ALUMINUM SULFATE AND ALUMS

Aluminum sulfate octadecahydrate [7784-31 -8]y Al2(S0 2 I8H2O, and its aqueous solutions are used primarily in the paper (qv) industry for sizing and as a flocculating agent in water (qv) and wastewater treatment. This material is often called papermakers alum or alum. Because this salt is precipitated from aqueous solution, aluminum sulfate hydrate [17927-65-0] Al2(SO 7/H2O, can have variable composition and is sometimes referred to as cake alum or patent alum. The solid commercial hydrate, generally written as the 18-hydrate, is typically dehydrated to correspond to from 17.0 —17.5% Al O where n = 13 14 (1,2). This dehydrated form is called dry alum, ground or lump. Aluminum sulfate solutions are typically 7.5—8.5% Al O and are known as liquid alum. 

Confusion arises in the nomenclature of alum because double salt compounds, M(I)A1(S0 2 where M is in the +1 oxidation state, have also traditionally been called alums. In particular, potassium aluminum sulfate [15007-61 -1] KJ ([ 0 2 referred to as ordinary alum or potash alum. 

Anhydrous aluminum sulfate [10043-01-3] Al2(SO, is a specialty item used in food applications. 

Over 50 acidic, basic, and neutral aluminum sulfate hydrates have been reported. Only a few of these are well characterized because the exact compositions depend on conditions of precipitation from solution. Variables such as supersaturation, nucleation and crystal growth rates, occlusion, nonequihbrium conditions, and hydrolysis can each play a role in the final composition. Commercial dry alum is likely not a single crystalline hydrate, but rather it contains significant amounts of amorphous material. 

Hydrates. Aluminum sulfate hydrates, Al2(SO 7/H2O, where n ranges from 0 to 27 have been reported (3—6). Relative decreasing vapor pressure studies indicate the presence of an octadecahydrate, hexadecahydrate, dodecahydrate, dihydrate, and the anhydrous salt, assuming that basic aluminum sulfates are not formed during the dehydration (3). 

Thermal analysis of the dehydration of Al2(SO 18H20 shows loss of 15 moles of water between 40°C and 250°C and 3 moles between 250°C 

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