CHAPTER VIII 



THE POLAROGRAPH 



PolarograpMc methods like oxidation-reduction j)otential measurements depend 

 upon electrode potential relationships but they are applied in a different manner. 



As already indicated in Chapter I, to measure an oxidation-reduction potential 

 an inert electrode is immersed in the system and an equal and opposite external 

 potential is applied so that no current flows in the system. With the Polarograph, 

 however, various potentials are applied to an electrode in the system and the current 

 flowing at each potential is measured. As the current flows hydrogen is liberated 

 at the cathode effecting reduction whilst an equivalent amount of oxidation occurs 

 at the anode if any oxidation-reduction systems are present. The magnitude of 

 the current flowing is a measure of the rate of these oxidation-reduction reactions and 

 hence, as will be shown later, of the concentration of oxidation-reduction systems 

 present. With the Polarograph therefore, there is the possibility not only of 

 recognising the qualitative nature of the oxidation-reduction systems present but 



MA 



Fig. 39 



Simplified diagram of polarographic apparatus, showing dropping mercury electrode, milliammeter 

 (MA) to measure current and source of variable applied electromotive force (V). Amplifying circuits 



are omitted 



also of their amount. When electrolytic current flows through such systems polar- 

 isation occurs and a small uniform electrode is required. Generally therefore the 

 inert metal wire or foil electrode used in oxidation-reduction potential measurements 

 is abandoned, and in its place drops of mercury continuously dropping from 

 a fine capillary tube are used. In general the mercury drops form the 

 cathode whilst the pool of mercury formed by the drops at the bottom of the vessel 

 can be used as the anode. Since any oxygen present in the system may be reduced at 

 the cathode and upset results the system is often deaerated by passing nitrogen or 

 hydrogen before measurements begin. 



Digrammatically the arrangement is represented in Fig. 39 ; by means of the 

 potentiometer P different potentials can be applied to the Cathode C and the current 

 flowing at each value of the potential can be measured by the galvanometer MA. 



If a hypothetical oxidation-reduction potential system is considered the voltage- 

 current curve obtained in the polarograph might be as given in Fig. 40. 



As the negative voltage applied is increased from a zero value the current 

 flowing increases very little until the point A is reached. The small current flowing 



