METHODS FOR QUANTITATIVE DETERMINATIONS. 427 



One c.c. of deci-normal sodium chloride solution, containing 

 0.005806 gramme of Nad, is the equivalent of: 



Gramme 



Silver, Ag 0.010712 



Silver nitrate, AgNO 3 0.016869 



Silver oxide, Ag 2 O 0.011506 



Deci-normal solution of potassium sulphocyanate ( Volhard's 

 solution). This solution, like the sodium chloride solution, is used 

 as a companion to silver nitrate ; it has the advantage that it can be 

 used in acid solutions, with ferric ammonium sulphate (ferric alum) 

 as indicator. Silver nitrate forms in the potassium sulphocyanate a 

 white precipitate of silver sulphocyanate : 



KCNS + AgNO 3 = AgCNS + KNO 3 . 



As indicator is used ferric alum, which produces with sulpho- 

 cyanate a deep brownish-red color, which, however, does not appear 

 permanently until all silver has been precipitated. 



As potassium sulphocyanate is rarely pure, 10 grammes, which is 

 about 3 per cent, more than the quantity required, are dissolved in 

 1000 c.c. of water. This solution has to be adjusted by standardizing 

 with deci-normal silver solution until equal volumes decompose one 

 another exactly. 



The sulphocyanate solution is used in the determination of the 

 amount of ferrous iodide in the saccharated salt and in the syrup. 



The operation is performed thus : To the solution of the ferrous 

 iodide are added nitric acid, ferric alum, and of deci-normal silver 

 nitrate solution a quantity more than sufficient to convert all iodine 

 into silver iodide. . The excess of silver nitrate present in the mix- 

 ture is determined by sulphocyanate solution. The ferric alum and 

 nitric acid must not be added until the silver nitrate has precipitated 

 all iodine, otherwise iodine will be liberated. This holds in all cases 

 where iodides are titrated. 



Gas-analysis. The analysis of gases is generally accomplished by measur- 

 ing gas volumes in graduated glass tubes (eudiometers) over mercury (in some 

 cases over water), noting carefully the pressure and temperature at which the 

 volume is determined. 



From gas mixtures, the various constituents present may often be eliminated 

 by causing them to be absorbed one after another by suitable agents. For 

 instance : From a measured volume of a mixture of nitrogen, oxygen, and 

 carbon dioxide, the latter compound may be removed by allowing the gas to 



