Potassium hydroxide
KOH identity, solid and aqueous forms, documented hydrates, analytical bases, and limits of product and hazard data.
Tags: chemicals, hydroxides, potassium
On this page
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 Membrane liquid Potassium hydroxide is KOH, CAS 1310-58-3, 56.11 g/mol. Fisher potassium hydroxide pellets
OxyChem lists aqueous, flake and crystal forms. Water and carbonate require separate data. OxyChem chlor-alkali product overview
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
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
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
| 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, 45% membrane specification, and Merck 105032 specification. 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, OxyChem membrane liquid, and Merck 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
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
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, ICSC 0357
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, ICSC
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, P250 product specification
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
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 provides broader process context. Its process scope does not establish KOH formulation performance.