DETECTION OF IMPURITIES. 437 



to a solution of 1 gramme of the salt in 10 c.c. of water. Since 0.1 c.c. of 5 

 sulphuric acid is capable of neutralizing 0.000686 gramme of potassium carbon- 

 ate, the whole quantity allowed would be neutralized by the addition of the 

 prescribed quantity of acid, and no red tint should be imparted to the heated 

 liquid by adding a few drops of phenolphthalein solution ; a red color would 

 indicate that more alkali carbonate was present in the weighed sample than 

 could be neutralized by the quantity of acid added. 



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 several alkaloids and also in a few 

 other cases. 



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

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

 medicinal purposes ? Which are the leading features adopted by the U. S. P. 

 in the identification of chemical preparations? State the reasons why the 

 U. S. P. describes the tests for impurities so minutely. Why can we not use 

 indiscriminately either one of a number of reagents or tests by which the pres- 

 ence of the same impurity may be indicated ? What is the principle applied 

 in the methods of the Pharmacopeia for the determination of a permitted 

 quantity of an impurity ? How can we decide the question whether a sample 

 of potassium acetate contains more than 1 per cent, of potassium chloride with- 

 out making a quantitative estimation of chlorine? 



