Electrolysis concentration changes during

The Hittorf method is based on measuring the concentration changes at the anode and cathode during electrolysis. These changes can be found by a sensitive analytical method, e.g. conductometrically for a suitable cell... 

The shape of the potential-time response is determined by the concentration changes of O and R at the electrode surface during electrolysis. The potential is related to Cq/Cr via the Nernst equation for a reversible system. The initial potential before current application is simply the rest potential or open-circuit potential (E ) of the solution, which reflects the initial Cq/C in solution. At the instant of current application, this ratio becomes finite and the potential changes to a value consistent with the Nernst equation. Early in the... 

In the above discussion we have assumed that the electrodes are inert—i.e., are not attacked as electrolysis proceeds—and that the ions M" and A" are simply deposited. If instead, for example, the anode were to pass into solution during electrolysis, the concentration changes would be correspondingly different the treatment, can readily be modified for this and other situations. 

Current Changes during an Electrolysis at Constant Applied Potential It is useful l4) consider the changes in current in the cell under discussion when the potential is held constant at -2.5 V throughout the electroly,sis. l. iider these conditions. the current decreases with time as a result of the depletion of copjter ions in the solution as well as the increase in cathodic concentration polarization. In... 

Fig. 1. Concentration changes near electrode surfaces during electrolysis. Quiet—unstirred solution flowing—electrode in flowing stream or well-stirred solution 81,2, etc.—diffusion layer thickness t—duration of electrolysis.

The first boundary condition describes how the concentration on an electrode surface changes during the electrolysis. Assume such potential is appUed to the electrode that the concentration of Ox on the surface immediately drops to zero. This potential is kept stationary during the electrolysis therefore, Co (0, t) = 0 for any t > 0. So, the first boundary condition characterizes the electrolysis regime. For other regime, this condition would be different, as would the solution. 

In establishing the character of a complex (or a double) salt, it is necessary to determine the nature and number of the constituent ions. It is usually simpler to determine if a given metal is in the positive or negative ion by investigating the changes of concentration of the liquid about the electrodes during electrolysis. Allowances have to be made for secondary changes. 

Another example of the effect of a change of concentration upon the cathodic process can be found in electrolysis of a solution of salts of copper and bismuth. As the respective deposition potentials, which practically equal the equilibrium potentials are fairly close (7c( — 0.34 V, 71 it, = 0.23 V) the two metals cannot he separated from each other electrolytically. On the addition of cyanide, however, Cu++ ions are converted into cupricyanide ions from which copper cannot be deposited prior the cathode reaches the potential Ttt u equalling to about — 1.0 V. As bismuth does not form cyanide complexes the resulting difference in potentials, 7ti — 7Cou — 1.23 V is a sufficient guarantee that during electrolysis only bismuth will be, preferentially deposited. 

A similar consideration can be applied to the cathodic processes. In a solution of mercuric nitrate bivalent mercury will bo reduced to univalent until the ratio of the respective activity of the mercurous salt formed and tho mercuric salt still remaining reaches the equilibrium value. During the course of further reaction the ratio of activities of both ions in the solution will not change any longer, and metallic mercury will be deposited. Therefrom, it is evident that mercuric nitrate cannot be quantitatively reduced to mercurous salt. Bivalent mercury can be reduced practically completely to univalent in the case of mercuric chloride. As the solubility of the mercurous chloride formed by the reduction and consequently also the concentration of Hg2+ ion is very small the equilibrium between the ions in the solution will be attained only then, when nearly all Hg++ ions will be reduced to univalent ones. On the other hand on reduction of the very slightly dissociated cyanide complex Hg(CN) the equilibrium between mercurous and mercuric ions is reached at the very beginning of electrolysis as soon as a hardly noticeable amount of Hg++ ions has been formed from that moment on metallic mercury will be deposited at the cathode with practically 100 p. o. yield. 

The chemical change occurring during the course of electrolysis is observable on or in the vicinity of the electrodes. In many cases such a change is a simple decomposition. If for example a dilute solution of hydrochloric acid is electrolysed (between platinum electrodes), hydrogen gas is liberated on the cathode and chlorine on the anode the concentration of hydrochloric acid in the solution decreases. 

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