# Oxalic acid

Oxalic acid identity, anhydrous and dihydrate forms, assay and water bases, source-specific physical properties, and evidence limits.

Reviewed: 2026-10-30

Tags: chemicals, organic-acids, oxalates

## Identity and supplied forms

Thermo Scientific **044410.30 anhydrous** oxalic acid lists **97.5–102.5% by aqueous acid-base titration**. Scharlab **AC1720 dihydrate** lists **99.5–102.5% by permanganometry**. Compare the method and hydrate basis before comparing assays. Oxalic acid, or ethanedioic acid, is represented by (COOH)₂. [ILO/WHO ICSC 0529, November 2009](https://chemicalsafety.ilo.org/dyn/icsc/showcard.display?p_card_id=0529&p_lang=en&p_version=2)

| Form | CAS number | Formula | Molar mass, g/mol | Source |
| --- | --- | --- | --- | --- |
| Anhydrous acid | 144-62-7 | C₂H₂O₄ | 90.03 | [Thermo Scientific 044410.30 specification](https://assets.thermofisher.com/chem-specs-pdf/retrievePdf?rootSku=044410&sku=044410.30) |
| Dihydrate | 6153-56-6 | C₂H₂O₄·2H₂O | 126.07 | [Scharlab AC1720 technical data sheet, version 2023.9.13](https://www.scharlabchina.com/documents/TDS/AC1720_TDS_EN.pdf) |

Derived chemical interpretation. The dot denotes two water molecules per acid formula unit in the ideal hydrate. It does not denote an aqueous concentration. The SDS reports an anhydrous molar mass of 90.04 g/mol. Calculations here use the specification's 90.03 g/mol. [Thermo Fisher 44410 US SDS, section 9](https://www.fishersci.com/store/msds?countryCode=US&language=en&partNumber=AA4441018&productDescription=OXALIC+ACID+ANHYDROUS+50G&vendorId=VN00024248).

## Grade, assay, and composition basis

These examples describe named analytical products, rather than all oxalic acid grades.

| Product | Published assay specification | Separately reported properties |
| --- | --- | --- |
| Thermo Scientific 044410.30, anhydrous, nominal 98% | 97.5% to 102.5%, aqueous acid-base titration | FTIR identity conforms. No water or individual impurity limits appear in this sheet. |
| Scharlab AC1720, dihydrate, ExpertQ, ACS, ISO, Reag. Ph Eur | 99.5% to 102.5%, permanganometric | Water-insoluble matter at most 0.005%, chloride at most 5 ppm, iron at most 2 ppm, and ignition residue at most 0.01%. |
| Sigma-Aldrich 75699, dihydrate, BioUltra | At least 99.0%, permanganate redox titration | Loss on drying 28% to 30%. Iron at most 2 mg/kg, sodium and potassium each at most 50 mg/kg. |

Sources are the [Thermo specification](https://assets.thermofisher.com/chem-specs-pdf/retrievePdf?rootSku=044410&sku=044410.30), the [Scharlab technical data sheet](https://www.scharlabchina.com/documents/TDS/AC1720_TDS_EN.pdf), and the [Sigma 75699 specification table](https://www.sigmaaldrich.cn/CN/en/specification-sheet/sigma/75699). The impurity examples above are a subset of the latter two tables.

Interpretive synthesis. Titrimetric assay, loss on drying, and individual impurity measurements describe different quantities. An analytical upper limit above 100% does not mean that a material has a mass fraction above 100%. These sheets do not explicitly identify a dry-substance correction for their assay entries. They therefore do not establish interchangeable dry-basis and as-supplied assay values. Loss on drying is a method-dependent mass change, rather than a water-specific result by definition. The Sigma table does not supply its drying conditions.

A historical certificate for Thermo product 44410, lot 10253065, reports 99.6% by aqueous acid-base titration and an FTIR identity result of conforming. The certificate was printed November 12, 2024. Its package-size note limits the results to that lot. This is neither a current-lot result nor a specification for every shipment. [Thermo lot 10253065 certificate](https://assets.fishersci.com/TFS-Assets/CCG/Alfa-Aesar/certificate/Certificate-of-Analysis/044410-10253065.pdf).

The US SDS declares oxalic acid above 95 weight percent. This is no substitute for lot assay, water, or impurity measurements. [Thermo Fisher 44410 SDS, section 3](https://www.fishersci.com/store/msds?countryCode=US&language=en&partNumber=AA4441018&productDescription=OXALIC+ACID+ANHYDROUS+50G&vendorId=VN00024248).

## Ideal formula-mass relationships

Nonprocedural derived chemistry. Using the formula masses above, ideal pure dihydrate contains `90.03 / 126.07 = 0.71413`, about 71.413% water-free acid by mass. Its ideal crystal-water fraction is about 28.587%. Equal numbers of acid formula units correspond to a dihydrate-to-anhydrous mass ratio of `126.07 / 90.03 = 1.40031`. These are stoichiometric identities, not measured product composition or preparation quantities. They assume pure stoichiometric forms, with no additional moisture or impurities.

The ideal water fraction lies within Sigma 75699's loss-on-drying interval. That numerical agreement does not prove that a drying result measures only crystal water or that any particular lot has the ideal composition. [Sigma 75699](https://www.sigmaaldrich.cn/CN/en/specification-sheet/sigma/75699).

## Physical properties and thermal interpretation

| Source and form | Reported data | Limits of the entry |
| --- | --- | --- |
| ICSC 0529, anhydrous formula | Density 1.9 g/cm³. Water solubility 9 to 10 g/100 mL at 20 °C. Melting entry 189.5 °C. | No density temperature or measurement method. The solubility volume is not explicitly identified as solvent or final solution volume. |
| Scharlab AC1720, dihydrate | Specific density 1.65 g/cm³. Bulk density approximately 900 kg/m³. Water solubility 102 g/L at 20 °C. Melting entry 101 °C. | No density temperature, thermal method, or explicit solubility denominator definition. |
| Supelco OX0270, dihydrate | Density 1.65 g/cm³ at 20 °C. Melting entry 98 to 100 °C, explicitly attributed to loss of crystal water. Boiling entry 149 to 160 °C at 1013 hPa, explicitly marked decomposition. | Undated product-page entries, without the thermal measurement method. |

Sources are [ICSC 0529](https://chemicalsafety.ilo.org/dyn/icsc/showcard.display?p_card_id=0529&p_lang=en&p_version=2), [Scharlab AC1720](https://www.scharlabchina.com/documents/TDS/AC1720_TDS_EN.pdf), and [Supelco OX0270](https://www.sigmaaldrich.com/US/en/product/mm/ox0270).

Thermo's historical anhydrous lot certificate reports a melting result of 183 °C with decomposition. Its US SDS instead lists a melting range of 189 to 191 °C and no decomposition-temperature data. Neither document supplies a method that reconciles the difference. [Lot certificate](https://assets.fishersci.com/TFS-Assets/CCG/Alfa-Aesar/certificate/Certificate-of-Analysis/044410-10253065.pdf), [SDS section 9](https://www.fishersci.com/store/msds?countryCode=US&language=en&partNumber=AA4441018&productDescription=OXALIC+ACID+ANHYDROUS+50G&vendorId=VN00024248).

Interpretive synthesis. Hydrate water loss, melting, and decomposition are distinct events. The reported temperatures do not establish a single universal thermal threshold or an ordinary boiling point for unchanged acid. Solid density and bulk powder density are also different properties. The solubility entries do not establish a common saturation curve because their concentration denominators and methods remain incomplete.

Sigma 75699's specification includes a clarity test at a stated mass and water volume under its solubility-method heading. It does not identify that test as a saturation measurement. Its approximately 1.0 pH entry belongs to that solution test, rather than to dry crystals or every aqueous concentration. [Sigma 75699](https://www.sigmaaldrich.cn/CN/en/specification-sheet/sigma/75699).

## Acid, oxalate, and metal salts

Derived chemical notation. Successive loss of the two acidic protons gives H₂C₂O₄, HC₂O₄⁻, and C₂O₄²⁻. The latter two are hydrogen oxalate and oxalate. These protonation states are different from the anhydrous-versus-dihydrate distinction. An oxalate salt includes counterions and may have its own crystal water, formula mass, and solubility. It is not the same supplied substance as the acid.

Interpretive synthesis. Acid dissociation, metal-complex formation, and precipitation of a metal oxalate describe different equilibria. Their quantitative treatment requires temperature, ionic medium, activity conventions, and constants for the particular species. This reference establishes no numerical dissociation or metal-binding constants. The notation alone predicts neither a metal-removal rate nor a surface-finish result. No alloy, coating, polymer, or formulated product compatibility is established here.

## Hazard and source scope

The Thermo Fisher 44410 US-English SDS, revision 5 of December 22, 2025, cites the US OSHA Hazard Communication Standard 2024. Section 2 classifies acute oral and dermal toxicity as category 4, serious eye damage as category 1, and repeated-exposure target-organ toxicity as category 2. Its listed target organs are kidney and liver. These are product- and jurisdiction-specific hazard statements, not worldwide classification or a compliance determination. [Thermo Fisher SDS, sections 1 and 2](https://www.fishersci.com/store/msds?countryCode=US&language=en&partNumber=AA4441018&productDescription=OXALIC+ACID+ANHYDROUS+50G&vendorId=VN00024248).

Source coverage is limited to the named manufacturer documents and ICSC 0529. Scharlab's sheet bears version 2023.9.13. Thermo's directly retrieved specification bears print date October 29, 2026. A search-index copy of the same endpoint showed `06/12/2025`; the directly retrieved PDF is the version used here. The Sigma specification and Supelco page expose no revision date for their cited tables. They were read on October 30, 2026 PDT. The supplier grade claims do not reproduce or independently verify paid compendial methods. No current-lot certificate, full impurity balance, polymorph comparison, or experimental speciation model is established here.

Related manufacturing context is [Surface-finishing processes](../../processes/finishing/index.md).