Electrokinetic Potential of Barium Sulphate

Abstract

THE electrokinetic potential of barium sulphate has been investigated repeatedly; but the results of various authors show no agreement either with respect to the sign of the potential or its magnitude. Most investigators, except Michaelis and Dolan¹, find the solid negatively charged, and Ruyssen² has recently reiterated his opinion that the pure salt has a negative charge in water. We have subjected this problem to a careful investigation³ and found that precipitated as well as recrystallized barium sulphate, ignited immediately before establishing contact with water, has a well-reproducible positive potential against its saturated solution in water and in alcohol–water mixtures, provided that traces of surface-active impurities, which may originate from rubber joints and are not always absent even in double-distilled water, are rigorously excluded. An all-glass streaming potential apparatus and a procedure of freeing distilled water from traces of surface-active impurities (distillation from acid potassium permanganate solution) has been described recently³. The barium sulphate crystals retain a constant positive potential on ageing in water, irrespective of the quantity of water that is streamed through the plug. However, when ordinary distilled water, or water that was in contact with rubber or filter paper (cf. Ruyssen's experimental method⁴), is streamed through a plug of barium sulphate crystals, the positive potential decreases and changes to negative. Even leaving the crystals with water in a vessel that is open to the laboratory air for several days may cause a decrease of the positive potential. Obviously surface-active impurities are strongly adsorbed on the surface of the barium sulphate crystals. However, the original positive potential is restored on ignition of the crystals to 500–700° C., or on treatment with hot concentrated nitric acid, whereby the impurities are destroyed.