable for the pentahydrate but you'll occasionally see a 19th-century source list both as if they were different chemicals — they aren't.

Photographic uses specific to the pentahydrate

While the chemistry is identical, the pentahydrate has a few practical advantages and disadvantages versus anhydrous:

• Slow-cooling fixer baths: the endothermic dissolution chills the bath, useful in summer when ambient temperature has crept above the recommended 18–22°C window. Anhydrous doesn't give this cooling effect.
• Crystal-form measurement: large transparent crystals can be eyeballed by approximate volume, useful when a balance isn't available. A "pinch" of pentahydrate gives a more reproducible weight than a "pinch" of fine anhydrous powder, which compacts unpredictably.
• Storage limitation: the pentahydrate effloresces in unsealed containers, gradually losing water and gaining dry crust. Recipes calibrated to fresh pentahydrate become slightly more concentrated over time. Anhydrous doesn't have this drift.
• Discontinued for some uses: most modern fixer concentrates (TF-3, TF-4, Kodak Rapid Fixer) use ammonium thiosulfate for higher fixing speed; pentahydrate is now used mainly in slow-acting traditional fixers and bleach baths.

Practical notes

• Storage: sealed glass or polyethylene container in a cool, dry place. Avoid metal containers; thiosulfate slowly attacks copper and brass. Indefinite shelf life if sealed; exposed crystals lose water over months.
• Solutions: aqueous fixer solutions made from either form are stable for weeks to months when sealed; faster spoilage in open trays from atmospheric oxidation to sulfate. Cloudy / discoloured solutions are exhausted and should be discarded.
• Measuring: weigh on a balance where possible. For volumetric measurement, packed pentahydrate crystals run roughly 1.5 g/mL by displaced volume; anhydrous powder runs roughly 1.0 g/mL by tap-packed volume.

Related compounds

• Sodium thiosulfate — the primary page covering fixer chemistry and uses (anhydrous is the standard reference form).
• Ammonium thiosulfate — modern rapid-fixer alternative; faster but more expensive.
• Sodium sulfite — fixer-bath companion that protects thiosulfate from oxidation.