Composition: The term "poly-aluminum chloride" or "PAC" refers to a class of soluble aluminum products in which aluminum chloride has been partly reacted with base. The relative amount of OH-, compared to the amount of Al, determines the basicity of a particular PAC product. The chemistry of PAC is often expressed in the form Aln(OH)mCl(3n-m). Basicity can be defined by the term m/(3n) in that equation. Because PAC is more expensive to produce than papermakers alum (aluminum sulfate) it is important to note the most important differences between these two products. Solutions of PAC are not as acidic as alum; consequently they do not tend to decrease the pH of the furnish nearly as much as an equivalent amount of alum. Another difference is that PAC is formulated so that it already contains some of the highly cationic oligomers of aluminum - materials that are especially effective for the modification of colloidal charges in a papermaking furnish. A particularly stable and important ionic species in PAC and related soluble aluminum chemicals has the formula Al12(OH)24AlO4(H2O)127+.
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