Sodium Metasilicate Na₂O and SiO₂ Content — Comparing Grades
Published Na₂O and SiO₂ minimums for anhydrous, granular and pentahydrate sodium metasilicate, why hydrate assays look lower, and how to normalize them before comparing quotations.
Published · Updated
Short answer
On our published specification, the granular (beaded) anhydrous grade carries SiO₂ ≥ 45.0% and Na₂O ≥ 50.0%, and the anhydrous powder grade SiO₂ ≥ 44.5% and Na₂O ≥ 49.5%. The pentahydrate carries Na₂O ≥ 28.5% and SiO₂ ≥ 27.5% on the Standard tier, because roughly 42% of its mass is water of crystallization rather than active material. The two are not directly comparable as printed: a hydrate assay always reads lower than an anhydrous assay for chemically equivalent material, and comparing them without normalizing for water is the most common error in quotation review.
Published assay minimums by grade
Each figure on this table states its basis. Read the full rule on the technical index.
| Property | Value | Basis |
|---|---|---|
| Anhydrous powder — SiO₂ / Na₂O | ≥ 44.5% / ≥ 49.5%Moisture ≤ 1.0% on this grade. | Published specification |
| Granules (beaded) — SiO₂ / Na₂O | ≥ 45.0% / ≥ 50.0%Same chemistry as the powder; particle size 18–60 mesh ≥ 93.0%. | Published specification |
| Pentahydrate — SiO₂ / Na₂O (Standard tier) | ≥ 27.5% / ≥ 28.5%As-shipped basis, water of crystallization included in the mass. | Published specification |
| Pentahydrate normalized to a water-free basis | ≈ 47.8% SiO₂ / ≈ 49.6% Na₂OMinimums divided by (1 − 0.425), using the theoretical water fraction of Na₂SiO₃·5H₂O. An arithmetic check for comparing offers — not a specification value. | Derived |
| Batch-measured assay | Reported per shipmentActual values normally sit above the published minimum; the COA is the figure to qualify against. | Batch COA |
What Na₂O and SiO₂ actually tell you
Na₂O is the conventional way of expressing the alkali content of a silicate and is the figure that tracks with the free alkalinity a formulation gains per unit dosed. SiO₂ expresses the silicate portion, which carries the sequestering, soil-suspending and surface-passivating behaviour rather than the caustic effect. A metasilicate specification quotes both because a buyer replacing a caustic source cares about the first, and a buyer replacing a builder or a corrosion-inhibiting component cares about the second.
This is also why the ratio matters more than either number alone. At the 1:1 molar ratio that defines metasilicate, the two arrive together in a fixed proportion. Any material offered as metasilicate whose Na₂O and SiO₂ imply a different ratio is either a different silicate or a blend, regardless of what the document is titled.
Normalizing a hydrate assay before comparing quotations
Water of crystallization accounts for about 42.5% of the pentahydrate's molecular mass (90.08 of 212.14 g/mol). Dividing the as-shipped Standard-tier minimums by the remaining 0.575 gives roughly 47.8% SiO₂ and 49.6% Na₂O — the same range as the anhydrous minimums. That arithmetic is what makes the two grades comparable on chemistry.
It does not make them comparable on landed cost. You are still paying freight, duty and handling on the water: about 1.74 tonnes of pentahydrate carries the active content of one tonne of anhydrous. Whether that trade is worth making depends on dissolution behaviour, dosing equipment and how alkalinity is controlled in the process — not on the assay percentage.
When quotations are compared across suppliers, normalize all of them to the same basis before ranking on price, and confirm that each quoted figure is a minimum rather than a typical value. A 'typical' number quoted against a competitor's minimum makes an offer look stronger than the commitment behind it.
Published minimum versus batch measurement
The published specification is a floor we commit to; the COA reports what a given batch measured. Qualification work should be run against COA values for the batches actually received, with the published minimum treated as the contractual limit rather than the expected value.
If your process has a narrower internal window than the published specification — a tighter Na₂O band, for example — state that window in the RFQ. It is a specification question to be settled before the first order, not something to be discovered from batch variation later.
How to verify this yourself
- Compare the Na₂O and SiO₂ figures on the batch COA against the grade specification, not against a general product brochure.
- Confirm whether a quoted assay is a minimum or a typical value before ranking offers on it.
- For hydrate grades, normalize the assay to a water-free basis before comparing against an anhydrous quotation.
- Where an internal window is narrower than the published minimum, state it in the RFQ so it can be confirmed before the first shipment.
What this page does not claim
- Published figures are specification minimums, not typical batch values; the COA is the record for a given shipment.
- The normalized water-free figures are arithmetic derived from the theoretical hydrate composition, and are not offered as a specification.
- This page does not state the alkalinity a given formulation will reach — that depends on dose, water and the rest of the system.
FAQ
What is the Na₂O content of sodium metasilicate?
Why does pentahydrate show a much lower assay than anhydrous?
How much pentahydrate replaces one tonne of anhydrous?
Is a higher Na₂O always better?
Ready to RFQ
A quotation needs grade and monthly quantity (MT), plus packing, destination port and buyer type. The form will not submit without these. Approved TDS, SDS and stamped COA samples are on Downloads; a shipment COA is issued per order. No prices are shown on the form.
- Grade
- Monthly quantity (MT)
- Packing
- Destination port
- Documents needed