Ca and Mg at pH 10 in presence of dye eriochrome black T are wine red. When completely complexed with EDTA, solution becomes blue. Mg must be present for satisfactory end-point and is added as MgEDTA. End point sharpness increases with pH, but high pH may cause precipitation of Ca(OH)2 or Mg(OH) and cause color changes of dye. pH of 10.0 – 0.1 is satisfactory compromise. Limit of 5 min for titration minimizes precipitation. Heavy metal interference is minimized by complexing with cyanide.

Method is applicable to drinking and surface waters and domestic and industrial wastes. To avoid large titration volumes, use aliquot containing <25 mg CaCO3.