1798
Anal. Chem. 1986, 58, 1798-1803
tained for the reference materials (stated as a range in column 6)is relatively large, this is not unusual for trace analytes at the parts-per-million level in solids. Undoubtedly, this dispersion can be partly attributed to a random variation of the blank level of nickel, the effect being most pronounced in the analysis of estuarine sediment and subbituminous coal, where the blank concentration contributed most to the total nickel concentration in the digests. Such blank complications are, however, not related to the electrochemical technique as such, and the uniform agreement of the mean observed and recommended nickel concentrations (columns 5 and 7) is taken as an indication that the CME approach is accurate for traces of nickel, even in quite composite materials. Current research is therefore focused on reducing the number of manipulations involved in the procedure and particularly on performing the analysis by means of a flow system with convenient medium exchange capability. &&try NO.HA, 95-45-4;NiA2,13478-93-8; HZO, 7732-18-5; Ni, 7440-02-0;Pd, 7440-05-3.
LITERATURE CITED (1) Cheek, G. T.; Nelson, R. F. Anal. Lett. 1978, 7 1 , 393-402. (2) COX. J. A.; Majda, M. A M . Chem. 1980, 52,861-864. (3) Lubert, K.-H.; Schnurrbusch, M.; Thomas, A. Anal. Chlm. Acta 1982, 144, 123-136. (4) Izutsu, K.; Nakamura, 7.; Takizawa, R.;Hanawa, H. Anal. Chim. Acfa 1983, 149, 147-155. (5) Guadalupe, A. R.; Abruna, H. D. Anal. Chem. 1985. 5 7 , 142-149. (6) Cox, J. A.; Kulesza, P. J. Anal. Chim. Acta 1983, 754,71-78.
(7) Wang. J.; Greene, 6.; Morgan, C. Anal. C h h . Acta 1984, 758, 15-22. (8) Prlce, J. F.; Baldwin, R. P. Anal. Chem. 1880, 52, 1940-1944. (9) Murray, R. W. In Electroenalytfcal Chemktry; Bard, A. J., Ed.; Marcel Dekker: New York, 1964; Vol. 13, pp 191-368. (10) Olson, C.; Adams, R. N. Anal. Chlm. Acta 1963, 2 9 , 358-363. (1 1) Vydra, F.; stuk, K.; JuOkovl, E. ElechochemlcalStripping Analysis; Ellis Horwood Limited. Sussex, England, 1976. (12) Brainlna, Kh. 2. Sfdpplng Voltammetry in Chemical Analysis; Wlley: New York, 1974. (13) Flora, C. J.; Nbboer, E. Anal. Chem. 1980, 52, 1013-1020. (14) Phlar, 6.; Valenta, P.; Nirmberg, H. W. Fresenlus‘Z. Anal. Chem. 1981, 307, 337-346. (15) Adeloju. S. 6.; Bond, A. M.; Briggs, M. H. Anal. Chim. Acta 1984, 164, 181-194. (16) Stary, J. The Solvent Extractlon of Metal Chelates; Pergamon: Oxford, 1964. (17) Ravichandran, K.; Baldwin, R. P. Anal. Chem. 1983, 55. 1586-1591. (18) Geno, P. W.; Ravichandran, K.; Baldwin, R. P. J . Nectronanal. Chem. 1985, 783, 155-166. (19) Skov, H. J.; Kryger, L. Anal. Chlm. Acta 1980, 722, 179-191. (20) Meites, L; Zuman, P. ElectrochemicalData; Wiley: New York, 1974; Part 1, Vol A. (21) Burger, K.; Dyrssen, D. Acta Chem. S c a d . 1983, 17, 1489-1501. (22) Peters, D. 0.;Hayes, J. M.; Hleftje, G. M. Chemlcal Seperatbns and Measurements; W. 6. Saunders: Phlladetphia, PA, 1974; pp 235-236. (23) Slllln, L. G.; Martell, A. E. Stabilly Constants of Metal-Ion Complexes; The Chemical Society: London, 1964; Specbl Publication No. 17.
RECEIVED for review December 12,1985. Accepted February 27,1986. Our work is currently supported by NATO Scientific Affairs Division (Grant 85/0679 for international collaboration) and by the Danish Natural Science Research Council (Grant 511-8033 for the computerized electroanalyticaldevice).
Uphill Transport Membrane Electrodes Masayuki Uto,Hitoshi Yoshida, Masao Sugawara, and Yoshio Umezawa* Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan
membrane electrode was constructed in whkh carrier-ed uparrtrampottol analytes la incorporated. The electrod. can boost mkdlvdy virtual concentratkn of rpeclfk anslyto8 by uphM trMIpMt against thdr concentratbn gradknt across a bul#4nuqu# m e m b r r r n e i n t o b l n n e r ~ solution, whore voluma la purposely made very small. Cd( I I ) , ~01p+, and cu(11) ion uph#l trru#port w a n e eiectrodes corwtructed here a8 Illudrstlve examples UtiRZe three different types of Input energhas, Le., complexatbn, concentration gradient, and redox, respectively, for upMll tranrpoil of each a w e . VdlSnmarlc detecflons were demondrated for Cd( I I ) and U022+Ion upMll transport electrodes, and a potentiometric detection for a Cu(1I) ion uphill transport memkaneelectrodelad#dercribed<emrrofkdmmntai behavlomand a posrlbk use for a new type of ebctrochemlcal sensor. A new
The mode of mass transfer often encountered in analytical chemistry is diffusion in which ions and molecules are transportad in general following their concentration gradient. On the contrary, there exists a different type of mass transfer called uphill and active transports in which chemical species are transported against their concentration gradients. This latter mode of mass transfer appears to be attractive in view of analytical applications. Here we describe novel voltammetric and potentiometric membrane electrodes that carry 0003-2700/86/035&1798$01.50/0
a mode of uphill transport in a coupled manner to diffusion and/or convection. The electrode consists of a liquid membrane through which ions can be selectively transported against their concentration gradient to be detected by an underlying electrode. As far as we know, no attempts have been made at coupling a carrier-mediated uphill transport to mass transfer by diffusion and convection for purposes of signal enhancement and a high degree of selectivity of the electrode response. In the present paper, uphill transport membrane electrodes for cadmium,uranyl, and copper ions are constructed and their fundamental behaviors are described in view of exploring a new type of electrochemical sensor.
THEORETICAL SECTION A carrier-mediated ion transport system consists of two aqueous solutions, Le., feed and receiving solutions, separated by a liquid membrane containing lipophilic carriers. When ions in a feed solution are transported into a receiving solution through a liquid membrane following a concentration gradient, the transport of ions continues until the final concentration of ions in the receiving solution, C,, becomes equal to that in the feed solution, Cf (C, = Cf). If one inputs “energy” such as light, redox energy, or concentration gradients of other ions into the transport system, ions can be pumped up from a feed solution to a receiving one against their concentration gradients. In biological systems the transport of metabolites or nutrients through cell membranes occurs in many cases against 0 1986 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986 A)
V,
=
V,
il m sol inner e Hg-f l e c t rode u t 1on
membrane
1799
smle
so l u t Ion
~
Active transport membrane electrode
Flgure 2. Tri-n-octylmethylammonium chloride (QCI) mediated Cd(I I) ion uphiil transport membrane system in conjuction with a mercury film electrode: inner solution, 1 M NH,NO,-NH,OH buffer (pH 9.7); sample solution, 1 M KCI acetate buffer (pH 5) Cd(I1).
+
Flgure 1. Relation between relative volume of feed vs. receiving solution in uphill transport membrane system and resultlng concentration change of ions in each solution: V,, volume of a feed solution; V,, volume of a receiving solution; C,, initial concentration of a metal ion in a feed solution; C,, concentration of a metal ion in a receiving solution transported across the membrane from a feed solution.
their concentration gradient, i.e., by the so-called active transport. Rosenberg (1) defined the active transport as a transfer of chemical matter from a lower to higher chemical potential (in the case of charged components, electrochemical potential). We can achieve active transport and uphill transport by using artificial liquid membrane systems (2,3). For driving uphill ion transport by an artificial liquid membrane system, we need to supply “energy” from outside, which is consumed with movement of ions against their concentration gradients. Shinkai et al. (4-6)used UV-vis light for the transport of potassium ions through liquid membranes incorporating a photoresponsive azobenzene-bridged crown ether. Takagi and ,co-workers utilized a redox energy for metal ion uphill transports mediated by carriers such as bathocuproine (7) and quaternary ammonium salt (8). Concentration gradients of other ions such as protons have also been utilized in metal ion uphill transport systems (2, 9, 10). Here we have tried to use three different types of “outer energies”: (i) energy of complexation for uphill transport of cadmium(II), (ii) energy of concentration difference of nitrate ions for uphill transport of uranyl ions, and (iii) redox energy for uphill transport of copper ions. The most important idea of the present study in incorporating uphill transport phenomena with an electrochemical !sensor is schematicallyshown in Figure 1. When the volumes (of the feed (V,) and receiving (V,) solutions are the same, an {increasein the analyte concentration does not occur even after complete uphill transport (Figure 1A). However, if Vf >> V , holds (Figure lB), an increase, if any, of the analyte concentration results even after incomplete or very little uphill transport. Thus, we can amplify or boost the virtual concentration of the analyte by keeping the volume of the inner ireceiving solution as small as possible compared to that of the feed solution. When the uphill transport membrane is to be applied for the selective separation of materials of interest, the consideration described above is of no use, because the net quantity of the materials transported is the only concern in that case rather than the increase in virtual concentration of the materials in a given cell volume, which is purposely made very small in volume. Thus, the idea given in Figure ‘1 is solely advantageous for the potential application of uphill
+
transport phenomena for membrane electrodes that are expected to function as a new type of electrochemical sensor. Figure 2 shows a cadmium ion “uphill” transport membrane system incorporated with an electrochemical redox process. The membrane component that mediates cadmium(I1) transport is an oil-soluble tri-n-octylmethylammonium chloride, known as one of the good extractants of metal-chloro complexes. Use of such quaternary ammonium salts for extraction (11) and membrane separation (12, 13) has been reported. An electrically neutral cadmium(II)-dichloro complex is extracted into the liquid membrane to form an ion pair of tetrachlorocadmate(I1) anion with the ammonium cation. The neutral ion pair complex thus formed diffuses to the other side of the membrane (i-e.,the inner solution), at which decomposition of the rather weak cadmium(I1)-chloro complex occurs owing to the absence, or lower concentration,of chloride ions in the inner solution and also the presence of ammonia, which complexes with Cd(I1). Complexation of Cd(I1) with ammonia lowers the concentration of the permeable Cd(II)-dichloro complex in the inner solution and shifts the steady state to a new one that helps in additional transport of the Cd(I1) complex. Similar uphill transport phenomena will also be presented for uranyl ions using a concentration gradient of nitrate ions as an external source of energy. UOZ2+ions in a nitric acid solution are extracted into the tri-n-butyl phosphate (TBP) membrane as a lipophilic uranyl complex U02(N0J2TBP2. The uranyl complex decomposes at the other side of the membrane, where nitrate ions are present at a lower concentration, and liberates both uranyl ions and nitrate ions in the receiving solution. In this membrane system, uphill transport of U022+ions is driven by co-transport of nitrate ions. Transport of protons also occurs in the form of HNO,TBP, and therefore U022+ions compete with protons for forming membrane-permeable lipophilic complexes. For driving a Cu(I1) ion uphill transport by redox energy, bathocuproine dissolved in the liquid membrane acts as a carrier for Cu(I1) ions. Cu(II) is reduced to Cu(1) by a reducing agent, hydroxylamine in this case, and extracted into the membrane phase as an ion pair of Cu(1)-bathocuproine chelate cations with chloride ions. Upon contact with an aqueous receiving solution, the Cu(1) ion pair complex is oxidized by dissolved oxygen in the receiving solution to dissociate into Cu(I1) and chloride ions.
EXPERIMENTAL SECTION Apparatus. Voltammogramswere recorded with Princeton Applied Research polarographic analyzer 174A (Princeton, NJ), in conjuction with a Rika Denki X-Y recorder RW-11 (Tokyo, Japan). A platinum wire served as an auxiliary electrode. A saturated calomel electrode, as the reference electrode, was immersed in the sample solution through a salt
1800
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
n
6
Figure 3. Structure of an uphill transport membrane electrode: 1, supported liquid membrane; 2, mercury film electrode; 3, inner filling solutlon; 4, a body of the electrode; 5, Teflon purge tube; 6, O-ring; 7, Teflon stopper.
bridge of 1M potassium nitrate solution with a Vycor tip. For potentiometric measurements, an Orion Research (Cambridge, MA) Model 701A digital ionalyzer was used. An Orion Cu(I1) ion-selectiveelectrode type 94-29 and a TOA (Tokyo, Japan) saturated calomel electrode HS-305D were used. A Shimadzu spectrophotometer UV-240 (Kyoto, Japan) with a 10-mm cell was used for the spectrophotometric determination of uranyl ions by a pyridylazoresorcinol method (14),and a Seiko I&E metal furnace atomic absorption spectrophotometer (AAS) SAS 727 (Tokyo, Japan) was used for the determination of Cd(I1) and Cu(II) ions. The pH of the solutions was measured with a Horiba glass electrode pH meter model F-7LC (Kyoto, Japan). Reagents. Tri-n-octylmethylammoniumchloride (Capriquat) was obtained from Dojin Chemical Laboratories (Kumamoto, Japan), diluted to half its initial concentration with chloroform, and shaken repeatedly with an aqueous 1 M sodium nitrate solution in order to transform it into the nitrate form. Tri-n-butyl phosphate was obtained from Wako Chemicals Co. (Tokyo, Japan) and bathocuproiene (2,9-dimethyl-4,7-diphenyl-l,l0-phenanthroline) from Dojin Chemical Laboratories, and they were used as received. A 0.02 M standard solution of uranyl(V1) was prepared by dissolving uranyl acetate, U02(CH3C00)2-2H20, in 60 mL of 6 M hydrochloric acid and making up to 500 ml'with water. An atomic absorption standard Cd solution (loo0 ppm) was daily diluted to prepare the Cd(I1) solution. A stock solution of Cu(I1) was prepared from the sulfate. Other reagents used were all of analytical reagent grade. Deionized and distilled water was used. An inner filling solution of 0.1 M ammonium nitrate-ammonia buffer solution, pH 9.7, was used for the Cd(I1) electrode, and for the UOZ2+electrode a mixture of 0.01 M nitric acid, 3 X vol % Triton X-100, and 5 X lo-* M mercurous nitrate was employed, which allows in situ formation of a mercury film on an underIying glassy carbon electrode. The addition of Triton X-100 in this case is to obtain a well-defined uranyl peak. An inner filling solution for a Cu(I1) ion transport membrane electrode was 0.5 M ammonium acetate solution. Construction of Uphill Transport Membrane Electrode. The uphill transport membrane electrode constructed is schematically shown in Figure 3. The supported liquid membrane consists of a 14-mm-diameter piece cut from Toyo
(Tokyo, Japan) poly(tetrafluoroethy1ene) membrane filter PT-100 (pore size 1 pm and mean thickness 80 pm), which was impregnated with carriers dissolved in chloroform. The membranes were kept overnight in a beaker covered with a watch glass in order to allow the evaporation of chloroform. The carrier used for cadmium ions was tri-n-octylmethylammonium chloride (Capriquat), and that for uranyl ions was a mixture of tri-n-butyl phosphate and tri-n-octylmethylammonium nitrate in a 2:l volume ratio. For a copper ion uphill transport membrane electrode, bathocuproine (6 mM) dissolved in Capriquat was used as a carrier. For cadmium and uranyl ion electrodes, the supported liquid membrane was attached to the body of an ammonia-sensitiveelectrode (Denki Kagaku Keiki, Tokyo, Japan) by means of an O-ring. A glassy carbon electrode used as an underlying electrode for the UO?+ electrode was polished to a mirror finish with alumina powder (0.05 pm) and washed thoroughly with water. A mercury film electrode (MFE) used for the Cd(I1) ion uphill transport membrane eletrode was prepared by depositing mercury onto a glassy carbon electrode at -0.4 V vs. SCE for 30 s in a 0.05 M perchloric acid4025 M mercurous nitrate solution and was transferred into the inner filling solution immediatly after rinsing with water and cautiously drying with a Kimwipe. The Cu(I1) ion uphill transport membrane electrode was constructed with a glass body, one end of which holds the supported liquid membrane fixed with a Teflon seal tape. A Cu(I1) ion selective electrode conditioned beforehand in a solution having the same composition as the inner solution was dipped in the inner solution. Also, the reference electrode was immersed in the same inner solution via a salt bridge of 1 M potassium nitrate solution with a Vycor tip. In the case of the uranyl ion electrode, the electrode was kept at an initial potential of -0.4 V for 3 min, in order to deposit a mercury film on an underlying glassy carbon electrode, and then the scan was started. After the end of each scan, the electrode potential was adjusted to +1.0 V and kept for 2 min to redissolve mercury from the MFE. When voltammograms are repeatedly measured without this procedure, the peak height decreased with the repetition of scans. After each transport experiment with the present membrane electrode, some amount of metal ions remained in the inner solution and possibly in the interior of the membrane phase even after it was washed with water. Therefore, we reconstructed the membrane electrode for each measurement, in order to keep the initial working conditions constant for a prototype experiment like the present case. This could be one of drawbacks of the present electrode, but we believe that this can be overcome if needed, for example, by using a flow system to replace an inner solution with a new one.
RESULTS AND DISCUSSION Uphill Transport Using Energy of Complexation. The intensity of a differential pulse anodic stripping voltammetric (DPASV) peak current of Cd(I1) at the uphill transport membrane electrode, described in the Theoretical Section, after immersing the electrode in 0.1 ppm Cd(I1) ion sample solution, was compared with that obtained with the same membrane electrode at time zero in which a given amount of Cd(I1) was spiked into the inner solution to give a concentration of 0.1 ppm. The results are shown in Table I. A net Cd(I1) concentration was amplified about 19 times as compared with the bulk sample concentration due to the uphill transport across the membrane of Cd(I1) ions into the small volume of inner solution. This is also confirmed by AAS for the same inner solution. The effect of the compositions of an inner filling solution on the amount of Cd(I1) ions transported is shown in Table 11. When ammonia concentration in the inner solution is increased, while keeping chloride concentration in the inner
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
. ,< .
Table I. Enhancement of DPASV Current Intensity at a Cadmium Ion Uphill Transport Membrane Electrode (See Text) Compared with Results by AAS
6.02
116
19.3
2.06
1801
20.6
230'
DPASV current obtained at Cd(I1) uphill transport membrane electrode at time zero with 0.1 ppm Cd(I1) spiked in the inner filling solution. The composition of the "sample" solution used in this particular case is 1 M KCl only. bDPASV current measured at Cd(I1) uphill transport membrane electrode in 0.1 ppm Cd(I1) (in 1 M KCl) sample solution after 4 h. 'Concentration of Cd(I1) transported into inner filling solution of the Cd(I1) uphill transport electrode from 0.1 ppm Cd(I1) (1M KCl) sample solution after 4 h determined by AAS. dConcentration of Cd(I1) in the sample soilution: [Cd],,,,. = 0.1 ppm (in 1 M KC1).
=.
4
c c
? a
V
'Table 11. Effect of Compositions of Inner Filling Solution (Receiving Solution) on the Amount of Cd(I1) Ions Transported through Cd(I1) Uphill Membrane (See Text)
case 1 2 3
composition of inner filling soln,' M KCl "3 0
1
1 1
0.01 1
amt of Cd(I1) transported,b PPm 17.6 0.2 13.0
"Sample (50 mL) conditions are 1 M KCl solution of pH 5 (acetate buffer) containing 1 ppm Cd(I1). The Volume of inner solution is 1.0 mL. bDetermined by AAS after 3.5 h. (Note that this value is zero at t = 0.)
V o l u w o f inner solution, ml Flgure 4. (1) DPASV peak current at a Cd(I1) ion uphill transport membrane electrode as a function of volume of the inner filling solution. (2) Concentration of Cd(I1) in the inner solution determlned by AAS for comparison. The peak current was measured at 3.5 h, and the AAS results were obtained at 5 h after immersing the electrode in the sample solution. Sample conditions (50 mL) are 1 r g mL-' Cd(I1) in 1 M KCi of pH 5 (acetate).
n
Table 111. Change in Virtual Concentration of Cd(I1) Ions in Inner Filling (Receiving) Solution Transported through Cd(I1) Uphill Membrane (See Text) with Different Volume of Inner Receiving Solution
vol of receiving s o h mL 0.5 1.0 1.5
[Cdlinner,(lP P ~[ C d l ~ , ( Pl . ~P ~[Cdlddd,'P P ~ 37.7 17.6 9.6
0.65 0.71 0.75
35 15
8 4 h
Determined by AAS after 3.5 h. Initial concentration of cadmium(I1) in 50-mL feed solution, [Cdli, is 1 ppm. 'Calculated from a decrease in the amount of Cd(I1) in the feed solution ac(cording to [CdlCdcd= ([Cd], - [CdIfd) x (volume of feed solution)/ (volume of receiving solution). solution the same as in the sample solution, the amount of Cd(I1) ions transported into the inner solution is significantly increased. The results indicate the transport system is driven by energy difference between complexation of Cd( 11) with ammonia at the inner side and complexation of Cd(I1) with chloride ions at t h e feed side. Therefore, an increase in concentration of chloride ions in the inner solution lowers the efficiency of transport of Cd(I1) ions (compare case 1with case 3 in Table 11). Volume Effect of Inner Solution. Figure 4 shows t h e dependence of the DPASV peak current at the Cd(I1) uphill transport membrane electrode on volume of the inner solution in t h e range from 0.5 to 1.5 mL. The smaller t h e volume of the inner solution is, the larger the observed current is, in accordance with the consideration described in the Theoretical Section (see Figure 1). This conclusion is consistent with the results by AAS measurements for the same system given in 'Table 111.
3.5 h
2 h
2 h lh
-
Blank I
I
I
I
I
-1,o
I
-0.5
E vs. SCE,
V
-1
I h N
.o
Blank
-0,5
E vs. SCE, V
Figure 5. DPAS voltammograms at a Cd(I1) ion uphiii transport membrane electrode in 1 M KCI solutlon (pH 5) containing (1, left) 0.1 r g mL-' and (2, right) 2 r g mL-' Cd(I1) ions: DPASV conditions are as follows: deposltion time, t,, = 2 min at -1.0 V vs. SCE; pulse amplitude, 50 mV; and pulse interval, 1 s. Volume of the inner solution is 1 mL.
Variation of Response w i t h Sample Analyte Concentrations. Differential pulse anodic stripping voltammograms of Cd(I1) at t h e uphill transport membrane electrode are shown in Figure 5. The peak height depends on the time after the electrode is immersed in the sample solution containing Cd(I1). The peak current measured at an arbitary chosen
1802
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8 , JULY 1986
!000 4 I
+ c
P U 2
50c)
1
[Cd(Il
)Is
-1
2
transport time of 3.5 h was plotted against the concentration of Cd(I1) in the sample solution as shown in Figure 6. The observed current is increased with the sample Cd(I1) concentration. This relation also holds when plotted at other shorter times, though peak current-analyte concentration profiles are not necessarily linear. At a Cd(I1) concentration above 1 ppm, two DPASV peaks are observed. This is due to adsorption of the quaternary ammonium chloride on an underlying mercury film electrode. Because of this adsorption effect, a surface concentration of chloride ions is significantly higher than in the bulk solution. Hence, both redox systems, CdC1,X-2/Cd(Hg)and Cd(NH3)2+/Cd(Hg),contribute to the anodic process observed, giving rise to two DPASV peaks. We confirmed this by measuring DPASV voltammograms at a conventional mercury film electrode in the ammonia buffer solution saturated with tri-n-octylmethylammonium chloride. When Cd(I1) concentration is 1 ppm, only a peak at -0.72 V corresponding to the CdC12x-2system appears, but when Cd(I1) concentration is increased to 20 ppm, an additional peak at -0.83 V corresponding to the Cd(NH3)x2+system appears. Selectivity. The “built-in” uphill transport membrane electrode can discriminate ions on the basis of their different permeability through the membrane. A detection of Cd(I1) in the presence of indium(II1) was tested. According to Pribil and Vesely (111, In(II1) is not extracted from the chloride media into a chloroform solution of tri-n-octylmethylammonium chloride. Nonpermeability of In(II1) through the membrane is therefore expected. Only a DPASV peak that corresponds to Cd(I1) is observed as shown in Figure 7, whereas with a conventional mercury film electrode, overlapped Cd(I1) and In(II1) peaks appear, which interfere mutually with their determinations. Uphill Transport Using a Concentration Gradient of Other Ions. An uphill transport membrane electrode for UOZ2+ions consisted of a tri-n-butyl phosphate mediated membrane transport system in conjunction with an underlying glassy carbon electrode (see Theoretical Section). With such an uphill transport membrane electrode, a differential pulse voltammetric peak is observed at ca. -0.42 V vs. SCE. Figure 8 illustrates a current-time profile of the electrode response using different volumes of the inner solution. As expected, the current obtained with a smaller inner volume is higher than with a larger volume. Similar results were obtained also by spectrophotometric measurements for the same chemical system. When sample UOZ2+ion concentrations are varied
-0.5
1
3
8
.
’
-0.5
-1 .O
E v s . SCE, V
, ppm
Flgure 6. (1) Current Intensities at WII) ion uphill transport electrode against different sample Cd(II) concentrations. (2) Concentration of Cd(I1) in the inner solution also measured by AAS for comparison. Sample conditions (50 mL) are 1 M KCI, pH 5 (acetate). Volume of the inner solution is 1.0 mL.
,
,o
E v s , SCE, V
Figure 7. Detection of 200 Mg L-’ Cd(I1)in the presence of 200 Hg L-’ indium(II1) in 1 M KCI solution of pH 5 (acetate buffer) with a (1) mercury film electrode and a (2) Cd(I1) ion uphill transport membrane
electrode. The voltammogram with the membrane electrode was recorded at 2 h after immersing it in the sample solution. Instrumental conditions for vottammetric measurements were the same as in Figure 5.
Time, h
Flguro 8. A current-time profile of a uranyl ion uphill transport membrane electrode using different volume of the inner solution: (1) 0.5 mL and (2) 1.0 mL. Sample conditions are (50 mL) 1 mM U02(CH3-
Conditions for differential pulse voitammetry are as follows: scan rate, 10 mV s-’; pulse ampliiude, 50 mV; and pulse interval, 1 s.
COO)* in 3 M ”OB.
between 0.5 and 2 mM, the steady current obtained becomes larger with increasing UOZ2+ion concentrations. Uphill Transport Using Redox Energy. An attempt was made to incorporate a copper ion uphill transport driven by redox energy with a potentiometric detection. The transport system described in the Theoretical Section concentrates Cu(I1) ions from a larger volume (50 mL) of a sample solution into a smaller volume (3 mL) of the inner solution by using redox energy as “input energy”. The sample solution was a mixture of 0.5 M ammonium acetate and 0.01 M potassium chloride in the presence of 1 mM hydroxylamine and a given amount of Cu(II) sulfate. The observed potential of the copper ion uphill transport membrane electrode shifts toward a more positive potential with time after immersing it in the sample solution of 1.0 X M Cu(I1) ions, Le., from 4.2 mV at time zero (Cu(I1) 0 mM in an inner solution) to 174 mV after 3 h. The potential change for the first 10 min is very small, and thereafter the potential becomes increasingly more positive. A steady potential is obtained after 3 h. A t this time,
-
Anal. Chem. 1966, 58, 1803-1806
we measured the concentration of copper ions in the inner solution by AAS. The AAS results showed that the Cu(I1) ion concentration is 3.6 X lo4 M,which is amplified 3.6 times as compared with the sample Cu(II) concentration. Thus, the present electrode boosts the virtual concentration of Cu(I1) ions by uphill transport using redox energy. The electrode potential becomes more positive when the sample Cu(I1) concentration is increased further.
CONCLUSION The “built-in” uphill transport liquid membrane electrode described here seems to be an attractive method for modifying sensitivity and selectivity of the electrochemical response. The small volume of an inner filling solution plays a key role in signal enhancement. Permeability of ions across the membrane contributes to a selective response of the electrode. When both the mercury film and reference electrodes are dipped in the inner filling solution as is the case for the proposed Cu(I1) electrode, an advantage of eliminating an ohmic drop through the membrane results, although an explicit coupling, if any, of uphill and active transports with an electric potential gradient across the membrane would be expected for a separate three-electrode system like the Cd(I1) and uranyl electrodes (Figure 3). The response time of the present electrode is rather slow, which will be improved by
1603
using a thinner membrane. The work is under way. Registry No. Cd, 7440-43-9;U, 7440-61-1;Cu, 7440-50-8.
LITERATURE CITED Rosenberg, T. Acta. Chem. S a n d . 1048, 2 , 14. Okahara, M.;Nakatsuji, Y. I n Biomimtk and Bioinorganic Chemisfry, Topics in Current Chemistry 728; Boscheke, F. L., Ed.; Springer-Verlag: Berlin, 1985;p 37. Bloch, R.; Kedem, 0.;Vofsl, D. Nature (London) 1063, 799. 802. Kumano. A.; Nlwa, 0.;Kajlyama, T.; Takayanagi, M.;Kano, K.; Shinkal, s. Chem. Lett. 1083, 1327. Shlnkai, S.; Mlnaml, T.; Kusano, U.; Manabe, 0. J . Am. Chem. SOC. 1882. 704. 1967. Shlnkal, T.;’Ishlhara, M.;Ueda, K.; Manabe, 0. J . Chem. Soc., Pekin Trans. 2 1085. 511. Ohkl, A.; Takagi, M.;Ueno, K. Chem. Lett. 1080, 1591. Ohki. A.; Hinoshlta, H.; Takagi, M.;Ueno, K. Sep. Sci. Technd. 1083, 78, 969. Choy, E. M.;Evans, D. F.; Cussler, E. L. J . Am. Chem. SOC.1974, 96, 7085. Bloch, R. Membrane Science and Technology; -. Flinn, J. E., Ed.; Plenum: New York, 1970;p 171. Pribll, R.; Vesely, V. Collect. Czech. Chem. Commun. 1072, 3 7 , 13. Danesi, P. R.; Hcrwitz, E. P.; Rickert, P. Sep. Sci. Techno/. 1082, 17,
1183. Danesi, P. R.; Chiarlria, R.; Castagnola, A. J . Memb. Sci. 1083, 14,
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RECEIVED for review August 28, 1985. Resubmitted January 28, 1986. Accepted February 28, 1986.
Polypyrrole Electrode as a Detector for Electroinactive Anions by Flow Injection Analysis Yoshihito Ikariyama and William R. Heineman* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172
A new electrochemlcal detector based on the repetltlve doping-undoplng of polypyrrole has been applled to the electrochemlcal determlnatlon of electrolnactlve anlons by flow Injection analysis. The electrode response for phosphate and carbonate was linear over the concentration range from 10 pM to 1 mM wtth a coemcid of variatbn of 1.2% for 250 pM phasphate and 2.5% for 1 mM carbonate. The electrode was stable for over 2 weeks In an anaeroblc atmosphere.
After its introduction by Nagy et a1 (l),flow injection analysis (FIA) has attracted considerable interest as a method for fast, repetitive, and reproducible analysis (2,3).Initially based on electroanalysis, the methodology has been further developed for this purpose (4,5)as well as extended to other modes of detection (6-8) such as absorbance (91, fluorescence (lo),and luminescence (11).Although amperometric detection for FIA and liquid chromatography (LCEC) has come into widespread use for the determination of electroactive subHtances (12-14), it is not directly applicable to the large inumber of compounds that do not undergo heterogeneous redox reactions at electrodes. This feature is advantageous in the sense that selectivity is improved by the absence of response to electroinactive interferences in the sample. Amperometric detection has been extended to some electroinactive substances by postcolumn reaction detectors (14), 0003-2700/S6/0358-1803$01.50/0
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Scheme I
Doping
Scheme 11
Undoping
immunoassay (151,and a triple step potential wave form for certain adsorbates (16). In recent years electrodes coated with conductive polymer films have been the subject of considerable interest (17). Conductive polymers such as polypyrrole and polythiophene are an especially important class of polymers (18-23),since ionic substances are easily and repeatedly incorporated into (doping) and released from (undoping) the polymers. The general concept of the doping-undoping property of polypyrrole is shown in Schemes I and 11. Conductive polymer coatings of this type have been used as a charge-storage material in rechargeable batteries (24),an organic electrode material (25),a protecting film on semiconductor electrodes to prevent photocorrosion (26),an ion gate membrane (23, 0 1986 Amerlcan Chemical Soclety
