428 ANALYTICAL CHEMISTRY. 



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. of mercury is 0.0000898 

 gramme. 1 c.c. of any other gas weighs as many times the weight of 1 c.c. 

 hydrogen as the molecule of this substance is heavier than that of hydrogen. 

 Thus the molecular weight of carbon dioxide is 21.835 times greater than that 

 of hydrogen, consequently 1 c.c. of carbon dioxide weighs 21. 835 times heavier 

 than 1 c.c. of hydrogen, or 0.0019608 gramme. 



It has been shown on pages 26 and 45 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 at the temperature of C. (32 F.) and a 

 pressure of 760 m.m. 



The reason for reducing volumes of gases to C. and 760 m.m. pressure, 

 known as normal temperature and pressure, is that the densities of gases are 

 given for these conditions. Therefore, to find the weight of any volume of 

 gas it must be reduced to normal temperature and pressure. 



A simple rule for reducing volumes of gases to C. is this : The volume of 

 a gas is proportional to its absolute temperature. The absolute temperature is 

 obtained by adding 273 to the reading of the centigrade scale. Thus, if a gas 

 measures 66 c.c. at 54.6 C., its volume at C. is found from the proportion : 



66 c.c. : [54.6 + 273] : : x : [0 + 273], 

 or, 



66 : 327.6 : : x : 273, 

 and 



In this reduction the pressure is supposed to remain constant. That is, the 

 volume of 55 c.c. at C. is still at the same pressure as the volume 66 c.c. was. 



To reduce a gas volume under any pressure to the volume it would occupy 

 if the pressure were changed to the normal i. e., to 760 m.m. use is made of 

 Boyle's law, viz., the product of the pressure times the corresponding volume 

 of a gas is always constant when the temperature is the same. This law is 

 expressed in the equation, PV=pv, where PV and pv are corresponding 

 pressures and volumes. 



If we assume that in the above case the volume of 55 c.c. is under a pressure 

 of 750 m.m., its volume at normal, or 760 m.m. pressure, is found by using the 

 equation : 



55 X 750 = x X 760, 



