The High Resistance Compensation Method (HRCM) was developed to measure the electrode potentials of high resistance electrodes. It is essentially similar to the classical Poggendorff compensation method, but instead of an ampere-meter, the pH-meter was introduced as an indicator of complete compensation. The method was applied to measurements of the electrode potentials of a single crystal rutile
(TiO2) electrode. Electrode potentials were converted to surface potentials, and the dependency of the surface potential on pH in aqueous medium was determined

Atiđa Selmani

Filip Šupljika

Ivan Leniček

Nikola Kallay

Tajana Preočanin

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 * Dedicated to Professor Zvonimir Maksić on the occasion of his 70th birthday. ** Author to whom correspondence should be addressed. (E-mail: nkallay@chem.pmf.hr) CROATICA CHEMICA ACTA CCACAA, ISSN-0011-1643, ISSN-1334-417X Croat. Chem. Acta 82 (1) (2009) 323-327. CCA-3323  Original Scientific Paper High Resistance Compensation Method for Surface  Potential Measurement* Nikola Kallay,a,** Tajana Preočanin,a Atiđa Selmani,a Filip Šupljika,a and Ivan Leničekb aLaboratory of Physical Chemistry, Department of Chemistry, Faculty of Science, University of Zagreb,  Horvatovac 102a, HR-10000 Zagreb, Croatia bDepartment of Electrical Engineering Fundamentals and Measurements, Faculty of Electrical Engineering and Computing, University of Zagreb, Unska 3, HR-10000 Zagreb, Croatia RECEIVED OCTOBER 10, 2008; REVISED DECEMBER 9, 2008; ACCEPTED DECEMBER 10, 2008  Abstract. The High Resistance Compensation Method (HRCM) was developed to measure the electrode potentials of high resistance electrodes. It is essentially similar to the classical Poggendorff compensation method, but instead of an ampere-meter, the pH-meter was introduced as an indicator of complete com-pensation. The method was applied to measurements of the electrode potentials of a single crystal rutile (TiO2) electrode. Electrode potentials were converted to surface potentials, and the dependency of the sur-face potential on pH in aqueous medium was determined. Keywords: single crystal electrode, high resistance compensation, surface potential, rutile, Poggendorff  INTRODUCTION Electrostatic potential at the inner plane of the Helm-holtz interfacial layer (0-plane), i.e., surface potential Ψ0, is one of the important variables affecting the inter-facial equilibrium of metal oxide aqueous systems. Several attempts have been made to measure the inner surface potential at the solid/liquid interface. First, the electrode potentials of metallic surfaces covered by metal oxides were measured and converted to surface potentials.1−3 The main problem with metal/metal oxide electrodes is the porosity of the oxide layer allowing direct contact of the aqueous solution with the metal. In the case of porosity, the measured potential is due to the redox equilibrium at the metal/aqueous electrolyte solu-tion interface determined by concentrations of interact-ing ions that depend on the solubility of the metal oxide. In fact, these electrodes behave as "electrodes of the second kind". Later on, measurements of the surface potential were performed by Ion Sensitive Field Effect Transistors (ISFET).4−7 A few years ago, the problem of porosity was originally solved by constructing an ice electrode.8 A compact ice layer was formed on the pla-tinum and the measured potential was a result of inter-facial reactions. This approach was followed by con-struction of single crystal electrodes (SCrE) of different oxides, but also of iron sulphide and silver chloride.9−14 To measure surface potential at the equilibrium condi-tion (equal ionic adsorption and desorption rates), no current should pass through the interface. The Poggen-dorff compensation method satisfies the requirements, but it cannot be applied due to the high resistivity of metal oxides. An approximation is the use of high resis-tance voltmeters (pH-meter), but still one cannot be sure that the measured response corresponds to the equili-brium condition. In this article, the development and implementation of the High Resistance Compensation Method (HRCM) is described, which is essentially the Poggendorff compensation method, but refined so that potentials of high resistance electrodes could be meas-ured. High Resistance Compensation Method The High Resistance Compensation Method (HRCM) is based on compensation of the electromotivity15 (electro-motive force) of the galvanic cell by counter voltage. The condition of compensation is the state in which the magnitudes of the electromotivity of the measured cell and the applied counter voltage are exactly equal. This condition is not detected by a high sensitivity ampere-meter, like in the case of the classical Poggendorff com-324 N. Kallay et al., High Resistance Compensation Method Croat. Chem. Acta 82 (2009) 323. pensation method, but by a high resistance millivoltme-ter, i.e., pH-meter. The setup is demonstrated in Figure 1. The procedure is simple. In the first step (D-mode), one measures the electomotivity directly by a high resistance millivoltmeter (pH-meter, 1012 Ω). This reading is not necessarily the equilibrium value, espe-cially if the internal resistance of the cell is not at least 100 times lower than the internal resistance of the pH-meter. In the second step (still D-mode), one adjusts the slide wire potentiometer so that the reading of the me-dium resistance millivoltmeter (1010 Ω) becomes ap-proximately equal to the value previously obtained by the high resistance millivoltmeter (pH-meter, 1012 Ω). In the third step, one switches to the HRCM-mode (C-mode) and adjusts the slide wire potentiometer so that the reading of the high resistance millivoltmeter (pH-meter, 1012 Ω) becomes zero millivolts. The electromo-tivity of the cell is then equal to the countervoltage as determined by the medium resistance millivoltmeter (1010 Ω). If the reading of the high resistance millivolt-meter (pH-meter, 1012 Ω) deviates from zero millivolts, one should take this into account and add this value to the reading of the medium resistance millivoltmeter (1010 Ω). The device is connected to a computer so that one may observe the change in electromotivity and its constancy over a time period, as well as data scattering. The HRCM will be analyzed and compared with conventional methods on the example of a high resis-tance galvanic cell of electromotivity (electromotive force) of 200 mV and the internal resistance of 1011 Ω. Misbalance of the Poggendorff bridge of 1 mV will result in electrical current of 9×10−16 A, which is too low to be detected. If one measures electromotivity with a pH-meter of internal resistance of 1012 Ω, the reading on the pH-meter will be 182 mV, i.e., 18 mV lower than the electromotivity. The reduction would be even higher for higher internal resistance of the galvanic cell. In the case of HRCM for misbalance of the bridge of 1 mV, the current will be 9.1×10−19 A, which will cause a re-duction in electromotivity of 0.09 mV. Since the sensi-tivity of the pH meter is significantly better than 1 mV, the error in evaluated electromotivity becomes negligi-ble. The HRCM will be demonstrated for the case of a single crystal rutile electrode.  EXPERIMENTAL Materials. All solutions were prepared using redistilled and decarbonated water: KNO3 (p.a., Kemika), HNO3 (0.1 mol dm−3, titrival, Riedel-de Haën), KOH (0.1 mol dm−3, titrival, Riedel-de Haën) and standard buffers (Riedel-de Haën). The single crystal rutile (TiO2) elec-trode was made using the single crystal (10 mm × 10 mm, thickness 0.5 mm) obtained from MaTecK GmbH. TiO2 is a semiconductor.16 The resistance of the used crystal was measured using U/I method of comparison with reference standard resistance of 1 GΩ, and was approximately 5 GΩ. The (0 0 1) plane was exposed to the electrolyte solution. The crystal surface was po-lished and cleaned with ethanol and redistilled water. Figure 1. High resistance compensation method (HRCM) formeasurements of electrode potential of the single crystal elec-trode (SCrE) of high resistance. The source of constant vol-tage of 3 V (U), slide wire potentiometer SWP, cell (E) withSCrE and reference electrode, two millivoltmeters: high resis-tance millivoltmeter, i.e., pH-meter (V, 1012 Ω) and mediumresistance millivoltmeter (V, 1010 Ω). Position D: direct mea-surement of electromotivity by the high resistance millivolt-meter, i.e., pH-meter (V, 1012 Ω). Position C: HRCM mode.  Figure 2. The single crystal rutile electrode for surface poten-tial measurements. N. Kallay et al., High Resistance Compensation Method 325 Croat. Chem. Acta 82 (2009) 323. An Ag|AgCl, Cl−(aq) electrode with a salt bridge (Metrohm, 6.0729.100) was used as a reference elec-trode. The salt bridge was filled with KNO3 solution (0.1 mol dm−3). Construction of the single crystal electrode is pre-sented in Figure 2. The single crystal is fixed in the plexiglass holder with epoxy resin. Inner side of the crystal is in contact with mercury, and mercury is in contact with graphite connected to the copper wire. Outer side of the metal oxide single crystal is exposed to the electrolyte aqueous solution. Acid-base Titrations. The system was thermostated at (25.0 ± 0.1) ºC. The ionic strength was controlled with KNO3. The solution (200 cm3) containing HNO3 (10−4 mol dm−3) and KNO3 (10−3 mol dm−3) was titrated with KOH solution (0.1 mol dm−3). At relatively low ionic strength it may be assumed that the point of zero charge does not deviate significantly from the isoelectric point, so that the latter may be used as the point of zero poten-tial.17 In order to measure the SCrE potential as a func-tion of pH, three electrodes were immersed: single crys-tal electrode (SCrE), Ag/AgCl, Cl− reference electrode and combined Ag/AgCl, Cl−/glass electrode. To avoid mutual effects through the public electrical network, two pH-meters powered by batteries were used (Me-trohm 826). The system was gently mixed with a mag-netic stirrer.  RESULTS Figure 3 demonstrates the measurement with the rutile electrode for original solution and for the first and second additions of the base. First, the original acidic solution (pH = 4.04) was monitored in the direct mode; the reading from pH meter (D1) was 233.0 mV. Then, the counter voltage of 233.0 mV was adjusted and ap-plied, and the reading in the HRCM mode was −2.8 mV. In the third step, the device was switched back to the direct mode and the D2 value was found to be 230.2 mV. The counter voltage of 233.0 mV was corrected for incomplete compensation (−2.8 mV) so that the SCrE electrode potential in the reversible condition was ob-tained as 230.2 mV. Scattering of the measured values was found to be within 1 mV and the readings were stable. After the first portion of the base (pH = 4.58), the SCrE potential was lowered so that D1 and D2 values were 208.1 and 206.2 mV, respectively. The HRCM value was obtained as 206.3 mV. The second portion showed similar behavior, but again the lower value of the SCrE potential was observed. It may be concluded that the direct mode and HRCM produce practically equal values, which is due to the fact that the pH meter employed in this study has sufficiently high internal resistance. Figure 4 displays the effect of pH on the surface potential at the rutile (0 0 1) plane exposed to aqueous solution. The electrode potentials of SCrE were con-verted to surface potentials by setting the value of the surface potential ψ0 to zero at the isoelectric point.17 For rutile (0 0 1) plane,18 pHiep  5.7. As expected on the basis of the Surface Com-plexation Model,19,20 rutile exhibited almost linear de-pendency of surface potential on pH, with the slope being lower in magnitude compared to the Nernst equa-tion. The slope is −50.97 mV, i.e., 13.6 % lower than the Nernstian. The points measured directly with the pH-meter coincide with the data obtained by HRCM.  Figure 2. The single crystal rutile electrode for surface poten-tial measurements.  Figure 3. Acid-base titration with the rutile SCr electrode (original solution, and also for two additions of the base to acidic solu-tion). Electrode potentials in the initial acidic solution (pH = 4.04), and after two additions (pH = 4.58 and pH = 5.18) were meas-ured in the direct mode (D1), by the high resistance compensation method (HRCM), and again checked by the direct mode (D2). 326 N. Kallay et al., High Resistance Compensation Method Croat. Chem. Acta 82 (2009) 323. DISCUSSION AND CONCLUSIONS In this article, we have demonstrated the potential of the HRCM developed for measurements of electrode poten-tials of high-resistivity electrodes. This method is espe-cially useful for measurements of surface potentials of crystal planes in contact with electrolyte solutions by means of single crystal electrodes. Even in the case when direct measurements using a high resistance milli-voltmeter (pH-meter) produce reproducible results, one cannot be sure that the measured values are equilibrium values, i.e., the values corresponding to equal rates of adsorption and desorption of ions at the interface. The high resistance compensation method solves the prob-lems related to direct measurements, but also those associated with the classical compensation Poggendorff method. The method may be used for any source of the electric potential but is especially useful for evaluation of the surface potential at the solid/liquid interface since it enables characterization of specific crystal planes, which is important for interpretation based on the ad-vanced MUSIC model.21 Most of the published data on charging the aqueous interfaces of crystals are obtained with colloid particles, thus yielding average properties of all planes exposed to the liquid medium. Such aver-age properties (electrokinetic potentials and surface charge densities) cannot be used for advanced theoreti-cal treatments based on consideration of the specific structure of a defined crystal plane. In addition to ad-vanced electrokinetic techniques, providing electroki-netic potentials of defined crystal planes,18 the corres-ponding surface potential data enable a better insight into interfacial equilibrium. It may be concluded that in order to ensure that data correspond to the equilibrium state at a specific crystal plane, the proposed High Re-sistance Compensation Method may serve as a useful tool. Acknowledgements. The financial support from the Ministry of Science, Education and Sports of the Republic of Croatia (project No. 119-1191342-2961) is gratefully acknowledged.  REFERENCES 1. N. G. H. Penners, L. K. Koopal, and J. Lyklema, Colloids Surf. A 21 (1986) 457. 2. M. J. Avena, O. R. Camara, and C. P. de Pauli, Colloids Surf. A 69 (1993) 217. 3. E. Kinoshita, F. Ingman, and G. Edwall, Electrochim. Acta 31 (1986) 29. 4. L. Bousse, N. F. de Rooij, and P. Bergveld, Surface Sci. 135 (1983) 479. 5. H. Van der Vlekkert, L. Bousse, and N. de Rooij, J. Colloid In-terface Sci. 122 (1988) 336. 6. J. F. Schneck, J. Colloid Interface Sci. 61 (1977) 569. 7. C. Cichos and Th. Geidel, Colloid Polym. Sci. 261 (1983) 947. 8. N. Kallay and D. Čakara, J. Colloid Interface Sci. 232 (2000) 81. 9. N. Kallay, Z. Dojnović, and A. Čop, J. Colloid Interface Sci. 286 (2005) 610. 10. T. Preočanin, W. Janusz, and N. Kallay, Colloids Surf. A 297 (2007) 30. 11. T. Preočanin, N. Kallay, and A. Čop, J. Colloid Interface Sci. 299 (2006) 772. 12. N. Kallay and T. Preočanin, J. Colloid Interface Sci. 318 (2008) 290. 13. N. Kallay, T. Preočanin, J. Marković, and D. Kovačević, Collo-ids Surf. A 306 (2007) 40. 14. S. V. Yanina and K. M. Rosso, Science 320 (2008) 218. 15. Vl. Simeon, Chem. Int. 26(6) (2004) 12. 16. U. Diebold, Surf. Sci. Rep. 48 (2003) 53. 17. N. Kallay, T. Preočanin, and T. Ivšić, J. Colloid Interface Sci. 309 (2007) 21. 18. J. W. Bullard and M. J. Cima, Langmuir 22 (2006) 10264. 19. M. A. Blesa and N. Kallay, Adv. Colloid Interface Sci. 28 (1988) 111. 20. R. E. G. van Hall, J. C. T. Eijkel, and P. Bergveld, Adv. Colloid Interface Sci. 69 (1996) 31. 21. J. P. Fitts, M. L. Machesky, D. J. Wesolowski, X. Shang, J. D. Kubicki, G. W. Flynn, T. F. Heinz, and K. B. Eisenthal, Chem. Phys. Lett. 411 (2005) 399.    Figure 4. Surface potential (Ψ0) of rutile (0 0 1 plane) as afunction of pH in aqueous KNO3 solution (Ic = 0.001 moldm−3) at 25 °C, as measured directly with the pH-meter (○),and by HRCM (◊). The Nernstian potential (ΨN) is representedby the dashed line. Surface potential Ψ0 was obtained from themeasured electrode potentials taking into account the isoelec-tric point for rutile (0 0 1) plane18 pHiep ≈ 5.7. N. Kallay et al., High Resistance Compensation Method 327 Croat. Chem. Acta 82 (2009) 323.  SAŽETAK Visokootporna kompenzacijska metoda za mjerenje površinskih potencijala Nikola Kallay,a Tajana Preočanin,a Atiđa Selmani,a Filip Šupljikaa i Ivan Leničekb aZavod za fizikalnu kemiju, Kemijski odsjek, Prirodoslovno-matematički fakultet, Sveučilište u Zagrebu,  Horvatovac 102a, HR-10000 Zagreb, Hrvatska bZavod za osnove elektrotehnike i električka mjerenja, Fakultet elektrotehnike i računarstva, Sveučilište u Zagrebu, Unska 3, HR-10000 Zagreb, Hrvatska Razvijena je visokootporna kompenzacijska metoda (HRCM) za mjerenje elektrodnih potencijala viskootpornih elektroda. To je u osnovi klasična Poggendorffova kompenzacijska metoda, gdje je umjesto ampermetra, pH-metar uključen kao indikator potpune kompenzacije. Metoda je primijenjena za mjerenje elektrodnog potencijala elektrode s monokristalom rutila (TiO2). Elektrodni potencijali su preračunati u površinske potencijale, te je odre-đena ovisnost površinskog potencijala o pH u vodenom mediju. 

High Resistance Compensation Method for Surface Potential Measurement

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