EXPERIMENT STATION BULLETINS. 389 



should not be exceeded, or variations in results will be obtained, due to 

 (lie escape of nnunonia. Finally the switch S is closed and the sliding 

 coiilact O moved aloni^j the bridj?e wire till Ihe sound in Ihe telephone 

 reccivei- reaches a minimum. The resistance in K should be arranged 

 so that this point falls near the middle of the bridge. After taking the 

 reading, the cell and the electrodes should be rinsed again two or three 

 times and a duplicate reading taken. 



It is general I3' quite dillicult to read the actual point of minimum 

 sound. This is usually obtained by taking tw^o readings, one on either 

 side of the minimum point, having the same intensity. The average of 

 the two readings should give the correct result. 



This bridge reading is all that is necessary for the determination of the 

 neutral point in ammonium citrate solution, since it is a constant func- 

 tion of the conductivity and may be used in place of it in plotting the 

 curve. Its use for this purpose does away with all mathematical calcu- 

 lations and makes the determination of the so-called "cell constant" un- 

 necessary. It is necessary only to keep the resistance in the box R the 

 same during each series of determinations and to be careful that the rela- 

 tive position of the electrodes is not disturbed by bending or striking 

 against the edge of the cell. 



The method of procedure as given by Hall is quite satisfactory. A 

 solution of citric acid is almost neutralized, the density being kept above 

 1.09. Small samples of this solution are then titrated with a dilute solu- 

 tion of ammonium hydroxide, using corallin as an indicator, to deter- 

 mine the approximate amount required to neutralize the remaining acid. 

 Definite quantities of the citrate solution are removed with a pipette 

 and transferred to clean, dry, volumetric flasks. To these portions of 

 the original solution varying quantities of the dilute ammonium 

 hydroxide are added in such a way that some of the flasks contain more 

 and some less than the approximate amount required for exact neutral- 

 ization as determined by the titration with corallin. These solutions are 

 then made up to the same volume with distilled water, placed in a 

 thermostat, the temperature of which is held constant, and allowed to 

 come to the temperature of the bath after which their resistances are 

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

 meters of ammonium hydroxide against the bridge readings gives a 

 curve from which may be read the exact amount of ammonium hydrox- 

 ide to neutralize the acid in a given quantity of the citrate solution. 



The following are some of the results obtained and illustrate the sharp- 

 ness with which the neutral point may be read: 



