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The procedure involves extraction of the copper by refluxing the milk powder with a solution of potassium thiocyanate in 95% acetone and determining the copper content of the extract by colorimetry with dihydroxyethyldithiocarbamic acid, after removing carotenoid pigments. The results agree with those obtained by the wet-digestion method but are higher than those yielded by dry-ashing. The advantages of the extraction method are (a) simplicity, (b) low copper content of the "blank", which, with proper precautions, may be negligible, and (c) a large enough sample can be used to permit visual colour comparison with standard solutions of Alizarin Yellow G.
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Boulet et al. (1945) studied this question.
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