CHEMICAL METHODS 947 



The tube containing the extracted gases is attached to the analytical 

 apparatus and, after passing through potassium hydroxide solution, con- 

 centrated sulfuric acid, ascarite and phosphoric anhydride, the gas enters 

 the analytical apparatus proper, the pressure of which has previously been 

 reduced to about 10"^ mm. of mercury. 



The technique for carrying out the analysis of the extracted gases in- 

 volves the combination of low-temperature fractionation and combustion 

 methods. The routine analysis has been standardized to include three 

 separate fractions. The first includes that portion of the gaseous mixture 

 which is non-condensable at the temperature of liquid nitrogen ( — 196°C.) 

 and consists mainly of air, methane, and hydrogen. The second fraction 

 includes that portion of the mixture which is not retained at a temperature 

 of — 145°C., but is condensable at the temperature of liquid nitrogen. The 

 last fraction is made up of the volatile soil constituents which are retained 

 at — 145°C. The middle fraction contains the ethane, propane, and butane 

 which may be evolved from the sample, while the last fraction is made up 

 of pentane and heavier hydrocarbons. The methane is determined by 

 combustion of the non-condensable mixture over a glowing platinum wire. 

 The water and carbon dioxide thus produced are collected and measured. 

 The water, in excess of that required to account for all the carbon dioxide 

 as methane, is calculated as hydrogen. The quantities of the other fractions, 

 heavier than methane, are determined by first measuring the initial volumes 

 present and then measuring the carbon dioxide resulting from combustion 

 with purified air. The increase in the final volume is used as a measure of 

 the quantities of these fractions initially present, the ethane-propane-butane 

 fraction being calculated as propane, while the heaviest fraction is calcu- 

 lated as hexane. 



Since the total volume of the gas sample initially prepared is known, 

 and since the various parts of the analytical apparatus are calibrated, permit- 

 ting the calculation of the volumes of gas used for the different determina- 

 tions, the volumes of the various constituents are readily determined. The 

 weights of the different fractions are calculated from the gas laws and the 

 final results expressed in parts per billion by weight (dry basis) of the soil 

 sample. 



The following equation, applicable to methane, illustrates the method 

 used in calculating the final results. 



Methane (parts per billion by weight) = 



V, .000714 



V • 10^ 



V2 Weight dry sample 



V = Total volume of gas sample in cubic centimeters. 

 J/^ = Volume, in cubic centimeters, of carbon dioxide pro- 

 duced upon combustion of methane fraction. 

 Vo — Volume, in cubic centimeters, of portion of gas sam- 

 ple used for methane determination. 

 .000714 = Weight, in grams, of 1 cubic centimeter of methane. 



