Medical brain notes 14

  INTRODUCTION A host of inorganic pharmaceutical substances essentially containing polyvalent and bivalent metal ions, for instance : Al 3+ , Ca 2+ , Mg 2+ , Bi 2+  and Zn 2+  were initially analysed quantitatively by the aid of  gravimetric   methods . These historically important procedures have become more or less obsolete since they are extremely   time consuming and tedious owing to several steps involved  e.g. , precipitation, filtration, washing, drying and finally ignition to a constant weight.   Thereafter, analysis was done by faster techniques based on oxalate-permanganate titrations which involved a precipitation procedure followed by quick titration of pre-heated solutions containing oxalate ions.   More recently the introduction of an analytical reagent disodium ethylene-diaminetetraacetate, invariably termed as  EDTA , an altogether latest titrimetric method has been used exclusively for the estimation of metals using...

  THEORY Complex  is a compound that is formed by the combination of a metal ion with a molecule that is   capable of donating electrons, for example : In the above two examples both Cu 2+  and Co 2+  form complexes with lone pair of electrons present in the neutral molecule ammonia  e.g. , N’’H 3 .   Chelate  is a complex that is formed by the combination of a polyvalent metal ion with a molecule   which essentially contains two or more groups that can donate electrons.   Specifically, disodium ethylenediaminetetraacetate (EDTA) reacts with  polyvalent metal ions  to result in the formation of a fairly stable  water-soluble complex,  or a  chelate compound .   It is, however, pertinent to mention here that the predominant state of the dissociated forms of EDTA ( viz  . Y 4– , HY 3–  , H 2 Y 2–  and H 3 Y – ) is solely dependent upon the pH of the medium at which complexation takes place : ...

  ASSAY METHODS The complexometric titrations involving various inorganic pharmaceutical substances may be catego-rized into  three  broad heads, namely : ( i ) Direct titration methods,   ( ii ) Masking and demasking agents, and   ( iii ) Residual titration methods.   1. DIRECT TITRATION METHODS   In direct titration, usually an appropriate buffer solution and a suitable indicator are added to the M 2+  (metal-ion) solution and subsequently the resulting solution is titrated with previously standardized disodium-EDTA until the indicator just changes colour. Sometimes, a simultaneous blank determination is also recommended to have a check for the presence of traces of metallic impurities in the reagents.   1.1. Preparation of 0.05 M Disodium Ethylenediamine Tetracetate Solution (Disodium Edetate 0.05 M)   Materials Required :  Disodium ethylenediaminetetracetate : 18.6 g.   Procedure :  Weigh accurately 18.6 g of disodium ...