Determination of the purity of a drug is an important exercise in the evaluation of drugs. Apparent correct identity does not always guarantee the purity of the drug as drugs of correct identity can be adulterated in a variety of ways (see under adulteration). Purity of a drug implies that the sample should not contain any other substances other than the substances constituting the drug or the presence of other related substances should not in any way exceed the maximum permissible limit. In assessing the purity of a drug, many macroscopical, microscopial, chemical and biological tests are employed. Determinations used for purity assessment include determination of the amount of foreign organic matter, ash and acid-insoluble ash values, extractive values, optical rotation, refractive index, saponification value, iodine value and various quantitative physical constants and numerical values.
Determination of Quality
Good quality, that is the presence of maximum quantity of the active constituents, is the most important criterion expected in a crude drug. Thus the ultimate aim of evaluation of a drug sample is to determine its quality. In order to do that both physico-chemical and biological methods are employed. Chemical assays and various physico-chemical determinations (gravimetric, titrimetric, colorimetric, spectroscopic, chromatographic, etc.) are very efficient incorrectly estimating the amount of chemical constituents in a crude drug. Biological and microbiological assays are also very useful in determining the qualities of certain drugs. These methods are valuable in cases of drugs, which cannot be assayed by chemical methods. In these inethods, use is made of the ability of a drug to being about a specific response in a living organism or tissue. Of course, the response of the organism must be measurable in terms of dose and concentration of the test drug.
As apparent from the foregoing discussion a complete evaluation of a drug sample may involve almost all the methods and techniques available for identification and assessment of purity and quality. (For details of the various methods and procedures, students are referred to standard practical books and Pharmacopoeias).