Coprecipitation of "foreign" ions

Mixtures of lithium and nickel hydroxides were prepared by impregnating nickel hydroxide with solutions of lithia in distilled water. Mixed gallium and nickel hydroxides were coprecipitated, by steam distillation, from solutions of Ni(OH)a and Ga203, 1.75 H2O in aqueous ammonia. These mixtures of hydroxides were dehydrated at 250° under vacuum (p = 10 torr). Preliminary experiments (40) have shown that incorporation of foreign ions does not occur at temperatures lower than 250° and that, in order to obtain a constant value of the electrical properties of doped samples, it is necessary to heat them at least 24 hours at 250° in vacuo. Incorporation is therefore a slow process at 250°. Dehydration is not complete at 250° NiO-f 10 at. % Li... 

The stoichiometry must be exact. Coprecipitation by solid-solution formation, foreign ion entrapment, and adsorption are possible sources of error. 

Non-lattice sites may play an important role in the incorporation of large foreign ions in crystal structures during coprecipitation Pingitore (Chapter 27) discusses the importance of these sites in the formation of coprecipitates of calcium carbonate containing Srz+ or Ba. White and Yee (Chapter 28) discuss the diffusion of alkali ions into defect structures in the surfaces of glasses and crystalline feldspars. 

Coprecipitation always occurs in connection with the precipitation of sparingly soluble compounds. Foreign ions to the insoluble compound are usually adsorbed on the surface of the precipitate. Coprecipitation is quantitative only for very small concentrations. 

As a result, less barium ion is required to complete the precipitation of the sulfate ion, and the calculated results are low. Some metal ions will complex the indicator and interfere. Foreign anions may coprecipitate as the barium salts to cause high results. Errors from chloride, bromide, and perchlorate are small, but nitrate causes large errors and must be absent. 

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