# Sodium metasilicate

Metasilicate identity, hydrate forms, oxide ratios and the limits of product-specific composition and physical-property data.

Reviewed: 2026-10-30

Tags: chemicals, silicates

## 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][usp]

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][oxy]

| Form | Conventional formula | CAS registry number | Evidence scope |
| --- | --- | --- | --- |
| Anhydrous | Na₂SiO₃ | 6834-92-0 | ICSC 0359 identifies the anhydrous substance and aliases disodium metasilicate and disodium trioxosilicate. [ICSC, April 2004][icsc] |
| Pentahydrate | Na₂SiO₃·5H₂O | 10213-79-3 | Manufacturer product identity. [Silmaco pentahydrate][m5web] |
| Nonahydrate | Na₂SiO₃·9H₂O | 13517-24-3 | Wako gives a formula mass of 284.20 and identifies product 199-02445. [Wako product page][wako] |

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][m5]

**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][m5], [anhydrous FE sheet][fe]

## Product composition, assay and impurities

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

| Named document | Reported composition and basis | Limitation |
| --- | --- | --- |
| Silmaco anhydrous powder FE, version 8, **18 June 2024** | Specification: Na₂O 48.0–51.0%, SiO₂ 44.4–47.4%, dry matter 93.5–97.5%. Oxide methods are derived from ISO 1692 and ISO 2124 respectively. | The separate typical-value table is explicitly informational: insolubles <0.5% (ISO 2122), Fe <100 ppm and Pb <2 ppm (ICP), CO₂ <5% (TIC). These are not its product specifications. [FE sheet][fe] |
| Silmaco granular anhydrous, version 4, **12 October 2010** | Na₂O 49.6–51.0%, SiO₂ 46.0–47.4%, dry matter 95.5–98.5%; molar ratio 0.94–0.98 and weight ratio 0.91–0.95. | Historical, form-specific specification; it does not replace the later FE powder document. [Granular sheet][m0] |
| Ankit P-28 pentahydrate, undated TDS | Na₂O and SiO₂ each 28–30%; dry matter 56–60%, defined as their sum; water-insoluble material 0.05% maximum. Oxide and insoluble entries cite IS-14212:1995. | The source names a method standard; that standard's full procedure was not reviewed here. [P-28 TDS][ankit] |
| Wako nonahydrate, code 19-0244 | Specification revised **3 April 2023**: assay minimum 98.0%, white to nearly white crystals, qualitative water-solubility test. The product page identifies assay by titration. | The retrieved PDF also bears **2026/10/31** at its top; that is separate from its printed revision date. No lot result is shown. [Wako specification][wakospec], [product page][wako] |

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][usp], [Wako][wako], [Silmaco nonahydrate][m9web]

## 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][oecd]

| Form | Density, g/cm³ | Bulk density, kg/m³ |
| --- | --- | --- |
| Anhydrous | 2.61; temperature unspecified | 1200 |
| Pentahydrate | 1.75 at 20 °C | 1000 |
| Nonahydrate | 1.65 at 20 °C | 800 |

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][fe], [granular sheet][m0]

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][ankit]

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][oecd] 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][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][icsc], [OxyChem, printed p. 12][oxy], [Silmaco nonahydrate][m9web]

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], [Wako SDS, pp. 3–4][wakosds]

## 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][wakosds]

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][s25]

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](../../processes/finishing/index.md) provides broader process context; it does not establish this substance's application conditions.

[usp]: https://doi.usp.org/USPFCC/FCC_F100568_03_01.html
[oxy]: https://www.oxy.com/siteassets/documents/chemicals/products/other-essentials/silicate.pdf
[icsc]: https://www.inchem.org/documents/icsc/icsc/eics0359.htm
[m5web]: https://silmaco.com/en/product/metasilicate-pentahydrate
[m5]: https://silmaco.com/sites/default/files/public/generated/files/products/TF-E-M5_1.pdf
[fe]: https://silmaco.com/sites/default/files/public/generated/files/products/TF-E-M0-FE%20v8_1.pdf
[m0]: https://silmaco.com/sites/default/files/public/generated/files/products/TF-E-M0v4_2.pdf
[ankit]: https://www.ankitsilicate.com/assets/docs/sodium-metasilicate-pentahydrate/TDS%20Sodium%20Metasilicate%20Pentahydrate%205H2O.pdf
[wako]: https://labchem-wako.fujifilm.com/asia/product/detail/W01W0119-0244.html
[wakospec]: https://labchem-wako.fujifilm.com/asia/product/spec_19-0244.pdf?jeAttribute=E
[wakosds]: https://labchem-wako.fujifilm.com/sds/W01W0119-0244JGHEEN.pdf
[m9web]: https://silmaco.com/en/product/metasilicate-nonahydrate
[oecd]: https://hpvchemicals.oecd.org/ui/handler.axd?id=3cb4f34b-2afa-4004-b447-521188909235
[s25]: https://www.oxy.com/link/e51eef06eb9c46b0b78c09a2090a30d4.aspx