LIMIT TESTS VIS-A-VIS QUANTITATIVE DETERMINATIONS
In general, limit tests are quantitative or semi-quantitative tests particularly put forward to identify and control invariably small quantities of impurity that are supposed to be present in a pharmaceutical substance. Obviously the amount of any single impurity present in an official substance is usually small, and therefore, the normal visible-reaction-response to any test for that impurity is also quite small. Hence, it is necessary and important to design the individual test in such a manner so as to avoid possible errors in the hands of various analysts. It may be achieved by taking into consideration the following three cardinal factors, namely :
(a) Specificity of the Tests : A test employed as a limit test should imply some sort of selective reaction with the trace impurity. It has been observed that a less specific test which limits a number of possible impurities rather instantly has a positive edge over the highly specific tests.
Exmaple : Contamination of Pb2+ and other heavy metal impurities in Alum is precipitated by thioacetamide as their respective sulphides at pH 3.5.
(b) Sensitivity : The extent of sensitivity stipulated in a limit test varies widely as per the standard laid down by a pharmacopoeia. The sensitivity is governed by a number of variable factors having a common objective to yield reproducible results, for instance :
(i) Gravimetric Analysis : The precipitation is guided by the concentration of the solute and of the precipitating reagent, reaction time, reaction temperature and the nature and amount of other substance(s) present in solution.
(ii) Colour Tests : The production of visible and distinct colouration may be achieved by ascertain-ing the requisite quantities of reagents and reactants, time period and above all the stability of the colour produced.
(c) Personal Errors : In fact, the personal errors must be avoided as far as possible as explained.