POLAROGRAPHY 



157 



It should be mentioned that the current flowing varies with the life of the drop 

 of mercury falling from the cathode capillary tube ; as it becomes larger the current 

 rises and when the mercury drop falls a new and smaller drop begins to form and 

 the current decreases again (curve T in fig. 41), but by using a galvanometer of long 

 period and incorporating a condenser in the circuit the variations in the current 

 with the life of the drop are largely eliminated and a relatively smooth curve (G) 

 is obtained. The mean current (M in fig. 41) is equal to six-sevenths of the maximum. 



Seconds 



Fig. 41 

 Current- time curves during life-time of drops of mercury 

 T — true curve 

 G — galvanometer readings 

 M — mean current 



Of course, when more than one oxidation-reduction system is present, there is 

 a number of steps on the voltammetric curve each corresponding to one of the 

 oxidation-reduction systems. 



EXPERIMENTAL METHODS 



The apparatus required for polarographic work consists first of the dropping 

 mercury electrode which is merely a container of mercury to which is attached a 

 capillary tube from which mercury drips continuously. The tip of the electrode 

 dips under the surface of the solution investigated contained in the electrode vessel 

 which has provisions for de-aeration, generally in the form of inlet and outlet tubes 

 through which nitrogen or hydrogen can be passed. Occasionally a small stationary 

 rotating or platinum electrode is used instead of the dropping electrode. The station- 

 ary electrode may be in the form of a pool of mercury at the bottom of the electrode 

 with a wire making contact and projecting from the vessel or a salt bridge attached 

 to an external half cell. By means of a convenient bridge circuit a variable, measured 

 voltage is applied across the electrodes and the current passing at different voltages 

 is measured by a sensitive galvanometer of long period to damp out the variations 

 in current during the life of the drop of mercury at the capillary tip of the dropping 

 electrode. The voltage applied is generally between -2 and + 2 volts and the 

 current should be measurable to 0-01 microamperes. 



In the simplest manually operated form of the polarograph a slide wire 

 potentiometer is used to apply the e.m.f. and a suitably shunted galvanometer 

 is used to measure the current flowing through the cell at each value of the applied 

 e.m.f. Generally, however, automatic recording devices are incorporated in the 

 polarograph. The voltage applied is varied continuously by means of a small electric 



