The reactions leading to the formation of crystalline Mg3(OH),CI*4H20 (phase S), Mg2(0H),CI .4Hz0 (phase 3), and Mg(OH)* are compared for the systems Mg0-MgCIZ-H20 and NaOH-MgCIZ-H20. The crystalline phases were determined by X-ray diffraction analysis. The concentration of the total magnesium and chloride in the solution and the pH of the solution determine the reaction product(s) in both systems. The influence of MgO reactivity and the molar ratio of reactants on the formation and stability of reaction products is discussed and the mechanism of the formation of phases 3 and 5 is explained. In the system Mg0-MgCI2-Hz0, MgO serves only to increase the concentration of total magnesium and the pH of the MgClz solution.

The precipitation process of solid phases Mg3(OH),Cb4H20 (phase 5), Mg2(OH)3CI.4H20 (phase 3), and Mg(OH)2 was followed by the addition of NaOH water solution in MgClz water solutions of different concentrations (0.001 to 4.8 rnol dm-3) and characterized by chemical, potentiometric, coulometric, and X-ray diffraction analyses. The concentration range in which the precipitation of solid phases occurs was determined. The phase distributions relative to the pH of solution and concentrations of magnesium and chloride were defined by the equilibrium diagram. The approximate solubility products of stable solid phases formed at different ionic strengths and at 293 K were determined.

Der Fällungsprozeß der festen Phasen (I)‐(III) aus dem homogenen Titelsystem durch Zusatz wäßriger NaOH‐Lösung zu wäßrigen MgCl2‐Lösungen unterschiedlicher Konzentrationen (0.001‐4.8 mol dm‐3) wird verfolgt und mit Hilfe chemischer, potentiometrischer, coulometrischer und röntgenographischer Analysen charakterisiert.

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