178 



"1 



CONTROL' UNIT 



^ % — PILOT WARNING 



MASTER SWITCH- 



LIGHTS 



TITRATION UNIT 



TITRATION UNIT 



STOPCOCK 

 MOTOR 



i 



CANNON PtUG 



CONTROL UNIT 



PILOT i !6HT 



CONTROL UNI 

 SWITCH 



Fig. 5. The Automatic Chlorinity Titrator (ACT-I) 



the potential of the electrode, measured against a suitable reference, will be 

 predictable at all stages of titration and will be related to the volume of silver 

 nitrate added as shown in figure 7. These data were obtained during a manual 

 titration of a sample of sea water by measuring the potential of the cell Ag: 

 AgCl(s) Cl"(aq) :: Ag+ :Ag after the addition of each increment of silver nitrate 

 solution. Figure 8 is a plot of A emf/A Cl^/oo against the mean chlorinity cal- 

 culated from the titration curve data. Volume of silver nitrate has been con- 

 verted to chlorinity units in this curve. The magnitude of the "derivative" in 

 the region of the equivalence point gives an indication of the precision that can be 

 expected in the chlorinity measurement for a given difference between the equiv- 

 alence point potential and any measured potential. For example, the maximum 

 slope is approximately 800 mv per chlorinity unit. Thus an error of +1 mv in 

 potential amounts to only +0.00125 CI °/oo, or, in order to obtain the precision 

 demanded of the Knudsen method, +0.01 CI o/oo, the potential measurement 

 must be good to +8 mv. 



Theoretically, the potential of the cell can be used as a measure of chlor- 

 ide ion concentration only when no current is drawn from the cell. Under these 

 conditions the electrodes are operated reversibly and the potential of a silver, 

 silver chloride electrode will be related to the concentration of chloride ions by 

 the familiar Nernst relation. In practice, it is impossible to measure the poten- 

 tial of a cell without drawing current from it. It becomes necessary then to find 

 to what extent the electrodes can be "worked" and retain the desired precision 

 and accuracy between the emf and concentration. Theory does not provide a 

 basis for making a reliable estimate of the extent to which "working electrodes" 



