Sodium metasilicate

Identity and scope

Silmaco’s historical granular anhydrous grade lists SiO₂/Na₂O molar ratio 0.94–0.98 and weight ratio 0.91–0.95. Nominal sodium metasilicate is Na₂SiO₃, one mole SiO₂ per mole Na₂O. The USP preview covers anhydrous material and pentahydrate. USP description

OxyChem separately lists liquid sodium silicates with roughly 1.6–3.3 SiO₂/Na₂O by weight. That broader product range has a separate identity. OxyChem handbook, printed p. 2

These are documented forms, not an exhaustive hydrate phase diagram. The formulas identify composition; they do not establish a molecular or polymeric species distribution in solution.

Oxide ratio and analytical basis

Silmaco's pentahydrate sheet explicitly uses SiO₂/Na₂O, with separate molar and weight ratios. Its version 7, dated 30 June 2008, specifies molar ratio 1.01–1.05 and weight ratio 0.98–1.02, both citing ISO 1689. The same sheet's introduction rounds the molar ratio to 1.0. Rounded identity language and a product specification range serve different purposes. Silmaco M5 sheet

Nonprocedural deduction: for oxide masses measured on the same sample basis,

• Weight ratio: Rw = mass(SiO₂) / mass(Na₂O).
• Molar ratio: Rn = [mass(SiO₂) / M(SiO₂)] / [mass(Na₂O) / M(Na₂O)].
• Therefore Rn = Rw × M(Na₂O) / M(SiO₂), where M is molar mass.

A molar ratio of one does not mean equal oxide masses. “Modulus” without the numerator, denominator and mass-versus-mole basis is ambiguous. A Na₂O/SiO₂ ratio reverses the order used above. These equations describe analytical composition, not solution speciation or performance.

Silmaco defines dry matter as Na₂O + SiO₂ in its M5 and anhydrous sheets. That definition is not an independent measurement of every non-water constituent. An oxide percentage, a hydrate assay, a dry-matter result and an SDS ingredient percentage are distinct quantities. M5 sheet, anhydrous FE sheet

Product composition, assay and impurities

Manufacturer documents show why the generic name cannot supply one purity number.

Silmaco's sheets say their specification parameters will appear on a certificate of analysis; they are not themselves named-lot certificates. No current or historical lot COA was verified for this article. An SDS composition declaration likewise does not establish a lot's assay, trace impurities or water content.

Nonprocedural deduction: using USP's anhydrous formula mass 122.06 and Wako's nonahydrate formula mass 284.20, an ideal Na₂SiO₃·9H₂O formula contains approximately (284.20 − 122.06) / 284.20 = 57.1% water by mass. Silmaco's undated nonahydrate page instead describes its commercial product as 54% water of crystallization and 46% dry matter. The difference remains unresolved; the product statement is not substituted for ideal stoichiometry, and no impurity or phase explanation is inferred. USP, Wako, Silmaco nonahydrate

Density, solubility and thermal properties

Density needs both a physical form and a measurement basis. The OECD SIDS dossier's metasilicate section, dated 3 February 2005, reports the following; its solid-density entries are rated “valid with restrictions,” while its bulk values are handbook data rated “not assignable.” OECD, pp. 189–190

The dossier does not identify the density measurement methods. These are not solution densities. Bulk density includes interparticle space. Silmaco FE specifies bulk density 0.90–1.20 g/cm³ without a method or temperature; its older granular sheet labels 1.000–1.300 g/cm³ only as “Density,” leaving the basis less explicit. FE sheet, granular sheet

The P-28 PDF pixels show bulk density 0.85–1.00, with unit printed gm/l; the search reader instead returned 0.85–1.1 with unit %. Neither supports silently rewriting the entry into g/cm³. This density value remains unresolved. Its pH 12–13 also lacks a stated solution concentration and temperature. P-28 TDS

The OECD dossier reports anhydrous water solubility 210 g/L at 20 °C and pentahydrate 610 g/L at 30 °C; both entries are manufacturer data rated “not assignable.” The g/L solvent-versus-final-solution basis and method are not specified there. OECD, p. 191 USP's public description gives anhydrous 270 g/L at 30 °C, so the temperature differs; this is not evidence of a same-condition contradiction. USP

For thermal identity, ICSC assigns 1089 °C to anhydrous metasilicate. OxyChem's handbook lists 72.2 °C for its pentahydrate and 1089 °C for S-25 anhydrous material. Silmaco's nonahydrate page lists 48 °C as a typical value. These source labels are not heating schedules or a complete account of hydrate dehydration and phase transitions. ICSC, OxyChem, printed p. 12, Silmaco nonahydrate

Wako's nonahydrate product page and its SDS section 9 instead print 1089 °C. This conflicts with the form-specific distinction above. The article retains it as a source discrepancy, not a verified nonahydrate transition. The SDS gives no decomposition temperature or density and describes water solubility qualitatively. Wako product page, Wako SDS, pp. 3–4

Hazard and evidence limits

Wako's English SDS for product 199-02445, revision 2.06, 13 September 2024, is written according to JIS Z 7253:2019. It classifies skin corrosion and serious eye damage as category 1, and specific target organ toxicity after single exposure as category 2 for the nervous system. Those are dated, product-specific Japanese-framework classifications, not a global compliance determination or performance evidence. Section 3's 98.0 weight-% ingredient entry is distinct from a certificate reporting a measured lot assay. Wako SDS, pp. 1–2

The sources establish terminology and bounded property reports. They do not establish a customer's material compatibility, formulation performance, purity requirements or approved application. No quantitative solution-density relationship, full hydrate phase diagram or aqueous species model is established here.

Source access was checked on 30 October 2026 PDT. Primary PDF pages supporting the tables were inspected visually as well as through extracted text. OxyChem's handbook has no visible revision date on the cited pages. Its cached 2023-06 S-25 sales specification was readable through the search service, but the original URL returned HTTP 404 and PDF pixels could not be retrieved; its numerical limits are therefore excluded here. An attempted OxyChem UNIFLO SDS download returned HTML rather than a PDF. Cached S-25 source URL

The USP source is a public monograph preview; the subscriber text was not reviewed. The OECD document is an agency assessment compiling older studies and handbook/manufacturer data, not a newly performed experiment. Original experimental reports underlying those property entries were not obtained. No paywalled analytical standard or unavailable document is treated as fully reviewed evidence.

Related reference

Surface-finishing processes provides broader process context; it does not establish this substance's application conditions.