DETECTION OF IMPURITIES. 227 



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

 a complete analysis, by which the absence of everything else 

 but the constituents of the pure substance is proved. 



Impurities derived from the materials used in the manufac- 

 ture of a substance (generally through an imperfect or incorrect 

 process of manufacture), or from the vessels used in the manu- 

 facture, are generally but few in number (in any one substance), 

 and their nature can, in most cases, be anticipated by one familiar 

 with the process of manufacture itself. 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 im- 

 purities can be detected. For one familiar with analytical chem- 

 istry it is an easy task to suggest, in most 'cases, the best and 

 quickest method by which the presence or absence of an im- 

 purity can be demonstrated; for one unacquainted with these 

 methods, it might be an impossibility to do so, even if a method 

 were given. 



For these reasons little stress has been laid upon the occur- 

 rence of impurities in the various chemical preparations here- 

 tofore considered ; moreover, the U. S. P. itself gives, in most 

 cases, directions for the detection of impurities, and, finally, 

 the analyst may avail himself of books specially treating of 

 the examination of chemicals. 



In order to give the student, and especially the beginner, a 

 guide in the examination for impurities, the following pages fur- 

 nish a few directions in regard to the impurities which ;may be 

 present in the more important chemical preparations, and also 

 some of the tests used for recognizing them. 



Examination of sulphuric, sulphurous, nitric, phosphoric, and 

 hydrochloric acids. The pure acids should be colorless, and upon 

 evaporation leave no residue whatever (phosphoric acid requires 

 red heat for complete evaporation). After being neutralized 

 with an alkali, and then slightly acidulated with hydrochloric 

 acid, they should give no precipitate with hydrosulphuric acid, 

 ammonium hydrate, sulphide, or carbonate. 



Sulphuric, phosphoric, and hydrochloric acids sometimes (nitric 



