EXPERIMENT STATION BULLETINS. 411 



JMTIAL ALKALI TITRATION CURVES 



In obtaining the first group of four No. O curves the whole titration 

 series was first set up and the pH values then determined, which re- 

 quired nearly four hours time for completion. The second group of four 

 No. O curves illustrate the results obtained from reading each unit as soon 

 as possible after bringing the Ca(OH)o in contact with the soil. The ob- 

 servable difference .in these two groups of curves is that the second 

 group shows marked butfering midway of the curves, which may in- 

 dicate the formation of a calcium salt. Points in these curves, how- 

 ever, can not be considered definite, as the reaction between soil and 

 Ca(OH)o progresses rapidly in the beginning. Tliese curves are use- 

 ful in making comparisons between them and the other alkali titration 

 curves. 



ALKALI TITRATION CURVES XOS. 1 A\l) 2 



As may be observed, these curves run closely together below pH 

 7.00. Curves No. 2, however, occupy a position slightly above curves 

 No. 1 below the neutral point; but they cross in the vicinity of pH 7.00 

 and lie below curves No. 1 above this point. The fall in i)H shown by 

 these alkali titration curves is considered due to the disappearance of 

 hydroxyl ions, indicating that above the neutral point tlie reaction is 

 still in progress. The slight rise in pH below the nentral point, shown 

 by curves No. 2, may be due to increased hydrolysis of soil compounds. 

 This phenomenon was also observed in the untreated soils and in the 

 acid titrations before tlie point of complete neutralization of soil bases. 

 pH readings on the No. 10 units taken at later periods than two days 

 are also shown on the graphs. These results seem to show that the re- 

 action on the acid side of the titration is practically complete in one 

 day; but in the presence of an excess of hyclroxyl ions, it is continuous 

 over a long period of time. 



Since the pH readings for the alkali titration curves were made in 

 open containers, it was thought necessary to check the results with 

 another experiment using Ca(OH)o and water without soil in alkali 

 concentrations representing excess of alkali over that required to bring 

 the soils to pH 7.00, in order to determine the effect of CO, absorption 

 from the air. This experiment was conducted in the same manner as 

 those for the soil — Ca(OH)o titrations except the soil quantities were 

 left out. The results are illustrated in Fig. 10. An examination of 

 these curves will show: (1) that the pH fell more rapidly in the dilute 

 solutions than in the more concentrated solutions, and (2) the rate 

 of fall in pH increases with time. These results are opposed to those 

 obtained when soil is present in the determinations; hence, it is as- 

 sumed that the soil — Ca(OH)o results are not due to absorption of CO, 

 from the air. 



