COLORIMETRIC DETERMINATION OF SELENIUM IN MINERAL PREMIXES

A method is described for determination of sodium selenite or sodium s elenate in mineral-based premixes, It is based on the formation of int ense-yellow piazselenol by Se(IV) and 3,3'-diaminobenzidine, Mineral p remixes typically contain calcium carbonate as a base material and mag nesium carbonate, silicon dioxide, and iron(III) oxide as minor compon ents or additives, In this method, the premix is digested briefly in n itric acid, diluted with water, and filtered to remove any iron(III) o xide, Ethylenediaminetetraacetic acid and HCl ape added to the filtrat e, which is heated to near boiling for 1 h to convert any selenate to selenite. After heating, the solution is buffered between pH 2 and 3 w ith NaOH and formic acid and treated with NH2OH and EDTA; any Se prese nt forms a complex with 3,3'-diaminobenzidine at 60 degrees C. The sol ution is made basic with NH4OH, and the piazselenol is extracted into toluene, The absorbance of the complex in dried toluene is measured at 420 nm, The method was validated independently by 2 laboratories, Sam ples analyzed included calcium carbonate fortified with 100, 200, and 300 mu g Se in the form of sodium selenite or sodium selenate, a calci um carbonate premix containing sodium selenite, a calcium carbonate pr emix containing sodium selenate, and a commercial premix; 5 replicates of each sample type were analyzed by each laboratory, Average recover ies ranged from 89 to 109% with coefficients of variation from 1.2 to 13.6%.