DETECTION OF IMPURITIES. 435 



The chemical tests given are sufficiently characteristic to leave no 

 doubt as to the true nature or identity of the substance. In order to 

 accomplish this object it is not necessary to apply all the analytical 

 reagents characteristic of the substance or its component parts, but 

 the U. S. P. selects from the often large number of known tests one, 

 or possibly a few, which answer best in the special case. 



For instance, while we have a number of tests, both for potassium 

 and iodine, the U. S. P., in the article on potassium iodide, gives but 

 one reaction for each of these elements. Yet these tests have been 

 selected with sufficient judgment to admit of no doubt regarding the 

 nature of the substance. 



Qualitative tests for impurities. These tests are in many cases 

 described minutely, i. e., the quantity to be taken of both the sub- 

 stance to be examined and the reagent to be added is stated. More- 

 over the amount of solvent (water, acid, etc.) to be used is mentioned, 

 and other details are given. The object of this exactness in describ- 

 ing the tests is not only to render the work easy for one not fully 

 familiar with analytical methods, but also, in some cases, to fix a 

 limit for the admissible quantity of an impurity. A certain reagent 

 may, in a concentrated solution, indicate the presence of a trace of 

 an impurity, while in a more dilute solution this reagent will fail to 

 detect it. The selection of the reagents used in certain tests is also 

 made with the view of establishing a sufficient purity for pharmaco- 

 poeial purposes of the article examined without demanding an absolute 

 purity. 



A few instances may help to illustrate these remarks : Potassium 

 can be precipitated from a solution of its salts by a number of re- 

 agents, which, however, differ widely in sensitiveness. Thus, tartaric 

 acid will cause the formation of a precipitate of potassium bitartrate 

 in a solution containing at least 0.1 per cent, of potassium ; in solu- 

 tions containing a less amount no precipitate is formed. Platinic 

 chloride is somewhat more sensitive than tartaric acid, and sodium 

 cobaltic nitrite, which is still more delicate, causes a precipitate in 

 solutions containing even as little as 0.04 per cent, of potassium. It 

 is evident that by using either one or the other of the three reagents 

 mentioned for the detection of potassium, this metal may or may not 

 be found, according to the quantity present in a solution. The 

 Pharmacopoeia, in directing the use of one of these reagents, limits 

 the amount of a permissible quantity of potassium according to the 

 sensitiveness of the reagent. 



