Acid-base chemistry of albite surfaces in aqueous solutions at standard temperature and pressure

Surface acidity of albite has been determined by potentiometric titration o f water-washed and unwashed powders at 23.5 degreesC and pH range of 2-9.5. Using NH4Cl as the background electrolyte the surface was titrated with HC l and NH4OH in both forward and backward directions. These titrants have th e advantage of possessing the same acid- and base-radicals as those of the background electrolyte salt. Acidimetric forward titration of unwashed albi te, which is also independent of dissociation of NH4+, shows that titration curves of various ionic strengths intersect at a pH near 3.93 (point of ze ro salt effect or pH(PZSE)). The back titrations did not yield a unique pH( PZSE). The pH values of point of zero charge (pH(ZPC)) calculated from alka limetric back titration experiments range from 6.75 to 8.14 depending on th e prewashed or unwashed nature of the albite and the ionic strength of the solution. During acidimetric back titration, the pH(ZPC) values vary from 4 .15 to 7.14 depending upon the same factors. Published reports on feldspar surface chemistry have relied upon back titration experiments, but these ex periments yielded doubtful and discrepant values of pH where the surface ch arge solely due to protonation is zero (point of zero net proton charge or pH(PZNPC)). These resulted due to a combination of factors that included di fferential dissolution of the mineral, precipitation of AI(OH)(3), adsorpti on of ions from the electrolyte salt, and the nature of prewashing. The two contrasting pretreatments produce a Si-rich feldspar surface at acidic pH and an Al-rich feldspar surface at alkaline pH that partly control the diff erence in results obtained from the back titrations that proceed from the t wo opposite ends of the pH scale. Therefore, only acidimetric forward titra tions with unwashed albite were used in this study, since these yield the m ost meaningful information on the acid-base chemistry of feldspar surface. The surface charge (sigma (S)) arising simply due to protonation is given b y molar concentrations of surface-adsorbed hydrogen ions, [H-Ad(+)] since s igmas = F X [H-Ad(+)]. However, sigmas obtained from calculated values of s urface-reacted total proton charge (sigma (SR)) is significantly affected b y charged species of Al, Si, Ca, and CO2 that are present in the solutions and Na+ double left right arrow H+ exchange that occurs on feldspar surface in solution. For all compositions, in the pH range studied, the net charge of the aqueous species that must be algebraically added to sigma (SR) to o btain sigma (s), is negative. Thus, the real values of [H-Ad(+)] must be lo wer than the apparent values of measured concentrations of surface-reacted H+ or [H-s(+)]. The magnitude of this correction is dependent upon the pH a t which the solution compositions are determined. When the net charges of t he dissolved species are estimated in the forward acid titration with unwas hed albite, the corrected surface charge versus pH curves of various ionic strengths show pH(PZNPC) = 4.38-4.72. The correction procedures assume bulk electrical neutrality of the solution and quasi-equilibrium conditions wit hin the time frame of the experiments and hence disregard the kinetics of d issolution. Since the estimates of the charge in this case are based on sol utions collected at alkaline pH, the corrections for charged aqueous specie s give slight overestimation of surface charge. Hence, for the unwashed albite, PHPZNPC = PHPZSE = 3.93 +/- 0.05. Below thi s pH, the amount of surface adsorbed [H+] is independent of pH. Consistent with this observation is a theoretically calculated adsorption isotherm wit h an assumed value of pH(PZNPC) = 3.93. The theoretically constructed curve further supports that the amount of [H-Ad(+)] is exceedingly small below p H(PZNPC). (C) 2001 Elsevier Science B.V. All rights reserved.