DETECTION OF IMPURITIES. 271 



sodium chloride, in which nitrogen dioxide is almost insoluble. Next are 

 introduced through the stopcock the measured (or weighed) quantity of ethyl 

 nitrite with a sufficient amount of solution of potassium iodide and sulphuric 

 acid. By the action of these agents nitrogen dioxide is liberated, and from the 

 volume obtained the quantity of nitrite present is calculated. The decom- 

 position is shown by the equation : 



C 2 H 5 N0 2 + KI + H 2 SO 4 = C 2 H 5 OH + I + KHSO 4 + NO. 



37. DETECTION OF IMPURITIES IN OFFICIAL INORGANIC 

 CHEMICAL PREPARATIONS. 



General remarks. Very little has been said, heretofore, about 

 impurities which may be present in the various chemical prepara- 

 tions, and this omission has been intentional, because it would have 

 increased the bulk of this book beyond the limits considered neces- 

 sary for the beginner. 



Impurities present in chemical preparations are either derived from 

 the materials used in their manufacture, or they have been intention- 

 ally added as adulterations. In regard to the last, no general rule 

 for detecting them can be given, the nature of the adulterating article 

 varying with the nature of the substance adulterated ; the general 



QUESTIONS. 351. Explain the principles which are made use of in gravi- 

 metric and volumetric determinations. 352. Give an outline of the operations 

 to be performed in the gravimetric determination of copper in cupric sulphate. 



353. What are normal and deci-normal solutions, and how are they made? 



354. What is the use of indicators in volumetric analysis? Mention some 

 indicators and explain their action. 355. Why is oxalic acid preferred in 

 preparing normal acid solution? What quantity of oxalic acid is contained 

 in a liter, and why is this quantity used ? 356. Suppose 2 grammes of crys- 

 tallized sodium carbonate require 14 c.c. of normal acid for neutralization: 

 What are the percentages of crystallized sodium carbonate and of pure sodium 

 carbonate contained in the specimen examined? 357. Ten grammes of dilute 

 hydrochloric acid require 35.5 c.c. of normal sodium hydroxide solution for 

 neutralization. What is the strength of this acid ? 358. Explain the action 

 of potassium permanganate and of potassium dichromate when used for volu- 

 metric purposes. 359. Which substances may be determined volumetrically 

 by solutions of iodine and sodium thiosulphate ? Explain the mode in which 

 the determinations by these agents are accomplished. 360. Suppose 1 gramme 

 of potassium iodide requires for titration 60 c.c. of deci-normal solution of 

 silver nitrate : What quantity of pure potassium iodide, is indicated by this 

 determination? 361. Describe in detail the volumetric determination of car- 

 bolic acid. 362. For what purposes is potassium sulphocyanate used volu- 

 metrically, and what is its action? 363. Explain the method used for the 

 analysis of ethyl nitrite. 



