Aluminum Sulfate
Properties
| State | Solid (white crystalline or powder) |
| Color | White |
| Solubility | Soluble in water (364 g/L at 20°C); acidic in solution |
| Melting Point | 770°C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Aluminum Sulfate
Aluminum sulfate is the workhorse coagulant of municipal water treatment, and the chemistry it does in a settling basin is genuinely elegant. Drop alum into raw water at a few tens of milligrams per liter and the Al³⁺ ions immediately start hydrolyzing into a polynuclear cascade — Al(H2O)6³⁺, Al(OH)(H2O)5²⁺, polynuclear clusters like [Al13O4(OH)24(H2O)12]⁷⁺, and ultimately amorphous aluminum hydroxide gel. That gel carries enough positive charge to neutralize the negative surface charge on suspended particles (clay, organic colloids, bacteria), letting them aggregate and settle out by gravity in the next stage of the plant. About 60% of all municipal water treatment worldwide uses aluminum sulfate or its polymerized derivatives as the primary coagulant, and the chemistry has been the same since the 1880s. Beyond water treatment, aluminum sulfate is what gets called 'papermaker's alum' in pulp and paper, where it precipitates rosin onto cellulose fibers to give paper its water-resistance, and 'mordant alum' in textile dyeing, where Al³⁺ binds dye molecules to fabric fibers. Gardeners use it to acidify soil — the hydrolysis releases protons that lower pH — which has the side effect of making aluminum bioavailable enough that hydrangea flowers shift from pink to blue when delphinidin in the petals coordinates the absorbed Al³⁺.
Where you'll encounter it
If you drink municipal tap water, you've drunk water that was clarified with aluminum sulfate. The treatment plant's job is to drop turbidity from raw-water levels (often 5–50 NTU) to below 1 NTU before chlorination, and aluminum sulfate floc is what does most of the work. In a chemistry teaching lab, the same compound is the source of the classic alum-crystal-growing demonstration — slow evaporation of a saturated KAl(SO4)2·12H2O solution gives the octahedral crystals that students take home. In food science, food-grade alum used to be the leavening agent in some baking powders before sodium aluminum phosphate took over that role; it still appears as a pickling firmer for cucumbers and as the astringent in some traditional aftershave products.
Common Uses
- Primary coagulant for municipal drinking-water treatment
- Papermaker's alum for rosin sizing in pulp and paper
- Mordant for fixing natural and acid dyes to textile fibers
- Soil acidifier for blue hydrangeas and acid-loving plants
- Pickling firmer in food-grade applications and traditional astringent
Safety Information
Solutions are acidic (pH around 2.5 for a 1 M solution) and irritate skin and eyes; concentrated solid causes mild burns on prolonged contact. Inhalation of dust irritates the respiratory tract. GHS H315, H319 — irritant only, not corrosive at the concentrations used industrially. Standard nitrile gloves and splash goggles are sufficient for routine handling.
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.