A NEW INDICATOR USED IN SUCCESSIVE CHELATOMETRIC TITRATION OF COPPER AND ZINC

The reagent,2-(3,5-dichloro-2-pyridylazo)-5-dimethylaminobenzene (abbreviated as 3,5-diCl-PADMAB hereafter) was studied for use as a new indicator in chelatometry.In an acetate buffer of pH 5.0,Zn(Ⅱ) in the presence of Cu(Ⅱ) which was masked with thiourea and ascorbic acid previously,was titrated with EDTA standard solution until the 3,5-diCl-PADMAB indicator turned its color from reddish violet to bright yellow.Then the Cu(Ⅱ) was demasked from its thiourea complex,and continued the titration of free Cu(Ⅱ) ion with EDTA solution until the indicator changed its color from bluish violet to yellowish green.Both of the color changes at the end point of titration were very sharp and distinct.RSD′s of the results obtained in analyzing two known samples of brass were found to be <0.1% for Cu and <0.4% for Zn.