Dissociation Quotient of Benzoic Acid in Aqueous ...

University of Nebraska - Lincoln

DigitalCommons@University of Nebraska - Lincoln Papers in the Earth and Atmospheric Sciences

Earth and Atmospheric Sciences, Department of

1-1-1995

Dissociation Quotient of Benzoic Acid in Aqueous Sodium Chloride Media to 250°C Richard Kettler University of Nebraska - Lincoln, [email protected]

David J. Wesolowski Oak Ridge National Laboratory

Donald A. Palmer University of Nebraska - Lincoln

Kettler, Richard; Wesolowski, David J.; and Palmer, Donald A., "Dissociation Quotient of Benzoic Acid in Aqueous Sodium Chloride Media to 250°C" (1995). Papers in the Earth and Atmospheric Sciences. Paper 197. http://digitalcommons.unl.edu/geosciencefacpub/197

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Journal of Solution Chemistry, Vol. 24, No.4, 1995

Dissociation Quotient of Benzoic Acid in Aqueous Sodium Chloride Media to 250°C Richard M. Kettler,! David J. Wesolowski,2 and Donald A. PalmerI> Received May 26, 1994; Revised January 16, 1995 The dissociation quotient of benzoic acid was determined potentiometrically in a concentration cell fitted with hydrogen electrodes. The hydrogen ion molality of benzoic acid/benzoate solutions was measured relative to a standard aqueous IiCI solution at seven temperatures from 5 to 250°C and at seven ionic strengths ranging from 0.1 to 5.0 molal (NaCI). The molal dissociation quotients and selected literature data were fitted in the isocoulombic (all anionic) form by a six-term equation. This treatment yielded the following thermodynamic quantities for the acid dissociation equilibrium at 25°C and 1 bar: 10gKa = -4.206±0.006, ,1Jl:=O.3±O.3 kJ-mol- 1 , .1S:=-79.6±1.0 J-moZ-l-K-l, and ,1C;;a = -207±5 J-moZ-l-K-l. A five-term equation derived to describe the dependence of the dissociation constant on solvent density is accurate to 250°C and 200 MPa. KEY WORDS: Benzoic acid; dissociation constant; ionization constant; potentiometry; ionic strength, temperature and pressure dependence; thermodynamics.

1. INTRODUCTION The ionization of benzoic acid is of great interest to the chemical and geological sciences. Much of the interest in the chemical community results from attempts to evaluate the validity of the Hammett equation(l) or other free-energy relationships.(2) These studies have established a discrepancy between thermodynamic properties derived from differentiation of 10gKa with respect to T and those measured using solution calorimetry.(3) IDepartment of Geology, University of Nebraska, Lincoln, NE 68588-0340. 2Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P. O. Box 2008, Building 4500S, Oak Ridge, TN 37831-6110. 385 0095-9782/94/0400-0385$07.50/0 ©1995 Plenum Publishing Corporation This article is a U.S. government work, and is not subject to copyright in the United States.

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Kettler, Wesolowski, and Palmer

Benzoic acid has been identified in a number of fluids of interest to the geological community including landfill leachates,(4,5) petrochemical wastewaters,(6) and the vadose waters found in forest soils,(7) and at a variety of hazardous and nuclear waste sites.(8,9) High temperature fluids, in particular those associated with petroleum, contain significant amounts of benzoic acid. These solutions include sedimentary basin brines,(lO,ll) oil shale retort and tar sand byproduct waters,(12,13) and waters present during hydrous pyrolysis of kerogen in the presence of limonite.(14) Benzoic acid complexes transition metal ions and is, therefore, of importance to research related to weathering, sedimentary diagenesis and the formation of ore deposits. The dissociation of benzoic acid follows the reaction (1)

The molal dissociation quotient for this reaction Q a is defined in terms of molal concentrations and is related to the equilibrium dissociation constant Ka as shown in Eq. (2). Qa

=

[H+][C 6 HsC02"] [C 6 HsC0 2H-]

=

Ka

YC6H5C02H

(2)

YH+YC6Hsco2

Equation (1) can be written in its basic. or anionic, form by combining it algebraically with the water dissociation reaction (3)

with the relevant concentration quotient defined as

= Kb YOn-YC6IISC02H awYc6Hsco2

(4)

where a w is the activity of water. Although data exist at temperatures ranging from 0 to 250°C (Table I), few measurements have been made in solutions of high ionic strength. The few studies performed at higher ionic strengths have used tetraalklyammonium salts as a supporting elcctrolyte(29.31) and are not directly relevant to aqueous solutions of geochemical interest. The dissociation quotients of benzoic acid have, therefore. been measured in aqueous NaCl media to ionic strengths of 5.0m and to temperatures of 250°C, the limit of thermal stability of benzoic acid in this experimental environment.

387

Dissociation Quotient of Benzoic Acid

Table I. Summary of the Thermodynamic Data for the Dissociation of Benzoic Acid at Saturated Vapor Pressurea pK. 4.223±0.0 1Od 4.204±0.005 d 4.224±0.005 d 4.266±0.010d 4.336±0.015 d 4.202 d.e 4.390 d,e 4.401 d,e 4.621 d,e 4.626 d,e 4.965 d,e 4.925 d,e 5.428 d,e 5.392 d,e

tlf{b

•

I1S/

I1C~;.c

0.251±0.021

4.01±0.15 2.51±0.15 0.44±0.15 -1.90±0.15 -3.29±0.17 -4.79±0.23 -5.95±0.27 -7.17±0.27 -9.08±0.36 4.218 4.208 4.202 4.205 4.210 4.219 4.202

0.627±0.10

78.2±0.3

-0.28±0.079

-81.35±0.2

0.42±0.042 d 0.63±0.21d

4.208 4.236 4.244 4.260 4.347 4.377 4.553 4.585 4.854 4.886 5.051

-78.2

-176±12

TeC) Ref. 5 25 50 70 90 25 99.9 100.0 150.2 150.3 200.2 200.2 250.9 251.0 5.0 12.5 25.0 40.0 50.0 65.0 75.0 85.0 100.0 15 20 25 30 35 40 25 25 25 25 25 50 50 100 100 150 150 200 200 225

15 15 15 15 15 16 16 16 16 16 16 16 16 16 17 17 17 17 17 17 17 17 17 18 18 18 18 18 18 19 20 21 22 22 22 22 22 22 22 22 22 22 22

388


Table I. Continued pKa

M1a b

/:t,.S/

/:t,.C~;a"

5.070 4.205 d 4.201 d 4.203 d 4.205 d

1.41 0.44 -0.60 -1.63

-75.7 -79.1 -82.4 -85.8

-198 -202 -205 -208

0.38±0.21d

3.080 -115 1.925 d

-143 0.494 d

-163 -1.138 d

-161 -2.749 d

-156 -4.306 d 4.216 d 4.209 d 4.205 d 4.201 d 4.202 d 4.208 d 4.211 d 4.218 d 4.222d 4.236 d 4.248 d 4.262 d 4.277d 4.293 d 4.311 d 4.330 d 4.351 d 4.373 d 4.395 d

4.214 4.212 4.213 4.215 4.221 4.232 4.241 4.203

0.502

-74.7

163

TeC) Ref. 225 20 25 30 35 25 5 10 15 20 25 30 35 40 45 50 55 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 15 20 25 30 35 40 45 25

22 23 23 23 23 24 25 25 25 25 25 25 25 25 25 25 25 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 27 27 27 27 27 27 27 28

389


Table I. Continued pK.

!1H a b

4.208 4.219 4.236 4.262 4.309 4.209

/).S/

/),C~;/

T(°C) Ref. 35 50 60 75 90 35

28 28 28 28 28 29

a 1 bar at T!> 100°C. b Units: kJ-mol-l. C Units: J-mol-I-K-I. d These values were combined with the results of this study in the fitting process. • Values extrapolated to saturation vapor pressure, see text.

2. EXPERIMENTAL 2.1

Materials

All solutions were prepared from reagent grade chemicals and distilled, deionized water (resistivity ~0.183 Mnm). Stock solutions of NaCl, NaOH and HCl were sparged and stored under argon in polypropylene containers. The test solutions were made using reagent grade benzoic acid (Mallinckrodt lot 0112 KVRK) combined with the NaOH and NaCl stock solutions and purified water. The protolytic purity of the benzoic acid was checked by titration against a standard NaOH solution. This titration yielded an equivalent weight for benzoic acid of 122.1 g-eq-l (cj. actual equivalent weight of 122.12). The high temperature cell was pressurized with ultrapure Matheson (99.999%) hydrogen. Two stock reference solutions with ionic strengths of 1 and 5 molal were prepared (ionic strength was determined from the stoichiometric molalities of all ions in solution) and diluted to the appropriate ionic strength prior to each experiment (Table 11). The molal ratios of HCl:NaCl in these stock solutions were 1:199 and 1:999 for the 1 and 5 molal solutions, respectively. Two stock test solutions with ionic strengths of 1 and 5 molal were prepared and diluted as necessary for the experimental runs (Table II). The 5 molal solution had a ratio of benzoic acid to N aCl of 1: 123 whereas the ratio in the 1 molal solution was I :23. Although equivalent formal concentrations of NaOH and benzoic acid were used in the 1 molal stock solution, sufficient NaOH was added to the 5m stock solution to produce a benzoate:benzoic acid ratio of 8: 1. As benzoic


390

Table II. Starting Molal Solution Compositions Ref. Cell

Test Cell

Expt.

m(NaCI)

2,8,13 9 10 11 14 15 16 18 19 21 22 23

0.9900 0.09898 0.4948 0.3300 0.09898 0.3299 0.4949 0.9970 0.9969 2.991 3.987 4.993

m(C 6 H sC0 2H)

m(NaOH)

m(NaCI)

0.02010 0.002010 0.01004 0.006699 0.002010 0.006697 0.01005 0.001996 0.001996 0.005988 0.007984 0.009998

0.01006 0.001006 0.005028 0.003353 0.001006 0.003352 0.005029 0.001777 0.001776 0.005330 0.007105 0.008898

0.9950 0.09950 0.4975 0.3317 0.09951 0.3317 0.4975 0.9950 0.9950 2.997 3.997 4.995

m(HCI) 0.005000 0.0005000 0.002500 0.001667 0.0005000 0.001666 0.002500 0.005000 0.005000 0.003000 0.004001 0.005000

acid is sparingly soluble in NaCI solutions, this high ratio allowed sufficient benzoic acid to be dissolved to perform the experiments. 2.2 Apparatus and Procedures

The static concentration cell used in this study has been described by Mesmer et al.,(32) with modifications described by Wesolowski et al. (33) and Kettler et al. (34) The solution compositions listed in Table II define the cell configuration Pt,H 2 1C6 HsC0 2 H,NaCI,NaOH II NaCI,HCll H20 Pt The porous Teflon liquid junction was saturated with stock reference solution prior to each experiment. The cell was assembled and purged at least five times by pressurizing the vessel to 1.5 MPa with hydrogen and venting to ambient room pressure. The solutions in the cell were stirred with Teflon-coated magnetic (Alnico-V) stirring bars during all phases of the experiments. The hydrogen pressure was then regulated to approximately 1.5 MPa and the system allowed to attain thermal equilibrium in either the oil or water baths. Equilibrium was considered to be achieved when the potential changed by less than 0.01 mV over a 4 minute period. After a stable potential was observed, the potential, temperature, and pressure were recorded. The bath temperature was then increased to the next setpoint and the data collection repeated. The cell was allowed to stabilize overnight (10 to 12 h) for those runs beginning at 5°C, whereas initial equilib-

391


rium was achieved in approximately 2.5 h for those runs beginning at 100°C and within 1 to 2 h at subsequent temperature increments.

3. RESULTS The observed potential (Table III) is related to the cell composition by the relationship E

= -(RT / F )In ([H+]t / [H+]r) -

E 1j

(5)

where square brackets denote the molality of the enclosed species and the subscripts r and t refer to the reference and test solutions, respectively; the ideal gas and Faraday constants are designated by R and F, respectively; and the absolute temperature is denoted by T. The use of matching ionic media in both the test and reference compartments minimizes the second term in Eq. (5), reduces the ratio of activity coefficients for minor ions to unity, and permits use of molal concentrations rather than activities. The second term in Eq. (5) is the approximation for the liquid junction potential (Table III) obtained from the Henderson equation [Eqs. (2 -12) of Baes and Mesmer.(35)] The limiting equivalent conductances for Na+, H+, OH-, and Cl- are taken from Quist and Marshall.