92 THE PHYSICAL PROPERTIES OF SOILS 



EIGHTEENTH EXERCISE 



CHROMIC ACID METHOD OF DETER- 

 MINING ORGANIC MATTER 



Many of the physical properties of soils are in- 

 fluenced by the amount of organic matter present. An 

 accurate method for its determination is important, 

 therefore, in the study of the physical properties of 

 soils. 



The determination of organic matter by dry combus- 

 tion usually gives results that are too high, owing to 

 the fact that combined water is driven off at the high 

 temperature necessary for the complete combustion of 

 the organic matter. The chromic acid, or wet com- 

 bustion, method does not drive off this combined water, 

 hence it is free from this objection. 



METHOD OF DETERMINATION 



The apparatus (T) consists of a round-bottomed 

 flask of about 400 cc. capacity in which the combustion 

 is effected. Connected to this flask is a train of ab- 

 sorption bulbs, so arranged that the CO:; produced by 

 the breaking down of the organic matter is absorbed, 

 and may be weighed at the close of the experiment. 



A carefully weighed sample of soil of about ten 

 grams is brought into the combustion flask, from five 

 to ten grams of pulverized potassium bichromate is 

 added, and the whole thoroughly mixed. The flask is 

 closed, and a gentle stream of air is drawn through 

 the entire system by means of an aspirator. After 

 about ten minutes, concentrated suli)huric acid (sp. gr. 

 about 1.83) is added by means of a dropping funnel. 



The combustion flask is slowly heated until the acid 

 begins to give off fumes. It is held at this temperature 



