[son io x itr5 090 x lo5 oio x io5

These calculations indicate that the [S04~] is reduced considerably. This example is an application of the common ion effect, which is used extensively in environmental engineering practice as well as in qualitative and quantitative analysis to accomplish essentially complete precipitation of desired ions. For example, hardness (Ca2+ and Mg2+) is removed by adding CO2- to precipitate CaC03(j) and OH" to precipitate Mg(OH)2(i).

Diverse Ion Effect

The diverse ion effect describes the adverse effect that unrelated ions often have upon the solubility of some relatively insoluble substances. Such ions, theoretically, play no part in the chemical equilibrium involved but often increase the solubility of desired precipitates to such an extent that undesirable results are obtained. Nitrate ion has such an effect on silver chloride. Because of the diverse ion effect, it is general practice to keep the concentration of extraneous ions as low as possible during qualitative and quantitative work.

The diverse ion effect can be explained by considering activity corrections to the solubility product. Using CaCOjO) as an example:

Since increasing ionic strength (increasing the concentration of unrelated ions) causes y to decrease to less than 1.0 (see Sec. 4.3), this increases the molar concentration of Ca2+ and CO|"" in solution at equilibrium.

Sometimes a common ion may serve very well up to certain concentrations, but when used at higher concentrations, it appears to have a diverse ion effect. In this case, the usual explanation is that complex ion formation is taking place, as described previously. Hydrochloric acid acts in such a manner when it is used as the agent to precipitate silver ion. When hydrochloric acid is added in excess, the soluble AgClj complex is formed:

Therefore, the amount of hydrochloric acid used should be carefully controlled.

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