628 APPENDIX 



boil ten minutes to expel carbon dioxid, cool, and titrate with standard 

 sodium hydroxid solution (of which i cc. is equivalent to 4 mg. of cal- 

 cium carbonate), using phenolphthalein as indicator. 



The acids and acid salts of the soil are difficultly soluble in water, but 

 by treating with a salt solution, as potassium nitrate, a double decomposi- 

 tion takes place, carrying acidity into solution. An equilibrium is reached, 

 however, before this reaction runs to an end, and if, after having drawn 

 off 125 cc. to titrate, 125 cc. of fresh potassium nitrate are added to the 

 bottle and the bottle again shaken for three hours, 125 cc. drawn off will 

 give a titration which is more than one half of the first. By continuing 

 this process until the last 125 cc. shows practically no acidity, we have a 

 series of titrations the sum of which represents the total acidity of the 

 100 g. of soil. It has been found by working with a number of different 

 soils, that as an average the sum of such a series is 2\ times the first ti- 

 tration. Consequently, when the sodium hydroxid is made up so that 

 i cc. is equivalent to 4 mg. of calcium carbonate, and 125 cc. (which 

 represents 50 g. of soil) are titrated, each o.i cc. required to neutralize 

 corresponds to i mg. of calcium carbonate required by the 100 g. of soil, 

 or to o.ooi per cent of calcium carbonate required by the soil tested. 



The titrations of duplicate samples should not differ more than 0.8 cc. 

 for soil samples requiring less than 100 cc. NaOH. 



Carbonate carbon. Carbonate carbon, when present, is determined 

 volumetrically in the apparatus used for total carbon, described and 

 illustrated in the Journal of the American Chemical Society, Vol. 26, 

 pages 294 and 1640, by treating the air-dried soil with dilute (i : i) hydro- 

 chloric acid and measuring the gas evolved both before and after absorp- 

 tion of carbon dioxid in an alkali pipette containing a 33 per cent solu- 

 tion of potassium hydroxid. The size of sample used for this test varies 

 (according to the amount of calcium carbonate present) between two and 

 ten grams. Duplicate tests of ordinary soils not very high in inorganic 

 carbon should check within 0.2 to 0.4 cc. These results are calculated 

 to and reported as calcium carbonate present. 



Corrections must be made for pressure and temperature, and absorp- 

 tion of carbon dioxid should be repeated to a constant reading; also 

 the gas should be allowed to stand for three minutes before the initial and 

 the final readings. 



Organic carbon. The total carbon of the soil is determined by 

 means of Parr's apparatus * as modified by Pettit 2 to contain an absorp- 

 tion pipette of potassium hydroxid. 



1 Jour. Am. Chem. Soc., Vol. 26, p. 294. 2 Ibid., p. 1640. 



