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Anal. Chem. 1984, 56,2360-2366
(47) Jamleson, J. C.; Ashton, F. E.; Frleson, A. D.; Chou, B. Can. J . Biochem. 1972, 5 0 , 871. (48) Adams, J. 6.; Wacher. A. Ciin. Chim. Acta 1968, 2 1 , 155. (49) Winzler, R. J. Methods Biochem. Anal. 1955, 2 , 279. (50) Gendler, S. J.; Dermer, A. 6.; Silverman, L. M.; Tokes, 2 . A. Cancer Res. 1982, 42, 4567-4573. (51) Rashld, S.A.; O’Quigley, J.; Axon, A. T. I?.;Cooper, E. H. Br. J . Cancer 1982, 45, 390-394. (52) Abramson, F. P. Ciin. Pharmacol. Ther. 1962, 32, 652-658. (53) Jackson, P. R.; Tucker, G. T.; Woods, H. F. Clin. Pharmacol. Ther. 1982, 3 2 , 295-302. (54) Martinez-Vea, A.; Gatell, J. M.; Segura, F.; Helman, C.; Elena, M.; Ballesta, A. M.; Mundo, M. R. Cancer (Amsterdam) 1982, 5 0 , 1783-1788. (55) Halsall, H. B.; Kirley, T. L. Arch. Biochem. Biophys. 1982, 216, 392-399. (56) Klrley, T. L.; Sprague, E. D.; Halsall, H. B. Biophys. Chem. 1982, 15, 209-216. (57) Wallace, J. A.; Halsall, H. 6. Biophys. J . 1963, 41, 406a. (58) Thaw, P. A.; Albutt, E. C. Ann. Ciln. Blochem. 1980, 17, 140-143. (59) Shlbata, K.; Ilubo, H.; Ishlbashl, H.; Tsuda, K. Blochlm. Biophys. Acta 1977, 495, 37-45. (60) Wlnzler, R. J.; Devor, A. W.; Mehl, J. W.; Smyth, I.M. J . Ciin. Invest. 1948, 2 7 , 609-616. (61) Johnson, A. M.; Schmld, K.; Alper, G. A. J . Ciin. Invest. 1969, 48, 2293-2299. (62) Staprans, 1.; Anderson, G. D.; Lurz, F. W.; Felts, J. M. Biochim. Biophys. Acta 1980, 617, 514-523. (63) Wang, H. P.; Chu, C. R. T. Clin. Chem. (Winston-Salem, N.C.) 1979, 2 5 , 546-549. (64) Hagenaars, A. M.; Kupers, A. J.; Nagel, J. I n “Immunoenzymatlc Assay Techniques”; Malvan, D., Ed.; Nijhoff: Netherlands, 1980; pp 16-44. (65) Ford, D. J.; Radin, R.; Pesce, A. J. Immunochemistry 1978, 15, 237-243. (66) Avrameas, S. Immunochemistry 1969, 6 , 43-52. (67) O’Kennedy, R. I f . J. Med. Sci. 1981, 92-96. (68) Motzok, I.; Branion, H. D. Biochem. J . 1959, 72, 177.
(69) Hamaguchi, Y.; Kato, K.; Ishlkawa, E.; Kobayashi, K.; Katunuma, N. FfBS Left. 1976. 6 9 . 11-14. (70) Kato, K.; HamaguchL’Y.; Okawa, S.;Ishlkawa, E.; Kobayashi, K.; Katunuma, N. J . Biochem. (Tokyo) 1977, 8 2 , 261-266. (71) Can, K.; Nlall, H. D.; Tregear, G. W. J. Lab. Clln. Med. 1967, 70, 820-830. (72) Bolton, A. E.; Hunter, W. M. Biochim. Biophys. Acta 1973, 329, 318-330. (73) Catt, K.; Tregear, G. W. Science (Washington, D . C . ) 1967, 158, 1570-1572. (74) Belanger, L.; Sylvester, C.; Dufour, D. Ciin. Chem. (Winston-Salem, N.C.) 1973, 48, 15-18. (75) Engvall, E.; Jonsson, K.; Perlmann, P. Biochim. Biophys. Acta 1971, 251, 427-434. (76) Pesce, A. J.; Ford, D. J.; Galzutls, M.; Pollak, V. E. Biochim. Biophys. Acta 1977. 492, 399-407. (77) Hedln, A.; Hammarstrom, S. Protides Bioi. Flu& 1981, 2 9 , 907-910. (78) Ochi, Y.; Fujlyama, Y.; Hosoda, S.;Mlyazakl, T.; Yoshimura, M.; Hachlya, T.; Kajlta, Y. Ciin. Chim. Acta 1982, 122, 145-160. (79) Mlyazakl, T.; Hachlya, T.; Kajlta, Y.; Ochi, Y.; Fujiyama, Y.; Hosoda. S. Ciin. Chim. Acta 1982, 122, 161-168. (80) Rudman, D.; Treadwell, P. E.; Vogler, W. R.; Howard, C. H.; Holllns, 6. Cancer Res. 1972, 32, 1951-1959. (81) Krotoskl, W. A.; Weimer, H. E. Int. J. Pept. Protein Res. 1963, 2 1 , 155-1 62.
RECEIVED for review January 23,1984. Resubmitted June 13, 1984. Accepted June 13,1984. This work was supported by NIH Grants A116753 and HD13207, NSF Grant CHE8217045, and a summer ACS Analytical Division Research Fellowship sponsored by the ACS Division of Analytical Chemists (M.J.D.). M.J.D. also acknowledges support as a Twitchell-Schubert-LowensteinFellow sponsored by the University of Cincinnati.
Selectivity of the Potentiometric Carbon Dioxide Gas-Sensing Electrode M. E. Lopez’ Department of Chemistry, University of Delaware, Newark, Delaware 19716
Experimental and theoretical Investigations of the potentlometric pCOp electrode have been employed to establish a steady-state model for both organic and Inorganic Interferences at this electrode. It Is shown that electrode response Is governed prlmarliy by the acldlty rather than the volatlilty of the interferents. With the proposed model, quantitative selectivity coefficient values could be calculated In good agreement with experimentally determined values.
The purpose of the present study was to evaluate the usefulness of the response model in predicting the degree of interference observed with the carbon dioxide electrode. The effect of membrane characteristics was also considered. Good quantitative agreement was shown between the theoretically calculated selectivity coefficients and experimentally determined values. Thus, the selectivity of the carbon dioxide electrode was found to depend on the acidity rather than the volatility of the compound. The anomalous response to relatively nonvolatile compounds was interpreted as nonequilibrium behavior.
The selectivity of the carbon dioxide electrode is of particular interest due its widespread use in clinical and industrial analyses. Interference from volatile inorganic and organic acids as well as from nonvolatile compounds has been reported (1-4). The nature of the interference has recently been shown to depend on the characteristics of the gas-permeable membrane (5). The first quantitative evaluation of the potential response of a gas-sensing electrode to interfering compounds was a recent study of volatile amine interference with the ammonia electrode (6). The proposed steady-state response model was based solely on the chemical equilibria in the electrolyte film.
EXPERIMENTAL SECTION Apparatus and Materials. The Orion Model 95-02 carbon dioxide electrodeis the potentiometricgas-sensing electrodewhich was studied. The electrode was assembled with either the Orion carbon dioxide membrane (No. 95-02-04) or the Orion ammonia membrane (No. 95-10-04)with its spacer assembly. The Orion carbon dioxide membrane is polyester mesh supported silicone rubber. The thickness of the silicone rubber (poly(methylvinylsi1oxane))is 0.009 cm and the overall thickness is 0.018 cm. The Orion ammonia membrane is microporous Teflon on a poly(ethy1ene) support. The thickness of the 0.2 wm pore size Teflon is 0.004 cm and the overall thickness is 0.018 cm. Potentiometric and pH measurements were made with a Corning Model 12 research meter and were recorded with a Heath/Zenith Model SR-204 strip-chart recorder. The glass-
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
jacketed cell was maintained at 25 "C by means of a Haake Model FS constant temperature circulator. A 0.5 cm thick circular Teflon spacer with an outer O-ring provided a seal between the outer body of the electrode and the inner wall of the sample cell. Sample solutions were stirred by means of a Teflon magnetic stir bar and a Fisher Flexa-Mix stirrer. A Corning semimicro combination pH electrode (No. 476050) was used to monitor the sample pH in constructing reverse acetic acid calibration curves. It was also used to check the linearity of the pH response of the Orion inner pH sensing element. A Beckman sodium glass electrode (No. 39278) and an Orion double-junction reference electrode (No. 90-02-00)with 10% potassium nitrate in the outer chamber were employed for Gran's plot titration of the sodium salts of carbonic acid, formic acid, acetic acid, and propionic acid. Reagents. Standard 0.1 M aqueoussolutionsof the compounds listed below were prepared with reagent grade chemicals and deionized water. Sodium salts of formic acid, propionic acid, salicylicacid, pyruvic acid, L-ascorbic acid, oxalacetic acid, L-malic acid, succinic acid, and lithium salts of DL-laCtiC acid and acetoacetic acid were obtained from the Sigma Chemical Co., St. Louis, MO. Sodium salts of oxalic acid, acetic acid, benzoic acid and sodium nitrite were obtained from Aldrich Chemical Co., Inc., Milwaukee, WI. Sodium bicarbonate and sodium sulfide were obtained from Fisher Scientific Co., Fair Lawn, NJ. Sodium sulfite was obtained from J. T. Baker Chemical Co., Phillipsburg, NJ. Ascarite, a carbon dioxide absorber, was purchased from Fisher Scientific Co., Fair Lawn, NJ. The carbonate content of the Orion carbon dioxide filling solution (No. 95-02-02)was determined to be 1.2 x M by standard addition using the Orion carbon dioxide electrode and a standard 0.10 M sodium bicarbonate solution. This filling solution was replaced by several prepared aqueous solutions of sodium bicarbonate and sodium chloride. A pH 1.9 bisulfate buffer, having an osmolarity of 0.2 M, w a prepared with sodium sulfate and standard hydrochloric acid. The filling solutions and buffers were prepared with freshly boiled deionized water which was cooled under an Ascarite drying tube or argon. Procedures Used To Obtain pH Response Characteristics of the Orion Inner pH-Sensing Element. The Corning Model 12 research pH meter was calibrated using the Corning semimicro combination pH electrode in pH 4 and pH 7 standard buffers. The pH of seven buffer solutions (I = 0.05) of pH 3 to pH 8 was alternately measured with the Corning pH electrodeand the Orion inner pH-sensing element. The Ag/AgCl reference element of the Corning pH electrode was employed as the reference electrode for both pH readings. Procedures Used i n Selectivity Studies. All potential measurements were made with the Orion Model 95-02carbon dioxide electrode immersed in a stirred sample solution which was maintained at 25.0 "C. The standard solution of the compound to be studied was progressively added with a Finnpipette to 10 mL of pH 1.9 bisulfate buffer. These additions were made through a small hole in the Teflon spacer which was flushed with the sample solution. Calibration curves were constructed by plotting the potential reading vs. the negative logarithm of the compound concentration. Each data point corresponded to a M. All 2-fold increase in concentration, from 5 X lo4 to 8 X potentials were recorded at steady state, as indicated by a plateau in the potential vs. time trace of the strip-chart recorder. Calibration curves for each compound were constructed with the silicone rubber and microporousTeflon membranes and a 0.01 M sodium bicarbonate in 0.09 M sodium chloride electrolyte solution. Calibration curves for the aliphatic carboxylic acids were then constructed with the microporous Teflon membrane and a 0.001 M sodium bicarbonate in 0.099 M sodium chloride electrolyte solution. Procedures Used To Construct Forward and Reverse Calibration Curves for Acetic Acid. A forward calibration curve for acetic acid was constructed, as above, using the microporous Teflon membrane and a 0.01 M sodium bicarbonate in 0.09 M sodium chloride electrolyte solution. A Corning semimicro combinationpH electrodewas then immersed in the sample solution. A reverse calibration curve was constructed by progressive additions of 1 M sodium hydroxide, monitored by the pH electrode. Each increase in sample pH corresponded to a
2381
Table I. Clinically Important Organic Acids Studied compd no.
a
compound
pK,
mg % blood"
1 2 3
pyruvic DL-lactic oxalic
2.49 3.86 1.25 (pKJ
0.35-0.92 8-17 0.2
5
L-malic
0.24-0.75
6 7
acetoacetic oxalacetic
8
L-ascorbic
3.40 (PK;) 5.20 (pKz) 3.58 2.15 (pK1) 4.06 (PKz) 4.10
0.31 0.38 1.1-1.8
Values found in ref 7. 50
0
E,mV
-50
-100
-150
I
3.30
I
I
I
I
3.00 2.70 2.40 2.10 - l o g [COMPOUND] , m o l / L
Flgure 1. Response of C02 sensor to C02 and clinically important organic acids using 0.01 M NaHCO, in 0.09 M NaCl filling solution. Numbers refer to compounds listed in Table I.
2-fold decrease in the effective concentration of un-ionized acetic acid. The electrode was then assembled with a microporous Teflon membrane and a 0.001 M sodium bicarbonate in 0.099M sodium chloride electrolyte solution. Forward and reverse acetic acid calibration curves were constructed following the above procedure. The carbon dioxide electrode was reassembled with a microporous Teflon membrane and the 0.01 M NaHC03in 0.09 M NaCl electrolyte solution. Argon, presaturated with water vapor, was bubbled into 10 mL of the bisulfate buffer with a syringe. The carbon dioxide electrode was then immersed in the buffer without the use of the Teflon spacer. Forward and reverse acetic acid calibration curves were constructed as above, maintaining a constant bubbling rate of argon in the sample solution. RESULTS AND DISCUSSION Selectivity to Clinically Important Acids. Table I lists the clinically important organic acids found in human blood (7). The selectivities of the Orion Model 95-02carbon dioxide electrode with the silicone rubber membrane and microporous Teflon membrane for these organic acids were found to be identical (Figure 1). There was no response (base line potential) to most of these acids. The small rapid response to acetoacetic acid (no. 6) and oxalacetic acid (no. 7) may be a response to carbon dioxide produced by decomposition of these acids in acidic media. The vapor pressure of pyruvic acid is 3.2 torr at 25 "C. No vapor pressure data for the remaining compounds could be found. Selectivity to Inorganic Acids. Figures 2 and 3 show the potential response of the Orion Model 95-02carbon dioxide electrode with the silicone rubber membrane and microporous
2362
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 250
15C 200 1 oc 150
50
E,mV
.//
100
E,mV
/
0
/
50
/ I
-50
/
0
.4
-100
-60
1 L. _ - _ I
.30
I
I
3.00 2.70 2.40 2.10 - l o g [ C O M P O U N D ] , rnol/L
Flgure 2. Response of CO, sensor with sillcone rubber membrane to (1) SO,, (2) NO,, (3) COP, and (4) H,S using 0.01 M NaHCO, in 0.09 M NaCl filling solution.
I
.30
1
-
3.00
1 -
2.70
I
I
2.40
2.10
-log [COMPOUND] , moi/L Flgure 4. Response of CO, sensor with silicone rubber membrane to (1) benzoic acid, (2) propionic acid, (3) carbonic acid (CO,), (4) formic add, and (5) acetic acid using 0.01 M NaHCO, in 0.09 M NaCi filling solution.
25C
200
i5 0
E,mV 100
50
0
-50 - 1 -
.30
I
I
I
3.00 2.70 2.40 2.10 - l o g [ COMPOUND], m o l / L
Flgure 3. Response of BO, sensw with microporous Teflon membrane to (1) SO,, (2) NO,, (3) CO,, and (4) H,S using 0.01 M NaHCO, in 0.69 M NaCi filling solution.
Teflon membrane, respectively, to carbon dioxide and other inorganic acids. The selectivities of the two C 0 2 sensors for these compounds at high concentrations are identical. The
response times of the carbon dioxide electrode with the microporous Teflon membrane were shorter, although of the same order of magnitude, as those of the carbon dioxide electrode with the silicone rubber membrane. All of the inorganic acids are gases at 26 "C and atmospheric pressure. The potential response to sulfur dioxide was the largest in magnitude; approximately 200 mV more positive than that to carbon dioxide at 8 X M concentrations. The response times of the two COz sensors for a 2-fold concentration increase of SO2 and C 0 2 a t 4 X lo-$ M were 1.5 min and 2 min, respectively. The potential response to hydrogen sulfide at high concentrations was smaller in magnitude than that to carbon dioxide. However, at low concentrations of the two compounds, the reverse was observed. The respective calibration curves intersect at 2 X lo9 M concentrations. The response times of the two C 0 2 sensors for the above concentration increase of HzS at 4 X M were both close to 30 s. The faster response to H2S and SO2 relative to C 0 2 correlates with their greater permeabilities in silicone rubber (8). In the case of the microporous Teflon membrane, there is no correlation of response times with vapor pressures. The gaseous anhydrides (NO, NOz,N20,and N204) above an acidified aqueous sodium nitrite solution are collectively referred to as NO, since their relative equilibrium distribution is uncertain. The potential response of the Orion Model 95-02 carbon dioxide electrode with the silicone rubber membrane to low concentrations of NO, was significantly reduced. In addition, the response time of this COz sensor to a 2-fold concentration increase of NO, at 4 X M was 7 times longer than that of the Orion Model 95-02 carbon dioxide electrode with the microporous Teflon membrane: 15 min vs. 2 min. ,The Orion Model 95-46 nitrogen oxide electrode is equipped with the microporous Teflon membrane (9). Selectivity to Organic Acids. Figure 4 shows the potential response of the Orion Model 95-02 carbon dioxide
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
2363
h b C
d
I
I
Figure 6. Schematic of CO, sensor: (a)bulk electrolyte, (b) pH glass tlp, (c)electrolyte film, (d) gas-permeable membrane, (e) sample so-
lution, (f) magnetic Teflon stir bar.
3.30
I
I
I
I
3.00
2.70
2.40
2.10
-log [COMPOUND], mol/L Flgm 5. Response of CO, sensor wlth microporous Teflon membrane to (1)formic acid, (2) acetic acid, (3)propionic acid, (4) benzolc acid, and (5) carbonlc acid (CO,) using 0.01 M NaHC03 In 0.09 M NaCl fllllng
solution. electrode with the silicone rubber membrane to the more volatile organic acids which were reported as interferents of the COz sensor. The potential response to benzoic acid (no. 1)ranged from 40 mV to 135 mV more positive than that to COz (no. 3). The response times for 2-fold concentration M and 4 X M were 30 min and 12 increases at 5 X min, respectively. There was no response to formic acid (no. 4) and acetic acid (no. 5). The potential response to propionic acid (no. 2) was observed only at concentrations above 2 X 10-3 M. As seen in Figure 5, the potential response of the Orion Model 95-02 carbon dioxide electrode with the microporous Teflon membrane to benzoic acid was observed only at the M). The potential highest concentration studied (8 X response was significantly reduced as compared to that observed with the silicone rubber membrane. In contrast, a very large potential response to high concentrationsof the aliphatic carboxylic acids was observed using the microporous Teflon membrane. The potential response to low concentrations of these acids was very small and the response times were excessively long, averaging 1h for a 2-fold concentration increase a t 5 X 10" M concentrations. Mechanistic Implications. Interference with the Orion Model 95-02 carbon dioxide electrode is defied as a potential response which results upon passage of a neutral species other than carbon dioxide across the gas-permeable membrane (Figure 6). The corresponding pH change in the aqueous sodium bicarbonate film is sensed by an inner pH glass electrode. The magnitude of the interference by a compound, HI, expressed as a selectivity coefficient,kij'&HI, is defined by the following equation (10):
= (H+)z/(H+)i (1) Subscripts 1and 2 refer to the hydrogen ion activities in the film upon electrode equilibration with the same nominal
concentrations of COzand HI in separate sample solutions. The potential response of the COz sensor to an acidified aqueous solution of carbon dioxide reflects the following hydrolysis reaction in the film:
C02 + H20
HC03- + H+ Interference by a compound, HI, initially reflects the titration of bicarbonate ion in the film.
HI + HCO3-
I- + COZ + HzO
Upon neutralization, the following hydrolysis predominates at higher sample concentrations of H I
HI
+ H20 + I- + H30+
An expression for the hydrogen ion activity in a sodium bicarbonate film containing a nominal activity of an interfering compound (HI) can be obtained by substitution of the following mass action expressions
(3) (4)
K, = (OH-)(H+)
(5)
into the charge balance equation for the film [Na+]
+ [H+] = [OH-] + [HC03-] + 2[C032-] + [I-]
(6) The resulting equation is a polynomial in hydrogen ion activity, (H+)2
&&,HI
K,, is a composite acidity constant for the hydration of C02 and the dissociation of carbonic acid. K,' is the acid disso-
2364
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
ciation constant for the interfering compound, HI, and I-, is its conjugate base. y is the activity coefficient. A similar expression can be derived for the hydrogen ion activity, (H+)l, in a sodium bicarbonate film containing a nominal activity of carbon dioxide by omitting the term for the dissociated form of the interferent, HI, in eq 6.
Table 11. Selectivity of Carbon Dioxide Electrodeo/Silicone Rubber Membrane
compound H2S
15260
COZ
54100
benzoic acid
NO, The activity of COZ, which is produced in the titration reaction of bicarbonate ion and HI in the film, can be evaluated by the following material balance equation:
The equilibrium distribution of the total moles of carbonate species, n,in the sample solution, the membrane, and the film is given by the three terms, respectively, on the right-hand side of eq 9. Although various carbonate species (COz,HCO,, C032-) exist in the film, only COz passes through the membrane into the sample solution until equal activities of C02 exist in the two solution phases. Subscripts s and f refer to the sample solution and film, respectively. The total moles of carbonate species, n, can be calculated from the analytical concentration of the sodium bicarbonate filling solution and the film volume, V, Vf is approximately 1X L as determined from the measured physical dimensions of the electrode. The moles of carbonate species in the sample solution and film are expressed in terms of the activity of COz and the reciprocal fractions of COz, Q,, and Qf, respectively. The moles of gaseous C02 within the membrane are also expressed in terms of the activity of C02 in solution by use of the ideal gas law and Henry's law. KH is the Henry's law constant for COz. The gas volume in the membrane, V,, is estimated to be 1 X 10" L in the microporous Teflon membrane. This was calculated from the percent porosity and total volume. The possible error in assuming the same value for Vf in the silicone rubber membrane is negligible since the total number of moles in the membrane, as given by the second term, is orders of magnitude smaller than that in either solution phase. The equilibrium distribution of the total moles of interferent, HI, is similarly given by eq 10
The total moles of the various species of the interferent (HI, I-), given by n', can be calculated provided that the analytical sample concentration and sample volume, V,, are known. Of the various species in solution, only HI equilibrates across the membrane. Q' is the reciprocal fraction which exists as HI in solution. Q and Q' are functions of the solution pH. Since the sample solutions in this study are buffered at pH 1.9, Q, and &,'are constants. Qf and Qf' are functions of the film pH which changes with the concentration of HI in the sample solution. Thus, eq 9 and 10 cannot be solved analytically.
&) -I
&f'=
(1
+
Simultaneous solution of eq 7,9, and 10 gives the hydrogen
P,"torr
SO2
5.1 890 3000
PK,
selectivity coeKbkf%2,HI found calcd
7.00 (PKJ 0.6 12.92 (pK,) 6.35 (pK,) 1 10.33 (pK,) 4.20 130 3.29 1900 1.76 (pK1) 6700 7.21 ( P K ~
0.7 1
135 1050 7530
M '0.01 M NaHC03in 0.09 M NaCl filling solution. b 8 X concentration of respective compound. Vapor pressure of pure compounds at 25 "C.
ion activity, (H+),, as well as the activities of HI and COzwhich exist in the sodium bicarbonate film upon electrode equilibration with a sample containing only the interferent. The pH response of the electrode to a sample containing only carbon dioxide, given by (H+)l,can likewise be obtained by simultaneous solution of eq 8 and 9. n,in this case, is the number of moles of carbonate species which are initially contained in the sample solution as well as the film. The selectivity coefficient, lzgZaI can be predicted and compared with experimentally determined values. The experimental selectivity coefficient is normally determined from the Eisenman-Nicolsky equation. log kE&,HI =
(E,- E , ) / S
(13)
Eland E2 are potential responses to the same nominal concentrations of HI and COz,respectively, in separate solutions. S is the slope of the experimental calibration curve constructed for COz. Tables I1 and I11 list experimentaland theoretical selectivity coefficients at 8 X M of the interferent and carbon dioxide. All of the experimental selectivity Coefficients except that of hydrogen sulfide are greater than unity, reflecting a larger potential response to the compound than to carbon dioxide. The degree of interference is seen to increase with the acidity of the compound. No correlation of the magnitude of the potential response with the volatility of the respective compound can be made. There is good quantitative agreement between the experimental and theoretical selectivity coefficients shown for both the silicone rubber membrane and microporousTeflon membrane. The only discrepancy appears for the NO, species. The larger observed response obtained with both membranes is probably caused by decomposition of nitrous acid in the presence of oxygen to produce nitric acid which is a stronger acid (11). A difference in the selectivity of the C02sensor with the two gas-permeable membranes is only observed for the organic acids. The more volatile aliphatic carboxylic acids are apparently insoluble in the silicone rubber membrane. Diffusion through the air-layer defined by the poly(tetrafluoroethy1ene) matrix of the microporous Teflon membrane appears to require that the compound have a vapor pressure above 5 torr and limited T bonding. This could explain the small response observed to benzoic acid. Deviations from Equilibrium Behavior. The good agreement shown in Table I11 between the experimental and theoretical selectivity coefficients for 8 X M concentrations of the carboxylic acids with the microporous Teflon membrane is somewhat surprising in view of the nonreproducible potential readings and long response times observed to l o ~ concentrations of these acids. A possible explanation for the
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984 2365
Table 111. Selectivity of Carbon Dioxide Electrodea/Microporous Teflon Membrane compound
P,C torr
KH,dL atm/mol
HZS
15260
10
COP
54100
29.8
propionic acid acetic acid formic acid NO,
4.1 16 40 890 3000
SO2
1.1 x 10-3 0.65
selectivity coeff,b found calcd
PK, 7-00Wi) 12.92 (pKz) 6.35 ( P K ~ 10.33 (PKJ 4.88 4.76 3.77 3.29 1.76 Wi) 7.21 (PG)
0.7
0.66
1
1
34 43 370 1800 7000
31 40 373 1050 7530
M concentration of respective compound. Vapor pressure of pure com0.01 M NaHC03 in 0.09 M NaCl filling solution. 8 X pounds at 25 "C. dHenry's law constant at 25 OC (ref 14);the value for acetic acid was calculated (ref 13). Table IV. Selectivity of Carbon Dioxide Electrode"/Dilute Filling Solutionb selectivity coeff,C kp6,,HI found calcd
compound formic acid acetic acid propionic acid
184 35.6 27.0
187 33.2 26.0
S,d mV/decade found calcd
49.0 57.3 57.7
45.9 54.8 55.2
"Microporous Teflon membrane. * 0.001 M NaHC03in 0.099M NaCl filling solution. c 8 X M concentration of respective compound. dSlope of line determined by five data points repreto 8 X M. senting 2-fold concentration increases from 5 X
100-
100 -
1 E,mV
3.30
3.00
2.70
2.40
2.10
COMPOUND] , m o l / L Figure 7. Response of C02 sensor with microporous Teflon membrane to (1) formic acld, (2) acetic acid, and (3)propionic acid using 0.001 M NaHCO, in 0.099 M NaCl fllllng solution. Solid llnes are theoretical response curves. -log[
long response times is an inherently low diffusion coefficient of these acids through the membrane. The 0.01 M NaHC03 in 0.09 M NaCl filling solution was replaced by a 0.01 M sodium acetate in 0.09 M NaCl electrolyte solution. The response times observed in constructing an acetic acid calibration curve with the latter fiiing solution were at lemt three times faster than that observed with the former solution. Thus, the diffusion of these organic acids through the Teflon membrane is not inherently slow. The 0.01 M NaHCO, in 0.09 M NaCl filling solution was then replaced by a 0.001 M NaHC0, in 0.099 M NaCl electrolyte solution. The osmolarities of the two solutions were the same. Calibration curves constructed for the carboxylic acids with the microporous Teflon membrane using the more dilute sodium bicarbonate filling solution are shown in Figure 7. The response times observed at low concentrations were at least 3 times faster than those observed by using the more concentrated filling solution. As shown in Table IV, there is excellent agreement between the selectivity coefficients and the slopes of the response curves. These results indicate that the anomalous behavior observed for low concentrations of
50 -
0-
-50
-
1
-100'3130
'
3.bo
5
2.;0
2140
'
2.io
-log [ C O M P O U N ~, rnol/L
Figure 8. Response of COP sensor with microporous Teflon meqbrane to acetic acid In (a) forward and (b) reverse directions using (1) 0.001 M NaHCO, in 0.099 M NaCl and (2) 0.01 M NaHCO, in 0.09 M NaCl
fllling solutions.
the carboxylic acids using the more concentrated sodium bicarbonate solution cannot be attributed to such phenomena as membrane adsorption (12) or gas-phase dimerization (13), the effects of which would be more pronounced using the more dilute filling solution. A steady-state response of a potentiometric gas-sensing electrode implies that an equilibrium distribution of the
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
respective curves lie closer together at low concentrations of acetic acid and, more importantly, enclose the theoretical response curve. No definitive explanation for these results can be given, nor could they be satisfactorily reproduced. However, the steady-state response model was shown to give the equilibrium response to low concentrations of these acids.
150r
CONCLUSIONS
E,mV
50-
I / O I'* -50
3.30
3.00
2.70
2.40
2.10
- l o g [COMPOUND], m O i / L
Flgure 9. Response of COPsensor wtth mlcroporous Teflon membrane to acetic acid In (a) forward and (b) reverse directions using 0.01 M NaHCO, in 0.09 M NaCl filling solution. Argon was bubbled In sample solution.
diffusing compound has been approached. A criterion for equilibrium is reversibility of the response. Figure 8 shows acetic acid calibration curves constructed in the forward and reverse directions. The respective curves obbtained by using the more dilute sodium bicarbonate solution lie closely above and below the theoretical response line. This implies that the calculated theoretical response is the equilibrium response of the C02sensor. However, the respective curves obtained by using the more concentrated sodium bicarbonate solution are widely separated at low concentrations of acetic acid and both lie below the theroetical response curve. These latter results suggest either that the response model is inadequate or that some phenomenon is causing irreversibility which becomes more pronounced at low concentrationsof acetic acid. Since the only difference between the two filling solutions is the concentration of sodium bicarbonate, the anomalous behavior observed using the more concentrated sodium bicarbonate solution may result from the corresponding higher concentration of carbon dioxide within the electrolyte film. Figure 9 shows the acetic calibration curves which were constructed in the forward and reverse directions by using the more concentrated sodium bicarbonate electrolyte solution while argon was bubbled through the sample solution. The
As can be judged from the good quantitative agreement between the theoretical and experimental values for the selectivity coefficients, the proposed steady-state response model accounts for the observed response of the carbon dioxide electrode to interfering compounds. Although a differential selectivity with membrane type was observed, the steady-state response is based solely on the chemical equilibria in the electrolyte film. Registry No. COz, 124-38-9;HzS,7783-06-4;NO,, 11104-93-1; SOz, 7446-09-5; pyruvic acid, 127-17-3; DL-laCtiC acid, 598-82-3; oxalic acid, 144-62-7;succinic acid, 110-15-6;L-malic acid, 97-67-6; acetoacetic acid, 541-50-4; oxalacetic acid, 328-42-7; L-ascorbic acid, 50-81-7;benzoic acid, 65-85-0;propionic acid, 79-09-4;acetic acid, 64-19-7; formic acid, 64-18-6.
LITERATURE CITED Orlon Reserach Instruction Manual, Model 95-02;Orion Research Inc., 1981. Cooiey, J. M.; Kratochvii, B. Can. J . Chem. 1978, 5 6 , 2452-2456. Qullbauk, Q. Q.; Shu, F. R. Anal. Chern. 1872, 44, 2161-2166. Midgiey, D. Analyst (London) 1875, 100, 386-399. Kobos, R. K.; Parks, S. J.; Meyerhoff, M. E. Anal. Chem. 1982, 5 4 ,
1976-1980. Lopez, M. E.; Rechnltz, Q. A. Anal. Chem. 1982, 5 4 , 2085-2089. Nordmann, J.; Nordmann, R. I n "Advances in Clinical Chemlstry"; Sobotka, H., Stewart, C. p., Eds.; Academic Press: New York, 1961; Vol. 4. Lynch, W. "Handbook of Silicone Rubber Fabrlcatlon"; Van NostrandRelnhold: New York, 1978. Orlon Research Instruction Manual, Model 95-46;Orion Research Inc., 1981. Analytlcai Chemistry Division, Commlssion on Anaiytlcai Nomenclature Pure Appl. Chem. 1981, 53, 1907-1912. Schwartz, S.E.; White, W. H. I n "Advances in Environmental Science and Engineering"; Gordon & Breach Science Publishers: New York, 1981;voi. 4. Pittman, A. 0. In "Fiuoropoiymers, High Polymer Series"; Wail, L. A., Ed.; WHey-Interscience: New York, 1972;Chapter 13. Kojima, K.; Tochigl, K. "Prediction of Vapor-Liquid Equilibria by the ASOQ Method, Physlcai Sciences Data 3";Eisevler: New York, 1979; Chapter 1. Hougen, 0. A,; Watson, K. M.; Ragatz, R. A. "Chemical Process Principles", 2nd ed.;Wiiey: New York, 1956;Part 1.
RECEIVED for review November 17,1983. Resubmitted May 4,1984. Accepted May 14,1984. We gratefully acknowledge support of NSF Grant CHE-8025625.
