Mass titration

Another key contribution of the Schwarz group was the recognition of the dramatic influence of oxide surfaces on bulk solution pH. In a landmark 1989 paper, Noh and Schwarz [7] demonstrated the method of mass titration, in which successive additions of oxide cause stepwise shifts in solution pH. This procedure is illustrated in Figure 6.7 [7], As indicated in Figure 6.1, the protonation-deprotonation chemistry of the surface hydroxyl groups is coupled to the liquid-phase pH. In mass titration, as the mass (or more appropriately, the surface area) of oxide in solution increases, the solution pH is brought to the PZC of the oxide, at which point no driving force for proton transfer exists... 

Figure 6.7 Mass titration of Noh and Schwarz. (From Noh, J.S., and Schwarz, J.A., J. Coll. Interf. Sci. 130,1989, 157.)...

Recently two methods applicable in low ionic strength conditions were developed (i) the mass titration method [38,39], and (ii) the adhesion method for solving the problem of sample conductivity [40-42]. 

Figure 4. Effect of acidic and basic contamination, u, on the results of mass titration with a hematite suspension [47]. u/mol g- ( ) - 10- , ( ) 0, (a) lO" , ( ) lo-. ...

The mass titration method is based on work by Noh and Schwarz who showed that addition of an oxide powder to an electrolyte solution changes the pH to a constant value which corresponds to the point of zero charge [38]. The method was experimentally and... 

Reaction enthalpies obtained from the temperature dependency of the pzc axe standard values corresponding to dilute systems. Figure 6. displays the results with hematite obtained by the mass titration method, which is suitable because of its applicability at low ionic strength. 

The method termed mass titration was originally described by Noh and Schwarz [59]. Dispersions containing 0.01, 0.1, 1,5, 10 and 20% of solid in deionized and outgased water are equilibrated for one day under nitrogen atmosphere. The pH is measured and plotted versus log (mass% of solid) and the plateau in the curve indicates the PZC. Addition of inert electrolyte was recommended (but not applied) in the original paper and the problem of optimum concentration of inert electrolyte was not addressed. The agreement between results obtained by mass titration and the CIP was good for alumina but problematic for titania. 

Although mass titration was originally designed as a method of determination of PZC, surface charging curves can also be calculated from mass titration data [60]. 

Mass titrations performed at different ionic strengths [61] with alumina give somewhat different results. While those performed at [NaCl] > 10 mol dm " were quite consistent and led to PZC similar to literature values, the titration in pure water gave pH 4.3 with 40% solid by weight. 

A completely different method is referred to as mass titration" in Ref. [64] Namely, a certain amount of powder is added to the solution of given pH and the dispersion is titrated with acid or base until the original pH value is reached. This procedure is very different from the mass titration discussed above, and it seems to be variation of potentiometric titration without correction for PZC = CIP, thus the results are reported as pH" in Table 3.1. 

Mass titration relies on the assumption that there is no acid or base associated with the solid surface, and this assumption is problematic for many specific materials (cf. the discussion concerning Figs. 3.1 and 3.3). Simulations of mass titration taking into account such acidic or basic impurities were carried out by Zalac and Kallay [65]. 

AI2O, A 1,0.5 A1,0., 7, LaRoche a, LaRoche, calcined Q, Asahi, high purity NaNOj 0.1 mol dm - NaNOj 0.1 mol dm" NaNOi NH4CI mass titration mass titration iep... 

FIG. 3.11 Distribution of PZC values reported for all types of AljOj in the literature. Dark gray columns cip , iep , and salt titration. White columns pH and mass titration. 

FejOs hematite, reagent 0.001 mol 1CM5 mass titration... 

TiO, anatase, reagent grade 10 " mol dm NaNOj 5 5 mass titration... 

Rgure 13.12 Mass titration curves for the determination of the zero point of charge of carbons after different severity of oxidation. (Reprinted from Ref. [76] with permission from Elsevier.)... 

The potentiometric mass titration method [657,658] produces results equivalent to those of the drift method described above. The same amount of base is added to three dispersions with different solid-to-liquid ratios and a constant ionic strength. The dispersions are titrated with acid, and the pH is recorded as a function of the amount of acid added. The intersection point of the obtained curves is taken as the PZC. In other words, the PZC is identified with the pH at which solid addition does not induce a change in pH. The drift method and mass titration are based on the same principle, the difference being that in potentiometric mass titration, the reagents are added in a different order. Potentiometric mass titration is affected by the acid or base associated with the powder in the same way as in the drift method and mass titration. The advantage of potentiometric mass titration over the drift method is that in the former the pH is measured only in buffered systems. 

In the mass titration method, the PZC is determined as the natnral pH of a concentrated dispersion. A detailed description of the experimental procedure can be found in [667], Mass titration become popular in the late 1980s [668,669], but the same method was already known in the 1960s as the pH drift method [183], Usually, a series of natural pH values of dispersions with increasing solid loads is reported, but only the natural pH of the most concentrated dispersion is actually used. The only role of the data points obtained at lower solid loads is to confirm that a plateau was reached in pH as a function of solid load that is, a further increase in the solid load is unlikely to bring about a change in pH. The mass titration method is based on the assumption that the solid does not contain acid, base, or other surface-active impurities. This is seldom the case, thus mass titration often produces erroneous PZCs. In this respect mass titration is similar to the potentiometric titration without correction illustrated in Figure 2.7, only the solid-to-liquid ratio is different. The experimental conditions in mass titration (solid-to-liquid ratio, time of equilibration, nature and concentration of electrolyte, and initial pH) can vary, but little attention has been paid to the possible effects of experimental conditions on the apparent PZC. The effect of an acid or base associated with solid particles on the course of mass titration was studied in [670], To this end, a series of artificially contaminated samples was prepared by the addition of an acid or base to a commercial powder. The apparent PZC of silicon nitride obtained in [671] by mass titration varied from 4.2 (extrapolated to zero time of equilibration) to 8.2 for time of equilibration longer than 20 days. The method termed mass titration was used in [672], but it was different from the method discussed above. 

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