selenous acid selenous acid selenous acid (or selenious acid) is the chemical compound with the formula h2seo3. structurally, it is more accurately described by o=se(oh)2. it is the principal oxoacid of selenium; the other being selenic acid. selenous acid names iupac names selenous acidselenic(iv) acid other names selenious acid identifiers cas number 7783-00-8 y 3d model (jsmol) interactive image chebi chebi:26642 y chembl chembl2009089 chemspider 1060 y drugbank db11127 echa infocard 100.029.067 ec number 231-974-7 kegg d05814 y pubchem cid 1091 rtecs number vs7175000 unii f6a27p4q4r y un number 3283 2630 comptox dashboard (epa) dtxsid9024300 inchi inchi=1s/h2o3se/c1-4(2)3/h(h2,1,2,3) ykey: mcahwihfghiesp-uhfffaoysa-n yinchi=1/h2o3se/c1-4(2)3/h(h2,1,2,3)key: mcahwihfghiesp-uhfffaoyaw smiles o()o properties chemical formula h2seo3 molar mass 128.984 g·mol−1 appearance white hygroscopic crystals density 3.0 g/cm3 melting point decomposes at 70 °c solubility in water very soluble solubility soluble in ethanol acidity (pka) pka1 = 2.46pka2 = 7.3 conjugate base hydrogenselenite magnetic susceptibility (χ) −45.4·10−6 cm3/mol pharmacology routes ofadministration intravenous infusion legal status us: ℞-only hazards ghs labelling: pictograms signal word danger hazard statements h301, h331, h373, h410 precautionary statements p260, p261, p264, p270, p271, p273, p301+p310, p304+p340, p311, p314, p321, p330, p391, p403+p233, p405, p501 related compounds other anions selenic acidhydrogen selenide other cations sodium selenite related compounds sulfurous acidtellurous acidpolonous acid except where otherwise noted, data are given for materials in their standard state (at 25 °c , 100 kpa). y verify (what is yn ?) infobox references selenious acidclinical dataahfs/drugs.commonographlicense data us dailymed: selenious_acid identifiersdrugbankdb11127comptox dashboard (epa)dtxsid9024300 echa infocard100.029.067 formation and properties selenous acid is analogous to sulfurous acid, but it is more readily isolated. selenous acid is easily formed upon the addition of selenium dioxide to water. as a crystalline solid, the compound can be seen as pyramidal molecules that are interconnected with hydrogen bonds. in solution it is a diprotic acid: h2seo3 ⇌ h+ + hseo−3 (pka = 2.62) hseo−3 ⇌ h+ + seo2−3 (pka = 8.32) it is moderately oxidizing in nature, but kinetically slow. in 1 m h+: h2seo3 + 4 h+ + 4 e− ⇌ se + 3 h2o (eo = +0.74 v) in 1 m oh−: seo2−3 + 4 e− + 3 h2o ⇌ se + 6 oh− (eo = −0.37 v) selenous acid is hygroscopic. uses the major use is in protecting and changing the color of steel, especially steel parts on firearms. the so-called cold-bluing process uses selenous acid, copper(ii) nitrate, and nitric acid to change the color of the steel from silver-grey to blue-grey or black. alternative procedures use copper sulfate and phosphoric acid instead. this process deposits a coating of copper selenide and is fundamentally different from other bluing processes which generate black iron oxide. some older razor blades were also made of blued steel. another use for selenious acid is the chemical darkening and patination of copper, brass and bronze, producing a rich dark brown color that can be further enhanced with mechanical abrasion. it is used in organic synthesis as an oxidizing agent for the synthesis of 1,2-dicarbonyl compounds, e.g. in laboratory preparation of glyoxal (ethane-1,2-dione) from acetaldehyde. selenious acid is a key component of the mecke reagent used for drug checking. medical selenous acid can supply the trace element indicated in people as a source of selenium. health effects like many selenium compounds, selenous acid is highly toxic in excessive quantities, and ingestion of any significant quantity of selenous acid is usually fatal, however it is an approved dietary source in proper amounts. symptoms of selenium poisoning can occur several hours after exposure, and may include stupor, nausea, severe hypotension and death.