Summary: Various crystalline and X-ray amorphous forms of aluminium hydroxide prepared from sulphate and chloride solutions by varying the mole ratio of NaOH to Al³⁺ have been characterized by chemical analysis and X-ray diffraction. When the NaOH/Al ratio is 3 or 3·3, crystalline Al(OH)₃ in the form of bayerite, nordstrandite, gibbsite, or mixtures is obtained in several hours. When the ratio is 2·75 or below, no Al(OH)₃ is evident and the products are X-ray amorphous and remain so even after ageing for six months. Chemical analysis indicates that the amorphous precipitates are basic aluminium sulphates or chlorides. In the 0 to 2·1 range of NaOH/Al, the composition of the products is constant, being approximately Al(OH)_2·2X_0·8. In the 2·1 to 2·75 range, a continuous series of basic salts, of composition ranging from Al(OH)_2·2X_0·8 to Al(OH)_2·75X_0·25, is obtained.

The following hypothesis is proposed to account for the occurrence of X-ray amorphous versus crystalline forms: When NaOH is added to an aluminium salt solution, the initial reaction yields Al(OH)2+ ions, which polymerize into stable 6-membered ring units of composition Al6(OH)126+ or multiples thereof (double and triple rings of composition Al10(OH)228+ and Al13(OH)309+), all with H₂O molecules on the surface. At NaOH/Al = 2 or nearly so, the inclusion of Al³⁺ in the basic ring units is complete; still further OH⁻ then transforms these ring units to larger polymers, giving rise to a continuous series of species. At NaOH/Al = 2·75 and below, the positively charged hydroxy-aluminium polymers repel one another unless joined together by the counter-anions to form basic aluminium salts, which are usually X-ray amorphous because of their highly hydrated state. At NaOH/Al = 3, the net positive charge per aluminium disappears and consequently the repulsion among polymers as well as their water of hydration becomes negligible; therefore, all the polymers cluster together and form crystalline Al(OH)₃ in a matter of hours or days, depending on the conditions of crystal growth.