STANDARD HALF CELLS 



33 



In these cases a triple-limbed cell is used, the extra central limb being plugged with 

 cotton wool and connected by rubber tubing to the aeration apparatus. Vacuum 

 vessels have been described for special purposes (Lehmann, 1930 ; Baimiberger, 

 1933). 



When investigating unstable systems Ball and Chen (1933) used an electrode 

 vessel through which reacting fluids were allowed to flow. By variation of the rate 

 of flow, the electrode potential could be measured at different intervals after mixing. 



Standard Half-Cell 



Any convenient standard half-cell may be used to complete the cell. The 

 potential of the calomel electrode, a common form of standard half-cell, depends 

 on the concentration of KCl used in setting up, the half-cell being : — 



Mercury : mercurous chloride, KCl solution : KCl solution 



Various concentrations of KCl have been employed, including tenth-normal, 

 normal, 3-5 normal and saturated KCl calomel electrodes. The author finds the 

 3-5 N. KCl electrode very satisfactory, since it has the low temperature coefficient 

 of the N/10 electrode, and alteration in concentration of KCl has only a slight effect, 

 as with the saturated KCl electrode. The following may be taken as the values of 

 the half-cells referred to the normal hydrogen electrodes :■ — 



TABLE 6 



These values are to be added to observed electrode potentials to obtain the 

 value of E^. 



Another very simple half-cell which may be set up in a few minutes is the 

 quinhydrone electrode. Any acidic buffer solution of known pH is taken, and a 

 little quinhydrone is shaken into it. A platinum electrode is immersed in the 

 solution and the cell is complete. It is essential that the pH of the buffer solution 

 be known exactly. Convenient buffers are : (i) 0-1 M. acetic — 0-1 M. sodium acetate, 

 which has pH 4-60, and (ii) 0-05 M. potassium hydrogen phthalate, which has 

 pH 3-974, also N/10 HCl is frequently used. 



The values of the quinhydrone electrode potential referred to the normal 

 hydrogen electrode are given in table 7. 



TABLE 7 

 Potentials of Quinhydrone Electrode 



