Sodium Sulfate Decahydrate
Properties
| State | Solid (large colorless monoclinic crystals) |
| Color | Colorless to white |
| Solubility | Soluble in water (solubility increases sharply with temperature up to 32.4 °C, then decreases above) |
| Melting Point | 32.4 °C (incongruent melting; releases water of crystallization to give Na2SO4 + saturated solution) |
| Boiling Point | Not applicable (loses water of crystallization) |
About Sodium Sulfate Decahydrate
Glauber's salt — sodium sulfate decahydrate, Na2SO4.10H2O, 322.195 g/mol — is the hydrated form Johann Rudolf Glauber first prepared in 1625 by reacting sulfuric acid with sodium chloride and called 'sal mirabilis' for what he believed were near-universal medicinal properties. The compound has two qualities that have kept it commercially relevant for four centuries. First, it is an incongruently melting hydrate: at 32.4 °C the crystal water releases and the solid 'melts' into a slurry of saturated Na2SO4 solution plus solid anhydrous Na2SO4. The phase change absorbs about 252 kJ/kg, putting Glauber's salt in the top tier of low-cost phase-change materials (PCMs) for thermal energy storage at near-room-temperature setpoints — solar wall panels, building thermal mass, off-peak chilled-water storage. Second, the dehydration-rehydration cycle is the classic teaching example for hydrate chemistry, neatly illustrating water of crystallization and Le Chatelier's principle. Mineralogically, the compound occurs as mirabilite in evaporite deposits where hot brine has cooled — Searles Lake in California, Karabogazgol in Turkmenistan, Saskatchewan's salt-lake belt. Industrially the decahydrate is mostly an intermediate; it is dehydrated to anhydrous Na2SO4 (thenardite) for use in detergent manufacturing, kraft pulping, and glass-fining.
Where you'll encounter it
If you have ever seen a Trombe wall or thermal-storage solar panel installation built in the 1970s or 1980s, the dark drums or sealed pipes inside the south-facing wall were probably filled with Glauber's salt — sometimes with a small amount of borax added as a nucleator to prevent supercooling. Daytime sun melts the salt at 32.4 °C, storing roughly 70 watt-hours per kilogram of latent heat; the salt resolidifies overnight and releases that heat back into the building. Architects Felix Trombe and Maria Telkes pioneered the design. In a teaching lab, Glauber's salt is the demonstration crystal for hydrate chemistry — heat a weighed sample in a crucible, watch the clear crystals lose 55.9% of their mass as water of crystallization driven off as steam, then add water to the white anhydrous residue and watch the temperature jump as the rehydration releases that stored enthalpy back. The 252 kJ/kg figure is large enough that the beaker physically warms in your hand.
Common Uses
- Phase-change material in solar-thermal walls and water-storage systems with 252 kJ/kg latent heat at 32.4 °C
- Off-peak thermal storage in HVAC chilled-water and ice-bank systems
- Crystal-water demonstration material in chemistry teaching labs
- Historical osmotic saline laxative under the trade name Glauber's salt
- Intermediate for production of anhydrous Na2SO4 used in detergents and kraft pulping
- Mineral source as mirabilite in Saskatchewan, California, and Turkmenistan evaporite deposits
- Laboratory cold-pack reagent (endothermic dissolution lowers temperature)
- Crystallization-experiment substrate for growing large clear crystals in school science demonstrations
Safety Information
Low toxicity. GHS: not classified as hazardous. No OSHA PEL specifically for sodium sulfate; OSHA particulates not otherwise regulated apply (15 mg/m3 total dust, 5 mg/m3 respirable). Ingestion of large quantities causes osmotic diarrhea — the basis for its historical use as a saline laxative — but is not systemically toxic. Acceptable for direct food contact in small amounts and listed as GRAS by the FDA for use in dietary supplements. Mild skin and eye irritation from concentrated solutions or dust; standard nuisance-dust precautions are sufficient. The decahydrate is stable up to 32.4 °C; above this it efflouresces, releasing water and converting to thenardite, so seal containers to prevent caking.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.