Anal. Chem. 1999, 71, 2518-2522
Application of Zetametry To Determine Concentrations of Acidic and Basic Impurities in Analytical Reagents Marek Kosmulski,*,† Patrik Eriksson, and Jarl B. Rosenholm
Department of Physical Chemistry, A° bo Akademi University, FIN-20500 A° bo, Finland
The sign and magnitude of the ζ potential of mineral oxides in allegedly pure organic solvents (high-quality chemical reagents) depends on the concentration of acidic and basic impurities, chiefly organic acids and amines. “Zetametric titration” with HClO4 makes it possible to estimate the concentration of basic impurities and offers an attractive alternative to other analytical methods. Titanium(IV) oxide has been chosen as the probe powder in view of its very low solubility and nearly neutral point of zero charge in aqueous systems. Some systems have a high buffer capacity and addition of acid and base has a minor effect on the ζ potential of titania. Specific adsorption of alkali metal cations on anatase can be also a significant problem. The response of ion-selective electrodes (ISE) in pure nonaqueous polar solvents is Nernstian down to 10-8 mol dm-3 HClO4, Bu4NF, and some metal chlorates (VII) with 10-2 mol dm-3 Bu4NClO4 to stabilize the ionic strength.1 In commercial reagents, however, the range of linear response is limited only down to ∼10-4 mol dm-3. The response of ISE in these commercial solvents can be quantitatively explained in terms of complexation of protons and metal ions by impurities. Potentiometry offers a possibility to determine the concentration of impurities, e.g., amines quantitatively.1 The applicability of the potentiometric method is, however, restricted to certain solvents by the limited availability of reference electrodes and other technical problems. In this paper, we introduce a variation of the potentiometric method, which employs mineral oxides as potentiometric sensors. SURFACE CHARGING OF MINERAL OXIDES The mechanism of surface charging of nonconducting materials has been discussed in detail in handbooks of colloid chemistry,2,3 and a special monograph on this topic is available.4 In the case of metal oxides, protons are the potential-determining ions pdi, and the sign of the electrokinetic potential (ζ potential) † On leave of absence from Technical University of Lublin, Poland. (1) Coetzee, J. F.; Desmukh, B. K.; Liao, C.-C. Chem. Rev. 1990, 90, 827-835. (2) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: Oxford, 1987; Vol. 1. (3) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: New York, 1995; Vol. II. (4) Hunter, R. J. Zeta Potential in Colloid Science; Academic Press: New York, 1981.
2518 Analytical Chemistry, Vol. 71, No. 13, July 1, 1999
depends primarily on the pH of the solution. When the concentration of strongly adsorbed species (surfactants, multivalent ions) is not too high, the isoelectric point, iep, of particular oxides is defined by the pH at which ζ ) 0. Listings of these characteristic pH values for different oxides can be found in the literature5 and they range from ∼2 for silica to ∼12 for magnesia. The sign of the electrokinetic potential of oxides can be easily determined using commercial equipment. Dozens of various zetameters are available on market; among different producers Coulter, Brookhaven, and Malvern offer the most common instruments. The ζ potential of TiO2, whose iep falls at nearly neutral pH,5 is negative in basic solutions and positive in acidic solutions. Certainly, a zetameter cannot be recommended as the tool to determine the pH of aqueous solution, since simpler and more reliable methods are available. However, the situation drastically changes when we deal with nonaqueous or nearly nonaqueous organic solvents. A glass electrode gives a Nernstian response in some organic solvents at a sufficiently high ionic strength1 (this is one limitation of the electrometric method), but for each solvent system the technical details (preparation of glass electrode) have to be developed individually, and success is not guaranteed. In addition to technical difficulties, there is also a problem with a common pH scale for different solvents, namely, what we can actually measure are the changes in potential, while the absolute value is a question of convention. Thus, zetametry can be an useful tool to determine the pH in nonaqueous media, especially at low ionic strengths or in other systems, where the electrometric methods do not work. We chose titania among many oxides to show some practical features of zetametry since this oxide is almost completely insoluble, its iep in water is at neutral pH, and it is commercially available as fine powder which forms dispersions stable against sedimentation. ELECTROKINETIC MEASUREMENTS IN NONAQUEOUS MEDIA. HISTORICAL SURVEY Already early studies at the turn of the century6 indicated that addition of acids and bases to dispersions of mineral oxides produced positively and negatively charged particles, respectively. These results were confirmed later using modern equipment.7 Thus, it should be rather clear that the sign of the ζ potentials of (5) Kosmulski, M. Langmuir 1997, 13, 6315-6320. (6) Dukhin, S. S. In Surface and Colloid Science; Matijevic, E., Ed.; WileyInterscience: New York, 1974; Vol. 7, pp 1-47. (7) Kosmulski, M.; Matijevic, E. Langmuir 1991, 7, 2066-2071. 10.1021/ac9806052 CCC: $18.00
© 1999 American Chemical Society Published on Web 05/14/1999
Table 1. Properties of Solvents
solvent
producer
grade
water (ppm) acids (label) bases (label) label Karl-Fisher (mol dm-3) (mol dm-3)
1-propanol Merck
Extra pure
