Potassium arsenate

Chemical forms and properties

These compounds differ by the number of acidic hydrogens on the arsenate moiety. In aqueous solution the species distribution shifts with pH, because arsenic acid has multiple dissociation steps with pKa values roughly near 2.2, 7.0 and 11.5. As a result, H2AsO4− and HAsO42− are commonly encountered at environmental pH values. The salts are generally crystalline, water-soluble to varying extent, and share the tetrahedral AsO4 geometry seen in related oxyanions.

Preparation, uses and examples

Potassium arsenates can be prepared by neutralizing arsenic acid with potassium hydroxide or by metathesis reactions between soluble potassium salts and arsenate sources. Historically they were used in pesticides, wood preservatives, glass and ceramic manufacture, and in some chemical analyses. Many of these applications have declined because safer alternatives became available and because of strict regulation. For historical context see industrial uses.

Safety, toxicity and environmental impact

Arsenate compounds are acutely toxic and chronic exposure is associated with serious health effects; arsenic compounds are widely recognized as hazardous and many are classified as carcinogenic. Disposal and handling require strict controls: use personal protective equipment, avoid inhalation and ingestion, and treat waste as hazardous chemical waste. Environmental mobility of arsenate in soils and water is influenced by pH, redox conditions and adsorption to mineral surfaces. More safety guidance is available at safety resources.

Distinctions and notable facts

• Arsenate (AsO43−) is chemically similar to phosphate (PO43−), so it can interfere with biochemical processes that normally use phosphate.
• Arsenite (AsO33−) differs in oxidation state and is generally more toxic and more mobile under reducing conditions.
• Regulation and monitoring of arsenic compounds in water, food and industrial emissions are common; consult local authorities for limits and remediation approaches: regulatory guidance.

Because of toxicity and environmental persistence, potassium arsenate is now largely of academic and historical interest rather than a common consumer chemical. Where it is encountered, strict laboratory and industrial controls are required to protect health and the environment.