80 LABORATORY MANUAL OF MICROBIOLOGY 



flat-bottomed flasks of 300-cubic centimeter capacity are used 

 for this purpose. The soil and the proper amount of organic 

 matter are introduced into the flasks; the moisture of the soil is 

 brought to the desired concentration and the flasks connected 

 with the respiration apparatus. The air, freed from CO2 as 

 before, is passed through the flasks over the surface of the soil. 

 The CO2 is absorbed in 100-cubic centimeter heavy-walled test- 

 tubes containing 25 to 50 cubic centimeters of the standard 

 barium hydroxide solution. The excess barium hydroxide is 

 then titrated back with standard oxalic acid solution (see also 

 Fig. 19, Exercise 58). 



DETERMINATION OF TOTAL CARBON 



Total carbon can be determined by (1) the various dry com- 

 bustion methods, including the bomb method (with Na202), and 

 (2) wet combustion methods, using a mixture of chromic and 

 sulphuric orchromic and phosphoric acids or permanganate and 

 sulphuric acid. The following method was found to give very 

 good results, especially with liquid cultures (see Fig. 4). 



A definite portion of liquid culture or soil, containing not 

 more than 100 to 120 milligrams of carbon, is placed in a 200- to 

 500-cubic centimeter round-bottomed Pyrex flask {B of Fig. 4). 

 In case of liquid materials, the water should be evaporated on a 

 steam bath before beginning the analysis. The flask is then 

 attached to the condenser D, and 10 cubic centimeters of an 

 oxidizing solution (85 grams chromic anhydride in 100 cubic 

 centimeters of water made up to 250 cubic centimeters with 85 

 per cent phosphoric acid) is introduced through the separatory 

 funnel C. Gentle suction is then applied and 25 to 40 cubic 

 centimeters of a mixture of equal parts of concentrated phos- 

 phoric and sulphuric acids are added. The stopcock in C is closed 

 and by the use of a low flame, flask B is heated as rapidly as 

 possible without developing pressure within. The gas is drawn 

 out through condenser D and through a U-tube E, which con- 

 tains a saturated solution of Ag2S04 and 5 per cent H2SO4 in 

 20-mesh pumice on the left side 1, and boiled concentrated H2SO4 

 in 20-mesh pumice on the right side 2. The Ag2S04 serves to 

 remove the chlorine and the H2SO4 the SO3 fumes. The gas is 

 then drawn through the absorption flask F and absorbed in a 

 measured quantity of O.bN NaOH solution. After flask B has 



