Environ. Sci. Technol. 2001, 35, 178-181
Determination of Henry’s Law Constants of Phenols by Pervaporation-Flow Injection Analysis SAMI Y. SHEIKHELDIN,† TERENCE J. CARDWELL,† ROBERT W. CATTRALL,† MARIA D. LUQUE DE CASTRO,‡ AND S P A S D . K O L E V * ,† Analytical Chemistry Laboratories, Department of Chemistry, La Trobe University, Bundoora, Victoria 3083, Australia, and Department of Analytical Chemistry, Faculty of Sciences, University of Cordoba, E-14004 Cordoba, Spain
A novel dynamic nonequilibrium technique for the determination of Henry’s law constant (HLC) of phenols based on pervaporation-flow injection (PFI) is described. A linear relationship between HLC and the amount of phenol measured by a detector in the acceptor line of a PFI system was demonstrated. This relationship was constructed using five frequently encountered phenols (phenol, 2,4dimethylphenol, 2,4-dichlorophenol, 2-chlorophenol, and 2,3dimethylphenol) and used for the determination of the HLC of three other phenols (2,4,6-trichlorophenol, 2-methylphenol, and 3-methylphenol). The HLC of all eight phenols were also determined by the single equilibrium static technique (SEST). Fairly good agreement was observed between both techniques regarding the HLC of 2,4,6trichlorophenol, 2-methylphenol, and 3-methylphenol. On the basis of the results obtained, it was concluded that the PFI technique offers considerable advantages over SEST in terms of precision, speed, labor intensity, and possibilities for automation.
Introduction Henry’s law constant (HLC), also known as the air-water partition coefficient, is a thermodynamic constant characterizing the ability of a solute to partition itself between air and water (1). Very often it is expressed as the ratio between the molar concentration of the solute in these two phases:
H ) CG/CL
(1)
where H is the Henry’s law constant (HLC), CG and CL are the molar concentrations of the solute in the gas and liquid phases, respectively. A number of experimental and theoretical methods for determining HLC have been developed (1, 2). The experimental methods include batch air stripping (3-6), trapping combined with solvent extraction (7), and equilibrium partitioning in closed systems (4, 8). All of them require the establishment of equilibrium between the gas and liquid * Corrseponding author phone: +61-3-94793747; fax: +61-394791399; e-mail:
[email protected]. † La Trobe University. ‡ University of Cordoba. 178
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phases and are often labor and time-consuming. In general, gas chromatography has been used widely in these methods for the determination of the equilibrium gas-phase concentration. The theoretical methods utilize quantitative structureproperty relationships (6) or thermodynamically based quantitative property-property relationships between vapor pressure on one hand and aqueous solubility data (9) or vapor pressure activity data (10) on the other. The technique called pervaporation-flow injection (PFI) (11) has been successfully applied for the direct quantitative determination of volatile and semivolatile solutes in liquid samples. In a PFI system, a sample containing a solute is injected into a donor stream upstream of a pervaporation module where the solute evaporates into the headspace and diffuses through a membrane into the acceptor stream where detection takes place. The solute concentration monitored by a detector in the acceptor stream is proportional to the solute concentration in the headspace, although not at equilibrium with the donor stream, it is expected to be related to the HLC of the solute. As a dynamic nonequilibrium technique, PFI should allow fast and reliable determination of HLC values of volatile and semivolatile solutes provided the relationship between their concentration in the acceptor stream and HLC is known. While developing a method for the on-line determination of phenol in environmental aqueous samples using a PFI system (12), it was noticed that a limited number of HLC experimental data for phenols at ambient temperature was available in the literature. These data are often difficult to compare and are scattered widely because of the different experimental procedures used. The aim of the present study was to explore the possibility of using PFI as a novel nonequilibrium technique for fast and reliable determination of HLC values of some phenols of environmental importance and to compare these values with results obtained by the traditional single equilibrium static technique (SEST).
Experimental Section Solution Preparation. The following reagents KNO3, NaOH (BDH, Australia), K4[Fe(CN)6] (BDH, Australia), phenol (99%), 2,4-dimethylphenol (99.5%), 2,4-dichlorophenol (99%), 2,4,6trichlorophenol (99.6%), 2-chlorophenol (99.5%) (Chem Services, PA), 2,3-dimethylphenol (99%, Aldrich Chemical Co., Milwaukee, WI), 2-methylphenol (99%, BDH, England), and 3-methylphenol (99%, BDH, England) were of analytical grade. The corresponding stock solutions were prepared in Nanopure deionized water (17.9 MΩ‚cm, Barnstead, USA). The donor stream was Nanopure deionized water, while a solution containing 0.1 M KNO3 and 0.01 M NaOH (pH 12) was used as the acceptor stream. Both the donor and acceptor solutions were degassed by sonication for 15 min. All aqueous phenol solutions used in the PFI (3.00 × 10-4 mol L-1) and the SEST (1.00 × 10-5 mol L-1) experiments as well as those used for calibrating the amperometric detector were prepared immediately before use by appropriate dilution of a freshly prepared 1000 mg L-1 aqueous stock solution of each phenol in water. Flow Systems. The PFI system used (Figure 1A) consisted of two peristaltic pumps with rate selector (Minipuls-3 Gilson, France), three injection valves (A, B, and C, type 50, Rheodyne Inc., CA), PTFE tubing (0.5 mm i.d.), and a homemade Perspex pervaporation unit (Figure 1B). Detection was performed in a flow-through amperometric wall-jet cell (Metrohm 656 electrochemical detector, Switzerland) incorporating a glassy carbon working electrode, an Ag/AgCl reference electrode 10.1021/es001406e CCC: $20.00
2001 American Chemical Society Published on Web 11/30/2000
A
B
FIGURE 1. Scheme of (A) the PFI system and (B) the donor and acceptor chambers (L ) 29.0 mm; W ) 5.0 mm; A ) 0.5 mm; D ) 5.0 mm; Φ ) 90°). (3.0 mol L-1 KCl internal solution), and a gold auxiliary electrode. Potentials were applied using a potentiostat (Metrohm 641 VA, Switzerland), and the detector output was acquired by a PC (Pentium 166 MHz) interfaced (PCL-818H, Advantech, Taiwan) to the potentiostat. The software written in MS Quick C 2.5 determined the peak maximum and calculated the peak area using the trapezoidal rule. The pervaporation unit with hexagonal donor and acceptor chambers shown in Figure 1B was similar in design to that previously used (12-14). The donor chamber was packed with a single layer of glass beads (3 mm diameter, Selby-Biolab, Australia) to improve the reproducibility and the sample throughput (14-16). The membrane was a 1.5 mm thick PTFE disk (diameter 40 mm) supplied by Trace Biotech AG, Germany. The donor and acceptor streams were maintained at the same temperature by immersing the pervaporation unit, the reservoirs of the donor and acceptor solutions, and most of the tubing into a thermostating water bath fitted with a thermoregulator (RATEK Instruments, Victoria, Australia) and a cooler (PBC-4 Neslab Instruments Inc., USA). The actual temperature of the donor stream was monitored at the outlet of the pervaporation unit (Figure 1A) using a homemade flow-through thermistor-based temperature probe (12, 13) connected to a conductivity meter (model 2100, TPS Pty Ltd, Australia). The thermistor was calibrated by passing water at a preselected temperature through the temperature probe directly from the thermostating bath. The acceptor line of the PFI system (Figure 1A) can be converted easily into a single-line flow injection (FI) system by setting injection valve B (Figure 1A) to the ‘inject’ position and using valve C (Figure 1A) for injecting the samples. This flow system was used to derive hydrodynamic voltammograms for all phenols, to study the influence of a number of system parameters on the detector peak area as discussed below, and to determine the concentration of the phenols in the solutions used in the SEST experiments. Hydrodynamic Voltammograms. Hydrodynamic voltammograms of all phenols (volume injected, 70 µL; concentration, 5.00 × 10-6 mol L-1) and of the electrolyte solution (0.1 M KNO3 and 0.01 M NaOH) were constructed in the 0-0.8
V potential range using the single-line FI system. The flow rate and the temperature were maintained at 0.88 mL min-1 and 20 °C, respectively. Influence of the System Parameters. The volume of the tubing connecting the injector to the amperometric detector in the single-line FI system was varied between 100 and 220 µL while keeping the flow rate constant (0.88 ( 0.01 mL min-1). The peak area corresponding to the injection of 70 µL 1.00 × 10-4 M K4[Fe(CN)6] solution was plotted against the tubing volume. Similar experiments on variation of the sample volume (70-130 µL), sample concentration (1.00 × 10-5-1.00 × 10-4 M K4[Fe(CN)6]), and flow rate (0.8-1.6 mL min-1) were performed also. PFI Measurements. The procedure for PFI measurements was initiated by first setting injection valve B (Figure 1A) to the inject position so that the acceptor chamber, which was part of the sample loop of valve B, was filled with the acceptor solution. Then, valve B was switched to the load position. As a result of this, the solution in the acceptor chamber of the pervaporation unit became static while the acceptor stream bypassing the pervaporation unit continued to flow through the detector, thus ensuring uninterrupted conditioning of the amperometric detector. Water samples containing one of the phenols studied were then injected (valve A, Figure 1A) into the donor stream. After 12 min, valve B (Figure 1A) was switched to the inject position, and the phenol was detected when flowing through the measuring cell at a flow rate of 0.88 mL min-1. Throughout all PFI measurements, valve C (Figure 1A) was set in the inject position. A constant liquid level in the donor chamber, which was very essential for obtaining reproducible results, was maintained throughout the pervaporation experiments with the help of a second peristaltic pump (pump 2, Figure 1A) in the donor line and a single layer of 3 mm glass beads at the bottom of the donor chamber. The liquid was maintained at a level where the beads were fully submerged. The flow rate in the donor line was maintained at 0.45 mL min-1. Calibration curves, expressed as peak area versus sample concentration injected for each individual phenol, were constructed by injecting a series of standards (70 µL) using valve C (Figure 1A). During the calibration process, valves A and B were set in the inject position, and all other parameters of the PFI system were the same as those during the pervaporation experiments. The performance of the glassy carbon electrode was checked by monitoring its response to aqueous 1.00 × 10-4 M K4[Fe(CN)6] solution injected directly into the acceptor stream via valve C (Figure 1A). SEST Experiments. These experiments were carried out by transferring 2.0 mL of a standard phenol solution in water (1.00 × 10-5 mol L-1) into a well-cleaned and dried 4-L umber bottle. The bottle was immediately well-capped and thermostated at 20 °C for a preselected time. The solute concentration was measured before inserting it into the bottle and after the waiting time using the single-line FI system (volume injected 70 µL, Figure 1A) outlined above. From the corresponding calibration curves obtained earlier, the sample concentrations were calculated. The waiting time for phenol was varied between 0.5 and 10 h to determine the minimum time required to establish equilibrium. The value obtained (2 h) was subsequently used for all SEST experiments.
Results and Discussion Calibration. The hydrodynamic voltammograms of the phenols and the background electrolyte used in this study are shown in Figure 2. On the basis of these results, the working electrode potential was selected as 0.65 V (vs Ag/ AgCl). The hydrodynamic voltammograms (Figure 2) indicate that either the diffusion coefficient or the kinetic parameters of the corresponding oxidation reaction or both are different for the different phenols. For this reason, a separate calibraVOL. 35, NO. 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Hydrodynamic voltammograms of the background electrolyte and phenols (sample volume, 70 µL; phenol concentration, 5.00 × 10-6 mol L-1; flow rate, 0.88 mL min-1; background electrolyte, 0.1 M KNO3 and 0.01 M NaOH; working electrode, glassy carbon).
TABLE 1. Calibration Range and Linearity (R 2) for All Phenols Studied Using the Single-Line FI System (Figure 1A) compound phenol 2,3-dimethylphenol 2,4-dimethylphenol 2,4-dichlorophenol 2-cholorophenol 3-methylphenol 2-methylphenol 2,4,6-trichlorophenol
calibration range (106 mol L-1) 0.57-1.41 0.66-1.32 0.81-1.88 118.3-473.2 564.6-2258.4 2.89-11.57 0.67-1.68 201.9-538.4
R2 0.997 0.988 0.999 0.982 0.975 0.995 0.972 0.990
tion curve was required for each individual phenol. These curves were linear in the concentration range used (Table 1). During the PFI experiments, the solute accumulates in the acceptor chamber of the pervaporation unit, which forms only part of the sample loop of valve B due to the connecting tubes (Figure 1A). The exact volume of the acceptor chamber under working conditions is difficult to measure because the flexible membrane stretches as a result of the pressure difference between the two chambers. For these reasons, valve C (Figure 1A) was used to inject a definite volume (70 µL) of the standards during calibration while setting valve B to the inject position (i.e., the acceptor channel operated in a single-line FI mode). However, the dispersion of the solute injected by valves B and C differs because of the different sample loop volumes of the two valves and the different volumes of the tubing connecting the valves to the detector. Since peak shape in FI systems depends on dispersion, peak height cannot be used for calibrating the detector in this case and peak area should be used instead. For a linear relationship between the concentration and the detector signal (C ) kS, where k is a constant and S is the detector signal), the peak area (∫∞0 S dt) multiplied by the flow rate and k will be equal to the amount of the analyte injected (17). Under such conditions, the peak area should be independent of dispersion, i.e., the geometric dimensions and shape of the flow-through sections of the FI system. This fact was confirmed in the single-line FI system outlined earlier by the experimental observation that, at constant flow rate (0.88 180
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TABLE 2. HLC Values Determined by SEST (This Study) and Other Methods (5, 6) at 20 °C versus Phenol Concentration in the Acceptor Line of the FPI Systema
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compound
HLC × 105 SEST
HLC × 105 cacceptor line × 106 (literature) (mol L-1)
phenol 2.20 ( 0.58 2.67 (5) 2,3-dimethylphenol 4.42 ( 0.77 3.02 (6) 2,4-dimethylphenol 6.23 ( 1.60 3.89 (6) 2,4-dichlorophenol 11.97 ( 3.95 2-chlorophenol 26.70 ( 5.84 45.7 (6)
1.39 ( 0.02 6.62 ( 0.10 8.06 ( 0.40 28.15 ( 0.58 51.08 ( 0.75
a The 90% confidence intervals for the SEST and PFI results are based on measurements in triplicate.
mL min-1) and constant loop volume, peak area was independent of the volume of the tubing (100-220 µL) connecting the injection valve to the detector. As expected (18), the peak area was found to decrease with increasing flow rate (0.8-1.6 mL min-1). For this reason in all subsequent experiments, special attention was paid to maintaining a constant flow rate throughout all FI and PFI experiments. As expected, the peak area was linearly dependent on the sample loop volume (70-130 µL) and concentration (1.00 × 10-51.00 × 10-4 M K4[Fe(CN)6]). SEST Experiments. Taking into account the vapor pressure of water at 20 °C [2.338 × 103 Pa (19)], it was estimated that even if the air in the umber bottle was absolutely dry, less than 3.5% of the 2.0-mL sample would evaporate. In reality, the air in the bottle was not dry, and this error was expected to be considerably smaller. For this reason, the liquid phase volume was assumed to be constant. The difference between the initial amount of the solute in the aqueous phase and that after the waiting time was assumed to be equivalent to that which had evaporated into the air enclosed by the bottle. It was found that the air/water equilibrium for phenol was established after approximately 2 h. Since phenol is the least volatile solute in this study, 2 h was adopted as the standard waiting time for all SEST experiments. The values of the HLC determined by SEST and their 90% confidence intervals are presented in Table 2. They were found to differ considerably from literature data (20% to 70%). These deviations can be attributed to the
trapping combined with solvent extraction (7)] makes PFI an attractive alternative to existing equilibrium techniques for the determination of HLC. On the basis of the results obtained, it can be concluded that, in the determination of the HLC of phenols, the PFI technique offers considerable advantages over existing equilibrium techniques in terms of precision, speed, labor intensity, and possibilities for automation. The PFI system is easily calibrated using at least two phenols of known HLC before applying it to the determination of HLC data for other phenols. It is expected that PFI can be used as a general technique for the determination of HLC data of other volatile and semivolatile solutes provided a suitable detector for the specific solute being investigated is available. Work in this direction is currently in progress.
Acknowledgments FIGURE 3. HLC (SEST) versus peak area (PFI) (b) and the corresponding best linear fit (s, eq 2). The error bars show the 90% confidence interval based on measurements in triplicate both by SEST and PFI (the experimental conditions are outlined in the text).
TABLE 3. HLC Values Determined by SEST (This Study) and Corresponding HLC Values Calculated by Equation 2a compound
HLC × 105 PFI (eq 2)
HLC × 105 SEST
3-methylphenol 2-methylphenol 2,4,6-trichlorophenol
2.63 ( 0.06 8.90 ( 0.06 17.28 ( 0.15
3.30 ( 0.53 8.18 ( 1.90 14.40 ( 3.99
a The 90% confidence intervals for the SEST and PFI results are based on measurements in triplicate.
vulnerability of the batch air stripping technique used (5, 6) to experimental error. PFI Experiments. The PFI results (Table 2) for phenol, 2,4-dimethylphenol, 2,4-dichlorophenol, 2-chlorophenol, and 2,3-dimethylphenol were found to be linearly correlated (eq 2) with the HLC data determined by SEST (Figure 3):
H ) 5.16C
(2)
where H is the HLC and C is the concentration of phenol detected in the acceptor stream. Equation 2 was used to calculate the HLC of 2,4,6trichlorophenol, 2-methylphenol, and 3-methylphenol utilizing the PFI data obtained (Table 3). The calculated values are in agreement with the SEST data, thus showing that PFI can be used successfully for the determination of HLC provided the corresponding linear relationship (eq 2) is known. The 90% confidence intervals for the PFI results (Table 2) are considerably smaller than those for the SEST results (Table 2), indicating that the PFI technique is much more precise. This fact together with the higher degree of automation of the PFI approach and its fast measurement time [12 min versus 2 h for SEST and 16 h for the method involving
The authors are grateful to the Australian Research Council for financial support and to La Trobe University for a scholarship for S.S.
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Received for review June 21, 2000. Revised manuscript received September 26, 2000. Accepted October 5, 2000. ES001406E
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