For powder characterization either characterize the powder directly - to get information on powder material or characterize suspension, which will give you information on this suspension. Results for the latter will always depend on solvent composition used and use dispersion intensity. That means for the same material you can produce very different results.
you should characterize the suspension you are interested in, e.g. if you envisage application in aqueous environment your choice should be water. Then poly- or pyrophoshate may be a good dispersant. At least in case of alumina it will reduce dissolution of ions from it.