Medical brain notes 14

  INTRODUCTION   Arrhenius’ definition of an  acid  is—‘ a substance which yields hydrogen ion (H + ) in an aqueous medium ’; and that of a   base   is— ‘a substance which yields hydroxy ions (OH – ) in an aqueous medium’. However, these definitions have  two  serious short-comings, they are :   ( a ) they lack explanation of the behaviour of acids and bases in non-aqueous media, and   ( b )  acidity  is associated with  hydrogen ion —a relatively simple particle ; whereas,  basicity  is associated with  hydroxyl ion —a relatively complex entity.     1. LOWRY AND BRONSTED’S THEORY OF ACIDS AND BASES   Just after the First World War in 1923, Bronsted and Bjerrum in Denmark and Lowry in Great Britain jointly put forward a more acceptable and satisfactory theory of acids and bases which is devoid of objec-tions earlier raised in Arrhenius’ definition.   According to Lowry and Bronsted’s theo...

  THEORY OF ACIDIMETRY Acidimetry, essentially involves the direct or residual titrimetric analysis of alkaline substances (bases) employing an aliquot of acid and is provided usually in the analytical control of a large number of substances included in the various  official compendia . Examples :   ( a )  Organic substances :  urea, sodium salicylate, diphenhydramine, emetine hydrochloride, meprobamate, paramethadione, pyrazinamide etc., and   ( b )  Inorganic substances :  sodium bicarbonate, milk of magnesia, ammonium chloride, calcium hydroxide, lithium carbonate, zinc oxide etc.   The  two  methods, namely : direct titration method and residual titration method are briefly discussed as under :   1. DIRECT TITRATION METHOD   It is an usual practice that when a solid substance is to be assayed, an aliquot quantity of the same may be weighed accurately and dissolved in sufficient water so that the resulting solution shou...

  ASSAY OF DRUGS A few typical examples of acidimetric titrations, employing  ‘direct titration method’  (DTM) and  ‘residual titration method’  (RTM) from the ‘ Pharmacopoeia of India ’ are described here :   1. DIRECT TITRATION METHOD   1.1. Sodium Carbonate (DTM)   Materials Required :  1 g of sodium carbonate ; 0.5 N sulphuric acid.   Procedure :  Weigh accurately about 1 g, dissolve in 20 ml of water (DW) and titrate with 0.5 N   sulphuric acid, using methyl orange solution as indicator. Each ml of 0.5 N sulphuric acid is equivalent to 0.42 g of NaHCO 3 .   Equation :   2NaHCO 3  + H 2 SO 4     →  Na 2 SO 4  + 2H 2 O + 2CO 2 ↑ 2(84.01)   It is evident from the above equation that 2 ml of NaHCO 3  is equivalent to 1 ml of H 2 SO 4 . Hence, 1 ml of NaHCO 3  is 1 equivalent and the equivalent weight is equal to the gram-molecular weight  i.e. , 84.01/ 2 = 42.0 g. One...