272 ANALYTICAL CHEMISTRY. 



properties of the substance to be examined for purity will, in most 

 cases, suggest the nature of those substances which possibly may have 

 been added, and for them a search has to be made, or, if necessary, a 

 complete analysis, by which is proved the absence of everything else 

 but the constituents of the pure substance. 



Impurities derived from the materials used in the manufacture of 

 a substance (generally through an imperfect or incorrect process of 

 manufacture), or from the vessels used in the manufacture, are usually 

 but few in number (in any one substance), and their nature can, in 

 most cases, be anticipated by one familiar with the process of manu- 

 facture. For one not acquainted with the mode of preparation it 

 would be a rather difficult task to study the nature of the impurities 

 which might possibly be present. 



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 Pharmacopoeia 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 no wise 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 Pharmacopoeia. 



