# Sodium hydroxide

NaOH identity, solid and aqueous forms, alkalinity reporting, grade-specific impurities and the limits of historical lot evidence.

Reviewed: 2026-10-30

Tags: chemicals, alkalis

## Identity and material form

OxyChem’s **50% membrane** NaOH lists **100 ppm NaCl maximum by weight**. Its **50% diaphragm** grade lists **1.10 wt% NaCl maximum**. The same nominal NaOH strength leaves a substantial chloride difference. [Membrane specification](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-soda-50-liquid-membrane.pdf) [Diaphragm specification](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-soda-50-liquid-diaphragm.pdf) Sodium hydroxide, or caustic soda, is NaOH, CAS 1310-73-2, about 40.00 g/mol. [NIOSH Pocket Guide, sodium hydroxide; page reviewed October 30, 2019](https://www.cdc.gov/niosh/npg/npgd0565.html)

Sigma-Aldrich 30620 is a pellet product. OxyChem’s liquids are aqueous and can absorb carbon dioxide. Supplied form alone leaves assay and carbonate content unresolved. [Sigma-Aldrich certificate, product 30620](https://www.sigmaaldrich.cn/certificates/Graphics/COfAInfo/fluka/pdf/PDF733412.pdf) [OxyChem caustic soda product overview](https://www.oxychem.com/chlor-alkali)

A documented hydrate identity is sodium hydroxide monohydrate, NaOH·H₂O: IUCr's July 1957 issue lists J. A. Wunderlich's crystal-structure paper, *Acta Crystallographica* 10, 462–463, DOI 10.1107/S0365110X57001541. This reference establishes the named research subject; the full paper was unavailable for this review. No lattice parameters, hydrate inventory or stability boundaries are adopted here. **Synthesis:** moisture in a pellet is not, by itself, evidence that the entire pellet is this crystalline monohydrate. [IUCr publisher issue record](https://journals.iucr.org/q/issues/1957/07/00/)

## Physical-property scope and units

NIOSH's solid-substance entry gives a melting point of 605 °F, boiling point of 2534 °F and specific gravity of 2.13. Its introduction defines boiling points at 1 atmosphere, and specific gravity at 68 °F relative to water at 39.2 °F unless otherwise noted. These values do not describe the thermal behavior or density of an arbitrary aqueous product. [NIOSH substance entry](https://www.cdc.gov/niosh/npg/npgd0565.html); [Pocket Guide property conventions, reviewed March 7, 2016](https://www.cdc.gov/niosh/npg/pgintrod.html#chemical-and-physical-properties)

The same entry labels solubility “111%.” The introduction defines its solubility convention at 68 °F and prints the denominator as g/100 mL, despite also calling it percent by weight. This compressed convention is retained as a source limitation; it is not interpreted as 111 g per 100 g of finished solution, or used to calculate a saturation mass fraction. No concentration-dependent solubility curve is established here. [NIOSH substance entry](https://www.cdc.gov/niosh/npg/npgd0565.html); [Pocket Guide conventions](https://www.cdc.gov/niosh/npg/pgintrod.html)

OxyChem's January 2026 handbook tables 1–2 give these 50.0 wt% NaOH aqueous reference rows at 60 °F, not lot measurements. The diaphragm row includes 1.000% NaCl. [OxyChem handbook, pp. 30–31](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-soda-handbook.pdf#page=30)

| Aqueous reference row at 60 °F | Specific gravity, as labeled | NaOH content, g/L | Solution density, lb/gal |
| --- | --- | --- | --- |
| Membrane grade, 50.0 wt% NaOH | 1.5298 | 764.780 | 12.760 |
| Diaphragm grade, 50.0 wt% NaOH, 1.000% NaCl | 1.5372 | 768.460 | 12.820 |

## Free hydroxide and total alkalinity

OxyChem distinguishes hydroxide from total alkalinity. Reporting total alkalinity as Na₂O equivalent does not identify a dissolved Na₂O ingredient. Its conversion is `% as NaOH = 1.2907 × % as Na₂O`. **Nonprocedural deduction:** 38.74% as Na₂O corresponds to approximately 50.00% as NaOH equivalent (38.74 × 1.2907 = 50.001718). This is a change of reporting basis, not a free-hydroxide measurement. [OxyChem handbook, pp. 45–48](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-soda-handbook.pdf#page=45)

The handbook warns that differences between separate total-alkalinity and hydroxide titrations can overstate low carbonate concentrations because of analytical error. **Synthesis:** carbonate is a separate analyte; an alkalinity difference alone is not proof of its concentration. [OxyChem handbook, p. 48](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-soda-handbook.pdf#page=48)

## Two named liquid-grade specifications

The following comparison uses OxyChem sales specifications CS 59059 and CS 44668, both marked 01/26. The ranges and maxima belong to these named products; they are neither generic NaOH properties nor customer acceptance targets. Each sheet separately lists total and hydroxide alkalinity. [Membrane specification](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-soda-50-liquid-membrane.pdf); [diaphragm specification](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-soda-50-liquid-diaphragm.pdf)

| Reported constituent/basis | 50% membrane | 50% diaphragm |
| --- | --- | --- |
| Total alkalinity as Na₂O, wt% | 38.36–39.50 | 38.00–39.60 |
| Hydroxide alkalinity as NaOH, wt% | 49.50–51.00 | 49.00–51.00 |
| Sodium carbonate as Na₂CO₃, wt%, maximum | 0.05 | 0.20 |
| Sodium chloride as NaCl, maximum | 100 ppm by weight | 1.10 wt% |
| Sodium chlorate as NaClO₃, maximum | 50.0 ppm by weight | 0.30 wt% |
| Sodium sulfate as Na₂SO₄, maximum | 25 ppm by weight | 0.075 wt% |
| Iron as Fe, ppm by weight, maximum | 3.0 | 9.0 |
| Copper as Cu, ppm by weight, maximum | 0.30 | 0.30 |
| Nickel as Ni, ppm by weight, maximum | 0.50 | 3.00 |

Both sheets state AWWA B501-19 and FCC chemical-test claims, together with a limitation concerning full FDA cGMP requirements. These are manufacturer statements about the specified products, not independent certification of every use. The older SDS instead names B511-10; this edition/identifier discrepancy is unresolved and is not silently harmonized. [Membrane sheet, footnotes](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-soda-50-liquid-membrane.pdf); [diaphragm sheet, footnotes](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-soda-50-liquid-diaphragm.pdf); [SDS M32415, section 1](https://sds.oxychem.com/DirectDocumentDownloader/Document?prd=M32415~~PDF~~MTR~~ANSI~~EN)

## Historical pellet assay and trace impurities

Sigma-Aldrich's two-page certificate for product 30620, batch STBK6493, identifies puriss. p.a./ACS reagent/reag. Ph. Eur. pellets, nominally ≥98%. It records release on February 17, 2022 and recommended retest in February 2025. The assay specification is 98.0–100.5%, with a reported result of 99.6%; carbonate as Na₂CO₃ is separately reported <1.0%. Selected trace results are iron <5 ppm, mercury <0.05 ppm, chloride <10 ppm and potassium <0.02%. These are bounded reports, not zero concentrations. [Sigma-Aldrich certificate, both pages](https://www.sigmaaldrich.cn/certificates/Graphics/COfAInfo/fluka/pdf/PDF733412.pdf)

The certificate does not state an assay procedure or water result. **Synthesis:** its 100.5% upper assay specification is not a physically literal mass fraction above unity, and 100 − 99.6 = 0.4 percentage points cannot be assigned to water. The certificate limits its conformity statement to release or subsequent retest; it does not establish current stock composition, continued suitability of this batch, or another supplier's grade. [Certificate, assay table and release disclaimer](https://www.sigmaaldrich.cn/certificates/Graphics/COfAInfo/fluka/pdf/PDF733412.pdf)

## SDS composition and regional hazard scope

OxyChem SDS M32415 revision 12, June 21, 2024, is labeled North America, English, and covers multiple liquid grades. Section 3 lists water 48.5–94.5%, NaOH 5.5–51.5% and NaCl 0–15%. **Synthesis:** these broad ingredient ranges are not a simultaneous formula for one lot and cannot replace the narrower sales specifications above. [SDS, sections 1 and 3](https://sds.oxychem.com/DirectDocumentDownloader/Document?prd=M32415~~PDF~~MTR~~ANSI~~EN)

Section 2 lists metal corrosivity category 1, skin corrosion category 1A and serious eye damage category 1, with acute aquatic category 3 identified under hazards not otherwise classified. This is a dated regional document's classification, not a worldwide compliance determination or evidence of process performance. [SDS, section 2](https://sds.oxychem.com/DirectDocumentDownloader/Document?prd=M32415~~PDF~~MTR~~ANSI~~EN)

## Coverage limits

The reviewed evidence distinguishes solid pellets, one bibliographically documented hydrate and named aqueous grades. It does not establish an exhaustive hydrate/phase diagram, concentrated-solution activities, a pH prediction, universal thermal values, or current lot water and impurity content. Solution concentration does not supply an activity model; product assay does not identify a crystal phase. The reference supplies no preparation, use or operating conditions.

The manufacturer PDFs were read directly, including the complete sales tables and footnotes, both certificate pages and the relevant handbook/SDS sections. IUCr coverage is limited to its publisher issue record; the monohydrate paper's full text was inaccessible. Research reviewed October 30, 2026 PDT; publication remains disabled.

## Related reference

[Surface-finishing processes](../../processes/finishing/index.md)