The short answer
Sodium sulphate is the sodium salt of sulphuric acid. Formula Na2SO4, CAS 7757-82-6, molecular weight 142.04 g/mol. It is white, odourless, neutral in solution and chemically about as unreactive as a common inorganic salt gets.
That inertness is the point. Sodium sulphate is bought precisely because it does not interfere — it carries bulk in a detergent powder, it changes the surface tension of a glass melt without joining the glass network, and it raises ionic strength in a dye bath without touching the dye.
Formula, molecular weight and identifiers
| Form | Formula | CAS | Molecular weight | Na2SO4 content |
|---|---|---|---|---|
| Anhydrous (thenardite, salt cake) | Na2SO4 | 7757-82-6 | 142.04 g/mol | 100 % |
| Decahydrate (Glauber's salt, mirabilite) | Na2SO4·10H2O | 7727-73-3 | 322.20 g/mol | 44.1 % |
Other names in trade: salt cake for the crude grade, disodium sulphate, sulphate of soda. The EC number is 231-820-9 and the HS code is normally 2833.11. As a food additive the anhydrous salt carries the number E514(i).
Anhydrous sodium sulphate and Glauber's salt
As with soda ash and washing soda, the difference between the two commercial forms is water of crystallisation, and the practical consequence is freight. The decahydrate is about 56 % water by mass, so a tonne of Glauber's salt delivers only about 441 kg of sodium sulphate. Nobody ships water across an ocean, so international trade is in the anhydrous form.
The decahydrate does have one genuinely useful property, covered below, and it remains the standard laboratory drying agent for organic solutions — anhydrous Na2SO4 is stirred into a wet organic layer, takes up water to become the decahydrate, and is filtered off. It is slower than magnesium sulphate but neutral and non-reactive, which is why it is preferred with sensitive compounds.
The 32.4 degree solubility peak
Sodium sulphate has an unusual solubility curve. Solubility rises steeply with temperature up to about 32.4 °C, reaching roughly 50 g per 100 mL, and then falls slightly as the water gets hotter. Almost no other common salt behaves this way.
The explanation is that below 32.4 °C the solid in equilibrium with the solution is the decahydrate, and above it the anhydrous salt. At 32.4 °C the decahydrate dissolves in its own water of crystallisation and the two forms swap over. Everything about the curve follows from that transition point.
Three consequences matter in practice:
- Phase change storage. The transition absorbs about 250 kJ per kilogram at a fixed 32.4 °C, which is close to comfortable room temperature. That makes Glauber's salt a classic solar heat store and building temperature buffer.
- Crystallisation control. A plant crystallising sodium sulphate has to hold either side of 32.4 °C deliberately, because drifting across it changes which solid comes out.
- Masonry damage. Sodium sulphate in porous stone or concrete cycles between the two hydrates with temperature and humidity, and the volume change breaks the pore walls. This is a standard accelerated durability test for building stone and a real cause of salt weathering.
Physical properties
| Property | Value (anhydrous) |
|---|---|
| Appearance | White crystalline powder or granules |
| Melting point | 884 °C |
| Density | 2.66 g/cm3 |
| Bulk density | Roughly 1.3 to 1.6 g/cm3 depending on grade |
| Solubility in water | About 19.5 g per 100 mL at 20 °C; about 50 g per 100 mL at 32.4 °C |
| pH of 5 % solution | Neutral, roughly 6 to 8 |
| Hygroscopicity | Anhydrous form readily absorbs moisture to form the decahydrate |
Natural and by-product sodium sulphate
Roughly half of world supply is mined or recovered from natural deposits and brines — thenardite and mirabilite beds, and sulphate-rich lake brines. The rest is recovered as a by-product from other processes, which is why quality varies so much between suppliers.
- Mannheim process. Sodium chloride and sulphuric acid react to give hydrogen chloride and sodium sulphate. Here the HCl is often the target product and the sulphate is the co-product.
- Viscose rayon and cellophane. The spin bath regeneration step produces large volumes of sodium sulphate.
- Resorcinol, battery recycling, chromate and silica production. Each throws off a sulphate stream that is crystallised and sold.
The origin matters commercially. By-product material can carry the impurity signature of whatever made it — chloride from the Mannheim route, organics from rayon, heavy metals from recycling streams — so a specification that names assay alone is not enough.
Commercial grades and what separates them
| Grade | Typical assay | What is controlled |
|---|---|---|
| Salt cake (crude) | 90 to 97 % | Assay and moisture only; for kraft pulping |
| Detergent grade | 99 %+ | Whiteness, low chloride and iron, controlled particle size, free flow |
| Glass grade | 99 %+ | Very low iron, controlled particle size for batch mixing |
| Textile / dyeing grade | 99 %+ | Low calcium, magnesium and iron; rapid dissolution |
| Analytical reagent (AR) | 99.5 %+ | Full trace-element profile against the monograph |
Iron is the recurring theme. A few tens of parts per million of iron tints glass green and dulls a white detergent powder, so the glass and detergent grades are effectively iron specifications wearing an assay figure.
Scapex Exports supplies sodium sulphate anhydrous and detergent grade sodium sulphate.
What sodium sulphate is used for
- Powder detergents. The largest single use. Sodium sulphate carries bulk, keeps the powder free-flowing, and gives the spray-drying slurry the right rheology. Concentrated and liquid formats have reduced this demand in developed markets but it remains dominant elsewhere.
- Glass. Added to the batch as a fining agent. It decomposes in the melt and the gas released sweeps out small bubbles, while the sulphate layer prevents a scum of silica forming on the surface.
- Kraft pulping. Salt cake makes up sulphur losses in the kraft recovery cycle, which is why the process was historically called the sulphate process even though the active cooking chemicals are sulphide and hydroxide.
- Textile dyeing. Used like sodium chloride to raise ionic strength and drive dye exhaustion, and preferred where chloride would corrode equipment or interfere with the dye.
- Chemical manufacture. Raw material for sodium sulphide, sodium silicate and other sodium chemicals.
- Thermal storage. Glauber's salt as a phase change material at 32.4 °C.
- Laboratory. Neutral drying agent for organic solutions.
Handling, packing and export documents
- Hazard. Not classified as a dangerous good. No UN number, general cargo. Dust is a mild eye and respiratory irritant; use dust extraction, goggles and a suitable mask when handling open material.
- Caking. The anhydrous salt is hygroscopic and will take up moisture to form the decahydrate, which sets hard. Keep bags sealed, store off the floor in a dry warehouse, and specify a moisture limit.
- Materials. Solutions are neutral and non-corrosive to carbon steel, stainless steel, HDPE and FRP.
- Segregation. Keep away from strong acids, which liberate sulphur dioxide from sulphate under heat, and from strong reducing agents.
- Packing. 25 kg and 50 kg PP laminated bags with inner liner, 1000 to 1250 kg jumbo bags, or bulk.
An export consignment should carry a certificate of analysis stating assay, moisture, water-insoluble matter, chloride, iron, calcium, magnesium, whiteness and particle size distribution; a 16-section GHS safety data sheet; commercial invoice and packing list; certificate of origin; and for food grade the E514 or FCC reference with heavy-metal limits and any Halal, Kosher or food-contact declarations the destination requires.
Enquire with a specification
Sodium sulphate anhydrous is supplied by Scapex Exports as a manufacturer and supplier from qualified Indian producers, in detergent, glass, textile, technical and analytical grades, with certificate of analysis, safety data sheet and full export documentation.
Send the grade, assay, iron and chloride limits, whiteness requirement, particle size, packing and destination port, and you will get a quotation written against that specification rather than a generic one.
Frequently asked questions
What is sodium sulfate?
Sodium sulfate is the sodium salt of sulfuric acid, formula Na2SO4, CAS 7757-82-6, molecular weight 142.04 g/mol. It is a white, odourless, neutral crystalline solid that dissolves in water without hydrolysing. Industrially it is used as a filler and processing aid in powder detergents, a fining agent in glass, a cooking chemical in kraft pulping and a levelling agent in textile dyeing.
What is the formula of sodium sulphate?
The anhydrous salt is Na2SO4, molecular weight 142.04 g/mol. The decahydrate, known as Glauber's salt, is Na2SO4 10H2O, CAS 7727-73-3, molecular weight 322.20 g/mol. Because that water makes up roughly 56 % of the weight, industry buys the anhydrous form and the decahydrate is mostly of interest for thermal storage and laboratory work.
What are the main uses of sodium sulphate?
The largest single use is as a filler and flow aid in spray-dried powder detergents. After that come glass manufacture, where it acts as a fining agent that removes scum from the melt; kraft paper pulping, where crude sodium sulphate is called salt cake; textile dyeing, where it levels and exhausts dyes; and the manufacture of other sodium chemicals.
What is Glauber's salt?
Glauber's salt is sodium sulphate decahydrate, Na2SO4 10H2O, named after the seventeenth-century chemist who first prepared it. It melts in its own water of crystallisation at about 32.4 degrees C, absorbing a large amount of heat, which makes it a well-known phase change material for storing solar heat and for passive temperature buffering in buildings.
Is sodium sulphate hazardous to ship?
No. Sodium sulphate is not classified as a dangerous good, has no UN number and ships as general cargo in bags, jumbo bags or bulk. It is a mild eye and respiratory irritant as a dust, so dust control and eye protection are appropriate, but it needs no dangerous goods declaration.
What is the colour of sodium sulphate?
Sodium Sulphate Anhydrous is supplied as a white crystalline powder.