# Potassium hydroxide

KOH identity, solid and aqueous forms, documented hydrates, analytical bases, and limits of product and hazard data.

Reviewed: 2026-10-30

Tags: chemicals, hydroxides, potassium

## Identity and forms

OxyChem Caustic Potash Crystal specifies **minimum 90 wt% hydroxide alkalinity as KOH**. Its **45% membrane liquid** specifies **45.00–46.00 wt% total alkalinity as KOH**. The alkalinity basis differs between the two specifications. [Crystal](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-potash-crystal.pdf) [Membrane liquid](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-potash-45-membrane.pdf) Potassium hydroxide is KOH, CAS 1310-58-3, 56.11 g/mol. [Fisher potassium hydroxide pellets](https://www.fishersci.com/shop/products/potassium-hydroxide-pellets-certified-acs-fisher-chemical-5/P2503)

OxyChem lists aqueous, flake and crystal forms. Water and carbonate require separate data. [OxyChem chlor-alkali product overview](https://www.oxychem.com/chlor-alkali/)

The ILO/WHO card describes a white hygroscopic solid and identifies its aqueous solution as a strong base. Hygroscopicity concerns water uptake. It does not, by itself, identify a crystalline hydrate or establish the water content of a particular sample. [ICSC 0357, May 2010](https://chemicalsafety.ilo.org/dyn/icsc/showcard.display?p_card_id=0357&p_lang=en&p_version=2)

## Hydrates and water

Rütter and Mootz reported crystal structures for the dihydrate KOH·2H₂O and tetrahydrate KOH·4H₂O in 1991. Their public abstract states that X-ray diffraction data were collected at −150 °C. The dihydrate result concerns a tetragonal substructure within a superstructure. The tetrahydrate is monoclinic. These are documented crystalline compositions, not labels for arbitrary aqueous concentrations. The accessible evidence here is the publisher's abstract and references, not the complete experimental paper. [Rütter and Mootz, 1991](https://onlinelibrary.wiley.com/doi/10.1002/zaac.19916010109)

Synthesis: a formula with a specified number of waters describes a stoichiometric hydrate. A commercial assay percentage alone cannot establish that hydrate's presence. Neither the word "crystal" on a product sheet nor a deficit below 100% assay supplies a phase analysis. This reference does not establish a complete hydrate inventory, a phase diagram, or equilibrium phase boundaries.

## What an analysis measures

OxyChem's handbook separates total alkalinity, hydroxide content, and carbonate analysis. Total alkalinity uses an acidimetric endpoint with modified methyl orange. Its hydroxide determination uses a separate carbonate-separation treatment and phenolphthalein endpoint. Results are expressed as KOH on a sample-mass basis. The handbook also describes carbonate gravimetry and notes its usual use of an inorganic-carbon analyzer. It identifies atmospheric carbon dioxide and moisture uptake as analytical concerns. These are method distinctions, not interchangeable names for one purity measurement. [OxyChem handbook, pages 41–47](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-potash-handbook.pdf)

| Term | Meaning and limitation |
| --- | --- |
| Hydroxide alkalinity or free hydroxide | An analytical result attributed to hydroxide under a stated method. It is not a direct measurement of hydroxide activity. |
| Total alkalinity, calculated as KOH | Acid-neutralizing capacity expressed on a KOH-equivalent basis. It is not an independent count of KOH formula units. |
| Carbonate, as K₂CO₃ | A separately reported constituent on a potassium-carbonate mass basis. A number reported as CO₃ has a different chemical mass basis. |
| Water | A separate composition quantity. Assay shortfall also permits carbonate and other constituents, so subtraction from 100% does not establish water. |
| Trace impurities | Elemental or compound-specific results. "As Fe," "as KCl," and "as K₂CO₃" identify different reported masses. |

The table is explanatory synthesis of the separate analytical fields in the [OxyChem crystal specification](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-potash-crystal.pdf), [45% membrane specification](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-potash-45-membrane.pdf), and [Merck 105032 specification](https://www.merckmillipore.com/CH/en/product/Potassium-hydroxide,MDA_CHEM-105032). It supplies no analytical procedure or solution activity model.

## Product specifications and historical results

The following examples retain each supplier's chemical basis. They describe the named products, not universal potassium hydroxide requirements.

| Source and product | Assay or alkalinity field | Selected separately specified constituents |
| --- | --- | --- |
| OxyChem Caustic Potash Crystal, CP 44713, 01/26 | Hydroxide alkalinity as KOH, minimum 90 wt.% | K₂CO₃ maximum 0.44 wt.%; KCl maximum 150 ppm by mass; Fe maximum 3 ppm by mass; Ni maximum 4 ppm by mass |
| OxyChem 45% Caustic Potash Membrane, EC-CP-44615, 01/26 | Total alkalinity as KOH, 45.00–46.00 wt.% | K₂CO₃ maximum 0.10 wt.%; KCl maximum 50 ppm by mass; Na maximum 2500 ppm by mass; Ni maximum 0.25 ppm by mass |
| Merck 105032 pellets, undated live product specification | Acidimetric KOH assay at least 85.0%; total alkalinity calculated as KOH, 85.0–100.5% | Carbonate as K₂CO₃ at most 2.0%; chloride as Cl at most 0.008% |

Sources for the respective rows: [OxyChem crystal](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-potash-crystal.pdf), [OxyChem membrane liquid](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-potash-45-membrane.pdf), and [Merck 105032](https://www.merckmillipore.com/CH/en/product/Potassium-hydroxide,MDA_CHEM-105032). Merck's total-alkalinity specification extends above 100%; the page does not explain that upper bound. It is not presented here as a literal mass fraction above unity.

A historical RCI Labscan certificate gives a different kind of evidence. Product AR1385, batch 21 04 0204, manufactured April 21, 2021, records an acidimetric assay of 86.2%. It reports K₂CO₃ below 2.0%, rather than an exact carbonate concentration. The printed best-before month is April 2026. These results describe that historical batch at certification, not a current shipment or the batch's present condition. The certificate does not report water. [RCI Labscan batch certificate, complete one-page table](https://rcilabscan.com/wp-content/uploads/2021/04/21040204.pdf)

Synthesis: a specification is a stated product boundary; a certificate result is evidence about an identified batch. A result preceded by `<` is bounded, not zero. Neither an assay minimum nor an SDS composition entry replaces a batch result.

## Concentration and physical-property bases

Mass percent means constituent mass per 100 units of mixture mass. Mass concentration, such as g/L, uses solution volume. Molar concentration uses moles per solution volume; molality uses moles per solvent mass. These definitions make the denominator consequential even when two labels both say "45%."

OxyChem's January 2026 handbook, Table 1, gives 655.65 g KOH/L and specific gravity 1.457 for the 45% row at 15.6 °C. Table 13 lists OxyChem's typical dry-product bulk densities of 60 lb/ft³ for flake and 75 lb/ft³ for crystal. These are not measured lot-specific or universal values. These bulk densities include the packed-particle volume and are not solution or single-crystal densities. The tables do not specify a measurement method or pressure. [OxyChem handbook, January 2026, pages 28 and 32](https://www.oxychem.com/siteassets/documents/chlor-alkali/caustic-potash-handbook.pdf)

Merck lists 2.04 g/cm³ at 20 °C for its solid product. Its solubility entry is 1130 g/L, without temperature or an explicit solvent-versus-solution volume denominator. ICSC instead reports water solubility as 110 g/100 mL at 25 °C. Neither entry explicitly resolves whether volume refers to water or final solution. These gaps prevent a numerical reconciliation or saturation calculation. [Merck physical properties](https://www.merckmillipore.com/CH/en/product/Potassium-hydroxide,MDA_CHEM-105032), [ICSC 0357](https://chemicalsafety.ilo.org/dyn/icsc/showcard.display?p_card_id=0357&p_lang=en&p_version=2)

Melting values also differ. Merck lists 360 °C, whereas ICSC lists 380 °C. Neither entry identifies a method, pressure, or phase-purity analysis. This evidence does not resolve the difference or justify averaging the values. [Merck](https://www.merckmillipore.com/CH/en/product/Potassium-hydroxide,MDA_CHEM-105032), [ICSC](https://chemicalsafety.ilo.org/dyn/icsc/showcard.display?p_card_id=0357&p_lang=en&p_version=2)

## SDS scope and evidence limits

Fisher's US SDS, revision 10 dated December 18, 2025, covers a list of catalog products including P250. Section 3 reports 100.0 weight percent potassium hydroxide, while the P250 product page specifies assay at least 85.0%. The documents therefore do not provide identical composition statements. The SDS disclaimer explicitly distinguishes its information from a quality specification. The difference is unresolved here; 100.0% is not treated as a measured lot assay. [Fisher US SDS, sections 1, 3, and 16](https://www.fishersci.com/store/msds?countryCode=US&language=en&partNumber=P2503&productDescription=POT+HYDROXIDE+CERT+ACS+3KG&vendorId=VN00033897), [P250 product specification](https://www.fishersci.com/shop/products/potassium-hydroxide-pellets-certified-acs-fisher-chemical-5/P2503)

That US SDS identifies metal corrosivity, acute oral toxicity, skin corrosion, serious eye damage, and respiratory irritation hazards. These dated product classifications do not establish global or concentration-independent compliance. [Fisher US SDS, sections 1–2](https://www.fishersci.com/store/msds?countryCode=US&language=en&partNumber=P2503&productDescription=POT+HYDROXIDE+CERT+ACS+3KG&vendorId=VN00033897)

The January 2026 OxyChem specifications and the handbook available through the legacy 2018-edition URL are separate documents. The current crystal PDF is dated January 2026; some search extraction still returns June 2023 text. Merck's live page has no visible specification revision date. Current-lot composition, complete hydrate equilibria, and a validated electrolyte activity model remain outside this evidence set. No process suitability or customer capability follows from these reference properties.

## Related reference

[Surface-finishing processes](../../processes/finishing/index.md) provides broader process context. Its process scope does not establish KOH formulation performance.