Most important is that the anions are stable at the

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Unformatted text preview: re stable at the same pH as the HT and stable with respect to hydrolysis. Another problem is the purity and crystallinity of the materials. For example, when preparing HTs with anions different than carbonate great care has to be taken to avoid contamination of CO2 because of the high stability of carbonate containing HTs, due to the formation of strong hydrogen bonds. Furthermore, line broadening in the XRD patterns and the possible presence of amorphous phases or adsorbed species at the HT surface may prevent definite answers. HTs are commonly prepared by coprecipitation. In order to coprecipitate two or more cations it is necessary to carry out the precipitation under the condition of supersaturation [1]. Usually supersaturation is achieved by physical (evaporation) or chemical (variation of pH etc.) methods. For the preparation of HTs the method of pH variation is most frequently used. The pH value must be chosen carefully because if it is too low not all the different metal ions will precipitate together and if it is too high the dissolution of one or more metal ions may occur. The pH value needed for the precipitation of HTs is not necessarily equal to the pH value of the precipitation of the most soluble metal hydroxide. For instance pure Al(OH)3 precipitates at pH 4.5 and 1. Introduction 10 ytrtrghrhrhhrhrhhfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffp Ni(OH)2 around pH 7 and Mg(OH)2 at pH 9.5 [1, 37], whereas Ni/Al-HT precipitates already at pH 4.0-4.5 and Mg/Al-HT at 7.7-8.5 as determined by potentiometric titration [1]. Generally HTs are synthesised at pH 7-10. Three variations of the method of pH variation have been reported in the literature [1, 2, 25]. 1.2.1.1. Coprecipitation by titration In the titration method the metal salt solution is titrated by a basic solution. By this method often not only solids of HT structure, but also other solid phases are formed, due to the precipitation of less soluble metal hydroxides prior to the precipitation of the HT. 1.2.1.2. Coprecipitation at high supersat...
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