Medical brain notes 14

  INTRODUCTION Gravimetric analysis  is an unique technique by means of which either an element or a compound is   obtained in its purest form through isolation and subsequent weighing. In order to achieve this, the element or compound is first and foremost separated from a specific portion of the pharmaceutical substance being determined and consequently the weight of the constituent in the given sample is calculated on the basis of the weight of the product.   However, in actual gravimetric analysis, the final weight of the product is usually accomplished by adopting anyone of the following standard methods, namely :   ( a ) Solvent extraction,   ( b ) Ignition or volatalization, and   (c) Precipitation from solution.   Gravimetric techniques are broadly based upon the quantitative precipitation of the respective cation or anion from a given solution in  two  different ways :   ( i ) as an insoluble compound that yields a residue ...

  THEORY The underlying principles and theories of gravimetric analysis are as stated below :   (i)            Law of mass action and reversible reactions,   ( ii ) Principle of solubility product, and   ( iii ) Common ion effect.   All the above three aspects shall be described briefly  vis-a-vis  their direct impact on the gravimetric analysis.     1. LAW OF MASS ACTION AND REVERSIBLE REACTIONS   A plethora of chemical reactions that are intimately associated with the quantitative analysis essen-tially belong to the class of reversible reactions. These reactions under certain prevailing experimental param-eters are made to proceed to completion, whereas in certain other conditions they may even attain equilibrium before completion. In the latter instance, erroneous results may creep in with regard to the pharmaceutical substance under estimation. Hence, it has become absolutely necessar...

  ASSAY METHODS A good number of pharmaceutical substances can be assayed gravimetrically. The gravimetric meth-ods adopted vary according to the nature of the substance under determination. However, most of the sub-stances being estimated gravimetrically fall into one or the other categories stated below, which would be discussed briefly with suitable examples :   ( a ) Substances assayed gravimetrically,   ( b ) Substances assayed after conversion :   ( i ) Substances assayed after conversion to Free Acid,   ( ii ) Substances assayed after conversion to Free Base,   ( iii ) Substances assayed after conversion to Free Compound, and   ( iv ) Substances assayed after conversion to Derivatives or Substitution Products.

  SUBSTANCES ASSAYED GRAVIMETRICALLY   A good number of pharmaceutical substances may be determined gravimetrically by obtaining their respective difficultly soluble salts as precipitates, weighing to a constant weight and finding the percentage purity of the substance in question.   A few typical examples are cited below so as to expatiate the procedure as well as the theoretical aspects.     1. Sodium Chloride   Materials Required :  Sodium chloride : 0.25 g ; 5% w/v silver nitrate in DW (+ 2-3) drops of conc. HNO 3  ; dilute nitric acid (6 N) ; asbestos fibre.   Theory :  The following reaction forms the basis for the calculation of the theoretical amount of silver   nitrate solution required as well as the purity of the given sample of NaCl. Thus, we have : As 0.2570 g of NaCl has been used (from experimental data); therefore, the exact amount of AgNO 3  required would be : Hence, the amount of AgNO 3  solution require...