EXPERIMENT STATION BULLETINS. 609 



justmeut of tlie solution. This was the electrical conductivity method 

 proposed by Hall and Bell (23) and later by Patten and the author 

 (42). It was subsequently modified by Hall (22) in an attempt to 

 make it practicable for routine work. It depends upon -the fact that 

 upon the addition of ammonia to a solution of citric acid (or vice versa) 

 the resistance of the solution to the passage of an electric current varies 

 progressively till the point of complete saturation of the acid (or base) 

 is reached, at which point, if the resistance and the amount of acid (or 

 base) be plotted, there will be a break in the curve. It was demonstrated 

 that accurate results could be obtained with the method but, because of 

 the apparatus required and real or imaginary difficulties in manipula- 

 tion, it has never found widespread use. 



In the titration method of Patten and Marti (41) which was pub- 

 lished in 1913 there appeared one which required neither special appa- 

 ratus nor difficult technic. This method "is based upon the work of 

 Schiff who found that ammonia unites with formaldehyde to form 

 hexamethylenetetramine according to the following equation : 



4NH3 4- 6CH,0 = N(CH„NCH,)3 + GH.O 



Schiff also found that the same reaction takes place between salts of 

 ammonia and formaldehyde and that the acid may then be titrated with 

 a standard solution of sodium or potassium hydroxide using phenol- 

 phthalein as indicator. With this method the equivalence of the acid 

 and alkali in a solution of ammonium citrate could be easily determined 

 and the composition of the solution adjusted accordingly," 



Rudnick and Latshaw (45) used this method to prepare a solution 

 which they compared with one x>repared by a procedure devised by 

 themselves. This was a variation of Hand's technic in which a series 

 of solutions of varying degrees of acidity and alkalinity were prepared 

 and that one which appeared to be the neutral one was selected for the 

 standard. The reaction of the reagent to be used was adjusted to this 

 using a colorimeter for their comparison. The solution prepared by 

 the former method has a ratio of ammonia to citric acid of 1 : 3.78G and 

 that made by the latter procedure had a ratio of 1 : 3.748. 



Bell and Co well (6) proposed two methods, one a chemical one and 

 one a physical. Tlie former was based upon the fact that ammonia is 

 soluble in chloroform while citric acid and ammonium citrate are not. 

 The solution to be tested was nearly neutralized. Aliquots were then 

 taken and mixed with increasing quantities of standard ammonia solu- 

 tion. They were then shaken out with chloroform. As soon as an 

 excess of ammonia was in the solution it could be detected in a sample 

 of the chloroform layer removed for the purpose. 



The second method proposed was based upon the determination of the 

 point at which heat ceases to be generated by the reaction between 

 the citric acid and ammonia. 



During the same year Hildebrand's (27) article on the use of the 

 hydrogen electrode appeared and in it he suggested the use of this 

 apparatus for the purpose of obtaining a truly neutral solution of 

 ammonium citrate. It was not until the following year, however, that 

 a detailed method of procedure was published. In this second article 

 with Eastman (15) he presented a scheme for the preparation of solu- 

 77 



