270 ANALYTICAL CHEMISTRY. 



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 titration equivalents of this solution for silver are the same 

 as those of deci-normal sodium chloride. 



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 

 remain in contact for a few hours with potassium hydroxide, which will absorb 

 all carbon dioxide, the diminution in volume indicating the quantity of carbon 

 dioxide originally present. The volume of oxygen may next be determined by 

 introducing a piece of phosphorus, which will gradually absorb the oxygen, 

 the remaining volume being pure nitrogen. 



In some cases gaseous constituents of liquids or solids are eliminated and 

 measured as gases. Thus, the carbon dioxide of carbonates, the nitrogen 

 dioxide evolved from nitrates, the nitrogen of urea and other nitrogenous 

 bodies, are instances of substances which are eliminated from solids in the 

 gaseous state and determined by direct measurement. 



The gas volume thus found is, in most cases, converted into parts by weight. 

 The basis of this calculation is the weight of 1 c.c. of hydrogen, which, at the 

 temperature of C. (32 F.) and a pressure of 760 mm., is 0.0000896 gramme. 

 1 c.c. of any other gas weighs as many more times as the molecule of this 

 substance is heavier than that of hydrogen. Thus the molecular weight of 

 carbon dioxide is 22 times greater than that of hydrogen, consequently 1 c.c. 

 of carbon dioxide weighs 22 times heavier than 1 c.c. of hydrogen, or 0.0019712 

 gramme. 



It has been shown on pages 21 and 25 that heat and pressure cause a regular 

 increase or decrease in volume. The data there given are used in calculating 

 the volume of the measured gas for the temperature of C. (32 F. ) and a 

 pressure of 760 m.m. 



Methods of gas-analysis have been adopted by the U. S. P. in the quantita- 

 tive determination of amyl nitrite and ethyl nitrite. The operation is per- 

 formed in an apparatus known as a nitrometer, consisting of two glass tubes held 

 in upright position and connected at the lower ends by a piece of rubber 

 tubing. One of the tubes is open, the other one is graduated and provided 

 with a glass stopcock near the upper end. In using the nitrometer for the 

 analysis of ethyl nitrite the graduated tube is filled with saturated solution of 



