Oct., 1963
COAGULATION EFFECTS O F HIGHLY CHllRGED COUNTERIONS
intermediate pH’s), 9-TPA: 1 X M , 12-TPA: 2 X M , and 12-.CMA: 9 X M . There is no obvious relationship between the basicity of these acids and C.S.C. The adsorption of these ions, which determines the c.s.c., depends on specific chemical factors in addition to charge. Some additional comments are warranted in the case of 12-TPA. This acid is known to be tribasic when solid, l9 113PW12040, but in dilute solutions shows a higher basicity suggesting the formulation H7PW12042.4*10,20Even a t lowest pH’s where the ionization would be expected to be repressed to some extent, the c.c.c of the 12-TPA is lower than that of tetrabasic 12-TSA. Since the latter is certainly completely ionized a t these concentration, the lower C.C.C. for 12-TPA indicates that species with charge higher than -4 must be present in dilute solution to an appreciably extent. We have suggested that these data indicate that in dilute solution two tungsten oxygen bridges of the 12-TPA may undergo hydrolysis with a subsequent increase in basicity. The irregularity in the coagulation curves of the 12-MCA (Fig. 6) and of the 12-TPA a t pH > 5 remains to be discussed. The ordinate of such coagulation curt.es is a measure of the rate of coagulation so that (19) J. F. Keggin, Proc. R o y SOC.(London), A144, 75 (1934); L. Pauling, J . Am. Chem. Soc., 61, 2868 (1929). (201 M. Kerker, J. Keller, J. Siau. a n d E. MatijeviC, Trans. Faraday SOL.,57, 780 (1961).
1999
oscillations of the type exhibited here must arise from several competitive processes which cause the rate of coagulation either to decrease or increase. Above the C.C.C. and below the C.S.C.the rate of coagulation will depend on the concentration of the free heteropoly ions in solution and 011the surface potential of the colloidal particles. Both quantities will depend, for a given initial concentration of the heteropoly acid, on the amount of the adsorbed acid. For a given particle size the relative ratio of the adsorbed to the free acid will change with the initial concentration of the heteropoly acid. I n addition, in sols in statu nascendi the size of primary particles may also be irifluenced by the concentration of the heteropoly acid. Thus, several of such competitive processes influencing the rate of coagulation may result in the oscillatory pattern. Similar effects have been observed earlier when silver bromide and silver iodide sols were coagulated by certain surface active agents.21 DISCUSSION A. W. NACMANS (Union Carbide Corp.).-Would you please cdmment on the nature of the driving force that gives rise to adsorption and charge reversal?
E. MATIJEV16.-wre believe that metal ions only when hydrolyzed reverse the charge and that this reversal of charge is caused by the chemisorption of hydrolyzed ions on the surface of lyophobic colloidal particles. (21) E. Matijevii. a n d R. €1. Ottewill, J . Colloid Sei., 13, 242 (1958).
