Anhydrous sodium sulphate is a white, neutral, water-soluble salt used at scale in powder detergents, textile dyeing, glass manufacture and pulping. Its defining physical characteristic is an unusual solubility curve: solubility rises steeply with temperature up to 32.4 °C and then falls slightly, because at that temperature the solid phase in equilibrium with the saturated solution changes from the decahydrate to the anhydrous salt. That single transition explains the product’s storage behaviour, its crystallisation behaviour, its historical name, and its use as a temperature reference point.

Substance identity

Related but distinct substances — do not conflate:

• Glauber’s salt, Na2SO4·10H2O — the decahydrate, a separate substance with its own CAS number. It contains roughly 44 % Na2SO4 by mass; the balance is water of crystallisation. It is not interchangeable with the anhydrous salt on a tonnage basis.
• Salt cake — a trade term for crude sodium sulphate, historically the by-product of hydrochloric acid manufacture from sodium chloride and sulphuric acid (the Mannheim process). Assay and impurity profile are lower and looser than refined grades.
• Mirabilite — the natural decahydrate mineral. Thenardite — the natural anhydrous mineral.

Commercial anhydrous sodium sulphate reaches the market both from natural sources (lake brines and evaporite deposits) and as a refined by-product of several industrial processes, including viscose/rayon production and various organic and inorganic syntheses. Route affects the trace impurity profile more than the assay.

Specification

Typical commercial ranges for refined anhydrous sodium sulphate. Market-typical figures, not guaranteed limits; the certificate of analysis for the grade and lot governs.

Grade nomenclature is not standardised across suppliers. Terms such as “detergent grade”, “high whiteness”, “glass grade” and “technical grade” describe intent rather than a defined limit set. Write the parameters you need into the specification with methods named; do not rely on the grade name.

Properties

The 32.4 °C transition, explained. Below that temperature the stable solid in contact with a saturated solution is the decahydrate; above it, the anhydrous salt. Because the decahydrate becomes progressively more soluble as it approaches the transition, the solubility curve climbs steeply to a peak there — and because the anhydrous salt’s solubility declines slightly with temperature, the curve turns over. Three practical consequences follow:

1. Retrograde solubility above the transition. Heating a near-saturated solution above 32.4 °C can precipitate solid rather than dissolve more. Heated evaporators and hot process lines carrying near-saturated sodium sulphate will scale.
2. Storage caking is a hydration event, not simple damp-clumping. Anhydrous material picking up moisture below the transition forms the decahydrate, with a large volume increase, and sets into a hard mass. Warehouse temperatures cycling across 32.4 °C in a humid store drive repeated hydration and dehydration and make caking worse.
3. The transition is sharp and reproducible, which is why it has served as a temperature reference point and why Glauber’s salt appears in latent-heat thermal-storage work as a phase-change material near ambient temperature.

Neutral in solution. Sulphate does not hydrolyse appreciably; sodium sulphate solutions are effectively neutral and add neither acidity nor alkalinity. This is why it functions as an inert electrolyte in dyeing and as a neutral filler in detergent powders.

Applications

• Powder detergents. Historically the largest single outlet. Sodium sulphate serves as a filler and processing aid: it standardises the active concentration, carries the formulation through spray-drying, controls bulk density and powder flow, prevents caking of the finished powder, and remains chemically inert toward surfactants and builders. Whiteness and low iron matter because the finished powder must look white.
• Textile dyeing. Used as the electrolyte in dyeing cellulosic fibres with direct and reactive dyes. Added neutral salt suppresses the electrostatic repulsion between the negatively charged dye and the negatively charged cellulose surface, driving dye exhaustion from the bath onto the fibre and improving levelness. Sodium sulphate is preferred over sodium chloride where chloride would corrode stainless-steel dyeing equipment. Calcium and magnesium content matters, because hardness ions can precipitate certain dyes.
• Glass manufacture — as a fining (refining) agent. Small additions to the batch aid removal of gas bubbles from the melt and help dissolve the siliceous scum that would otherwise float on the glass surface. This is a distinct role from soda ash, which is the bulk sodium source; sodium sulphate is a minor batch component used for melt quality.
• Pulp and paper. Sodium sulphate was the traditional make-up chemical of the kraft (“sulphate”) process, reduced to sodium sulphide in the recovery furnace. Modern mills have largely replaced it with other make-up chemistry, but the process name and the residual use remain.
• Other outlets: starch manufacture; leather tanning auxiliaries; a raw material for sodium sulphide and other sodium chemicals; a filler and diluent in a range of formulated powders; latent-heat thermal storage (as the decahydrate).

Handling and storage

Hazard profile. Sodium sulphate is among the more benign bulk industrial salts and is not generally classified as hazardous. Dust causes mechanical irritation of the eyes and respiratory tract. The supplier’s current safety data sheet governs classification, controls and first aid.

Moisture and caking — the principal handling issue. Anhydrous material converts to the decahydrate on contact with moisture, expanding and setting into a hard mass that may require mechanical breaking. Store sealed, under cover, off the floor, in a dry space; use moisture-barrier packaging in humid climates; avoid condensation cycles and avoid warehouse temperatures cycling across the 32.4 °C transition. Rotate stock. Caked material is not usually off-specification material — it is hydrated material — but it is unusable without breaking, and the hydrated fraction assays lower on a Na2SO4 basis until re-dried.

Flow and dust. Bulk density is high and standard grades flow reasonably, but fine grades dust. Enclose transfer points and extract at source.

Incompatibilities.

• Strong reducing agents at high temperature — sodium sulphate is reduced to sodium sulphide. Not a concern at ambient; relevant in furnace and calcination contexts.
• Strong acids in the presence of any sulphide contamination — potential hydrogen sulphide release. Refined grades should not contain sulphide; crude salt cake is a different matter.
• Aluminium and other reactive metals in contact with moist salt — general galvanic and chloride-assisted corrosion risk in wet storage, particularly where chloride is present.
• Concrete floors and steelwork tolerate the dry salt; damp sodium sulphate in porous masonry is a known cause of salt crystallisation damage, through repeated hydration and dehydration cycling.

Packaging. Typically supplied in PE-lined bags or FIBCs, or in bulk. Moisture-barrier packaging is a technical requirement, not a preference.

Notes

• Always confirm anhydrous versus decahydrate on the contract. The decahydrate contains roughly 44 % Na2SO4 by mass. A per-tonne comparison between the two is meaningless without stating which one is quoted.
• Whiteness is not purity. It is an appearance parameter driven mainly by iron and other trace chromophores, relevant to detergent and textile buyers and irrelevant to most other users. Its measurement method must be named for the figure to mean anything.
• The 32.4 °C transition is the single most useful fact about this product in practice. Storage caking, evaporator scaling and phase-change-material behaviour all trace back to it.
• Do not specify a food or potable-water grade by assay alone. Where food, feed or drinking-water contact is involved, the applicable purity monograph in the destination market and its heavy-metal limits must be named on the specification, and compliance must appear on the certificate of analysis.
• Agronomic aspects of this material are covered at /fertilizer-products/anhydrous-sodium-sulphate.html.

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Frequently asked questions

What is the practical difference between anhydrous sodium sulphate and Glauber’s salt (the decahydrate)?

Both are the same salt and differ only in bound water. Anhydrous Na2SO4 has a molar mass of 142.04 g/mol, while the decahydrate Na2SO4·10H2O is 322.20 g/mol. That means a tonne of decahydrate contains roughly 441 kg of Na2SO4 and about 559 kg of water. They also behave differently in storage: the decahydrate melts in its own water of crystallisation near 32.4 °C, so it is a poor choice for warm warehouses and for dry blending. For most industrial uses — detergent filler, glass batch, textile dyeing and dye standardisation — the anhydrous grade is what gets specified, because it stays free-flowing and you are not paying freight on water. Buy the hydrate only when a process specifically calls for it.

What do the individual lines on a typical specification sheet actually control?

Na2SO4 assay is the active content on a dry basis and sets how much salt you really receive. Moisture counts twice: it dilutes the assay and it seeds caking. Chloride is the line most often ignored, yet it corrodes stainless steel and attacks glass-furnace refractory, and a high figure usually signals brine or by-product origin. Iron drives colour — it tints a glass melt and shows as specks in detergent powder. Water-insoluble matter is the filterable residue that blocks nozzles and leaves haze in dye baths. Whiteness index is an optical grading used mainly for detergent grade. The pH of a dilute solution should sit close to neutral, since sodium sulphate is a neutral salt; a strongly alkaline reading points to residual carbonate or hydroxide from the production route.

How do I compare two offers fairly when the quoted prices per tonne look similar?

Compare cost per unit of contained Na2SO4 delivered, not price per tonne of product. Three corrections do most of the work. First, normalise the assay: divide the quoted price by the guaranteed minimum Na2SO4 percentage. Second, deduct moisture, which is water you pay for and then ship. Third, confirm whether the quotation is anhydrous or decahydrate, because the hydrate is roughly 56% water by mass, so a cheap-looking hydrate price is normally the more expensive one once converted. After that, look at bulk density and packaging, since low-density powder fills a container by volume before it reaches payload, and at the chloride and iron limits, which decide whether the material is usable in your process at all.

Why does anhydrous sodium sulphate cake in the warehouse, and what actually prevents it?

Anhydrous sodium sulphate is a genuine desiccant — it takes up water and converts to the decahydrate, and that hydration is what welds a bag into a solid block. The transition between the two forms sits at about 32.4 °C. Below that temperature, in humid air, the anhydrous form is the less stable one and will slowly hydrate; above it the anhydrous form is stable. This is why caking complaints cluster in cool, damp stores rather than hot ones. The controls are ordinary warehouse discipline: keep bags sealed and off bare concrete, limit temperature cycling across the transition point, avoid opening more bulk than a shift will consume, and do not stack pallets high enough for pressure to consolidate a partly hydrated layer.

What should it not be stored next to or mixed with?

In dry storage the material is chemically unremarkable; the real risks are moisture pickup and cross-contamination. Keep it away from deliquescent salts sharing the same store, and keep chloride-bearing materials away from glass- or detergent-grade stock, because chloride is a specified limit. At high temperature and in contact with strong reducing agents such as carbon or aluminium powder, sulfate can be reduced to sulfide, so it does not belong beside those in a hot process area. In solution the important incompatibility is calcium: mixing a sulfate solution with calcium nitrate or with hard water precipitates calcium sulfate, which scales heat exchangers and plugs filters and drip emitters. If a fertigation programme uses both, they belong in separate stock tanks and must be diluted before they meet.

When is sodium sulphate the wrong product to buy?

For most agricultural situations it is the wrong sulfur source, and that is worth saying plainly. Na2SO4 is about 22.6% sulfur, but it is also about 32.4% sodium, and the sodium stays behind in the soil. Repeated application raises exchangeable sodium, which disperses clay, damages structure and infiltration, and adds to the salinity load — the opposite of what a sulfur correction is meant to achieve. On sodium-sensitive crops, on heavy or already sodic soils, and where irrigation water is itself saline, choose a sulfur source that carries a useful cation instead: gypsum, ammonium sulfate, potassium sulfate, magnesium sulfate or elemental sulfur. It is also the wrong purchase whenever your specification demands low iron or low chloride and the offered grade does not guarantee them.

Powder, crystal or granular — does the physical form matter if the chemistry is identical?

It matters for handling rather than chemistry. Fine powder dissolves fastest and blends into detergent bases without a separate dissolving step, but it dusts, bridges in hoppers and has the lowest bulk density, so it costs more per tonne to move and store. Coarse crystal and granular material flows better, dusts less and loads a container closer to payload, but dissolves more slowly in cold water. For blenders the decisive issue is segregation: if the sodium sulphate is granular while the other components are powder, or the reverse, the mix will separate during handling and transport, and the bag your customer opens will not match the one you sampled. Match particle size across the whole blend rather than chasing the cheapest single component.

Related substances

• Soda ash and sodium sulphate are usually purchased together for glass and detergent work, where one supplies the sodium flux and the other the refining or filler function.
• Sodium sulphate and ammonium sulphate carry the same sulphate ion, but only one of them brings a plant nutrient cation with it – which is why sodium sulphate is a process chemical and not a sulphur fertilizer.
• Both salts are traded in defined hydration states, and both show how the hydrate you actually buy determines caking behaviour, storage and the real content per tonne.