180 



>, 0.250 



Equivalence point r'' 



4 8 12 16 20 



Volume Ag NO3 Solution 



Fig. 7. Potentiometric titration curve 

 of sea water with silver nitrate. Indi- 

 cator electrode was AgrAgCl. Reference 

 electrode was Ag:Ag 



4 8 12 



Chlorlnity (7oo) 



Fig. 8. Derivative curve for potentio- 

 metric titration of sea water with silver 

 nitrate. Maximum of this curve cor- 

 responds to equivalence point shown in 

 Figure 7. 



destroy this relationship. However, 

 the qualitative effects of variations in 

 several factors that can be controlled 

 experimentally are known. When cur- 

 rent is drawn from a cell, the electrodes are polarized. The result is similar 

 to that obtained by energizing electrodes in a conductivity cell with a D.C, signal 

 or A.C. of poor wave form. In the case of the cell in the ACT-I, silver chlor- 

 ide is either made or destroyed at the indicator electrode and silver either plated 

 out or put into solution at the reference electrode, depending on the direction of 

 current flow. In either case the potential of the electrodes will change. Actu- 

 ally, drawing current from the cell is of secondary importance. The important 

 variable is the current density at the electrodes. It is possible to reduce polar- 

 ization effects by decreasing the current density. This can be achieved either 

 by increasing the area of the electrodes or, if the sensitivity of the measuring 

 equipment will permit, by decreasing the current drawn from the cell. 



In the Lingane (1948) apparatus the circuit was arranged so that a current 

 of 4.65 microamperes per volt would flow through the cell. Under these condi- 

 tions Lingane concludes, 'the current is so small that the 'electrolysis error' at 

 the indicator electrode is negligible. " Although this does not give a complete 

 indication of polarization effects, the precision and accuracy of his titration 

 suggest that they are small. 



In the present titrator circuit the current drain on the electrodes is held 

 to less than 0.005 nnicroampere, and the apparent impedance of the input system 

 is of the order of 20 megohms. A circuit having these characteristics when 

 coupled with the electrodes presently used in the titrator effectively eliminates 

 electrolysis errors and potential drift due to "working electrodes. " 



