A critical study has been made on the masking of manganese and iron by their precipitation as Fe,[Fe(CN),] and Mn,[Fe(CN),J with K4[Fe(CN),] in the presence of ascorbic acid and potassium chloride for the complexometric titration of calcium plus magnesium with EDTA a t p H 10. The optimum acid concentration and temperature for the selective and quantitative precipitation of Mn (I I) and Fe( I I) liexacyanoferrates (I I), which are granular and nonadsorbant metallochromic indicators, are found to be 1-2.5 N hydrochloric acid and 25-50 O C , respectively. I n a solution with an acidity of lower than 1 N, the hesacyanoferrates(I1) of Mn(I1) and Fe(I1) coprecipitate calcium and magnesium hexacyanoferrates(I1). At 70 O C , the amount of coprecipitation increases. Up to 08.75 and 19.60 mg of manganese and iron, respectively, can be successfully inaslced in coiiiplesonietric titration. For the titration of calcium alone at pH 12, the interference of manganese and iron has been overcome by osidising Mn(I1) -TEA to Mn(II1) -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 K,[Fe(CS),], a non-toxic reagent. For calcium, TEA alone is used for masking both manganese and iron.

A critical study has been made on the masking of titanium and de-masking of the Ti0 -EDTA (ethylenediaminetetraacetic acid) complex with tartaric acid. It reveals that tartaric acid de-masks Ti0 -EDTA selectively and quantitatively with the release of EDTA at pH 5.3 from a solution at 60-70 "C containing Ti0 -EDTA, Al -EDTA and Fe -EDTA complexes. The release of EDTA from Al -EDTA is, however, effected by the usual practice of de-masking with sodium fluoride or ammonium fluoride. Based on these observations, procedures are given for the direct complexometric determination of aluminium, titanium and iron present in the same analyte by stepwise indirect titration with EDTA, which does not involve prior separation of the metals. The method is, therefore, very simple, rapid and accurate and has been applied successfully to determine aluminium and moderate to low amounts of titanium and iron present in common aluminosilicate materials. Results for the determinations compare favourably with both the certified values and values obtained by other standard methods.

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