# Sodium hypophosphite

Anhydrous and monohydrate identity, assay and phosphorus bases, product-specific impurities, and limits of physical-property evidence.

Reviewed: 2026-10-30

Tags: chemicals, sodium-salts, hypophosphites

## Identity and related phosphorus salts

Sodium hypophosphite, also called sodium phosphinate, has **CAS 7681-53-0 for the anhydrous salt** and **CAS 10039-56-2 for the monohydrate**. They share EC 231-669-9. The hydrate has a separate assay basis, illustrated by Merck’s 99.0–104.0% titration specification as NaH₂PO₂·H₂O. [Sigma-Aldrich 04434](https://www.merckmillipore.com/ER/en/product/sigald/04434) [Merck 104646](https://www.merckmillipore.com/INTL/en/product/Sodium-phosphinate-monohydrate,MDA_CHEM-104646)

| Form | Formula | CAS number | Formula mass |
| --- | --- | --- | --- |
| Anhydrous | NaH₂PO₂ | 7681-53-0 | 87.98 g/mol |
| Monohydrate | NaH₂PO₂·H₂O | 10039-56-2 | 105.99 g/mol |

These are the identities stated for [04434](https://www.merckmillipore.com/ER/en/product/sigald/04434) and [S5012](https://www.sigmaaldrich.com/US/en/product/aldrich/s5012), respectively. A commercial name without its formula and assay basis leaves the mass basis unresolved.

Hypophosphite, phosphite and phosphate are distinct anions. For comparison, a documented sodium phosphite is Na₂HPO₃·5H₂O, while dibasic sodium phosphate is Na₂HPO₄. Neither is another name for NaH₂PO₂. [Sigma-Aldrich phosphite 04283](https://www.sigmaaldrich.com/US/en/product/sial/04283), [phosphate RDD022](https://www.sigmaaldrich.com/US/en/product/sigma/rdd022). **Chemical deduction:** charge balance gives phosphorus formal oxidation states +1 in H₂PO₂⁻, +3 in HPO₃²⁻ and +5 in HPO₄²⁻. Equal total phosphorus therefore does not establish equal chemical identity or behavior.

## Formula mass, water and phosphorus equivalents

**Nonprocedural stoichiometric deductions:** for ideal pure monohydrate, the anhydrous-salt equivalent is 87.98/105.99 = **83.01% by mass**. The remaining **16.99%** is stoichiometric crystal water. One mole of either formula contains one mole of phosphorus. Using 30.974 g/mol for P gives approximately **35.21% P** for ideal anhydrous salt and **29.22% P** for ideal monohydrate. These are formula-based quantities, not measured water, purity or available reducing capacity. The formula masses are those above; phosphorus atomic weight is from [CIAAW's phosphorus entry](https://ciaaw.org/phosphorus.htm).

A report “as P,” “as NaH₂PO₂,” or “as NaH₂PO₂·H₂O” consequently has a different denominator. Total-P analysis also includes phosphorus in phosphite or phosphate impurities; it cannot by itself quantify hypophosphite. This follows from the distinct formulas, not from an assumed solution speciation or activity model.

Thermo Scientific A13385.36 illustrates an independent water/assay basis: its original one-page specification, printed **2026-10-29**, lists **12–17% water by Karl Fischer titration** and **≥97.0% assay on a dry-weight basis**, with the assay method marked unspecified. A nominal monohydrate name thus does not make every supplied sample exactly 16.99% water. Nor does a dry-basis assay establish the as-received hypophosphite percentage without the applicable assay definition and measured water result. [Thermo specification](https://assets.thermofisher.com/chem-specs-pdf/retrievePdf?rootSku=A13385&sku=A13385.36).

## Product specifications and historical lot evidence

Anhydrous 04434 is described as puriss., meeting the analytical specification of BPC63, with **98–101% assay**. Its listed limits include heavy metals **≤0.001% as Pb**, free alkali **≤0.2% as CaO**, chloride and sulfate **≤200 mg/kg each**, arsenic **≤2 mg/kg** and iron **≤10 mg/kg**. “As Pb” and “as CaO” are reporting equivalents, not proof of those molecular species. The page does not give the assay method. These are product specifications, not a lot's measured impurity profile. [04434 product data](https://www.merckmillipore.com/ER/en/product/sigald/04434).

Glentham GX0223 batch **308XGI**, specification v1.0, was manufactured **2024-11-06**. Its monohydrate-based certificate was printed **2026-10-30 21:31:44**, timezone unstated: [original certificate](https://www.glentham.com/en/products/product/coa/?batch=308XGI&code=GX0223).

| Property | Specification | Batch 308XGI |
| --- | --- | --- |
| Appearance | Colourless crystals or white crystalline powder | Colourless crystals |
| Solubility, 10% in water | Clear, colourless | Clear, colourless |
| pH | 6.0–8.0 | 6.7 |
| Sulfate; chloride | ≤200 ppm each | ≤20 ppm each |
| Lead; arsenic | ≤2 ppm each | ≤1 ppm each |
| Iron | ≤5 ppm | ≤1 ppm |
| Calcium | ≤50 ppm | 15 ppm |
| Assay, titration, as NaH₂PO₂·H₂O | 99.0–104.0% | 103.0% |

Conformance concerns quality release. Assay uncertainty and detailed titration method are absent: **103.0% is an assay result, not a mass fraction exceeding unity**. Its retest and print dates do not establish present condition or another lot's composition. [GX0223 certificate and footer](https://www.glentham.com/en/products/product/coa/?batch=308XGI&code=GX0223).

Fluka/Solstice 30410 separately lists a combined **phosphate, phosphite (as PO₄) limit of 0.5%**. This reporting equivalent neither identifies their individual proportions nor measures total hypophosphite. The undated catalog also contains inconsistent synonym fields (Antiformin; Hyclorite), which are not adopted here. This source is limited to its explicitly labeled impurity specification. [30410 catalog](https://lab.solstice.com/shop/sodium-hypophosphite-monohydrate-30410).

## Density and solubility are source-specific

| Evidence | Reported property | Interpretation limit |
| --- | --- | --- |
| Anhydrous 04434 | Density 1.77 g/cm³ at 20 °C; water solubility 909 g/L at 30 °C | Neither measurement method nor the solubility volume convention is stated. [Source](https://www.merckmillipore.com/ER/en/product/sigald/04434). |
| Monohydrate Merck 104646 | Density 1.81 g/cm³; bulk density 1000 kg/m³; solubility 1100 g/L | The product page omits density temperature and solubility temperature/method. Bulk density includes interparticle space and is not crystal density. [Source](https://www.merckmillipore.com/INTL/en/product/Sodium-phosphinate-monohydrate,MDA_CHEM-104646). |
| Monohydrate S5012 | Water: 100 mg/mL, clear and colorless | This is a stated concentration/appearance observation, without a saturation claim. [Source](https://www.sigmaaldrich.com/US/en/product/aldrich/s5012). |

**Interpretive synthesis:** differing solid form, temperature and unspecified methods prevent a direct reconciliation of 909 and 1100 g/L. A clear solution at 100 mg/mL does not contradict a larger reported solubility. None of these entries supplies a solution-density curve or converts g/L into mass percent; solvent volume and final solution volume are not interchangeable.

## Thermal evidence and dated hazard scope

Thermo's **ACR22279 SDS, version 7, revised 2022-03-28**, identifies the monohydrate and reports **238 °C** under melting point/range and **>238 °C** under decomposition temperature; its density fields contain no data. The relevant page does not identify the thermal test method. A melting-field entry alone does not establish a clean equilibrium melting transition. [Original SDS, pp. 1 and 4](https://assets.thermofisher.com/DirectWebViewer/private/document.aspx?prd=ACR22279~~PDF~~MTR~~CGV4~~EN~~2022-04-19+05%3A04%3A46~~Sodium+hypophosphite+monohydrate~~).

Merck's **104646 GB SDS, version 2.5, revised 2018-07-14**, instead separates **>100 °C**, described as loss of crystal water, from **>248 °C for anhydrous substance**. These are web-reader-accessible historical entries; direct PDF retrieval was forbidden, so their original page pixels were unavailable for this review. Their differing event labels and unspecified methods leave the thermal discrepancy unresolved. They do not establish a universal decomposition onset or a permissible temperature. [Merck SDS, §9](https://www.merckmillipore.com/Web-AE-Site/en_US/-/USD/ShowDocument-File?Country=GB&DocumentId=104646_SDS_GB_EN.PDF&DocumentType=MSD&DocumentUID=339265&Language=EN&Origin=null&ProductSKU=MDA_CHEM-104646).

That same 2018 GB document states that the substance was not classified as hazardous under Regulation (EC) No 1272/2008. This is a dated, product- and jurisdiction-specific classification statement, not a current global compliance conclusion or evidence of process suitability. [Merck SDS, §§1–2](https://www.merckmillipore.com/Web-AE-Site/en_US/-/USD/ShowDocument-File?Country=GB&DocumentId=104646_SDS_GB_EN.PDF&DocumentType=MSD&DocumentUID=339265&Language=EN&Origin=null&ProductSKU=MDA_CHEM-104646).

Kobayashi's original 1960 paper distinguishes furnace temperature from specimen temperature and describes nitrogen-atmosphere and air observations separately. It uses thermogravimetry, X-ray diffraction and ion-exchange chromatography, with figures and notes identifying different analytical observations. This supports treating event, atmosphere and measurement location as part of the claim; it does not reconcile the later SDS numbers. No experimental heating sequence or application performance is inferred here. [Kobayashi, *Nippon kagaku zassi* 81, 1838–1840, DOI 10.1246/nikkashi1948.81.12_1838](https://www.jstage.jst.go.jp/article/nikkashi1948/81/12/81_12_1838/_pdf).

## Evidence coverage and limits

Research reviewed **2026-10-30 PDT**. The Thermo specification, Thermo SDS and Glentham certificate were downloaded from their publishers, read as text and visually inspected at the cited pages, including table headings and footers. Kobayashi's Japanese original was inspected across all three pages, including figures, captions and notes; its interpretation here is qualitative. Dynamic generation matters: the web-reader's Thermo specification carried a **2026-05-28** print date, whereas the retrieved PDF carried **2026-10-29**; the cited values agreed. The Glentham web-reader likewise retained an older print date. A print timestamp is not a new analytical test.

The Merck/Sigma catalog facts above were available through the web reader; direct requests returned access errors or timeouts. No current lot result, complete phosphite/phosphate impurity balance, solution activity model or modern thermal dataset reconciling the cited SDS entries was established. The generic substance reference does not establish formulation compatibility, manufacturing performance or customer qualification.