A critical study has been made on the masking of manganese and iron by their precipitation as Fe₂[Fe(CN)₆] and Mn₂[Fe(CN)₆] with K₄[Fe(CN)₆] in the presence of ascorbic acid and potassium chloride for the complexometric titration of calcium plus magnesium with EDTA at pH 10. The optimum acid concentration and temperature for the selective and quantitative precipitation of Mn(II) and Fe(II) hexacyanoferrates(II), which are granular and non-adsorbant metallochromic indicators, are found to be 1–2.5 N hydrochloric acid and 25–50 °C, respectively. In a solution with an acidity of lower than 1 N, the hexacyanoferrates(II) of Mn(II) and Fe(II) coprecipitate calcium and magnesium hexacyanoferrates(II). At 70 °C, the amount of coprecipitation increases. Up to 68.75 and 19.60 mg of manganese and iron, respectively, can be successfully masked in complexometric titration. For the titration of calcium alone at pH 12, the interference of manganese and iron has been overcome by oxidising Mn(II)- TEA to Mn(III)- TEA, which is stable against EDTA titration with air bubbling. Iron forms a quantitative Fe - TEA complex almost instantaneously at pH 12. A simple procedure has been devised for the titration of Ca + Mg with EDTA after masking manganese and iron with K₄[Fe(CN)₆], a non-toxic reagent. For calcium, TEA alone is used for masking both manganese and iron.