Ionic Crystals in Acidic and Alkaline Media

Dissolution of Ionic Crystals in Acidic and Alkaline Media (5). All ionic crystals are not soluble and many insoluble substances are etched In acidic and alkaline media. The ideas of dissolution in terms of double layer potentials can be extended to such situations in which it is considered that protons or hydroxyl ions of an acid or an alkali are complexingagents for anions or cations, respectively. For example, in the dissolution of MgO in aqueous HCI, H+ ions are the complexing agents for 0 such that OH and HjO are produced. 

Assuming that, for the dissolution of a hydroxide crystal, the rate of removal of hydroxide ions as water is much greater than that of OH so that 

This equation shows that the potential 4 increases with ion concentration. Substitution of Eq. 25 into Eq. 24a yields  

The dissolution mechanism of an oxide may be complicated. The following situations are possible  

Surface oxide ions may first hydrate to form OH ions. The dissolution mechanism then is like that for a hydroxide. 

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