Sodium Metasilicate CAS Numbers and Chemical Identity

Identity of the solid silicates we manufacture

Each figure on this table states its basis. Read the full rule on the technical index.

Why the hydrate changes the CAS number but not the chemistry

Anhydrous and hydrated sodium metasilicate share the same anion and the same silica-to-alkali ratio; what differs is the water of crystallization bound in the solid. Because a hydrate is a distinct crystalline substance rather than a wet version of the same one, chemical registries assign it its own CAS number. That is why a document set can be internally consistent and still fail an incoming-goods check — the number on the SDS refers to the hydrate actually shipped, while the number on the purchase order refers to the anhydrous form.

The practical consequence is on the assay, not on the chemistry. Roughly 42% of the pentahydrate's mass is water of crystallization, so its declared SiO₂ and Na₂O percentages are far lower than the anhydrous figures even when the underlying material is equivalent on a water-free basis. Any comparison across hydrates has to be normalized before it means anything.

Metasilicate is not sodium silicate

Sodium silicate is a family of compositions described by a modulus — the molar ratio of SiO₂ to Na₂O, typically 2.0 to 3.5 in commercial liquid grades. Sodium metasilicate is the specific member of that family where the ratio is 1:1, supplied as a solid rather than a solution. Higher-modulus silicates behave as binders and silica sources; the 1:1 metasilicate behaves as an alkaline builder. Substituting one for the other changes free alkalinity, dissolution behaviour and the mass you have to dose.

In practice the mismatch usually surfaces one of two ways: a specification written as 'sodium silicate' that quotes an Na₂O of around 50%, which only a solid metasilicate reaches; or a request for liquid water glass answered with a solid metasilicate quotation. Both are identity problems, not price problems.

Settling an identity question before quotation

Three data points close almost every identity question: the CAS number, the chemical formula including any water of crystallization, and one assay figure — Na₂O or SiO₂ — with the basis stated. If those three agree across the purchase specification, the SDS and the COA, the material is identified. If any one disagrees, the specification should be corrected before a grade is recommended.

Where an end customer's specification cannot be changed, send the specification as written and state the discrepancy explicitly in the RFQ. It is faster to resolve an identity mismatch in writing before shipment than to unwind it at an incoming-goods inspection.

FAQ