Calcium reference change values

Solochrome dark blue or calcon ( C.1.15705). This is sometimes referred to as eriochrome blue black RC it is in fact sodium l-(2-hydroxy-l-naphthylazo)-2-naphthol-4-sulphonate. The dyestuff has two ionisable phenolic hydrogen atoms the protons ionise stepwise with pK values of 7.4 and 13.5 respectively. An important application of the indicator is in the complexometric titration of calcium in the presence of magnesium this must be carried out at a pH of about 12.3 (obtained, for example, with a diethylamine buffer 5 mL for every 100 mL of solution) in order to avoid the interference of magnesium. Under these conditions magnesium is precipitated quantitatively as the hydroxide. The colour change is from pink to pure blue. 

Karpov et al. (1980) demonstrated the isomorphism 3Ca 3Na + Ce after refinement of the multiplicities, which permitted the introduction of calcium atoms into the Na2 site of model 1 and in the Nal site of model 2 . This isomorphous substitution requires a change of the number of atoms from three to four, and vice versa. The substitution 2Ca Na-I-Ce, which is usually observed in rare earth minerals, does not require any change of the number of atoms. However, Karpov et al. (1980) did not refer to the latter type of substitution, and any concrete values of the result of the refinement of multiplicities are lacking in their paper. Therefore, it is impossible to decide which kind of substitution occurs in the structure. 

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