Beryllon Ⅱ [8-oxynaphthalene-3,6-disulfonic acid-l-azo-2'(-1',8'-dioxynaph-thalene-3',6'-disulfonic acid)tetra sodium salt] is recommended as a substitute for Eriochrome black T in the complexometric determination of the sum of calcium and magnesium,such as in the case of limestone,dolomite etc.At pH=10.0,the end point is easily detected as the color changes from blue-violet to violet-red.Distinct from Eriochrome black T,titanium,iron and aluminum do not form any colored complex at the recommended pH.Therefore,calcium and magnesium can be titrated directly with EDTA in presence of these metals by using potassium-sodium tartrate as masking agent.