Anal. Chem. 1997, 69, 3272-3276
Conductometric Detection of Anions of Weak Acids in Chemically Suppressed Ion Chromatography Arety Caliamanis, Malcolm J. McCormick, and Peter D. Carpenter*
Department of Applied Chemistry, Royal Melbourne Institute of Technology, City Campus, GPO Box 2476V Melbourne, Victoria 3001, Australia
A commercial micromembrane suppressor, usually used to chemically suppress eluent conductance in ion chromatography, has been successfully used to effect ion replacement reactions in suppressed eluent streams. For 10 mM fluoride and 100 µM acetate, there were net decreases in conductance upon conversion of the acids to the sodium salts, showing that these concentrations were below their critical point concentrations (CPCs), defined here as the formal concentration above which a specified conjugate salt has a higher conductance than the same formal concentration of the weak acid. For 10 mM carbonate and 10 µM borate, there were net increases in conductance, showing that these concentrations were above their respective CPCs. The most effective of several ion replacement reagents investigated was EDTA at pH 11, which, for 5.0 mM borate, produced 250and 1400-fold increases in peak height and peak area, respectively, compared with a normal IC system and low background conductance. The applicability of the system used in this work to cation and anion replacement reactions in general is discussed. Anions are commonly determined by ion chromatography (IC) using conductance detection, often with chemical suppression, which converts the anions of interest to their conjugate acids. For strong acids this is an advantage, but weak acids are weakly ionized and can give lower detector response. Enhanced detection of ions is usually attempted via ion replacement or preconcentration techniques.1 Several novel approaches have been used previously in attempts to overcome the weak acid anion problem. Okada and Kuwamoto2 simply used nonsuppressed conditions, while Rocklin et al.3 and Tanaka and Fritz4 used ion exclusion chromatography rather than ion exchange chromatography. Tanaka and Fritz4 obtained a 10-fold increase in sensitivity for carbon dioxide and bicarbonate using two postcolumn ion exchange columns to convert the carbonic acid first to potassium bicarbonate and then to potassium hydroxide. There have been several approaches to ion replace-
ment as a means of improving detection of weak acids in chemically suppressed IC.5-10 One of the more successful approaches was that of Berglund and Dasgupta,8,9 who installed a purpose-built membrane converter after the detector to convert acids to salts, which were then detected with a second conductance detector. The sensitivity for strong acids decreased by a factor of ∼2, while for weak acids the signal was more than 1 order of magnitude greater. Subsequently, Berglund et al.10 reintroduced NaOH after the first detector as a means of converting the weak acids back to more-conducting salts. This novel approach provided microgram-per-liter level detection across the whole pKa range. There are two main points to consider when attempting to convert a weak acid to a conjugate salt in order to improve detection. First, the weak acid analyte is, by definition, weakly ionized11 and, therefore, weakly able to enter into ion exchange reactions upon which this process is dependent. Second, conductance is a bulk property dependent on both the anions and cations.12 Since the hydrogen ion has a much larger molar ionic conductance in aqueous systems than any other cation,12 converting a fully ionized acid (HX) to a salt (e.g., NaX) would always lower conductance. However, complete conversion of partially ionized acid to fully dissociated NaX could increase conductance. For example, a very strong acid such as hydrochloric acid could be converted to sodium chloride with a consequent decrease in sensitivity, while boric acid, which is a very weak acid (pKa ) 9.14), would be converted to sodium borate. This conversion will increase conductance (and hence detector response) only if the concentration of the analyte is above some “critical point” (Figure 1). This critical point concentration (CPC) will depend on the molar ionic conductance of hydrogen ions and whatever replacement ion is used, the degree of ionization of the weak acid, and the extent of conversion of the weak acid to the conjugate salt. The extent of conversion will be dependent upon the pH of the
* To whom correspondence should be addressed. E-mail: Carpenter@ RMIT.edu.au. (1) Weiss, J. Ion Chromatography, 2nd ed.; VCH: Weinheim, 1995; Chapters 3 and 8. (2) Okada, T.; Kuwamoto, T. Anal. Chem. 1985, 57, 829-833. (3) Rocklin, R. D.; Slingsby, R. W.; Pohl, C. A. J. Liq. Chromatogr. 1986, 9 (4), 757-775. (4) Tanaka, K.; Fritz, J. S. Anal. Chem. 1987, 59, 708-712.
(5) Tanaka, S.; Yasue, K.; Katsura, N.; Tanno, Y.; Hashimoto, Y. Bunseki Kagaku 1988, 37, 665-670. (6) El Khatib, E. A. Z. Pflanzenernaehr. Bodenkd. 1990, 153 (3), 201-205. (7) Okada, T.; Dasgupta, P. K. Anal. Chem. 1989, 61, 548-554. (8) Berglund, I.; Dasgupta, P. K. Anal. Chem. 1991, 63, 2175-2183. (9) Berglund, I.; Dasgupta, P. K. Anal. Chem. 1992, 64, 3007-3012. (10) Berglund, I.; Dasgupta, P. K.; Lopez, J. L.; Nara, O. Anal. Chem. 1993, 65, 1192-1198. (11) Atkins P. W. Physical Chemistry, 4th ed; Oxford University Press: Oxford, 1990; Chapter 9. (12) Atkins P. W. Physical Chemistry, 4th ed; Oxford University Press: Oxford, 1990; Chapter 25.
3272 Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
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Figure 1. Ideal variation in solution conductance versus formal concentration of a weak acid (HX, solid line) and a conjugate salt (NaX, dashed line). The point at which the two curves cross is the critical point concentration (defined in the text).
reagent used to effect the ion replacement and the pKa of the analyte acid. Higher reagent pH values should help ionize the weak acid and thus facilitate conversion to the salt. Complete conversion of the acid to the conjugate salt would be ideal. In this paper, we report preliminary experiments which investigated the feasibility of using a second commercially available micromembrane suppressor before the detector as an ion exchange reactor (IER) to convert the weak acids to conjugate salts (Figure 2). The first suppressor was used for chemical suppression of the eluent in the usual way. EXPERIMENTAL SECTION Apparatus. Ion chromatography was carried out using a Dionex 2010i ion chromatograph, a Dionex GP40 programmable gradient pump, a Dionex Ion Pac AG4A-SC guard column in series with a Dionex Ion Pac AS4A-SC 4 mm column, followed by either one or two Dionex anion micromembrane suppressors (AMMSII, P/N 043074) and then the Dionex conductance detector. A Shimadzu C-R3A computing integrator was used to acquire peak heights and peak areas. An eluent flow rate of 1.0 mL/min was used throughout. Samples were filtered and injected through 0.45 µm Nylon 66 membrane filters (Alltech Assoc. Aust. Pty. Ltd.) into a 50 µL injection loop. At least three replicate injections of each sample were made and the results averaged. Regenerant or IER reagent (see below) solutions were forced through the micromembrane suppressors under 5 psi pressure (1.2 mL/min). A Metrohm 650 pH meter (calibrated over a pH range of 4-7, using Activon capsule buffers), a Pt 100 temperature probe, and a combined glass/silver, silver chloride electrode were utilized to measure the pH of solutions. Reagents. Reagent grade water was obtained from a Milli-Q system (Millipore Corp.). All chemicals were BDH analytical reagent grade, unless otherwise specified, and all glassware was A grade. All standard solutions, eluents, and reagents were prepared in Milli-Q water, degassed under vacuum, and filtered through 0.45 µm membrane filters (Millipore cellulose acetate) prior to use. Standard solutions included sodium fluoride (as dried NaF, 50 mM), acetate (as crystalline CH3COONa, 10 mM), carbonic acid (as Na2CO3‚10H2O, 25 mM), and boric acid (as B(OH)3, 100 and 1000 mM). Eluent was sodium hydroxide (5 mM). Reagents included sulfuric acid (12.5 mM), sodium
Figure 2. Schematic diagram of the instrumental setup, showing a second micromembrane supressor being used as an ion exchange reactor. The fate of weak and strong acid anions as they pass through the system is also shown.
hydroxide (1-30 mM), and EDTA (as Na2(EDTA), 100 mM). Analyte standards were prepared in 5 mM NaOH to eliminate the water dip. RESULTS AND DISCUSSION Ion-Exchange Reactor (IER). The anion micromembrane suppressors used in this work are based on permeable cation exchange membranes which are used to effect exchange of eluent cations on one side of the membrane for hydrogen ions supplied from the other side.13 In the IER system used here (Figure 2), the suppressed eluent stream flows across one side of the membrane in the second suppressor (the IER), while on the other side is what we have termed an IER reagent, which supplied cations to effect the desired ion replacement. The effect of the IER on peak height and width (dispersion) was observed by using Milli-Q water as IER reagent. For fluoride (pKa ) 3.45) and acetate (pKa ) 4.75) solutions, there was a general decrease in peak heights and an increase in widths (of ∼18%). This degree of dispersion was considered acceptable for this work. There were also increases in peak area of ∼10% which were not initially expected, since the fluoride and acetate calibration graphs were reasonably linear (concentration range, 10-1000 mM, r2 > 98%). However, closer inspection of these graphs revealed slight negative curvature, which probably occurred because of the decreasing ionization of the weak acid with (13) Weiss, J. Ion Chromatography, 2nd ed.; VCH: Weinheim, 1995; Chapter 3.
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Table 1. Comparison of Average Peak Heights Obtained for Acetate and Fluoride Solutions Using the Normal System and Using the IER System with Milli-Q Water and 1 mM NaOH as IER Reagent IER system anion determined
normal IC system
Milli-Q water reagent
1 mM NaOH reagent
fluoride, 50 µM fluoride, 500 µM acetate, 50 µM acetate, 500 µM
74 200 704 000 235 000 1 465 000
73 800 642 000 199 000 1 271 000
17 200 181 400 46 500 920 300
increasing concentration.11 Hence, if a concentration is, e.g., halved by dispersion, then the resultant conductance will be slightly more than half of the initial conductance. As dispersion led to dilution of the analyte plugs, increases in area were observed. NaOH as IER Reagent. There are at least two factors that need to be considered when selecting the IER reagent. First, high pH should maximize ionization of the analyte acid and hence maximize ion exchange. Second, high concentrations of the replacement cation (in this work Na+) in the IER reagent should help maximize ion exchange, hence our initial choice of NaOH as IER reagent. Fluoride peak heights showed steady decreases amounting to 57%, 41%, and 20%, respectively, for 0.5, 1.0, and 2.0 mM fluoride solutions as the NaOH concentration was increased from 1 mM (pH 11) to 25 mM and then remained constant up to 30 mM (pH 12.5). Similar trends were observed for peak areas. These results show several things. First, the minimum NaOH reagent concentration needed to effect maximum conversion of up to 2.0 mM fluoride to NaF was 25 mM under the conditions used. Second, the decreases in heights (and areas) that were found as conversion from HF to NaF proceeded with increasing NaOH concentration clearly showed that 2.0 mM fluoride was below the CPC (Figure 1). Third, the significant reduction in the percentage peak height decrease (from 57% to 20%) as fluoride concentration increased is confirmation that the CPC was being approached as fluoride concentration increased. A 10 mM fluoride solution showed a 13% decrease in height over the same hydroxide concentration range and hence was also below the CPC. These percentage decreases are relative to the heights obtained with 1 mM NaOH. The peak heights obtained for all NaOH concentrations used were less than the results obtained using the normal system or with Milli-Q water as the IER reagent. For example, there was a ∼77% decrease in peak height for 50 µM fluoride when Milli-Q water was replaced with 1 mM NaOH as the IER reagent (Table 1). Results similar to the fluoride results were obtained for acetate solutions between 10 and 100 µM14 and are not given here. The results showed that 100 µM acetate was below the CPC. At low anion concentrations, the extent of HX ionization is large, and hence so is the contribution of hydrated protons to the measured conductance. Conversion from HX to NaX therefore causes a large drop in conductance. At higher anion concentrations, the acid is less ionized, and so conversion of HX to NaX becomes beneficial. No further investigation into the use of NaOH as the IER reagent was undertaken because, as the concentration of NaOH (14) Caliamanis A. B. App.Sc. (Honours) Thesis, RMIT, Melbourne, 1995.
3274 Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
Figure 3. Relationship between the average peak height and the pH of the IER reagent (10 mM EDTA) for (A) [, 50, 9, 100, 2, 500, and ×, 1000 µM acetate solutions and (B) 10 mM carbonate solution.
was increased from 1 mM (pH 11) to 30 mM (pH 12.5), the background conductance increased exponentially from ∼5 to 65 µS, suggesting that hydroxide was leaking across the membrane into the eluent stream, despite the negative charge on the cation exchange membrane. Background conductances in the range 5-25 µS were common in our laboratory, and higher values were considered undesirable. EDTA as IER Reagent. Due to the background problems encountered using NaOH, it was decided to test the effectiveness of EDTA (ethylenediaminetetraacetic acid), which has a large complex anion that should not cross the membrane as easily as hydroxide, as an IER reagent. A 10 mM disodium EDTA solution had a pH of ∼5. As the pH was increased to 11 (by the addition of NaOH), the background conductance increased from ∼2 to 7 µS. Over pH 11, the background conductance increased dramatically, so solutions of pH above 11 were not used. An additional advantage to using Na2(EDTA) was that the concentration of Na+ ions was not totally dependent upon the amount of NaOH added. For example, at pH 11, the NaOH and EDTA IER reagents had 1 and 42 mM of Na+ ions present, respectively. Hence, for a given pH, higher concentrations of Na+ ions would be available to help force the ion exchange reaction. Peak heights and areas of acetate solutions (10-1000 µM) were measured with EDTA as IER reagent at pH 5, 7, 9, and 11. The results shown in Figure 3a are typical. Peak heights were lower at each pH compared with those from the normal system by at least 47%, more than could be explained by the increased dispersion, suggesting that these acetate solutions were all below the CPC. Peak heights for each concentration decreased as the EDTA pH increased from 5 to 7. The decrease varied from ∼10% at 1000 µM to ∼35% at 10 µM. These results are consistent with there being increased conversion of acetic acid to sodium acetate at the higher pH, again provided the solutions were below the CPC. However, the small increases in peak heights from pH 9
Table 2. Conductivity detector Responses as a Function of Borate Concentration for the Normal System and for the IER System Using a Range of IER Reagents (A) Detector Response in Integrator Height and Area Units (×104) IER system normal system [borate] 10 µM 25 µM 50 µM 75 µM 100 µM 250 µM 500 µM 750 µM 1000 µM 1.0 mM 2.5 mM 5.0 mM 7.5 mM 10 mM 25 mM 50 mM 75 mM 100 mM
height
0.14 1.20 4.19 8.21 13.0 31.7 40.8 47.9 44.1
NaOH, 1.0 mM
area
height
area
2.35 9.14 29.7 59.1 98.6 278 405 550 470
5.98 5.80 7.79 13.0 14.6 38.2 65.2 88.2 110 109 183 266 318 350 434 465 544 607
117 110 173 243 298 758 1 290 1 750 2 200 2 350 4 150 7 160 9 970 12 100 24 800 39 800 50 600 57 600
EDTA, pH 5 height
1.02 8.42 18.9 26.2 31.7 50.0 66.6 89.0 96.6
EDTA, pH 7
EDTA, pH 9
EDTA, pH 11
area
height
area
height
area
height
area
5.95 90.2 293 521 749 2070 4030 5720 7420
2.51 7.65 13.4 17.2 18.3 44.3 70.9 94.0 115 104 190 275 324 367 446 479 530 609
51.7 241 283 354 377 926 1 480 1 990 2 470 2 390 4 520 7 620 10 300 13 200 25 200 40 500 51 700 59 100
2.89 6.67 11.8 15.2 17.3 44.9 85.0 101 118 108 200 285 336 377 454 481 526 604
56.4 154 262 317 348 926 1 760 2 140 2 470 2 360 4 600 7 740 10 600 13 200 25 900 40 600 51 700 58 400
2.75 7.17 13.0 16.3 17.6 50.7 85.4 115 142 125 222 315 375 409 783 511 584 647
27.7 198 313 318 328 944 1 630 2 150 2 710 2 890 5 280 8 820 12 000 14 600 27 600 43 300 54 600 62 100
(B) Detector Response Expressed as Ratios of Height and Area Obtained in the IER System to Those Obtained with the Normal System IER system NaOH, 1.0 mM [borate] 1.0 mM 2.5 mM 5.0 mM 7.5 mM 10 mM 25 mM 50 mM 75 mM 100 mM
EDTA, pH 5
EDTA pH 7
EDTA, pH 9
EDTA, pH 11
height
area
height
area
height
area
height
area
height
area
765 153 63.5 38.7 26.9 13.7 11.4 11.4 13.8
1000 454 241 169 123 89.2 98.3 92.1 123
7.16 7.04 4.50 3.19 2.44 1.57 1.63 1.86 2.19
2.53 9.87 9.86 8.82 7.60 7.44 9.95 10.4 15.8
728 159 65.7 39.4 28.2 14.1 11.8 11.1 13.8
1017 495 256 174 134 90.7 99.9 94.1 126
758 167 68.1 41.0 29.0 14.3 11.8 11.0 13.7
1008 504 261 180 134 93.1 100 94.1 124
877 186 75.4 45.7 31.4 24.7 12.5 12.2 14.7
1231 578 297 202 148 99.2 107 99.3 132
to 11 (Figure 3a) were consistently seen across the acetate concentration range and seem anomalous. In contrast to acetate above, and fluoride with NaOH as IER reagent, the peak height of 10 mM sodium carbonate (pKa1 ) 6.37, pKa2 ) 10.32) increased steadily over the EDTA pH range 5-11, resulting in a net 52% increase in height at pH 11, compared with the normal system (Figure 3b). Peak height at pH 5 was 20% lower than that with the normal system. These results show several things. First, the decrease in peak height at pH 5 was of the magnitude expected from increased dispersion with the IER system. Second, the increases in heights (and areas) that were found with increasing pH clearly showed that 10 mM carbonate was above the CPC. Third, conversion of carbonic acid to a sodium salt was clearly incomplete at low pH and might not have been complete even at pH 11. Solutions of higher pH were not used because of the unacceptable increase in background conductance. A much weaker acid, boric acid (pKa ) 9.14), for which the conversion to its sodium salt would be expected to have an even greater effect on sensitivity, was then tested with the normal system and with each of the IER reagents. Borate solutions in the range 1.0 (detection limit)-100 mM were used initially. The results (Table 2) show several things. First, spectacular increases were obtained in height (and area), relative to those with the normal system, using each of the IER reagents with even
the lowest borate concentration. This clearly showed that 1.0 mM borate was above the CPC. Second, typical increases in height (area), relative to those with the normal system, decreased from ∼760- to ∼40-fold (from ∼1000- to ∼200-fold) as the concentration of boric acid increased from 1.0 to between 10 and 25 mM, and then increased and leveled off at ∼10-fold (∼100-fold) up to 100 mM of borate (Table 2B). However, graphs of peak height or area versus concentration showed significant curvature at lower concentrations and a marked decrease in curvature above ∼10 mM (Figure 4a). Even EDTA at pH ∼5, which had linear correlation coefficients of 92% for height and 99% for area, still displayed this behavior. These deviations from linearity can be attributed to a combination of factors: (i) As the pH of EDTA reagent was increased, the peaks obtained for the higher concentration borate solutions became broader and flatter. Above 75 mM, the borate peaks split, indicating column overloading, affecting peak heights more than areas. (ii) The capacity of the IER reagent was exceeded above ∼10 mM borate. Capacity is a function of reagent concentration and also of residence time in the IER. For the 10 mM EDTA solution at pH 11, [Na+] was 42 mM, and there was a maximum of 60 mM protonatable sites (not all of which would be protonated under the conditions used). Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
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and improved linearity of the peak height or area versus concentration graphs (r2 ) 91-99%), although some negative curvature was still evident. The detection limit decreased from 1.0 mM to 10 µM borate, indicating that even 10 µM was above the CPC for borate.
Figure 4. Relationship between the average peak height and (A) the pH of the IER reagent (10 mM EDTA) for [, 100 µM, 9, 500 µM, 2, 1000 µM, ×, 10 mM, , 50 mM, and b, 100 mM borate solutions * and (B) borate concentration for IER reagent (10 mM EDTA) at [, pH 5, 9, 7, 2, 9, and ×, 11.
(iii) At the higher borate concentrations (above ∼10 mM), the boric acid concentration was high enough to suppress ionization sufficiently for ion replacement to be incomplete. Whatever the cause, the IER reagents used were not effective above ∼10 mM borate (Figure 4a). Third, the most effective IER reagent tested was EDTA at pH 11, although the improvement from pH 7 and 9 to 11 was marginal (Figure 4b). For a 5.0 mM borate solution, peak height and area increased by 250- and 1400fold, respectively, when using EDTA at pH 11, compared with those with the normal system, and the detection limit dropped from 1 mM to 10 µM. However, EDTA at pH ∼5 was significantly poorer as IER reagent than were EDTA solutions at higher pH, although the peak height trends were similar, with percentage increases in heights decreasing from 600% to 60% for the concentration range 1.0-10 mM and then increased to 140% at 100 mM borate (Figure 4b). Borate solutions in the range 10-1000 µM were then tested with each of the IER reagents (Table 2B). No peaks were detected with EDTA at pH ∼5 as IER reagent. The results obtained for the other IER reagents showed improved Gaussian peak shape
3276 Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
CONCLUSIONS The IER system of analysis developed proved to be very beneficial for boric acid, a very weak acid, which showed a dramatic increase in sensitivity. In contrast, carbonic acid only showed a moderate (52%) increase in height, while hydrofluoric and acetic acids showed substantial decreases (of approximately 5-fold). The CPCs of these acids clearly depend on their pKa. There would be considerable advantage in knowing or being able to predict the CPC of particular weak acid/conjugate salt pairs because the effect of ion replacement can then be predicted for a given analysis. This is presently under investigation. In addition, the relative importance of pH and replacement ion concentration in effecting ion replacements involving weak acids is not yet known. Current work is focused on this and optimizing IER reagent, related conditions (such as concentration and flow rate), and replacement ion. For example, potassium, with a higher molar ionic conductance than sodium,11 would further improve sensitivity. The fact that ion replacement can be readily achieved using commercial equipment means that the determination of trace amounts of weak acids such as boric acid can be enhanced without the need to build special apparatus. It is also conceivable that further ion replacement reactions could be effected by additional membrane suppressors placed in series to achieve even greater sensitivity increases, as has been done using ion exchange columns.4 A particularly interesting application of the approach we have demonstrated would be the determination of borate in a complex matrix such as seawater, because the IER system would greatly enhance the sensitivity of borate but at the same time decrease that of chloride. ACKNOWLEDGMENT We thank Andrew Hind for technical assistance performed as part of an undergraduate project in 1992. A.C. would like to acknowledge financial support given by an RMIT Postgraduate Research Scholarship. Received for review December 31, 1996. Accepted May 22, 1997. AC961307C X
Abstract published in Advance ACS Abstracts, July, 1; 1997.
