174 STATE BOARD OF AGRICULTURE. 



in agricultural work demauds. however, that some convenient means 

 be devised for preparing the necessan- reagent. 



As is well known, the variation in electrical conductivity offers a 

 means of titrating some solutions with which ordinary indicators fail 

 and it occurred to one of us, that, possibly this might offer a way of 

 overcoming the difficulty in making a neutral solution of ammonium 

 citrate. The method depends upon the velocities of the hydrogen and 

 hydroxy] ions. If alkali is added to an acid solution, i. e., a solution 

 containing free hydrogen ions, the conductivity decreases as the amount 

 of alkali is increased. The reason for this is, that the rapidly moving 

 hydrogen ion of the acid is removed by union with the hydroxyl ion of 

 the base and the current-carrying power of the solution is thrown more 

 and more upon the slower moving anions and cations of the acid and 

 alkali radicles respectively. After passing the neutral point however, 

 the conductivity increases owing to the addition of an excess of the 

 rapidly moving hydroxyl ions. Hence, by plotting the conductivity (or 

 resistance or bridge readings) against the amount of alkali and get- 

 ting several readings on each side of the neutral point a curve can be 

 plotted which will show a minimum or maximum at the point of 

 neutrality of the solution. From this curve the exact amount of alkali 

 required to neutralize a given -quantity of the solution can be predicted. 

 ^Ve found that the method worked ver}^ well and no trouble was ex- 

 perienced in determining quite accurately the amount of alkali required 

 to neutralize the acid present in the solution used. The method is 

 easily carried out and requires but little more time than the ordinai'y 

 method of neutralizing with coralline as an indicator. 



The method of procedure was as follows : A solution of citric acid 

 was almost neutralized with ammonia, care being taken to keep the 

 density below 1.09. Small samples of this solution were then titrated 

 with a dilute solution of ammonia (about 3%) to determine the ap- 

 proximate amount required to neutralize the remaining acid. Definite 

 (piantities of the citrate solution were then removed with a pipette and 

 transferred to clean dry flasks. To these portions of the original solu- 

 tion, varying quantities of the dilute ammonia solution were added in 

 such a way that several contained more and several less than the ap- 

 proximate amount required for exact neutralization, as determined by 

 the titration with coralline. These solutions were then placed in a 

 thermostat the temperature of which was held constant and allowed to 

 come to the temperature of the bath, after which their resistances were 

 measured by the wheatstone bridge method. Plotting the cubic centi- 

 meters of ammonia added against the bridge readings, gave a curve from 

 which could be read the aihount of ammonia required to exactly neutral- 

 ize the acid remaining in a given quantity of the citrate solution. 



The following tables give the results of two such titrations, 100 c. 

 e. of acid solution being used for each determination. Solution I was 

 almost saturated. Solution 2 was diluted so that the addition of the 

 required amount of ammonia brought the density up, almost, to 1.09. 



