Spectrophotometric determination of cobalt(II) and cobalt(III) after separation in ammonia solution by precipitating cobalt(II) with acetylacetone

Abstract

Cobalt(II) and cobalt(III) are determined spectrophotometrically in ammonia and in ammonia-ammonium hydrogen carbonate solutions in the presence of Ni(II), Fe(II) and Fe(III). Co(II) is separated by precipitation with acetylacetone (acac) as the [Co(acac)₂(NH₃)₂] complex. Co(III) remains in the solution. Co(II) is determined in the precipitate with ammonium thiocyanate or with concentrated hydrochloric acid. Co(III) is determined either from the total cobalt content or in the filtrate as an amminecobalt(III) complex. The Fe(III) interferences can be removed by extracting the acetylacetonatoiron(III) complex into chloroform. The Ni(II) interferences are compensated for by using reference cobalt-free solutions with the same Ni content as the sample in the spectrophotometric measurements. The relative standard deviation of the results for a Co concentration of 0.1–0.5 g l^–1 is ca. 3%.