DETECTION OF IMPURITIES. 433 



39. DETECTION OF IMPURITIES IN OFFICIAL INOKGANIC 

 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 

 properties of the substance to be examined for purity will, in most 

 cases, suggest the nature of those substances which possibly may have 

 been added, and for them a search has to be made, or, if necessary, a 

 complete analysis, by which is proved the absence of everything else 

 but the constituents of the pure substance. 



Impurities derived from the materials used in the manufacture of 

 a substance (generally through an imperfect or incorrect process of 

 manufacture), or from the vessels used in the manufacture, are usually 

 but few in number (in any one substance), and their nature can, in 

 most cases, be anticipated by one familiar with the process of manu- 

 facture. For one not acquainted with the mode of preparation it 

 would be a rather difficult task to study the nature of the impurities 

 which might possibly be present. 



their action. 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? Suppose 2 grammes of crystallized 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 specimens 

 examined. Ten grammes of dilute hydrochloric acid require 35.5 c.c. of nor- 

 mal sodium hydroxide solution for neutralization ; what is the strength of 

 this acid? Explain the action of potassium permanganate and of potassium 

 dichromate when used for volumetric purposes. Which substances may be 

 determined volumetrically by solutions of iodine and sodium thiosulphate? 

 Explain the mode in which the determinations by these agents are accom- 

 plished. 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? Describe in detail the volumetric 

 determination of carbolic acid. For what purposes is potassium sulphocya- 

 nate used volumetrically, and what is its action ? Explain the method used 

 for the analysis of ethyl nitrite. 

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