434 ANALYTICAL CHEMISTRY. 



The same remarks apply to the methods by which the impurities 

 can be detected. One familiar with analytical chemistry can easily 

 find, in most cases, a good method by which the presence or absence 

 of an impurity can be demonstrated ; but to one unacquainted with 

 chemistry it might be an impossibility to detect impurities, even if 

 a method were given. 



For these reasons little stress has been laid upon the occurrence of 

 impurities in the various chemical preparations heretofore considered. 

 Moreover, the U. S. P. gives, in most cases, directions for the detec- 

 tion of impurities, so explicit that anyone acquainted with analytical 

 operations will find no difficulty in performing these tests satisfac- 

 torily. 



However, while the Pharmacopeia gives exact instructions how to 

 manipulate, it furnishes no explanations why certain methods have 

 been adopted, or why certain operations are to be performed. It is 

 for this reason, and for the special benefit of the beginner, that a few 

 paragraphs are devoted to the consideration of official methods for 

 testing the chemical preparations of the U. S. P. 



Official chemicals and their purity. Absolute purity of chemi- 

 cals is essential in some cases, as, for instance, when they are intended 

 as reagents ; such chemicals are commercially designated as C. P. 

 (chemically pure). For the majority of medicinal chemicals, how- 

 ever, such absolute purity is unnecessary, as the small proportion of 

 harmless impurities present in nowise interferes* with the therapeutic 

 action of the substance, and a demand for absolute purity, which 

 greatly enhances the cost of chemicals, is therefore unreasonable and 

 not required by the Pharmacopeia. 



The presence of a small fraction of one per cent, of sodium chloride 

 in many official chemicals cannot be looked upon as objectionable, 

 while the same amount of arsenic would render the preparation unfit 

 for medicinal use. 



The methods used by the Pharmacopoeia to determine the qualit/ 

 of a chemical preparation may be divided into four classes, as follows : 

 1. Tests as to identity ; 2. Qualitative tests for impurities ; 3. Quan- 

 titative tests for the limit of impurities ; 4. Quantitative determina- 

 tion of the chief constituent. 



Tests as to identity. These tests are partly of a physical, partly 

 of a chemical character. They include, in the physical part, the 

 examination of the appearance, color, crystalline structure, specific 

 gravity, fusing-point, boiling-point, etc. 



