Determination of ionized magnesium with ion-selective electrodes

Abstract

The role of magnesium is acknowledged in human physiology even though it is incompletely understood,a nd measuremenot f ionized magnesium by ion-selective electrode analysers is becoming commoner as recent research has alerted clinicians to its importance. Standardisation of ionized magnesium in blood samples in the form of a reference method is required. A prototype reference cell, which was developed for the reference method for ionized calcium, has been tested and found to perform well for magnesium. Selectivity coefficients, and the effects of pH and of proteins on various magnesium selective membranes have been determined. None of these membranes showed sufficient selectivity for magnesium over calcium and sodium for use with serum samples without simultaneous measurement of calcium. Knowledgeo f magnesiums peciationis requiredf or a full understanding of its role in physiology. Stability constants for magnesium and calcium with various ligands have been determined by using a new method in which a pH electrode and a Mg (or, Ca) electrode have been employed simultaneously in alkalimetric titrations. The' results were analysed using the program SUPERQUAD. In general, agreement was very good between values obtained from titrations with Mg (or Ca) electrodes and from pH. The protonation constants of the ligands were also determined and agree well with literaturev alues.R esultsf or Mg-citrate,l actate,g lycinate,a spartatea nd glutamatec omplexationc onstantsc omparew ell with recentlyp ublishedd ata. New systems investigated were Mg-pyroglutamate (Mg-5-oxo-2- pyrrolidinecarboxylic acid) and Mg-pyridoxine (Mg-3-hydroxy-4,5- bis(hydroxymethyl)-2-methylpyridine). Also HEPES (N-(2-hydroxyl) piperazine-N-ethanesulfoniacc id), used in calibration standardsa s a pH buffer, was found not to complex magnesium at physiological pH.