CHELOMETRIC MICRO-DETERMINATION OF RARE EARTHS

In this communication, the use of Xylenol Orange, Methylthymol Blue, 4-(2'-thiazoly- lazo)-resorcinol (TAR), Arsenazo I, Stilbazo, and Alizarin Red S-Methylene Blue as indica- tors for the direct chelometric micro-determination of rare earths are compared. The results are summarized as follows: Indicator Optimum acidity Titratable range, in pH in RE_xO_y Relative error, % E. P. colour change Among the metal indicators studied, Xylenol Orange and TAR, are preferred for the direct chelometric micro-determination of rare earths. Because they can be used over a wider titratable range (5--400 μg RE_xO_y). However the former (X. O.) is found to be better than the latter (TAR), because it can be used over a wider pH range (4.3--6.2) and the colour change is sharper. The interferences of diverse ions and their elimination for the direct chelometric micro-determination of rare earths with Xylenol Orange and TAR as indicators is also reported. Besides, six more indicators, namely Eriochrome Black T, Eriochrome Blue Black R (C. J. 202), Bromopyrogallol red, Chromeazurol S (C. I. 723), 2-hydroxyl-4-sulfonaphthalene- azo-resorcinol, and 4-(2-pyridylazo) resorcinol have been tested and found to be inferior to the above mentioned ones for this very purpose.