STANDARD POTENTIALS 17 



there must be a standard of reference, some zero value to which all other potentials 

 are referred ; this is the normal hydrogen electrode to which the potential volts is 

 ascribed. The next difficulty is to decide what potentials are to be regarded as positive 

 and which negative. Here, unfortunately, there is confusion and conflict. In this 

 work, and in nearly all biochemical investigations, oxidising systems are regarded as 

 being more positive in potential than any more reducing system. Since oxidising 

 systems are those which take uj? electrons more readily, i.e., attract negative electricity 

 they are regarded as being more positive, thus : — 



Fe©®© + € -> Fe®® 



so that ferric iron is regarded as more positive than ferrous iron. Unfortunately, 

 writers on thermodynamics frequently have adopted the opposite convention (Lewis 

 and Randall, 1923) and regard the potential as the tendency of electricity to pass from 

 the right to the left of the cell, i.e., from the electrolyte to the electrode. 



In Table 1 are given values of the standard oxidation-reduction potentials of a 

 number of inorganic systems. Potentials are referred to the normal hydrogen elec- 

 trode as zero and oxidising systems are positive in potential compared with more 

 reducing systems. The data are derived mainly from Lewis and Randall (1923), 

 Latimer (1938), De Vries (1947) and Glasstone (1947). 



Summary of Chapter I 



Oxidation and reduction processes are defined in terms of electron migrations ; 

 oxidising properties are due to a tendency to take up electrons, and reducing pro- 

 perties to a tendency to part with electrons. When one substance is oxidised (parts 

 with electrons) another is always simultaneously reduced (takes up the electrons 

 liberated). This electronic concept has suggested a method of studying quantita- 

 tively those reversible oxidation-reduction processes which are of vital importance 

 to the living cell. The method is the measurement of electrode potentials which 

 enable the degree of oxidation or reduction of a system to be gauged, and which 

 render possible the grading of oxidising and reducing systems according to their 

 intensity levels. 



In a reversible oxidation-reduction system the electrode potential depends upon 

 the relative amounts present of the oxidised and reduced forms of the substance 

 studied. If a solution contains a mixture of the oxidised and reduced forms Xq 

 and Xr, the reactants in the reversible oxidation-reduction system : — 



Xr ^ Xq + n electrons 



then the electrode potential (E^) at 30°C. will be given by the equation : — 



E, = Eo + --logp^^ 



where Eo = constant for the system ; [X^] and [Xr] are the concentrations of 

 the oxidised and reduced forms respectively. 



It follows from this equation that the more oxidising a system is the higher 

 (more positive) will be the electrode potential and more reducing systems will have 

 lower (or more negative) potentials. Conversely the potential is dependent solely 



