Sodium Metasilicate Pentahydrate vs Anhydrous — Which Grade to Buy

Key Specification Differences

The decisive difference is water of crystallization. Anhydrous sodium metasilicate carries high actives — SiO₂ ≥ 45.0% and Na₂O ≥ 50.0% — at molecular weight 122.06 g/mol. The pentahydrate binds five water molecules per unit (MW 212.14 g/mol), so on a mass basis its actives are lower: Na₂O ≥ 28.5% and SiO₂ ≥ 27.5%. About 42% of the pentahydrate's weight is bound water.

Both grades share Fe ≤ 0.02%, supporting colour stability in white and light-coloured end products. Because the actives differ, the correct comparison between a quotation for each grade is cost per unit of active silicate delivered, not price per tonne of material.

The Number That Matters: Silicate per Tonne, Not Price per Tonne

A tonne of pentahydrate is about 420 kg of water. If your process ends up in solution anyway, that water costs you nothing at the plant — but you paid ocean freight on it. On a long route the freight on the water can be a meaningful share of the landed cost, and that is the case for anhydrous. On a short route with cheap freight, pentahydrate usually wins on landed cost per kilogram of silicate.

Run the comparison per kilogram of Na₂SiO₃ delivered, not per tonne of product. Ask for both grades on one RFQ and the quotation will show the figures side by side.

Handling, Dissolution and Hot-Climate Storage

Anhydrous sodium metasilicate is a low-moisture grade (moisture ≤ 1.0%) supplied as white powder, granules or beads. It dissolves more slowly than the pentahydrate, releases more heat when it does, and stays stable in hot storage. It is the grade for dry-blend systems where introducing water should be minimised, and for goods that will sit in a container through a tropical port or a hot inland warehouse.

Pentahydrate is a white crystalline granule or powder that dissolves quickly at ambient temperature with mild heat release, which is why liquid cleaners, ceramics slips, water-treatment blends and metal-cleaning baths specify it. Its weakness is heat: above roughly 72 °C it begins to melt in its own crystal water and cakes, and a closed container in tropical sun can reach that. For a pentahydrate shipment into a hot climate, ask for pallets and shrink wrap and plan to move it out of the container promptly on arrival.

How to Choose Between Them

Choose pentahydrate when you dissolve it in water at room temperature and want it in solution within minutes; when your formulation is a liquid — degreaser, floor cleaner, bottle wash, boiler treatment; when your warehouse stays below normal summer temperatures and turns stock over within months; and when freight is a small part of your landed cost.

Choose anhydrous when you dry-blend it into a powder or tablet, where added water would cake the mix; when the goods will pass through a hot port or warehouse; when you pay freight on a long route and want to move fewer tonnes of water; and when your dosing system feeds granules or beads by weight and needs free-flowing material.

Switching From One Grade to the Other

Because anhydrous carries about 1.74 times the silicate of pentahydrate, replace 1 kg of pentahydrate with about 0.58 kg of anhydrous to keep the same silicate dose — then confirm on a plant trial, because dissolving speed and heat release differ. If your process water is cold, anhydrous may need agitation or warm water to dissolve completely.

Both grades ship in 25 kg bags or 1,000 kg jumbo bags and can travel in the same container packed as separate batches. Loading per 20-foot container depends on packaging rather than grade: 27 tonnes loose or about 22 tonnes palletised in 25 kg bags. When sending an RFQ, state the grade (or both), CAS number, target actives, particle size or appearance, packing, and destination port so quotations are directly comparable.

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