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Characterization of Ph in Liquid Mixtures of Methanol/H2O/CO2

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Characterization of Ph in Liquid Mixtures of Methanol/H2O/CO2 Anal. Chem. 2000, 72, 475-480 Characterization of pH in Liquid Mixtures of Methanol/H2O/CO2 Dong Wen and Susan V. Olesik* Department of Chemistry, The Ohio State University, 120 West 18th Avenue, Columbus, Ohio 43210 The presence of H2O and CO2 in enhanced-fluidity liquids up to 20 mol %, diminished resolution was observed between changes the pH in these mixtures due to the formation of ammeline (pKa 4.2) and hydroxyatrazine (pKa 4.9). Buffered - carbonic acid. The acid base equilibria in enhanced- mixtures of methanol/H2O/CO2 mobile phases were also used fluidity liquids will also be affected by the reduction in to improve the chiral resolution of 1,1′-binaphthyl-2,2′diyl-hydrogen the dielectric constant with the addition of CO . The pH 7 2 phosphate. When up to 20 mol % CO2 was added to a methanol/ of enhanced-fluidity liquid mixtures at room temperature ammonium nitrate mixture, the resolution increased from 0.45 to was determined from the UV/visible absorption spectra 0.78. Higher efficiency and longer retention time were observed of several pH indicators. pH values of methanol/H O/CO 2 2 as well. Obviously, it is imperative to investigate pH variation of mixtures with CO proportions as high as 19.2 mol % are 2 enhanced-fluidity liquid mixtures in which both H O and CO are reported. The effect of adding buffer to methanol/H O/ 2 2 2 present. CO2 mixtures on pH was also studied. It was also shown Potentiometric methods are capable of pH measurements that pressure variation did not significantly influence the 8-13 pH of enhanced-fluidity liquids. under high-pressure conditions. However, the standardization under such conditions is not always stable.12 Difficulties in probing Enhanced-fluidity liquid mixtures include high-polarity solvents a tightly sealed system under elevated pressure still exist, such as alcohols and water combined with liquefied gases, such especially when gaseous components are involved. 1,2 as CO2. These mixtures provide the advantages of high solvent Besides traditional potentiometric pH measurements, spectro- strength and fast mass transfer inherited from liquids and liquefied photometric experiments can also be used to measure the acidity gases, respectively. Enhanced-fluidity liquid mixtures can be used of the system of interest.14-18 Xiang et al.17 determined pH values to advantage for a range of separation problems.1-3 For example, for aqueous sulfuric acid and sulfuric acid/ammonia mixtures when methanol/CO2 and methanol/H2O/CO2 liquid mixtures under sub- to supercritical conditions (pressure up to 400 atm, were used as mobile phases for chromatographic separations, temperature up to 400 °C) using the changes in the UV/visible advantages such as improved separation efficiency, increased spectrum of acridine. Toews et al.16 used the changes in the optimum linear velocity, lower pressure drop, and decreased spectrum of bromophenol blue to measure pH in aqueous 1,2 analysis time were observed. Methanol/H2O/CO2 mixtures were solutions that were heated to 70 °C and pressurized with CO2 up also used effectively as extraction solvents for polar solutes. to 200 atm. pH values ranging from 2.80 to 2.95 were reported Substantially improved extraction recoveries and increased extrac- for water in contact with supercritical CO . tion rates were previously reported for phenolic pollutants and 2 This study describes spectrophotometric determinations of pH phenyoxyacid herbicides on house dust,3,4 as well as atrazine and its metabolites in sediments.5 in methanol/H2O/CO2 mixtures. pH values of these mixtures with Enhanced-fluidity liquid mixtures of high polarity often include CO2 content up to 19.2 mol % are reported. The effect of the addition of aqueous buffer on pH control in the methanol/H O/ H2O as one of the components. If combined with CO2, the pH of 2 the mixtures will change due to the formation of carbonic acid. CO2 mixtures was studied. In reversed-phase high-performance liquid chromatography (HPLC), the pH of the mobile phase is varied to control the charge of (7) Sun, Q.; Olesik, S. V. Anal. Chem. 1999, 71, 2139. ionizable analytes. A recent reversed-phase HPLC separation of (8) Niedrach, L. W. Science 1980, 207, 1200. (9) Ding, K.; Seyfried, W. E. Science 1996, 272, 1634. triazine herbicides with high pKa values was profoundly affected (10) Lvov, S. N.; Gao, H.; Macdonald, D. D. J. Electroanal. Chem. 1998, 443, by the presence of CO2 and H2O in an enhanced-fluidity liquid 186. 6 mobile phase. With the proportion of CO2 in the mobile phase (11) Waard, C. D.; Milliams, D. E. Corrosion 1975, 31, 177. (12) Crolet, J. L.; Bonis, M. R. Corrosion 1983, 39, 39. * To whom correspondence should be addressed. Phone: (614)292-0733. (13) Miyasaka, A.; Denpo, K.; Ogawa, H. ISIJ Int. 1989, 29, 85. Fax: (614)292-1685. E-mail: [email protected]. (14) Neumann, R. C.; Kauzmann, W.; Zipp, A. J. Phys. Chem. 1973, 77, 2687. (1) Cui, Y.; Olesik, S. V. Anal. Chem. 1991, 63, 1812. (15) Xiang, T.; Johnston, K. P. J. Phys. Chem. 1994, 98, 7915. (2) Lee, S. T.; Olesik, S. V. Anal. Chem. 1994, 66, 4498. (16) Toews, K. L.; Shroll, R.; Wai, C. M. Anal. Chem. 1995, 67, 4040. (3) Reighard, T. S.; Olesik, S. V. Anal. Chem. 1996, 68, 3612. (17) Xiang, T.; Johnston, K. P.; Wofford, W. T.; Gloyna, E. F. Ind. Eng. Chem. (4) Monserrate, M.; Olesik, S. V. J. Chromatogr. Sci. 1997, 35, 82. Res. 1996, 35, 4788. (5) Shows, M.; Olesik, S. V. J. Chromatogr. Sci., in press. (18) Park, S. N.; Kim, C. S.; Kim, M. H.; Lee, I.; Kim, K. J. Chem. Soc., Faraday (6) Yuan, H.; Olesik, S. V. Anal. Chem. 1998, 70, 1595. Trans. 1998, 94, 1421. 10.1021/ac9905219 CCC: $19.00 © 2000 American Chemical Society Analytical Chemistry, Vol. 72, No. 3, February 1, 2000 475 Published on Web 12/22/1999 THEORY It is therefore reasonable to believe that changes in dielectric The dissociation of a pH indicator, a weak acid, can be constant will follow the same pattern for methanol/H2O/CO2 observed through its UV/visible absorbance spectra. If the acidic mixtures. Using dielectric constants of methanol/H2O mixtures - 26 form (HIn) and the basic form (In ) of the indicator absorb in the literature and the mole fraction of CO2 in the mixture, ultraviolet or visible light, their absorbance ratio is proportional the dielectric constants for the studied methanol/H2O/CO2 + to the [H ] of the solvation medium. mixtures were calculated. Since no more than 20 mol % CO2 was - + added into methanol/H2O, the dissociation of a weak acid in AHIn/AIn- ∝ [HIn]/[In ] ∝ [H ] (1) methanol/H2O/CO2 can be approximated by that in methanol/ H2O of the same dielectric constant. 27 By making the assumption that the concentration and activity Bosch et al. generated equations that allow the calculation of the hydrogen ion are approximately equal (i.e., [H+] is low), of pKa values in methanol/H2O mixtures for commonly used the above relationship can be used to determine the pH of a buffers. The pH value of a buffer in a given methanol/H2O mixture - solution. could then be readily determined using the Henderson Hassel- bach equation. To probe the pH variation in methanol/H2O/CO2 mixtures, the relationship between the A /A - ratio of a pH indicator and HIn In pH ) pK + log[conjugate]/[acid] (4) [H+] must be established first. This implies knowledge of weak a acid dissociation in methanol/H2O/CO2 mixtures, which is not available in the literature. - Calibration curves, plots of AHIn/AIn for a given indicator It is obvious that carbonic acid will form in methanol/H2O/ + versus [H ], were made using methanol/H2O mixtures with the CO2 mixtures. Because it is nonpolar, the addition of liquid CO2 same dielectric constant as the studied methanol/H2O/CO2 will lower the dielectric constant of the resultant mixture as well. mixtures and were used to determine the [H+] of the methanol/ The Born equation (eq 2) is often used15,17,19 to described the net H2O/CO2 mixtures. z 2 z 2 Δ ) C 1 + A - HA EXPERIMENTAL SECTION G° (2) rH O+ rA rHA Materials. HPLC-grade methanol was obtained from J. T. [ 3 ] Baker (Phillipsburg, NJ). H2O was distilled and deionized by a NANOpure II system (Sybron/Barnstead, Boston, MA). The change in Gibbs energy per mole in an ionization equilibrium of resistivity of the deionized H2O was 17.3-17.8 MΩ. SFE/SFC- an acid, HA, where zi is the charge of the ion, C is a constant, r grade CO was obtained from Air Product and Chemicals 2 is the radius of the respective solvated species, and is the (Allentown, PA). dielectric constant of the solvent. Since the Gibbs energy of a Tetrabromophenolphthalein ethyl ester potassium salt, thymol reaction is related to its equilibrium constant, the variation of the blue, and bromophenol blue (ACS reagent) from Aldrich Chemical dissociation constant, Ka, in mixed solvents is a function of the Co. (Milwaukee, WI), potassium dihydrogen phosphate (HPLC dielectric constant of the mixture using eq 3, where R is the gas grade, 99.6%) and dipotassium hydrogen phosphate (ACS certified, 2 2 99.2%) from Fisher Scientific (Fairlawn, NJ), glacial acetic acid z z - ) C 1 + A - HA (99.8%) and phosphoric acid (85%) from Mallinckrodt Chemical RT ln KA (3) rH O+ rA rHA [ 3 ] (St. Louis, MO), and sodium acetate (ACS reagent, 99.8%) from Jenneile Enterprises (Cincinnati, OH) were used as received. Sodium carbonate from Jenneile Enterprises was dried at 200 constant and T is absolute temperature.
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