IMPORTANCE OF SURFACE-COMPOSITION ON POIN TS OF ZERO SALT EFFECT AND ZERO NET PROTON CHARGE MEANING

Titration curves of the iron oxide coated fraction of Osorno soil carr ied out at 25-degrees-C in different KCl background solutions exhibite d a crossover point (named point of zero salt effect, PZSE), near the point of zero net proton charge (PZNPC). Synthetic aluminosilicates an d natural deferrated colloids samples presented a PZSE, where a net pr oton charge exists. The Constant Capacitance model describes the titra tion curves of the iron oxide coated colloids at ionic strengths betwe en 1.0 x 10(-3) and 1.0 M, when the PZSE is taken equal to the PZNPC T he model also describes the curves of the deferrated colloids, but it must be assumed that the PZNPC is assumed to be the initial pH value o f the titration. The PZNPC depends on ionic strength, and is different of PZSE. From experimental data and MCC fitting it is concluded that PZSE is similar or equal to PZNPC in iron coated samples due to the ho mogeneity of the active surface sites, dominated by FeOH groups. The l ack of congruence between the PZSE and the PZNPC in deferrated colloid s is related to the AlOH/SiOH ratio of surface groups, and it is expla ined by the differences of their dissociation constants.