Magnesium aluminium hydroxocarbonate hydrates were coprecipitated from mixed metal nitrate solutions, a t total Cfi~ = 0.2 M and Mg/Al, = 1 ratio, with four sodium hydrogen carbonate-sodium carbonate solutions (of p H 8.1 to 11.5) a t ambient temperature. The course of precipitation was monitored by potentiometric (pH) titration, and the compositions of the primary and final precipitates were determined by chemical analysis, infrared spectrophotometry and X-ray diffraction.
Precipitation generally occurred through three stages, primary precipitation (of low CO, aluminium hydroxocarbonates) at low pH with evolution of carbon dioxide, their dissolution by complexing to form hydroxocarbonatoaluniinate anions and then secondary precipitation of the final coprecipitate a t higher pHs. The final product from coprecipitation by sodium hydrogen carbonate solution (pH 8.1) was mainly the magnesium hydroxocarbonatoaluminate 'MAHC I' ; the final products from coprecipitation by sodium hydrogen carbonate-sodium carbonate solutions (pH 9.4 and 10.3) were 'MAHC I'/ 'MAHC 11' mixture and 'MAHC II'/'MAHC I' mixture whereas the final product from coprecipitation by sodium carbonate solution (pH 11.5) was a complex mixture if 'MAHC 11' with 'MAHC I' and 'MAHC III'; 'MAHC I' was probably Mg2[Al,(OH)I,(C0,),] . h H,O, 'MAHC 11' was probably Mg[Al,(OH),(CO,),]h H,O whereas 'MAHC 111' was probably Mg[Al,(OH),CO,]. h H,O. 'MAHC 11' -Mg[A1,(OH),(C03)2]h H,O, and 'MAHC 111' -Mg[A12(OH)&O.J * h H2O.