ABSTRACTThe sharpness of the end point during the titration for magnesium or calcium using the EDTA titrimetric method is improved by using the free acid form of CI Mordant Black 11 buffered to pH 10 with ammonium hydroxide rather than its sodium salt. Apparently, the ammonium salt of the dye sorbs more strongly in the blue region than does the corresponding sodium salt increasing the end point resolution; hence use of the free acid rather than the commercially available sodium salt is suggested as the end point indicator. Methods for converting the commercial dye to the free acid are given.