Chromogenic Behaviour of Eriochrome Cyanine R.C. in the Spectrophotometric Determination of Scandium, Yttrium and Lanthanum

Sulphohydroxydimethyl fuchsone dicarboxylic acid (Eriochrome Cyanine R.C.) has been described as a reagent for the spectrophotometric determination of scandium, yttrium and lanthanum. The red coloured chelates formed have a composition 1 : 2 (metal : reagent) in all the cases. The colour of the dye changes with change in \(p\)H. The \(\lambda\)_\(max\) of scandium chelate is 535 mµ (at \(p\)H 5.0), of yttrium chelate is 530 mµ (at \(p\)H 6.0) and of lanthanum chelate 480 mµ (at \(p\)H 6.2). However the wavelength of study for lanthanum was selected at 500 mµ. A four fold excess of the reagent is required for full colour development. Other characteristics, i.e., range for adherence to Beer's law, range for effective photometric determination, \(p\)H range of stability and molecular extinction coefficient have also been determined.