140 UNITY AND DIVERSITY IN BIOCHEMISTRY 



chloride solution an electrode made of a noble metal (platinum, for 

 example) at whose surface no chemical reaction occurs. By means of an 

 agar bridge, we can connect the solution with another solution capable of 

 giving or receiving electrons. In this solution we will also place a platinum 

 electrode and we will join the two electrodes by a wire together with a 

 potentiometer in the circuit. If the electron pressures of the two solutions 

 are different there will be an electron flow which will be revealed by the 

 passage of an electric current. If we know the electron pressure of the 

 reference solution, and measure the potential difference and direction of 

 flow of the current, then we have a basis on which to calculate the electron 

 pressure of the first solution. As a reference solution we may take what is 

 commonly known as the normal hydrogen electrode, that is, a normal solu- 

 tion of hydrochloric acid saturated with hydrogen gas. The e.m.f. of the 

 system is measured with a potentiometer. The electron pressure (which 

 depends upon the ratio between the concentrations of the oxidized and re- 

 duced substance) is called the oxidation-reduction (or redox) potential (E). 

 The relation between E and the concentrations of the oxidized substance 

 and the reduced substance is given by the equation : 



RT (oxidized form) 

 nF (reduced form) 

 in which, 



R is the gas constant; 



T is the absolute temperature ; 



n is the number of electrons involved when the substance passes from 



the oxidized to the reduced state (for Fe+++ <^ Fe++, « = 1); 

 Eo is a special constant for each particular system. 

 Furthermore, In signifies log to the base e and F is the Faraday {F = 

 96,500 J, and since 1 cal = 4-18 ], F= 23,098 cal). 



If the concentration of the oxidized form (Ox) is equal to the concen- 

 tration of the reduced form (Red), 



(Ox) 

 {Ox) = {Red) and In -j^^ = 0. 



RT 



£:=£„+ -J- XO=Eo. 



To measure Eo, it is only necessary to measure E when {Ox) = {Red). 

 Eo is termed the standard oxidation-reduction potential, it is the potential 

 which is measured against the hydrogen electrode at the pH of a normal 

 solution of HCl (pH = 0, since log 1 = 0). It has become customary to 

 state the potential at, or around, pH 7-0 and this standard potential is 

 designated by the symbol Eo'. 



