The short answer
Aluminium sulphate is Al2(SO4)3, CAS 10043-01-3 anhydrous, molecular weight 342.15 g/mol. What actually ships is the hydrate, and the hydrate is where the confusion starts.
It is the workhorse coagulant of municipal and industrial water treatment, the sizing agent of the paper industry, and the traditional mordant of the dye house. Almost all of that rests on one reaction: dissolve it in water and it produces a dense, sticky aluminium hydroxide floc that sweeps suspended solids out of suspension.
Formula, molecular weight and identifiers
| Form | Formula | CAS | Molecular weight |
|---|---|---|---|
| Anhydrous | Al2(SO4)3 | 10043-01-3 | 342.15 g/mol |
| Tetradecahydrate | Al2(SO4)3·14H2O | 16828-11-8 | 594.39 g/mol |
| Octadecahydrate | Al2(SO4)3·18H2O | 7784-31-8 | 666.42 g/mol |
Other names: filter alum, papermaker's alum, sulphate of alumina, cake alum, patent alum. HS code 2833.22. The EC number is 233-135-0.
Where the 342.15 comes from: two aluminium at 26.98, three sulphur at 32.06, twelve oxygen at 16.00 — 53.96 + 96.18 + 192.00 = 342.14, rounded to 342.15 with the standard atomic weights.
Why commercial alum is sold as percent Al2O3
Commercial aluminium sulphate does not crystallise at a single, reliable degree of hydration. Depending on how it is cooled and stored it sits somewhere around 14 to 18 molecules of water, and it loses water slowly on the warehouse floor. Quoting it as a hydrate formula would therefore be meaningless.
So the trade quotes the aluminium content as Al2O3 instead. A typical ferric grade is 17 % Al2O3 min; non-ferric grades run 17 % or a little lower. That single number is what you actually buy, because it is what does the coagulating, and it is independent of how much water happens to be in the crystal that week.
Three consequences follow:
- Never compare offers on tonnage. Compare on Al2O3. A 17 % material against a 15 % material is 13 % more active alumina per tonne.
- Dose rates are quoted as product, not as alumina. Changing supplier at a different Al2O3 means re-jarring the dose.
- Storage matters. Slabs and lumps left open take up or lose moisture, which shifts the assay and cakes the material.
Aluminium sulphate is not the same as alum
Strictly, an alum is a double sulphate of a monovalent and a trivalent metal with twelve waters of crystallisation. Aluminium sulphate has no second cation and is not one.
| Material | Formula | CAS | Molecular weight |
|---|---|---|---|
| Aluminium sulphate | Al2(SO4)3 | 10043-01-3 | 342.15 |
| Potash alum | KAl(SO4)2·12H2O | 7784-24-9 | 474.39 |
| Ammonium alum | NH4Al(SO4)2·12H2O | 7784-25-0 | 453.33 |
In practice the water industry says “alum” and means aluminium sulphate, and the pickling, baking and traditional dyeing trades say “alum” and mean potash alum. When a specification just says alum, ask which.
Physical properties
| Property | Value |
|---|---|
| Appearance | White to off-white slabs, lumps, kibbled granules or powder; ferric grade is often pale tan |
| Odour | Odourless |
| Taste | Sweetish, strongly astringent |
| Solubility in water | About 36 g per 100 mL at 20 °C, rising steeply with temperature |
| pH of 1 % solution | Acidic, roughly 3 |
| Density | About 2.67 g/cm3 (anhydrous) |
| Behaviour on heating | Loses water of crystallisation, then decomposes above about 770 °C to alumina and sulphur oxides |
How it clarifies water
Raw water carries clay, silt and organic colour as colloidal particles that all hold a negative surface charge, repel each other and therefore never settle. Aluminium sulphate fixes that in two ways at once.
The trivalent aluminium ion neutralises that negative charge so the particles can approach and stick together. At the same time, the aluminium hydrolyses and precipitates as aluminium hydroxide, which comes down as a voluminous floc that physically sweeps particles out of the water column on the way. The simplified reaction with natural bicarbonate alkalinity is:
Al2(SO4)3·14H2O + 3Ca(HCO3)2 → 2Al(OH)3↓ + 3CaSO4 + 6CO2 + 14H2O
Two operating facts fall straight out of that equation. It consumes alkalinity — roughly 0.5 mg of alkalinity as CaCO3 per mg of alum dosed — so soft waters need lime or soda ash alongside. And aluminium hydroxide is only sparingly soluble in a window of about pH 6.0 to 7.8; outside it the floc redissolves and the dose is wasted. Every alum plant that stops working is worth checking for pH before anything else.
Ferric and non-ferric grades
| Grade | Al2O3 | Iron as Fe2O3 | Where it is used |
|---|---|---|---|
| Ferric (commercial) | 17 % min | Up to about 0.5 % | Effluent, paper, textiles, construction |
| Non-ferric | 17 % min | Below about 0.005 % | Potable water, fine paper, cosmetics, pharma |
The difference is the raw material. Ferric grade is made by digesting bauxite in sulphuric acid, and the iron in the ore comes with it. Non-ferric is made from purified aluminium hydroxide, which costs more and is why the two grades trade at different prices.
Iron matters for three reasons: it puts colour into treated drinking water, it yellows paper and textile substrates, and in cosmetic and pharmaceutical use it is simply not permitted at ferric-grade levels. If the end use touches anything visible or ingestible, specify non-ferric and put an Fe2O3 limit in the purchase order.
What aluminium sulphate is used for
- Water and wastewater treatment. The dominant use. Coagulation and clarification of drinking water, phosphate removal in sewage works, and solids removal in industrial effluent.
- Paper. Sets rosin size onto the fibre, controls pitch, fixes dyes and retention aids, and holds the wet-end pH — the reason it is called papermaker's alum.
- Textiles. Mordant for natural and some synthetic dyes, improving fastness by anchoring dye to fibre.
- Leather. Alum tanning and pickling of hides.
- Construction. Accelerator and waterproofing admixture in concrete and cement grouts.
- Fire fighting. Traditional component of foam extinguishing systems, reacting with bicarbonate to generate the foam.
- Agriculture. Lowering soil pH, and controlling phosphorus release in poultry litter.
Scapex Exports supplies aluminium sulphate in ferric and non-ferric grades alongside the related coagulants aluminium chlorohydrate (ACH) and ammonium aluminium sulphate.
Handling, packing and export documents
- Hazard. The solid is not normally classified as a dangerous good for transport, and ships as general cargo. Confirm the position for solutions and for your destination with your forwarder, as concentrated solutions can be classed as corrosive.
- Health. Dust and solutions irritate eyes, skin and the respiratory tract. Goggles, gloves and dust extraction when handling open material.
- Materials. Solutions are acidic and attack carbon steel and concrete. Use FRP, HDPE, rubber-lined steel or suitable stainless; acid-resistant lining for bunds and sumps.
- Segregation. Keep away from alkalis, which precipitate aluminium hydroxide and block lines, and from strong oxidisers.
- Storage. Dry and covered. Slabs and lumps cake if they take up moisture; kibbled and granular forms are easier to dose but need the same dry store.
- Packing. 25 kg and 50 kg HDPE laminated bags, 1000 to 1250 kg jumbo bags, or bulk. Solution grades in IBCs and road tankers.
An export consignment should carry a certificate of analysis stating Al2O3 content, free acid, iron as Fe2O3, water-insoluble matter, heavy metals and particle size; a 16-section GHS safety data sheet; commercial invoice and packing list; certificate of origin; and for potable water use the NSF, EN 878 or equivalent conformity reference the destination requires.
Enquire with a specification
Aluminium sulphate is supplied by Scapex Exports as a manufacturer and supplier from qualified Indian producers, in ferric and non-ferric grades, as slab, kibbled, granular or powder, and as solution, with certificate of analysis, safety data sheet and full export documentation.
Send the grade, minimum Al2O3, iron limit, physical form, packing and destination port, and you will get a quotation written against that specification rather than a generic one.
Frequently asked questions
What is the chemical formula of aluminium sulphate?
Al2(SO4)3 — two aluminium ions balancing three sulphate ions. The anhydrous salt has CAS number 10043-01-3 and a molecular weight of 342.15 g/mol. Commercial material is hydrated: the octadecahydrate Al2(SO4)3 18H2O has CAS 7784-31-8 and a molecular weight of 666.42 g/mol.
What is the molecular weight of aluminium sulphate?
342.15 g/mol for the anhydrous salt. Add water of crystallisation and the figure rises: 594.39 for the tetradecahydrate and 666.42 for the octadecahydrate. That is why you cannot compare two offers on tonnage — a tonne of 18-hydrate carries only about 513 kg of actual Al2(SO4)3.
Is aluminium sulphate the same as alum?
Not strictly, though the trade uses the words interchangeably. True alum is a double sulphate: potash alum is KAl(SO4)2 12H2O, CAS 7784-24-9, molecular weight 474.39, and ammonium alum is the ammonium equivalent. Aluminium sulphate is a simple salt with no second cation. Water treatment 'alum' or 'filter alum' almost always means aluminium sulphate.
What is the difference between ferric and non-ferric alum?
Iron content, and it decides where the material can be used. Ferric grade is made from bauxite and carries iron as an impurity, typically up to about 0.5 % as Fe2O3; it is fine for effluent, paper and textiles. Non-ferric grade is made from purified aluminium hydroxide and holds iron below roughly 0.005 %, which is what potable water, paper whiteness and cosmetic use require.
Why does aluminium sulphate lower the pH of water?
Because the aluminium ion hydrolyses. When Al2(SO4)3 dissolves, the aluminium reacts with water to form aluminium hydroxide floc and releases hydrogen ions, which is exactly the reaction that does the coagulating. The alkalinity in the raw water neutralises those hydrogen ions, so a soft, low-alkalinity water needs lime or soda ash dosed alongside the alum or the pH falls out of the effective range.
What is Aluminum Sulfate?
Aluminum Sulfate (CAS 10043-01-3) is supplied as a white granular/lump solid, with the molecular formula Al2O12S3 and a molecular weight of 342.15 g/mol. Its main applications include water treatment and paper sizing. Scapex Exports exports and supplies it from India in 17% Al2O3 Technical grade, with a batch Certificate of Analysis (COA) and MSDS.
What is Aluminum Sulfate used for?
The main applications of Aluminum Sulfate are water treatment and paper sizing.