Deposition of Sparingly Soluble Salts on Electrodes

A precipitation of insoluble salts on an electrode surface can be initiated and controlled by varying the concentration of metal ions. The most effective method is electrodeposition of mercuric and mercurous salts induced by an anodic polarization of a mercury drop electrode in a solution of anions [32-35]  

The insoluble salt Hg2X2 is precipitated as a submonomolecular layer on the mercury electrode surface. It is assumed that no lateral interactions between the deposited particles exist. At equilibrium, this redox reaction satisfies the Nernst equation [33,36]  

The redox reaction (Eq. II.7.18) can be considered as an electrosorption reaction in which the ligand X is adsorbed to the electrode surface by forming a more or less polarized covalent bond with mercury atoms [37, 38]. When die quantity of adsorbed ligand is less than a monolayer, the adsorbate on the surface behaves as a two-dimensional gas. The adsorption constant of the ligand depends on the electrode potential  

The adsorbate is stripped off by cathodic polarization of the mercury electrode. Apart from inorganic anions [32-35], the method can be used for the accumulation of various sulfur-containing organic molecules such as thiols [32, 36, 39-41], thioethers [32], thiopentone [42] and phenothiazines [43]. 

Manganese dioxide can be accumulated on the surface of a platinum disk electrode by oxidation of Mn ions at 0.9 V vs SCE in 0.1 mol/L NH4CI [4,44]. The precipitate is subsequently reduced back to Mn at 0.3 V. This reaction is the best example of electroprecipitation on noble metal electrodes [2,45,46]. 

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