388 



STATE BOARD OF AGRICULTURE. 



The coating wliieli is formed should be black and velvety. It is quite 

 essential that a pr()i>ei' coating of platinum black hv oblained. for upon 

 the condition of the electrodes dei>ends the sharpness of the end point. 

 Then tJie electrodes ai-e washed with wann distilled water and replaced in 

 the cell with dilute IIo SO4 substituted for the i>latinizing solution. The 

 cuiTent should be passed for about the same length of time but need not 

 be reversed so often. The electrodes are then wa.shed several times with 

 wann distilled water and tested to be sure that all soluble material has 

 been removed. This is done by taking several bridge readings, as de- 

 scribed below, with distilled water in the cell. These readings should re- 

 main constant if the cell is clean, otherwise there will be a gradual nse 

 of resistance. 



For deteiTuining the resistance of a solution the apparatus is con- 

 nected as shown in the diagram (Fig. 1). 



AB=bridge wire, 

 C^sliding contact, 

 Rr=resistance box, 

 X=:electrolytic cell. 

 I =introduction coil, 

 T=telephone receiver, 

 S:=:switch or telegra])h key. 



F. 



The cell and electrodes should first be rinsed I wo m- three times with 

 the solution to be tested. A quantity of the soluticm is then ]»laced in the 

 cell X. and air Inibbles shaken out by moving the electrodes n]> and 

 down and tlu^ connect i<»ns made as indicated. 



Before taking the reading the solution must be allowed to come to the 

 temiKM'ature of the li;ith. If the llasks containing the origiujil solution 

 nre kept in the Icith. tliis will t;il<e only a short time, ollicrwise the cell 

 and contenis shonid be left in the bath from seven lo ten minutes. With 

 citrate solution containing an excess of ammonium hydroxide this period 



