ANALYTICAL CHEMISTRY. 



Again, in testing for arsenic, the chemist has his choice between a 

 number of more or less delicate tests. Gutzeit's test is so sensitive 

 that by means of it arsenic can be detected in a solution containing 

 only 0.000001 gramme of arsenous oxide in a cubic centimeter. This 

 test would be, therefore, by far too severe when applied to a number 

 of pharmaceutical preparations, for which reason the Pharmacopoeia 

 directs in many cases the less sensitive hydrogen sulphide test. 



Quantitative tests for the limit of impurities. While, as above 

 stated, even the qualitative tests are often so made as to be to some 

 extent of a quantitative character, the U. S. P. recommends in many 

 cases methods by which a stated limit of an impurity can be detected 

 without the necessity of determining by quantitative analysis the 

 actual amount of the impurity present. 



Formerly it was, and to some extent it is now, customary to limit 

 the amount of a permissible quantity of an impurity by referring to 

 the intensity of the reaction. In case the impurity was to be detected 

 by precipitation (as, for instance, sulphates or chlorides in potassium 

 nitrate) it was stated that the respective reagents used for the detec- 

 tion (in the case named, barium chloride or silver nitrate) should not 

 produce more than a very slight precipitate, or turbidity, or cloudi- 

 ness, etc. These descriptions are, of course, very indefinite, and the 

 conclusion arrived at depends largely upon the individuality of the 

 observer. 



In order to obviate this uncertainty the U. S. P. has introduced a 

 number of more exact methods. These depend upon the addition of 

 a definite quantity of a reagent capable of eliminating a certain quan- 

 tity of the impurity from a given quantity of the substance to be 

 examined. In thus examining a preparation the impurity may or 

 may not be present ; if present, the permissible quantity will be re- 

 moved by the operation, and if originally not present in larger quan- 

 tity, the substance will now be found free from the impurity, while 

 if present in larger proportions than can be removed by the quantity 

 of reagent added, the excess can be detected by appropriate tests. 



If an excess of impurity is thus discovered, regardless of the fact 

 whether the excess be large or small, the substance examined does 

 not come up to the pharmacopoeial requirements. 



Thus, in potassium bromide, the pharmacopoeial limit of potassium carbon- 

 ate is 0.068 per cent. In order to determine whether or not this limit is ex- 

 ceeded, the Pharmacopeia directs the addition of 0.1 c.c. of sulphuric acid 



