ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 11, pp. 1883−1889. © Pleiades Publishing, Ltd., 2011. Original Russian Text © A.A. Kiprianov, N.M. Pankratova, I.A. Ponomarev, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 11, pp. 1797−1803.
APPLIED ELECTROCHEMISTRY AND CORROSION PROTECTION OF METALS
Potentiometric pH Sensors for Measurements in Fluoride-Containing Solutions A. A. Kiprianov, N. M. Pankratova, and I. A. Ponomarev St. Petersburg State University, St. Petersburg, Russia Received December 23, 2010
Abstract—The results of tests in fluoride-containing solutions were presented for an antimony electrode, solidstate polyvinyl chloride-graphite electrodes modified by a quinone-hydroquinone series system, and electrodes made of silicate glasses, in particular, of commercial formulations. A technique and criteria were proposed for comparison of the performance characteristics of different glass electrodes in fluoride-containing solutions, and samples most resistant to hydrofluoric acid were identified. DOI: 10.1134/S1070427211110085
Some industrial applications, in particular wastewater analysis, often require pH measurement sensors resistant to fluoride-containing solutions. Conventional glass electrodes commonly used for pH measurements in process fluids are unsuitable for this purpose. The available publications recommend that pH in fluoride-containing solutions be measured with the use of pH sensors that either have long been known or are currently under development, e.g., metal oxide [1, 2] and quinhydrone [3] electrodes, membrane electrodes based on a liquid ion-exchanger or on a neutral carrier [4, 5], phosphate glass electrodes [6], and electrodes made of specially formulated silicate glasses [7, 8]. However, the instruments proposed are often unfit for specific tasks. For example, such interesting developments as plasticized membrane electrodes and phosphate glass electrodes are unsuitable for application under real process conditions. They exhibit low selectivity, poor potential stability, and incomplete realization of hydrogen function. At the same time, glass electrodes that are currently used most successfully under process conditions are not resistant to hydrofluoric acid attacks. In this connection, much effort has been dedicated in recent decades to development of special glass formulations with enhanced resistance to HF, but no criteria were provided for evaluating the performance characteristics of the fluoride-resistant glass sensors proposed. For silicate glass
membrane electrodes the stability in fluoride-containing solutions is determined by the fluoride concentration and pH of medium, as well as by the residence time of electrode in a specific solution. The versatility of the problem to be solved poses the main difficulty in selection of appropriate indicating electrode, and the lack of a universal tool motivates further research and development aimed to create new structures and adapt the existing ones for special usage. As to the type of the reference electrode and the design of the electrochemical cell, there is also no obvious choice either, because the commonly used silicate materials are susceptible to etching in fluoride-containing solutions. In view of the above-said, in particular in regard to development of new formulations of electrode glasses, we carried out here comparative tests of three pH sensors most promising for fluoride applications: an antimony electrode, a solid-state polyvinyl chloride-graphite electrode modified by a quinone-hydroquinone series system, and electrodes prepared from various silicate glass formulations. EXPERIMENTAL The electrode properties were examined by the potentiometric method at room temperature with the use of an mV/pH meter (I-130, Status-2) in a charge-transfer
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electrochemical cell with a double salt bridge: Ag|AgCl, KCl (sat.) || Measured solution | pH Measurement electrode
As reference electrode served an EVL-1M3 saturated silver chloride electrode. Considering the fact that the experiments were carried out in aggressive media, special attention was paid to the salt bridge immersed into the measured solution. To this end, several types of devices with different porous diaphragm materials were used, specifically those based on zirconia ceramics, quartz fiber, and chemically resistant (polished) rubber. According to [1, 2], the performance characteristics of an antimony electrode are unaffected by the presence of fluoride ions, which phenomenon was also observed experimentally. The electrode potential is described by the following equation: 2.3RT Esb = E°sb –––––– pH, F
Based on the calibration plots obtained for an antimony electrode with the use of standard buffer solutions, the coefficient of ∂E/∂pH was estimated at 54.4 mV, and the standard electrode potential E°Sb exp, at 160 mV, which is close to theoretical value (152 mV [9]). These values are observed in a narrow pH range (4.01–6.86), and within these limits pH can be determined accurately to within 0.2 pH units, which is consistent with the data from [1, 10]. The plots constructed for a broad pH range (1.68–9.18) are prominently nonlinear, and their parameters are far from theoretical (∂E/∂pH = 49 mV, E°Sb exp = 180 mV). Poor reproducibility of the potential and deviations in the acidic region are due to the presence in the buffer solution of potassium tetraoxalate able of complexing with antimony [1, 10]. The range within which an antimony electrode displays a linear response can be extended to more acidic solutions via use of other than standard special buffer systems that do not contain ions possessing a complexing power. Realization of the hydrogen function by an antimony electrode in the acidic region is limited by pH = 2–2.5 [1]. Deviations from linearity in strongly acidic and alkaline regions are due to the occurrence of potential-forming systems with different standard potential values, which is associated with possible formation of various antimony compounds, e.g., Sb2O3, Sb(OH)3, HSbO2, Sb2O4, Sb2O5 [10]. Thus, with special calibra-
tion solutions a linear response is displayed within a very limited range which does not exceed 5 pH units (2 < pH < 7). Also, the behavior of an antimony electrode is affected by the presence of redox systems and anions possessing a complexing power, as well as of certain cations, e.g., copper deposited on the antimony surface [1, 7]. In view of the above-mentioned shortcomings, an antimony electrode is typically rejected for routine pH measurements, but under regular renewal of its surface it is in demand for special applications under severe conditions (acidic media, high fluoride content), which do not require high accuracy of measurements It should be noted that application of an antimony electrode is possible only with special instruments that allow adjusting the coordinate of the isopotential point. Solid polyvinyl chloride-graphite electrodes modified with a quinine–hydroquinone series system (hereinafter, quinhydrone electrodes) were prepared by the technique proposed in [3]. The procedure consisted in mixing quinhydrone with graphite and polyvinyl chloride powder in a 1 : 6 : 1 ratio. Chemically pure quinhydrone was prerecrystallized from an aqueous solution; tetrahydrofuran served as solvent for the mixture. The latter was made into a homogeneous plastic mass from which cylindrical feedstocks for membranes were formed. After drying at 40°C for 1 day and polishing, they were glued into the electrode body together with a copper electrical lead. It was noted that the presence of fluoride ions does not affect the potential of a quinhydrone electrode, while the influence is exerted by redox systems. According to data from [11], the theoretical function of quinhydrone electrodes is virtually unaffected by redox systems when in concentrations under 5 × 10–4 g-equiv l–1 if their oxidation potentials in examined solutions differ from that of the quinine–hydroquinone system by no greater than 100 mV. The potential of a quinhydrone electrode varies with pH as 2.3RT aC6H4O2 2.3RT Eqh = E°qh + ––––– log –––––––– – ––––––– pH. 2F aC6H4(OH)2 F
Figure 1 shows the calibration plot (curve 1), averaged over the values obtained for several quinhydrone electrodes with standard buffer solutions on different days. The experimental value ∂E/∂pH = 58 mV is close to theoretical; the standard electrode potential E°qh exp was well reproducible and was estimated at 490 ± 2 mV vs. saturated silver chloride electrode [E°sc (20°C) =
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201 mV], which corresponds to the data reported in [12] (E°qh = 696 ± 1 mV). Good reproducibility of the E° values allows interchangeability of electrodes fabricated by the same technology and their calibration using the same solution with a known pH value. Compared to an antimony electrode, a quinhydrone electrode exhibits a hydrogen function over a broader pH range; these electrodes can be used at pH < 8. In solutions with higher pH values the response of such electrodes is impaired because of the specific properties of the potential-forming system: Hydroquinone is a weak acid which dissociates via proton detachment (pK1 = 9.8, pK2 = 11.4 [12]). Curves 2 and 3 in Fig. 1 show that a 6-h residence of the electrode in solution leads to impaired response: (∂Е/∂pH)exp decreases to 54 mV, and E°qh exp, to 474 mV. After 71 hours of residence these values significantly changed: ∂E/∂pH decreased to 23 mV, and E°qh exp, to 383 mV. Thus, the response of the quinhydrone electrode gets negligible. The impaired response observed upon prolonged residence of electrode in solution is due to a faster elimination of the reduced (hydroquinone) species from the redox system compared to the oxidized (quinone) species [11]. The shortcoming identified pose limitations on the use of quinhydrone electrodes in the flow-though mode, but they are suitable for analytical applications and not very prolonged measurements, provided thorough renewal of the surface (regaining calibration). Like in the case with an antimony electrode, commercial application of a quinhydrone electrode requires the use of special pH-meters which allow adjusting the coordinate of the isopotential point.. It is a well known fact that, in solutions free from hydrofluoric acid, a glass electrode significantly outperforms other known sensors used for pH measurements. However, unlike antimony and quinhydrone electrodes, it is susceptible to corrosive attacks by hydrofluoric acid: Its application in the presence of fluoride ions is limited and depends on many factors. All the glass electrodes examined by us were prepared from different alkali metal silicate glasses, in particular, from commercial formulations. The glasses were synthesized, and the electrodes were prepared, by known techniques [13, 14]; the glass blanks were filled with an acetate buffer solution containing a NaCl addition, whereupon silver chloride electrodes were placed therein. The glass formulations were designated in this study as H1, H2, H3, etc. The prepared electrodes exhibited theoretical slopes at pH
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within 1.68–12.45: ∂E/∂pH = 58 ± 1 mV. The potentiometric stability of the glass electrodes was examined by titration of sodium fluoride solutions of different concentrations (ranging from 0.018 to 0.45 M) with hydrochloric acid solutions. The initial volume of the NaF solution in all the experiments was 150 ml. In the course of the experiment the electrode potentials were recorded at regular (~2-min) intervals; pH of the solution was monitored with a quinhydrone electrode. Figure 2a shows the potentiometric titration curves which suggest that the electrodes prepared from glasses of different compositions behave differently: Upon addition of certain amounts of acid the potential exhibited deviation from a hypothetical curve, a potential jump, whose position depends on the electrode glass composition. Figure 2b shows that, at the fluoride concentration chosen, the potential of the electrodes made of different glass formulations exhibited a linear pH dependence with a close to theoretical slope up to a certain pH value, after which it abruptly changed. The pH values at which the potential deviates from the initially linear plot can serve as a criterion for comparison of the fluoride resistances of the electrodes prepared from different glass brands. A linear E–pH plot observed after the potential jump remains to be studied in detail, but the results of the tests with different glass formulations suggest that, generally, the
Fig. 1. Calibration plots for the electrode based on a quinine– hydroquinone series system. (E) Potential, mV. Residence time in the buffer solution with pH 4.01, h: (1) 0, (2) 6, and (3) 71.
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–log cH+ Fig. 3. Potential Е, mV, vs. –log cH+ parameter in a NaF + HCl (cNaF = 0.45 M) solution for the electrodes made of different glass formulations. Glass brand: (1) H3, (2) H10, (3) H5, (4) H9, (5) H1, (6) H12, (7) H7, and (8) H2; the same for Fig. 6.
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Fig. 2. Glass electrodes potentials E, mV, vs. (a) volume V, ml, of the added HCl + NaF solution (cHCl = 0.605, cNaF = 0.09 M) in a NaF solution (cNaF = 0.09 M) and (b) pH in the NaF + HCl solution (cNaF = 0.09 M). Electrode made of (1) H10 and (2) H12 glass. (a): (3) a hypothetic fluoride-resistant electrode (theoretical curve) and (b) a fragment of the calibration plot for the (1) H1 and (2) H12 glass electrode in buffer solutions free from fluoride ions.
slope of this plot is markedly different from theoretical. Figure 3 shows the Е–(–log cH+) dependences for the electrodes examined. The choice of these specific coordinates was dictated by the fact that a quinhydrone electrode was employed only in selected experiments. Hence, the calculated –log cH+ parameter was used instead of pH ≡ –log аH+, because for the examined systems containing different moieties (Na+, F–, H+, Cl–, HF, HF2–, H2F2) the calculation of the pH parameter is complicated by uncertain values of the activity coefficients. The results of calculations at pH <5 were validated by measurements in a model CH3COONa–HCl system. The deviation of the points from a straight line at close to neutral pH values is associated with inaccuracy of the calculation. The highest resistance to fluoride-containing solutions was exhibited by H2 and H7 glasses for which the potential jump was observed at the lowest pH values, and the lowest resistance, by the electrodes prepared from commercial glasses H1 (no. 20) and H3 (UST). To confirm realization of hydrogen function up to the potential jump in the E–V(E–pH) dependences and impairment of the response after the potential jump we carried out experiments in a flow-through cell. The flow rate of solution through the cell was kept constant within
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Fig. 4. Relative changes in the potentials E, mV, for (1) glass H2 electrode and (2) quinhydrone electrode as correlated with the changes in pH for the (a) 0.018 and (b) 0.45 M NaF solution.
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Fig. 5. pH dependence of the potential of the (1) glass H2 electrode and (2) quinhydrone electrode in NaF + HCl solution. cNaF, M: (а) 0.018 and (b) 0.45.
5–10 ml min–1. In each experiment the fluoride concentration was kept constant; pH of solutions was changed in a jumpwise manner. Before and after the experiment a buffer solution without fluoride was passed through the cell. Figures 4 and 5 correlate the changes in the potential of the electrode prepared from H2 glass, whose fluoride resistance was the highest (curve 1), with the potentials of a calibrated quinhydrone electrode that occurred in the same flow-through cell (curve 2). Figure 4 shows that
the potential jumps and times required for steady-state values to be established in the solution with the fluoride concentration of 0.018 M are identical for the two types of electrodes, and the Е–pH plots (Fig. 5a) constructed with the use of these values are linear and have the theoretical slope. An increase in the fluoride concentration to 0.45 M (Fig. 4b) prevents realization of the complete function of the glass electrode: In Fig. 5b small deviations from linearity are observed already at pH 5.6, and at pH
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–log cF– [cF–] = M Fig. 6. Comparison of the experimental results with the data available from prospectus [7]. (1, 2) The stability limit for (1) conventional and (2) fluoride-resistant glass electrodes and (3) limiting fluoride concentration of the process nitrogencontaining wastewater coming to the inlet chamber of the neutralization tank at a fertilizer production enterprise. Glass brand: (1) H3, (2) H10, (3) H5, (4) H9, (5) H1, (6) H12, (7) H7, and (8) H2.
3.6 they become fairly significant. This fact is consistent with the potential jump in the earlier obtained E–pH plot (Fig. 3), for which an abrupt change in potential is observed at pH 4.2. The agreement of the results of the two different experiments confirms the validity of the criterion (the position of the potential jump in the E–pH curve) selected for comparing the fluoride resistances of the electrodes. To determine how the residence time of the electrode in a fluoride solution affects the electrode potential, a prolonged experiment was carried out. The electrodes were placed into a cell filled with a NaF + HCl solution characterized by the sodium fluoride concentration of 0.09 M and pH 4.5 (initial solution), whereupon the potentials were fixed, and the cell was left to stand for a period ranging from several hours to several days. Next, the solution was replaced by a fresh one equivalent to the initial solution in composition and concentration (to eliminate the effect of solution alkalization due to release of alkali components out of glass), and the potential was measured. The solution replacement procedure was repeated several times over a 2-week period. The experiments showed that, for the electrodes prepared from poorly stable glasses, the potential changed abruptly in the first few hours of the experiment, while the potential of the glass H7 electrodes did not change, thereby confirming the preservation of hydrogen function by the electrode.
The shift to positive values of the potential, observed for the H12 glass electrode, which is in contrast with the opposite trend observed for the other glass electrodes, can be explained by formation on the membrane surface of a LaF3 protective layer preventing the release of alkaline components into solution [15, 16]. Our results show that the electrodes prepared form different silicate glass formulations are suitable for pH measurements in fluoride-containing solutions, though within certain pH and fluoride concentration ranges. The limits within glass electrodes are able of functioning at a specific fluoride concentration are determined by the pH values at which the E–pH dependence exhibits positive deviations from realization of complete hydrogen function by the electrode (Figs. 2b and 3). Figure 6 shows a diagram illustrating the limits of applicability of different electrodes used for pH measurements in fluoride-containing solutions [7]. Curve 1 delineates the stability region for conventional glass electrodes, and curve 2, that for HF-resistant glass electrodes offered by Mettler Toledo. According to data from [7], the use of glass electrodes under more severe conditions in the area to the right of curve 2 is unacceptable. Using the criterion proposed in this study for assessing the performance limits for glass electrodes (the position of the potential jump in the E–pH dependence), the points corresponding to the glasses examined were plotted in Fig. 6. The criteria used in [7] for the construction of the dependences are obviously somewhat different from those chosen in this study, but the data obtained are in principle consistent with the scheme proposed. It is also seen that, in fluoride resistance, H2 and H7 glasses are markedly superior to commercial glasses H1 and H3 used to manufacture conventional glass electrodes (e.g., ESP04-14 and ESP-01-14) which are extensively applied in media free from fluoride ions. Comparison of curve 2 (Fig. 6) with dotted line 3 shows that the maximal fluoride concentration (6.7 × 10–4 M) in the process nitrogen-containing wastewater coming to the inlet chamber of the neutralization tank at a fertilizer production enterprise is 1.5 orders of magnitude lower than the minimal fluoride concentration in the solutions tested by us (1.8 × 10–2 M). However, Fig. 5a shows that, already at the fluoride concentration of 1.8 × 10–2 M, the glass H2 electrode is capable for realizing complete hydrogen function throughout the pH range examined (pH > 3.5). Based on our results, H2 and H7 glass electrodes can be suggested for testing in real processes.
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The tests with the three types of electrodes, carried out in this study, showed that, for pH measurement applications, the use of glass electrodes is preferable for solutions in which pH and fluoride concentration do not extend beyond the functioning limits of the electrodes selected. Under severe conditions (high fluoride content and low pH) the response of glass electrode is rapidly impaired, and its use is unacceptable. In that case, solidstate polyvinyl chloride-graphite sensors modified with a quinine–hydroquinone series system are suitable for brief measurements. A major advantage offered by this type of electrodes consists in good reproducibility of the calibration straight line for different quinhydrone electrodes, which affords their interchangeability and calibration with the use of the same point. For prolonged measurements under severe conditions it is necessary to use an antimony electrode. The use of both an antimony or a quinhydrone electrode requires application of instruments that allows adjusting the coordinate of the isopotential point, as well as preverification of the absence of competing redox systems in the solution examined. CONCLUSIONS (1) A comparative assessment of the functioning of an antimony electrode, solid-state polyvinyl chloridegraphite electrodes modified with a quinone-hydroquinone series system, and electrodes prepared from different glass formulations was carried out. The possibilities and limits for application of these electrodes in fluoridecontaining solutions were identified. (2) A technique was presented for comparative examination of the potentiometric properties of glass electrodes to be used for pH measurements in fluoride-containing solutions. (3) Criteria for assessing the relative stability of glass electrodes in fluoride-containing solutions were suggested. Glasses suitable for making electrodes that are most resistant to hydrofluoric acid were indicated. REFERENCES 1. Bates, R.G., Determination of pH: Theory and Practice, New York: Wiley, 1964.
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2. Metrohm USA Inc. Combined Antimony Electrode; http:// www.metrohmusa.com/Products/Titration/Electrodes/pHElectrodes/60421100.html. 3. Kazak, A.S., Rodionova, S.A., Trofimov, M.A., and Pendin, A.A., Zh. Anal. Khim., 1996, vol. 51, no. 9, pp. 970–974. 4. Han, Won-Sik, Park, Myon-Yong, Chung, Koo-Chun, et al., Electroanalysis, 2000, vol. 13, issue 11, pp. 955–959. 5. Lutov, V.M. and Mikhelson, K.N., Sens. Actuators, B: Chemical, 1994, vol. 19, issues 1–3, pp. 400–403. 6. Nomura, Tsuyoshi and Nakagava, Genkichi, Chem. Soc. Jpn., 1983, vol. 56, pp. 3632–3634. 7. Thornton, Mettler Toledo, Application Notes, Measurement of Conductivity and pH in Semiconductor HF Etching Solutions & Wastewater, 2008. http://us.mt.com/global/ en/home/supportive_content/application_editorials/ App-Note-THOR-Measurement-of-Conductivity-pH. rxHgAwXLlLnPBMDSzq--.MediaFileComponent.html/ AN-Measurement_Conductivity__pH_0708.pdf. 8. Thornton, Mettler Toledo, Application Bulletin, Measurement of pH in Fluoride-Containing Processes, 2004. http://my-mt.com/thornton/pdf_files/bulletins/Hydrofluoric_Acid.pdf. 9. Mishchenko, K.P. and Ravdel’, A.A., Kratkii spravochnik fiziko-khimicheskikh velichin (Concise Reference Book of Physicochemical Quantities), Leningrad: Khimiya, 1972. 10. Stock, J.T., Purdy, W.C., and Garcia, L.M., Chem. Rev., 1958, vol. 58, pp. 611–626. 11. Kazak, A.S., Rodionova, S.A., Trofimov, M.A., and Pendin, A.A., Elektrokhimiya, 1996, vol. 32, no. 7, pp. 887–890. 12. Trofimov, M.A., Puzanov, V.V., Kazak, A.S., and Pendin, A.A., in Ionnyi obmen i ionometriya: Mezhvuzovskii sbornik statei (Ion Exchange and Ionometry: Interuniversity Coll.), St. Petersburg: St-Peterburg. Gos. Univ., 2000, issue 10, pp. 215–228. 13. Karpukhina, N.G., Study of the Bulk and Electrode Properties of Halogen-Containing Alkali Metal Silicate Glasses, Cand. Sci. Dissertation, St.-Petersburg, 2001. 14. Ives, D. and Janz, G., Reference Electrodes: Theory and Practice, New York: Academic, 1961. 15. Bobrov, V.S., Zh. Prikl. Khim., 1978, vol. 51, no. 10, pp. 2237–2242. 16. Bach, H., Baucke, F.G.K., and Krause, D., Electrochemistry of Glasses and Glass Melts, Including Glass Electrodes, Berlin: Springer, 2001.
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