Chelatometric titration of calcium and magnesium in the presence of a large amount of magnesium

A chelatometric titration method was recommended to the determination of calcium and magnesium in the coexistence of a large amount of the latter. The outline of the procedure is as follows: Three aliquots of the sample solution are taken. A preliminary titration of calcium in the first aliquot is made with ethylene glycol bis (2-aminoethyl ether)-N, N, N', N'-tetraacetic acid (EGTA) at about pH 13 using either 2-hydroxy-1-(2'-hydroxy-4'-sulfo-1'-naphthylazo)-3naphthoic acid (NN) or Calcichrome as an indicator. The volume of EGTA consumed in the above titration and further 0.5 ml are added to the second aliquot of the solution and then a precise direct determination of magnesium is carried out by means of the titration with ethylenediaminetetraacetic acid (EDTA) at pH 9.510.0 using Eriochrome Black T (EBT) as an indicator. In the third aliquot the sum of calcium and magnesium is titrated with EDTA at pH 9.510.0 using EBT as an indicator. The accurate value of calcium is calculated from the difference between the second and the third titration.According to this method calcium and magnesium in the samples containing a large amount of magnesium such as magnesia clinker, dolomite, dolomitic limestone, etc. are determined with satisfactory precision.