Point of zero salt effect

Potentiometric titration denotes a change in the pH of any given clay or soil suspension as a function of base or acid added. Generally, three types of potentiometric titration curves are produced (Fig, 3.36). The first type, represented by Figure 3.36a, shows a common crossover point for all three potentiometric curves, representing three different concentrations of an indifferent electrolyte (i.e, NaN03). The crossover point of the titrations is known as the point of zero salt effect (PZSE). The intercept of the dotted line with the titration lines is known as the pH of zero titration (PZT). For a pure oxide,... 

Point of Zero Salt Effect (PZSE), 156,159 Point of Zero Titration (PZT), 156, 159 Polarity of Molecules, 16 Potassium, 7,103, 117,118 Feldspars, 104,127 Micas, 104,120,126 Fixation in clays, 175 Potential-determining ions, 148, 159 Precipitation reactions, 65 Primary minerals, 102, 104 Chemical formula, 104 Proton, 23,138,146 Activity, 45 Pyrite, 260... 

This is often referred to as the isoelectric point. It is the condition where particles do not move in an applied electric field. If one wants to specify that the pzc is established solely due to binding of or OH", one may specify point of zero net proton charge (or condition) (pznpc). Furthermore we can define a point of zero salt effect (pzse)... 

The effects of electrolyte concentration. The effects of electrolyte concentration on anion reaction depend on the pH and thus on the charge carried by the surface. At sufficiently low pH, increasing electrolyte concentration decreases adsorption. At a higher pH, it increases it (Fig. 5). There is thus a pH at which electrolyte concentration has little effect. For anions, this point of zero salt effect occurs at a lower pH than the point of zero charge of the oxide. For the data in Fig 5., the point of zero salt effect on phosphate adsorption was just above pH 4, whereas the pzc of the non phosphated oxide was near pH 8. That is, reaction with the anion has decreased the charge on the oxide. 

Figure 10. Effect of pH and solution concentration of sodium chloride on the sorption of phosphate at four concentrations of back-ground electrolyte in solution. The point of zero salt effect for an intermediate concentration of P has been shown by an asterisk. The lines are drawn from a fitted model [38].

In terms of equation (1), this would mean a normal distribution of log (Kj). However, it is also possible that there would be heterogeneity in the term of equation(l). Evidence for this is obtained from a study of the effect of salt concentration on adsorption of anions. Fig. 10 shows that there are effects of salt concentration on phosphate sorption by a soil. These effects are analogous to those on phosphate sorption by goethite at low pH, increasing salt concentration decreases sorption at higher pH it increases sorption. There is thus an intermediate pH at which salt concentration has no effect. The value of this point of zero salt effect decreases with increasing sorption as would be expected. However, it occurs at a higher pH than the point of zero salt effect on pH (Fig.ll). 

Figure 11. Effect of level of addition of phosphate on the point of zero salt effect on phosphate sorption and on the point of zero salt effect for pH [38].

This type of PZC is called the point of zero salt effect because it is the unique pH at which the addition of electrolyte does not produce a net gain in either or OH adsorption. 

Barrow, N. J., and A. S. Ellis. 1986b. Testing a mechanistic model. V. The points of zero salt effect for phosphate retention, for zinc retention and for acid/alkah titration of a soU. Journal of Soil Science 37, no. 2 303-310. doi 10.1111/j.l365-2389.1986.tb00032.x. 

Pressure of gas i Point of zero net charge Point of zero net proton charge Point of zero net intrinsic charge Point of zero salt effect Isoelectric point Dipolar moment of species i... 

An example for extensive determinations of different points of zero charge (note that point of zero charge in this general context pertains even to particular pH values with nonzero surface-charge density) is available tfom Woods et al. [22]. For boehmite, these authors found that the point of zero salt effect (PZSE, determined by salt addition), the CIP, and the isoelectric point (lEP, determined by microelectrophoresis) did not coincide [22]. The pristine point of... 

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