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Magncsirim alumiiiiiim hydroxides woro coprecipitatcd from different mixed rrietal cation solutions -a t total CAI = 0.1 M and Mg/AI, ratios from 1 to 6 -with sodium hydroxide solut,iori at ambicnt temperature, with diffcrent prc-ageing conditions for tho aluminiiim hydroxidc prc-precipitate. The coprccipitatioris were monitored by poteritiometric ( p H ) titratiori and the final precipitate compositions were examined by chemical analysis, infrarcd spcctrophotometry and thermal analysis.Magnesium hydroxide was coprecipitated onto completely recrystallised aluminium hydroxidc as a simple mixture.Gencrally, with no to t,hree days pre-ageing, microcrystalline aluminium hydroxide was first precipitatcd at pH about 4; this then partially redissolved o n further addition of sodium hydroxide (to form hydroxoaluminate anion) and inagneaium alumiriium hydroxidc coprccipitates were formcd continuously a t pHs from 8. liitroductiuiiMagnesiuiii hydroxide -alu uiiniuiii hydroxide coprecipitate powders have iiiany useful industrial applications. High surface-area adsorbents and seini-conductor catalysts for many organic syntheses are prepared by the controlled thernial decoiiipositiori of these powders (ASILMORI': ; CIAEKE ; NURIO). 'l'he refractory niagnesiuni aluitiinatc (Spinel) can he prepared by the dehydration of suitable coprecipitates and then solid-state reaction of the aiiiorphous magnesia-alumina 'proto-spinel' product (BRAT-TON ; IVANUVA) ; KAMESWARI; MORTLANU). These powders also have important iiiedical applications in gastric anti-acid formulations (Anon. Chattanooga ; Anon. Heruies ; Anon. Keystone ; Anon. Kyowa ; SCHWANK). Previous experimental studies on the coprecipitations from mixed cation solutions with sodium hydroxide and the conipositions and structures of the coprecipitates have been reported by inorganic chemists (CHALYI; FEITKNECHT) and by soil chemists and geologists (BRINDLEY ; OASTIJCHE ; MASCOLO) ; these latter workers have proposed a brucite-aluminium hydroxide solid solution composition with a 'hydrotalcite' structure.

'1 series of aluminium hydroxocarbonate hydrates were prepared by precipitation from ttluminium nitrate solution, with five sodium hydrogen carbonate-sodium carbonate solu-!,ions of different pH, a t ambient temperature. The course of precipitation was monitored '3y pH measurement and the final precipitate compositions were determined by chemical .tnalysis, infra-red spectrophotometry, X-ray diffraction and thermal analysis.Precipitation generally occurred through three stages, primary precipitation of materials with low carbonate content a t low pH with evolution of carbon dioxide, their dissolution to form hydroxcarbonato anions and then secondary reprecipitation of the final products a t higher pH. These materials were mixtures of polyhydroxoaluminium carbonate hydrates of general composition A1,(OH)3,-2CO3. hH,O (where n = 2, 4, 6 and h = 6-8); their CO, content increased with increasing pH and carbonate anion content of the precipitant solution.

Magnesium liydroxoalurniiiate hydrates were coprecipitatetl from different mixed metal cation solutions a t Mg/Al, ratiticv froin 1 t o 4 by ammonium hydroxide, The coprecipitations were monitored by potentiornetric titration and the final precipitate compositions were exainiiied by chemical analysis, X-ray diffraction, infra-red spectrophotornetry and tliernial analysis. The process of coprecipitation was similar to that for coprecipitation with sotliurn hydroxide but large excess of ammonium hydroxide was required for cornplete reaction a t pHs from about 8 to 10.At Mg/Al, = I , the main phase was probably Mg(H,O)h [Al(0H),l2; at Mg/Al, = 2 , the rimin phase was probably Mg,(H,O), [Al,(OH),,]; at Mg/A12 = 4, the main phase was probably (MgOH,) (H20)* [A12(OH)J.

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