Electrodes ofthe Second Kind

The case where an insoluble intermediate compound forms is more complicated, though, in our opinion, more interesting. In this relation, let us continue with the analysis of the system that was considered in Section 2.2. Let us make an additional assumption that insoluble hydroxide MOH (or the product of its decomposition - M2O) may form within it. In this case, the maximum M ion concentration in the solution [M ] is defined by the solubility product of the said compound or the expression equivalent to it  

1) For the sake of simplicity stoichiometric coefficients of the compounds are not given. 

Let us imagine that equilibrium [M ], which formed in the second state (see Section 2.2), is larger than its critical value corresponding to expression (2.20). Then, a certain part of M(I) is to change into an insoluble compound. The decrease In the amount of this component in the solution phase will upset the equilibria that were reached before causing its relevant shifts. All the macroprocesses of the system will come to a stop after a complete (total) equilibrium has been reached, which includes in the case under consideration the following  

In case of the equilibrium under study, the composition of the complex system may differ in essence from the earlier described one when the electrode of the first kind is formed. Now, by means of Eq. (2.20), concentration of ions M of the intermediate oxidation number is described and, having inserted the value in Eq. (2.17), is found. As from the electrochemical point of view, ligand is considered to be inert, its total concentration remains unchanged during the processes. Then, the equilibrium concentration of free ligand [L] can be found from Eq. (2.15). Then, with known [M ], and [L] values and the stability 

It is not difficult to evaluate the amount of the insoluble product in (moles), 

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