DETECTION OF IMPURITIES. 275 



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. 



For instance, the Pharmacopoeia fixes the limit of potassium chloride 

 in potassium carbonate at 0.15 per cent., and in order to determine 

 whether this limit is exceeded or not the Pharmacopoeia directs the 

 addition of 0.1 c.c. of deci-normal silver nitrate solution to a solution 

 of 05 gramme of potassium carbonate in 6 c.c. of diluted nitric acid 

 and 4 c.c. of water. After removing the precipitated silver chloride 

 by filtration no precipitate should be produced in the filtrate by the 

 further addition of silver nitrate solution. The limit of many im- 

 purities, which can be separated by precipitation in definite quantity, 

 is thus determined. 



In other cases the limited quantity of an impurity may be deter- 

 mined without the formation of a precipitate, as, for instance, an 

 alkaline impurity in an otherwise neutral salt by the addition of a 

 standard acid. 



Thus, in potassium bromide, the pharmacopoeial limit of potassium 

 carbonate is 1.38 per cent. In order to determine whether or not 

 this limit is exceeded, the Pharmacopoeia directs the addition of 0.2 

 c.c. of normal sulphuric acid to a solution of 1 gramme of the salt 

 in 100 c.c. of water. Since 0.1 c.c. of normal sulphuric acid is capa- 

 ble of neutralizing 0.0069 gramme of potassium carbonate, the whole 

 quantity allowed, 1.38 per cent, or 0.0138 gramme, would be neu- 

 tralized by the addition of the prescribed quantity of acid, and no 

 red tint should be imparted to the liquid by adding a few drops of 

 phenol-phtalein 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. 



By methods similar to those described, the limit of many other 

 impurities is determined, as, for instance, the limit of sulphuric acid 

 by removing it with barium chloride, or that of carbonates in caustic 

 alkalies by lime-water. 



