4-(3,5-dichloro-2-pyridylazo)-1,3-diaminobenzene as a metallochromic indicator for the complexometric determination of Cu(II) with EDTA

In slightly acid solutions Cu(II) and 4-(3,5-dichloro-2-pyridylazo)-1,3-diaminobenzene (3,5-C12PADAB) form a red-purple complex( = 5.0 x 10(4)1.mole(-). cm(-1) at 547 nm). The complex is readily broken by EDTA, with a sharp change of colour to the yellow of the free reagent (Deltalambda(max) = 110 nm at pH 3.5). This colour contrast and the reversibility of the formation reaction, together with the solubility of the complex in water, make the pyridylazo reagent useful as a metallochromic indicator for copper titration, at pH 2.7-5.5. The Cu(II)-3,5-C12PADAB indicator system has been satisfactorily applied to the determination of sulphur in cast iron, "useful oxides" (Al(2)O(3) + Fe(2)O(3)) in samples of commercial aluminium sulphate, and total phosphorus in a synthetic mixed fertilizer.