Sodium hexanitritocobaltate(III)

Sodium hexanitritocobaltate(III) is inorganic compound with the formula Na3[Co(NO2)6]. The anion of this yellow-coloured salt consists of the transition metal nitrite complex [Co(NO2)6](3-). It was a reagent for the qualitative test for potassium and ammonium ions.

Synthesis and reactions
The compound is prepared by oxidation of cobalt(II) salts in the presence of sodium nitrite:
 * 4 [Co(H2O)6](NO3)2 + O2 + 24 NaNO2 → 4 Na3[Co(NO2)6] + 8 NaNO3 + 4 NaOH + 22 H2O

Application for analysis of potassium
Although the sodium cobaltinitrite is soluble in water, it forms the basis of a quantitative determination of potassium, thallium, and ammonium ions. Under the recommended reaction conditions the insoluble double salt, K2Na[Co(NO2)6]*H2O is precipitated and weighed. In geochemical analysis, sodium cobaltinitrite is used to distinguish alkali feldspars from plagioclase feldspars in thin section.