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 :
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 should have more or less the same equivalent concentration as that of the acid used in the titration. Methyl orange (pH range = 3.0 to 4.4) is the indicator of choice for obvious reasons, as phenolphthalein and most other indicators are instantly affected by the carbonic acid (H2CO3) generated in the reaction which ultimately cause a change in colour even before the reaction attains completion.
Residual titration or back titration is normally employed in the following two situations, namely :
Case I : when a chemical reaction proceeds rather slowly or sluggishly, and
Case II : when the substance under determination fails to give a sharp and distinctly visible end-point with an indicator by direct titration.
In usual practice, the residual titration is accomplished by allowing to dissolve the substance under estimation in an accurately measured quantity of a standard solution of known strength present in excess and subsequently titrating the excess of the latter with another previously standardized solution. A good number of examples of this particular method shall be discussed in subsequent exercises.