276 ANALYTICAL CHEMISTRY. 



Quantitative determination of the principal constituent. 

 These determinations are made in the majority of cases volumetric- 

 ally, and require no special explanation here, as the methods have 

 been fully considered in the previous chapter. Gravimetric methods 

 are used in the determination of the alkaloids of cinchona and opium, 

 and also in a few other cases. 



QUESTIONS. 364. What are the sources of the impurities found in chemical 

 preparations ? 365. Why is it not obligatory to use chemically pure chemicals 

 for medicinal purposes? 366. Which are the leading features adopted by the 

 U. S. P. in the identification of chemical preparations? 367. State the reasons 

 why the U. S. P. describes the tests for impurities so minutely. 368. Why can 

 we not use indiscriminately either one of a number of reagents or tests by which 

 the presence of the same impurity may be indicated ? 369. What is the prin- 

 ciple applied in the methods of the Pharmacopoeia for the determination of a 

 permitted quantity of an impurity? 370. How can we decide the question 

 whether a sample of potassium acetate contains more than 1 per cent, of potas- 

 sium chloride without making a quantitative estimation of chlorine ? 



