558 



Journal of Agricultural Research 



Vol. X, No. II 



the same way the equilibrium between carbonates and bicarbonates can 

 be changed by modifying the partial pressure of the carbon dioxid in the 

 gas phase. In a study of soil solutions a system whose stability is 

 dependent upon the gas phase is often of more importance than one con- 

 sisting wholly of stable salts. Equilibrium conditions are, furthermore, 

 not necessarily reached in any soil solution. The movement of the soil 

 moisture may remove the salt which is formed in one place and carry it to 

 another in a rather unstable condition 



The presence of the carbon dioxid in the atmospheric air is due in 

 large part to the action of the bacteria and other organisms upon the 

 organic matter of the soil. If a soil solution containing sodium chlorid, for 

 example, be in contact with the solid calcium carbonate and the bacteria 

 begin to generate carbon dioxid, the gas will appear and pass through the 

 soil solution in its way through the soil toward the outside air. In other 

 words, the equilibrium between sodium chlorid and calcium carbonate, 

 so far as soil conditions are concerned, may be determined by a much 

 higher partial pressure of carbon dioxid than is present in atmospheric air. 



REACTION BETWEEN SODIUM SALTS AND CALCIUM CARBONATE IN 

 PRESENCE OF CARBON DIOXID AT ATMOSPHERIC PRESSURE 



as 



eooo 



Solutions of sodium salts with the solid calcium carbonate were prepared 

 before described and brought to equilibrium with carbon dioxid. This 



was accomplished by 

 passing carbon dioxid 

 into the solutions until 

 the gas that was read- 

 ily absorbed had been 

 taken up. The shaker 

 bottles ( I -quart milk 

 bottles) , which were 

 about half full of the 

 solutions, were then 

 filled with carbon di- 

 oxid, tightly stoppered, 

 and shaken until final 

 equilibrium was 

 reached. This filling of 

 the bottle with carbon 

 dioxid and shaking it 

 had to be repeated a 

 great many times and 

 over a period of several 

 days. The final product represented equihbrium between the sodium 

 salt and calcium carbonate in an atmosphere approaching pure carbon 

 dioxid. Portions of the solution were withdrawn and titrations made 

 with standard sulphuric acid. The titration figure of No. i , in which 



O 5 /O 'S ^O ^S 30 SS 'K) '35 SO 55 60 



Fig. II. — Graphs showing the concentration of sodium bicarbonate 

 in the systems discussed above when in equilibrium with carbon 

 dioxid at approximately atmospheric pressure. 



