Magnesium complexometric analysis

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

Ethylenediamine tetraacetic acid (EDTA) was introduced originally as a water-softener and as a textile dyeing assistant because of its ability to form very stable, water soluble complexes with many metal ions, including calcium and magnesium. The equilibria involved in chelation of metal ions by EDTA and related ligands have been exhaustively studied, notably by G. Schwarzenbach and his colleagues, and provide the basis for complexometric methods of chemical analysis. EDTA and its metal complexes have also become probably the most familiar examples of agents used in chelation therapy. 

The method described here is based on the difference between measurements of total alkaline earths by complexometric titration with EDTA (ethylenediamine-N,N,N, N -tetra-acetic acid) and selective measurement of calcium described in Section 11.2.1. The simultaneous EDTA titration of calcium, strontium and magnesium involves Eriochrome Black T (EBT) as indicator and was originally applied to seawater analysis by Voipio (1959) and Pate and Robinson (1961). To eliminate subjective errors in the determination of the endpoint, Culkin and Cox (1966) used photometric endpoint detection. A slight modification of this procedure, including the standardization of EDTA by magnesium is reported here. 

For most work in pharmaceutical analysis classical methods have now been superseded by complexometric titration. Determination of calcium by itself presents little difficulty and two basic procedures are applicable, (i) titration at pH 10 in ammonia buffer using solochrome black as indicator and (ii) titration at pH 12 to 13, either in diethylamine using alizarin black (diadem chrome black) or in potassium hydroxide solution using Patton and Reeder s indicator. Provided a small quantity of complexed magnesium is included for titration (i) sharp and satisfactory end-points are obtained by these methods and there is no significant difference in the results obtained at the two pH values if a pure calcium compound is titrated. Difference in the two results would be expected if the calcium salts were contaminated with magnesium, when the titration at pH 10 would include both ions and that at the higher value the calcium only. 

One of the most significant advances in chemical analysis as applied to pharmaceuticals during the last decade is undoubtedly the introduction and development of the complexometric titration. The ability of certain amino-polycarboxylic acids to react stoichiometrically and instantaneously with certain metal ions was first recognised and described by Schwarzenbach in 1945. Later, the same author, together with co-workers, described the first metal indicator, murexide, and then, perhaps the most important of all, Eriochrome Black T (usually referred to in this country as solo-chrome black). This was followed quite shortly by the first description of the now classic use of the complexometric titration for the determination of temporary and permanent hardness in water. It was some time, however, before metal indicators capable of functioning at an acid pH were developed with the availability of such indicators a rapid increase in the application of complexometric titrations took place and there are now few metal ions that are not capable of determination by this means. In the present book reference will be found to the use of complexometric methods for determination of aluminium, bismuth, calcium, copper, iron, lead, magnesium, manganese, mercury and zinc. In addition, indirect methods are described for the determination of certain anions such as fluoride, phosphate and sulphate. 

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