ACID-BASE CHEMISTRY OF ALBITE SURFACES IN AQUEOUS SOLUTIONS AT STANDARD TEMPERATURE AND PRESSURE

Abstract

Surface acidity of albite has been determined by potentiometric titration of water-washed and unwashed powders at 23.5°C and pH range of 2-9.5. Using NH4Cl as the background electrolyte the surface was titrated with HCl and NH4OH in both forward and backward directions. These titrants have the advantage of possessing the same acid- and base-radicals as those of the background electrolyte salt. Acidimetric forward titration of unwashed albite, 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 zero salt effect or pHPZSE). The back titrations did not yield a unique pHPZSE. The pH values of point of zero charge (pHZPC) calculated from alkalimetric back titration experiments range from 6.75 to 8.14 depending on the prewashed or unwashed nature of the albite and the ionic strength of the solution. During acidimetric back titration, the pHZPC 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 experiments yielded doubtful and discrepant values of pH where the surface charge solely due to protonation is zero (point of zero net proton charge or pHPZNPC). These resulted due to a combination of factors that included differential dissolution of the mineral, precipitation of Al(OH)3, adsorption 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 difference in results obtained from the back titrations that proceed from the two opposite ends of the pH scale. Therefore, only acidimetric forward titrations with unwashed albite were used in this study, since these yield the most meaningful information on the acid-base chemistry of feldspar surface. The surface charge (σS) arising simply due to protonation is given by molar concentrations of surface-adsorbed hydrogen ions, [HAd+] since σS=Fx[HAd+]. However, σS obtained from calculated values of surface-reacted total proton charge (σSR) is significantly affected by charged species of Al, Si, Ca, and CO2 that are present in the solutions and Na+#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 σSR to obtain σS, is negative. Thus, the real values of [HAd+] must be lower than the apparent values of measured concentrations of surface-reacted H+ or [HS+]. The magnitude of this correction is dependent upon the pH at which the solution compositions are determined. When the net charges of the dissolved species are estimated in the forward acid titration with unwashed albite, the corrected surface charge versus pH curves of various ionic strengths show pHPZNPC=4.38-4.72. The correction procedures assume bulk electrical neutrality of the solution and quasi-equilibrium conditions within the time frame of the experiments and hence disregard the kinetics of dissolution. Since the estimates of the charge in this case are based on solutions collected at alkaline pH, the corrections for charged aqueous species give slight overestimation of surface charge. Hence, for the unwashed albite, pHPZNPC=pHPZSE=3.93+/-0.05. Below this pH, the amount of surface adsorbed [H+] is independent of pH. Consistent with this observation is a theoretically calculated adsorption isotherm with an assumed value of pHPZNPC=3.93. The theoretically constructed curve further supports that the amount of [HAd+] is exceedingly small below pHPZNPC.