EXPERIMENT STATION BULLETINS. 413 



the acid curves represent the resultant concentrations of hydrogen ions 

 of the several reactions involved. Points obtained from the No. 10 

 units taken at later periods than two days are indicated on the graphs. 

 An important conclusion that may be drawn from these acid curves, 

 and one bearing on the object of this experimentation, is that in presence 

 of an excess of acid reagent the soil acid reaction goes to a certain point 

 and remains quite constant for several days, at least so far as may be 

 determined by measuring the concentration of hydrogen ions. If this 

 be true, then the line of separation between the more soluble soil bases 

 and the insoluble soil bases is quite marked and the quantity of active 

 or easily soluble soil bases may be quite accurately determined by 

 direct titration with dilute, strong acids. This point will be taken 

 up again later under methods for determining active bases and excess 

 acids in soils. 



BACK TITRATION CURVES NOS. 3 AND 4 



The alkali back titrations were all made wMth Ca(OH)o, but the 

 acid back titrations shown in the first group of curves were made with 

 ALClfi and in the second group with HCl. In performing these back 

 titrations a quantity of acid or alkali equivalent to the previous treat- 

 ment was added to each unit, and the volume of the liquid in each 

 unit was increased above 50 ec. by the volume of the reagent used. In 

 case of the back titrations with acids, the volume increases from 50 cc. 

 to 59 cc. over the series, but with the alkali, from 50 cc. to approximately 

 73 cc. because in the latter case more of a saturated solution of Ca(0H)2 

 was required to equal the 0.1 N equivalent. Evidence will be introduced 

 later to show that this increase in volume could have had only a slight 

 effect on changing the pH of soil suspensions. Gillespie and Hurst (2), 

 and Sharp and Hoagland (7) have also published data to this effect. 

 Allowing for changes in pH produced by volume increases, changes in 

 hydrolysis and solubilities, it may be said that in these back titrations 

 the pH comes back closely to that of the untreated soils. Apparently 

 the Ca(OH)o, wholly or in part, forms salts with weak soil acids and 

 these salts may be back titrated closely to the point of chemical equival- 

 ence with strong acids. 



Considering the acid titrations, salts were formed with soil bases, 

 but a quantity of alkali equivalent to the acid used in the initial treat- 

 ments brought the pH of the mixtures closely to that of the untreated 

 soils. The acid salts formed might be back titrated, but it would be 

 difficult to account for the quantities of alkali used equivalent to the 

 neutral salts formed unless it were assumed that in forming these 

 neutral salts, soil bases were split off from weak acids and these acids 

 took up the alkali in the back titrations. 



THE TEMPERATURE FACTOR 



Temperature control was not provided for individual determinations. 

 An attempt was made, however, to control the temperature within cer- 

 tain limits of the room in which the work was done by regulating the 

 ventilation. This room was situated on the north side of a brick build- 



