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Vitamin B1 ion-selective microelectrode based on a liquid-liquid interface at the tip of a micropipette
ANALYTICA CHIMICA ACTA ELSEVTER
Analytica
Chimica Acta 3 12(1995) 329-335
Vitamin B, ion-selective microelectrode based on a liquid-liquid interface at the tip of a micropipette Bo Huang, Bazhang Yu, Peibiao Li Department
of Chemistry,
Received 5 December
*, Mian
Jiang, Yongsheng Bi, Shufang Wu
Wuhan Unirvrsi~.
Wuhm 430072.
China
1994; revised 19 April 1995; accepted 72 April lY95
Abstract A vitamin B, ion-selective electrode based on 1,2-dichloroethane-water (DCE/W) and nitrobenzene-water (NB/W) interfaces supported at the tip of a dual-section micropipette was examined by using cyclic voltammetry. asymmetric sweep rate cyclic voltammetry and stripping voltammetry. We have determined trace vitamin B, down to 4.6 i( 10Yh mol I _’ in aqueous sample solution. The response mechanism has been discussed and some relevant data of vitamin Bl transfer across NB/W and DCE/W interfaces were also obtained. K~~wo~k Ion selective electrodes; Voltammetry;
Vitamin B,; Liquid-liquid
1. Introduction Electrochemical studies on the interface between two immiscible electrolyte solutions (ITIES) can provide useful information about the mechanism of liquid membrane ion-selective electrodes [l-3]. When a polarizable ITIES functions as an ion-selective electrode, it can be used for both voltammetry and potentiometry [4-61. In recent years, studies on the ITIES supported at the tip of a micropipette have attracted much attention [7-121. Senda et al. [13] has reported an amperometric ion-selective microelectrode based on a nitrobenzene-water interface which was successfully used to monitor trace acetylcholine ion in biological fluid. Some work on vitamin B, ion-selective electrode has demonstrated that a stable Nernstian response of protonized vitamin B, could be obtained in acidic aqueous media (pH(4) [14,15].
. Corresponding
author.
0003.2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI OClCl3-2670(95)00217-O
microinterface
In our study, we constructed an ion-selective microelectrode based on the 1,2-dichloroethane-water (DCE/W) and ni‘tro b enzene-water (NB/W) interface supported at the tip of a dual-section micropipette with the advantages of a short shank, flat cross-section and rigidity [16]. This kind of electrode was easy to fabricate and to renew. As a new technique, it has been used not only to illustrate the transfer mechanism by cyclic voltammetry, asymmetric sweep rate cyclic voltammetry and polarography, but also to detect trace vitamin B, (a well-known important necessity in our bodies [17]) in aqueous sample solution with stripping voltammetry.
2. Experimental 2.1. Reagents and solutions The aqueous base electrolyte solutions were prepared from 0.01 mol l- ’ LiCl and citric acid-
330
B. Huang et al. /Analytica
Na,HPO, buffer (CPN) solution (pH 2.2-8.0), and the organic solutions from nitrobenzene (NB) or 1,2-dichloroethane (DCE) and tetrabutylammonium tetraphenylborate (TBATPB) [18]. Vitamin B, (thiamine chloride hydrochloride, Shanghai Biochemicals Factory) was of biochemical grade and other chemicals were of analytical-reagent grade or better. Doubly distilled water was used in the experiment. 2.2. Microelectrode and cell The construction of the electrochemical be represented by [19] (RE2)
AgbgCl]O.Ol
TBATPB(O)(hitamin
M TBACl(W’)]O.Ol
cell can
M
Chimica Acta 312 (1995) 329-335
experiment was (20 f 2°C).
carried
out at room
temperature
2.3. Apparatus A four-electrode current scan polarographic system [20] for both ascending water electrode (AWE) and stationary water electrode @WE) was laboratory-built which was similar to that used by Yoshida and Freiser [21]. The voltammetric and chronoamperometric experiments were performed employing a four-electrode instrumental system [ 161. The applied voltages were produced from a Model 402 potentiostat (Yanbian Electrochemistry Instrumental Factory). A Type 3086 X-Y recorder (the Fourth Instrumental Factory, Shichuan) was also used.
B, test soln. (W)!AgCl!Ag
(RE1) The dual-section micropipette was prepared by hand using a glass tube (ca. 6 mm i.d.) and the tip diameter ((65 pm i.d.1 was measured by an Abbe comparator which is extensively used to measure the spectral interval width in emission spectrometry. This micropipette was filled with a 1,Zdichloroethane solution (0) of 0.01 mol 1-l TBATPB to a depth of 2 cm from the narrow end by a water pump or a microsyringe of 10 ml. Then an aqueous solution (W’) of 0.01 mol 1-t TBACl was placed over the organic solution in the micropipette, an Ag/AgCl electrode and a platinum electrode sealed within a thin glass tube were immersed into W’ and 0 phase, respectively. The cell concerned was placed in a grounded shielding box during the experimental process. The
3. Results and discussion 3.1. Polarographic behaviour of vitamin B, Current scan polarography at the AWE proceeded as follows [20]. The aqueous electrolyte solutions ascended dropwise through the organic solution from a poly(tetrafluoroethylene) (PTFE) capillary with a mean flow rate and drop time. The current between CE, and CE, was scanned, and the potential difference between RE, and RE, was recorded following correction of the iR drop. The correction was achieved by using the blank voltammetric curve as an indication of proper correction [21]. In solutions of low acidity (pH)5), the thiazole ring in the vitamin B, molecule will open according to:
H
S-
B. Huang et al. /Analytica Table 1 The limiting currents and Nernstian
331
Chimica Acta 312 (1995) .729-335
response slopes S at different
PH PH I,D (nA) I,N (nA) S” (mV) SN (mV)
1.0 2.0 3.7 3.7 4.2 4.2 30.0 31.1 29.0 30.6
2.2 3.6 4.1 34.0 33.8
3.0 3.5 3.9 36.4 -
4.0 3.2 3.6 35.7
Note, D represents the data measured NB/W, respectively.
5.0 2.8 3.0 46.0 43.2
6.0 2.3 2.5 53.0 50.4
from DCE/W
7.0 2.0 2.1 56.5 54.6
8.0 1.X 1.0 58.0 55.5
and N from
However, vitamin B, is stable up to pH 8 in the citric acid-Na,HPO, buffer solution [15]. Our experiment showed that the limiting currents at pH)7.0 and pH(2.2 were corresponding to the transfer of the uni- and divalent protonized vitamin B, across the DCE/W interface, respectively. The half-wave potential of vitamin Bf+ was 0.41 V. The limiting currents and Nemstian response slopes at various acidity are summarized in Table 1. The slope (S) of vitamin B, can be represented by Eq. 1: S=S,6++S#+
(1)
Here S, and S2 were equal to 58.0 mV for vitamin B: (pH 8.0) and 30.0 mV for vitamin Bif (pH l.O>, S+ and 8’+ denoted distribution coefficients of these two ions in vitamin B,, respectively. The distribution curve is shown in Fig. 1. From the data listed in Table 1 and Eq. 2: a++ s’+=
1
the dissociation constant of solutions, p K,, = 5.01, was agreement with the reported value can also be calculated pK,,=pH+log(I-12)/(Z,-Z)
(2) vitamin B, in aqueous obtained and nearly in value of 5.04 [15]. This by Eq. 3: (3)
Here, I, and I, represent the limiting currents at pH 1 and pH 8 (see Table 11, respectively. The limiting current of the polarogram was proportional to the square root of the height of the water reservoir. The half-wave potential E1,2 was independent of the concentration of vitamin B, in the aqueous phase. The limiting currents were proportional to the concentration of vitamin B, in water in the range of 5 X lo-’ to 5 X 10e4 mol 1-l. The
1
2
3
4
5
6
7
8
PH Fig. 1. Dependence
ot 6 on pH.
results mentioned above suggested that the transfer of vitamin B, from W to DCE was controlled by the diffusion of vitamin B, in W. 3.2. Voltammetric
behauiour of Litumin B,
In this experiment, we investigated the transfer of vitamin Bf+ across the DCE/W microinterface supported at a pipette with the tip inner diameter of 32 and 64 pm. At pH)5, the voltammetric wave of vitamin B, was not evident, hence we could not discuss the results in this pH range.
Table 2 Results of the asymmetric
sweep rate cyclic voltammetry.
vc (mV 5-l ) vv tmV s-‘)
10 20 50 80 200
R
20
50
80
100
150
14.20 8.35 7.56 6.88 3.94
22.83 14.72 12.20 10.82 7.50
27.20 19.47 16.26 14.26 10.82
32.00 22.60 18.44 16.38 12.38
37.83 42.33 0.9974 25.70 31.70 0.9960 22.93 25.06 0.9968 14.38 21.94 0.9978 14.94 17.56 0.9978
vc = Constant sweep correlation coefficient
rate; vQ = various of I, - ~4”.
sweep
200
rate;
R = linear
B. Huang et al. /Analytica
332
The cell can be represented (RE,)
Ag!AgCl(O.Ol
TBATPB
Chimica Acta 312 (1995) 329-335
by: 12.5
M TBACl (W’)]O.Ol M
(NB or DCE)()O.Ol M buffer + xM
vitamin B, (W) (AgCl IAg (RE, )
0
With regard to a liquid-liquid interface supported at the tip of a micropipette, Stewart et al. [22] have defined the movement of ions towards the interface and from inside the pipette as ingress and egress transfer. Voltammogram for vitamin Bf+ at 6 mV s-’ [16] showed that the ingress transfer of vitamin Bf+ from the W to the NB phase displayed a sigmoidal shaped current curve as observed at a solid microelectrode. While the peak current Zr agreed with the egress transfer of vitamin B:+ from the NB to the W phase. The limiting current I,, was smaller than Zr due to the concentrating effect (or pre-concentration) of vitamin B,2+ from W into the little volumetric NB in the capillary during this slow forward scan. The half-wave potential of 0.43 V coincided with the results observed in the polarogram of vitamin B,2+. Thus the formal transfer potential EzsW + ” and the Gibbs energy G,c;),w+ NB were calculated as 234 mV and -45.2 kJ mol-’ for E,‘IvW+ DCE vitamin BF+, respectively. Accordingly, and GzsW + DCE were calculated as 207 mV and - 39.9 kJ mol-‘. The diffusion coefficient D” was also obtained for vitamin Bi+ ion (in W) as 6.7 X lop6 cm2 s- ’ based on the steady current (4.2 nA). Logarithmic analysis of this S-curve gave a line with a slope of 30.6 mV decade-‘. This result was not fully equivalent to the expected value for a reversible two-charge transfer according to the following equation: E=E,,,+(RZ’/nF)ln[(Z,-Z)/Z]
(4)
The slight deviation of the slope from what we had expected could be explained by the residual iR drop effect of the highly resistive microelectrode. This iR drawback could be eliminated by offering a proper compensating potential electrically. However, we usually chose a little smaller compensation than the complete one in order to avoid the oscillation of system. The Z-E curves of vitamin Bff exhibited a pair of peaks as the potential sweep rates increased, currents for both of the forward and the reverse scan
12.5 H
-25.0
-37.5
0.2
0.3
0.4
EN Fig. 2. Asymmetric sweep rate cyclic voltammogram of vitamin B:+ (DCE/W). @ = 64 pm COME); concentration of vitamin B, =1X10-” mol I-‘; initial potential, Ei =0.2.5 V; negative limit potential, E, = 0.18 V; positive limit potential, E, = 0.47 V; constant sweep rate: 10 mV s-‘; various sweep rate (a --t f): 20, 50, 80, 100, 150, 200 mV s- ’
tended to be identical. This phenomenon can be considered as the concentrating effect of vitamin Bf+ ion is not important during the forward scan rate u)40 mV SK’ m . this experiment. We found that the dependence of current Zr, on the sweep rate did not increase as in the classical manner with u’/~ and did not coincide with the ordinary behaviour of the peak height in linear potential sweep voltammetry at an electrode of conventional size. This phenomenon was also observed on the micropipette interface and was explained in [22]. In this case, asymmetric sweep rate cyclic voltammetry [22,23] was introduced in our study. This technique provided a constant potential sweep rate in one direction and various sweep rates in another, therefore keeping the same amount of ions inside the capillary. It was performed by an asymmetric waveform software in our experiment [23]. Fig. 2 showed the asymmetric sweep rate voltammogram for vitamin By+ ion transfer. It could be seen that the S-shaped curves in each scan cycle coincided well
333
B. Huang et al. /Analytica Chimica Acta 312 (1995) .129-335
(4
1
1.25 nA
1
. cl
40
80
(-3
(4 t
b)
Fig. 3. Chronoamperometric curve of vitamin B, (DCE/W). Cell, (~~,)~g/Agci lo.01M TBACI(W') lo.01M TBATPB~DCE) IIO.01M LiCl+ 0.01 M citric acid-Na,HPO, (pH 2.2) + 2X 10eJ M vitamin B, (W) 1AgCl/Ag (RE,); curve (a), forward scan current: curve (b), reverse scan current; curve (cl, cyclic voltammetric curve (v = 30 mV SK’ ).
and the peak height for reverse scan obeyed the dependence on the square root of sweep rate which could be exploited to demonstrate that linear diffusion controlled the egress transfer of vitamin B:+ ion across the DCE/W microinterface. The relevant data (peak current I,> are summarized in Table 2. Thus the asymmetric sweep rate cyclic voltammetry provided its advantage in this system over the classical cyclic voltammetry.
linear diffusion controlled transfer (F,) of vitamin B*+ back from DCE to the W phase. When the 1 sampling time t(40 s, curve (a) decreased sharply as time elapsed, while at t)40 s, the decrease tendency was very smooth. This suggested that the concentration of vitamin Bf+ on the interface inside the microelectrode was nearly zero. Based on the above experiment, we found that it took 15 s to reach the steady state (following the spherical diffusion mode F,) for the voltammetric study of this microelectrode, thus the pre-electrolysis time 7; needs to be at least 15 s for stripping voltammetry. 3.4. Stripping
rroltammetry
The voltammetric curve of vitamin Bff at 6 mV s-l showed that the cathodic current was rather larger compared with the limiting current of the anodic current. This interesting fact was explained by the elevated-concentration effect [19] which was similar to the ordinary stripping voltammetry effect.
60
40 a F 0 T
3.3. Chronoamperometry 20 Fig. 3 shows current-potential curves when applying a potential step E,-E, (or &-Et) to the cell. Curve (b) corresponds to the S-shaped voltammetric curve (c), whose current stemmed from spherical diffusion controlled transfer (F,) of vitamin B:+ from the bulk of W into the cylinder of DCE in the capillary. For about 15 s, curve (b) gradually tended to be parallel to the time curve. When applying E,-E, to the cell, the reverse chronoamperometric curve (a), however, corresponded to peak-shaped voltammetric curve (c) whose current resulted from
4 Cl1 iS4 M Fig. 4. Dependence of stripping current on the concentration of vitamin Bf+ in aqueous phase. v = 20 mV s-’ : preconcentration potential: 0.46 V: preconcentration time T, (I + 4) = 30, 60. 300. 600 s.
B. Huang et al./Analyticn Chimica Acta 312 (1995) 329-335
334
0.1
0.2
0.3
0.4
0.5
EN Fig. 5. Stripping voltammogram of vitamin Bf+ (DCE/W). potential: 0.43 V; preconcenv = 20 mV s-l, pre-concentration tration time Ti (a + f) = 0, 30, 60, 120, 300, 600 s.
During the ingress transfer [22], a higher concentration area formed around the inside of an interface in contrast to the bulk concentration outside. When applying an appropriate potential step which corresponded to the limiting current (forward scan> for a period of pre-concentration time and followed by the reverse linear potential scan, a large peak current was obtained. Thus trace vitamin B, was detected down to 4.6 X 10e6 mol 1-l with a linear range of 5 X lop5 to 1 X 10m3 mol 1-l by this microinterface electrode (Fig. 4). The detection limit for vitamin B, was lower than (within 1 order of magnitude of) continuous flow chemiluminescence 1241 and HPLC-UV determination [25], equivalent to HPLCatmospheric pressure chemical ionization-mass spectrometry [26] and derivative spectrophotometry [27]. Fig. 5 displays the stripping voltammogram of 1 X 10m3 mol 1-l vitamin B,. It gave good linearity between peak current Zr, and the square root of pre-concentration time Ti. In the potential range concerned, the stripping peak current increased with Ei as well as the scan rate (v= 20-200 mV s-r>. The selectivity of this microelectrode was evaluated. Measurement of 1 X 10m4 mol 1-l vitamin B, was not affected by addition of 5 X 1O-3 mol 1-l vitamin B,, 1 X lop3 mol 1-l vitamin B,, 0.05 mol 1-l ascorbic acid (vitamin c) and 0.01 mol 1-l Mg’+, Ca*+ or K+. Such compounds did not show
apparent transfer peak currents under these conditions, but higher concentrations (higher than the concentration of the supporting electrolyte) might lead to distortion of the vitamin B, peak. Aqueous solution (pH 2.2) containing 1 X lop4 mol 1-r vitamin B, was electrolyzed during 5 min at 0.46 V. The standard deviation of the stripping currents was estimated as 0.2 nA when successively determined six times. The average relative standard deviation was 3.8%. Recovery experiments were performed by adding 1.5 X 10m4 mol 1-l vitamin B,, the average recovery was 103%. Based on the above experiments, it can be concluded that this vitamin B, ion-selective microelectrode based on a liquid-liquid interface supported at the tip of a micropipette could be a useful tool for studying the ion transfer mechanism and for analytical applications. The half-wave potential of vitamin B, in voltammetry corresponds to the one in current scan polarography. Asymmetric sweep rate cyclic voltammetry proves the occurrence of linear diffusion controlled-egress transfer of vitamin B, across the microinterface. Trace vitamin B, can be determined satisfactorily by stripping voltammetry.
Acknowledgements The authors gratefully acknowledge the support for this research by a doctoral special grant and the Trans-Century Training Programme Foundation for the Talents from the State Commission of Education of the People’s Republic of China.
References Ill J.D. Reid, P. Vanysek and R.P. Buck, J. Electroanal. Chem., 161 (1984) 1.
I21 B. Hundhammer,
H.J. Seidlitz, S. Becker and S.K. Dhawan, J. Electroanal. Chem., 180 (1984) 355. 131 M. Senda, Rev. Polarogr., 30 (1984) 19. 141 T. Kakutani, T. Ohkouchi, T. Osakai and M. Senda, Anal. Sci., 1 (1985) 219. bl P. Vanysek, Anal. Chem., 62 (1990) 832A. [61P. Vanysek, I.C. Hemandez and J.-J. Xu, Microchem. J., 41 (1990) 336. [71 G. Taylor and H.H.J. Girault, J. Electroanal. Chem., 208 (1986) 179.
B. Huang et al. /Analytica Chimica Acta 312 (1995) .329-335 [S] J.A. Campbell and H.H. Girault, J. Electroanal. Chem., 266 (1984) 465. [9] AA. Stewart, Y. Shao, CM. Pereira and H.H. Girault, J. Electroanal. Chem., 305 (1994) 135. [lo] Y. Shao, M.C. Osborne and H.H. Girault, J. Electroanal. Chcm., 318 (1991) 101. [I l] Y. Shao and H.H. Girault, J. Electroanal. Chem., 334 (1992) 203. [12] M.C. Osborne. Y. Shao, C.M. Pereira and H.H. Girault. J. Electroanal. Chem., 364 (1994) 155. [13] M. Senda. T. Kakutani, T. Osakai and T. Ohkiouchi, The 1st Bioelectroanalytical Symposium, Matrafured, Budapest, 19x7, p. 353. [14] S. Kihara, Z. Yoshida and T. Fujinaga, Bunseki Kagaku, 31 (1982) 297. [15] S.-Z. Yao, G.-L. Shen and X.-M. Xu, Yaoxue Xuebao, 18 (lY83) 612. [16] B.-Z. Yu, B. Huang, P.-B. Li, Microchem. J., 52 (1995) 10.
335
[17] R.A. Nicholson and I. Shain, Anal. Chem., 36 (1964) 706. [18] C. Gavach and F. Henry, J. Electroanal. Chem., 54 (1974) 361. [19] T. Ohkiouchi. T. Kakutani, T. Osakai, M. Senda, Anal. Sci., 7 (1991) 371. [20] Y.-H. Shao, P.-B. Li and Z.-F. Zao, Fenxi Yiqi, No. 4 (1987) 34. [21] Z. Yoshida and H. Freiser, J. Electroanal. Chem., 162 (1984) 307. [22] A.A. Stewart, G. Taylor and H.H. Girault. J. Electroanal. Chem., 296 (1990) 491. [23] B.-Z. Yu, B. Huang and P.-B. Li, Huaxuc Tongbao, No. 10 (1991) 37. [24] N. Grekas and A.C. Calokerinos, Talanta. 37 (1990) 1043. [25] X.-Y. Wang, Sepu, 11 (1993) 364. [26] 1.J. Hewala, Anal. Lett., 26 (1993) 2217. [27] K. Yamanaka, S. Horimoto, M. Matsuoka and K. Banno, Chromatographia, 39 (1994) 91.
Analytica
Chimica Acta 3 12(1995) 329-335
Vitamin B, ion-selective microelectrode based on a liquid-liquid interface at the tip of a micropipette Bo Huang, Bazhang Yu, Peibiao Li Department
of Chemistry,
Received 5 December
*, Mian
Jiang, Yongsheng Bi, Shufang Wu
Wuhan Unirvrsi~.
Wuhm 430072.
China
1994; revised 19 April 1995; accepted 72 April lY95
Abstract A vitamin B, ion-selective electrode based on 1,2-dichloroethane-water (DCE/W) and nitrobenzene-water (NB/W) interfaces supported at the tip of a dual-section micropipette was examined by using cyclic voltammetry. asymmetric sweep rate cyclic voltammetry and stripping voltammetry. We have determined trace vitamin B, down to 4.6 i( 10Yh mol I _’ in aqueous sample solution. The response mechanism has been discussed and some relevant data of vitamin Bl transfer across NB/W and DCE/W interfaces were also obtained. K~~wo~k Ion selective electrodes; Voltammetry;
Vitamin B,; Liquid-liquid
1. Introduction Electrochemical studies on the interface between two immiscible electrolyte solutions (ITIES) can provide useful information about the mechanism of liquid membrane ion-selective electrodes [l-3]. When a polarizable ITIES functions as an ion-selective electrode, it can be used for both voltammetry and potentiometry [4-61. In recent years, studies on the ITIES supported at the tip of a micropipette have attracted much attention [7-121. Senda et al. [13] has reported an amperometric ion-selective microelectrode based on a nitrobenzene-water interface which was successfully used to monitor trace acetylcholine ion in biological fluid. Some work on vitamin B, ion-selective electrode has demonstrated that a stable Nernstian response of protonized vitamin B, could be obtained in acidic aqueous media (pH(4) [14,15].
. Corresponding
author.
0003.2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI OClCl3-2670(95)00217-O
microinterface
In our study, we constructed an ion-selective microelectrode based on the 1,2-dichloroethane-water (DCE/W) and ni‘tro b enzene-water (NB/W) interface supported at the tip of a dual-section micropipette with the advantages of a short shank, flat cross-section and rigidity [16]. This kind of electrode was easy to fabricate and to renew. As a new technique, it has been used not only to illustrate the transfer mechanism by cyclic voltammetry, asymmetric sweep rate cyclic voltammetry and polarography, but also to detect trace vitamin B, (a well-known important necessity in our bodies [17]) in aqueous sample solution with stripping voltammetry.
2. Experimental 2.1. Reagents and solutions The aqueous base electrolyte solutions were prepared from 0.01 mol l- ’ LiCl and citric acid-
330
B. Huang et al. /Analytica
Na,HPO, buffer (CPN) solution (pH 2.2-8.0), and the organic solutions from nitrobenzene (NB) or 1,2-dichloroethane (DCE) and tetrabutylammonium tetraphenylborate (TBATPB) [18]. Vitamin B, (thiamine chloride hydrochloride, Shanghai Biochemicals Factory) was of biochemical grade and other chemicals were of analytical-reagent grade or better. Doubly distilled water was used in the experiment. 2.2. Microelectrode and cell The construction of the electrochemical be represented by [19] (RE2)
AgbgCl]O.Ol
TBATPB(O)(hitamin
M TBACl(W’)]O.Ol
cell can
M
Chimica Acta 312 (1995) 329-335
experiment was (20 f 2°C).
carried
out at room
temperature
2.3. Apparatus A four-electrode current scan polarographic system [20] for both ascending water electrode (AWE) and stationary water electrode @WE) was laboratory-built which was similar to that used by Yoshida and Freiser [21]. The voltammetric and chronoamperometric experiments were performed employing a four-electrode instrumental system [ 161. The applied voltages were produced from a Model 402 potentiostat (Yanbian Electrochemistry Instrumental Factory). A Type 3086 X-Y recorder (the Fourth Instrumental Factory, Shichuan) was also used.
B, test soln. (W)!AgCl!Ag
(RE1) The dual-section micropipette was prepared by hand using a glass tube (ca. 6 mm i.d.) and the tip diameter ((65 pm i.d.1 was measured by an Abbe comparator which is extensively used to measure the spectral interval width in emission spectrometry. This micropipette was filled with a 1,Zdichloroethane solution (0) of 0.01 mol 1-l TBATPB to a depth of 2 cm from the narrow end by a water pump or a microsyringe of 10 ml. Then an aqueous solution (W’) of 0.01 mol 1-t TBACl was placed over the organic solution in the micropipette, an Ag/AgCl electrode and a platinum electrode sealed within a thin glass tube were immersed into W’ and 0 phase, respectively. The cell concerned was placed in a grounded shielding box during the experimental process. The
3. Results and discussion 3.1. Polarographic behaviour of vitamin B, Current scan polarography at the AWE proceeded as follows [20]. The aqueous electrolyte solutions ascended dropwise through the organic solution from a poly(tetrafluoroethylene) (PTFE) capillary with a mean flow rate and drop time. The current between CE, and CE, was scanned, and the potential difference between RE, and RE, was recorded following correction of the iR drop. The correction was achieved by using the blank voltammetric curve as an indication of proper correction [21]. In solutions of low acidity (pH)5), the thiazole ring in the vitamin B, molecule will open according to:
H
S-
B. Huang et al. /Analytica Table 1 The limiting currents and Nernstian
331
Chimica Acta 312 (1995) .729-335
response slopes S at different
PH PH I,D (nA) I,N (nA) S” (mV) SN (mV)
1.0 2.0 3.7 3.7 4.2 4.2 30.0 31.1 29.0 30.6
2.2 3.6 4.1 34.0 33.8
3.0 3.5 3.9 36.4 -
4.0 3.2 3.6 35.7
Note, D represents the data measured NB/W, respectively.
5.0 2.8 3.0 46.0 43.2
6.0 2.3 2.5 53.0 50.4
from DCE/W
7.0 2.0 2.1 56.5 54.6
8.0 1.X 1.0 58.0 55.5
and N from
However, vitamin B, is stable up to pH 8 in the citric acid-Na,HPO, buffer solution [15]. Our experiment showed that the limiting currents at pH)7.0 and pH(2.2 were corresponding to the transfer of the uni- and divalent protonized vitamin B, across the DCE/W interface, respectively. The half-wave potential of vitamin Bf+ was 0.41 V. The limiting currents and Nemstian response slopes at various acidity are summarized in Table 1. The slope (S) of vitamin B, can be represented by Eq. 1: S=S,6++S#+
(1)
Here S, and S2 were equal to 58.0 mV for vitamin B: (pH 8.0) and 30.0 mV for vitamin Bif (pH l.O>, S+ and 8’+ denoted distribution coefficients of these two ions in vitamin B,, respectively. The distribution curve is shown in Fig. 1. From the data listed in Table 1 and Eq. 2: a++ s’+=
1
the dissociation constant of solutions, p K,, = 5.01, was agreement with the reported value can also be calculated pK,,=pH+log(I-12)/(Z,-Z)
(2) vitamin B, in aqueous obtained and nearly in value of 5.04 [15]. This by Eq. 3: (3)
Here, I, and I, represent the limiting currents at pH 1 and pH 8 (see Table 11, respectively. The limiting current of the polarogram was proportional to the square root of the height of the water reservoir. The half-wave potential E1,2 was independent of the concentration of vitamin B, in the aqueous phase. The limiting currents were proportional to the concentration of vitamin B, in water in the range of 5 X lo-’ to 5 X 10e4 mol 1-l. The
1
2
3
4
5
6
7
8
PH Fig. 1. Dependence
ot 6 on pH.
results mentioned above suggested that the transfer of vitamin B, from W to DCE was controlled by the diffusion of vitamin B, in W. 3.2. Voltammetric
behauiour of Litumin B,
In this experiment, we investigated the transfer of vitamin Bf+ across the DCE/W microinterface supported at a pipette with the tip inner diameter of 32 and 64 pm. At pH)5, the voltammetric wave of vitamin B, was not evident, hence we could not discuss the results in this pH range.
Table 2 Results of the asymmetric
sweep rate cyclic voltammetry.
vc (mV 5-l ) vv tmV s-‘)
10 20 50 80 200
R
20
50
80
100
150
14.20 8.35 7.56 6.88 3.94
22.83 14.72 12.20 10.82 7.50
27.20 19.47 16.26 14.26 10.82
32.00 22.60 18.44 16.38 12.38
37.83 42.33 0.9974 25.70 31.70 0.9960 22.93 25.06 0.9968 14.38 21.94 0.9978 14.94 17.56 0.9978
vc = Constant sweep correlation coefficient
rate; vQ = various of I, - ~4”.
sweep
200
rate;
R = linear
B. Huang et al. /Analytica
332
The cell can be represented (RE,)
Ag!AgCl(O.Ol
TBATPB
Chimica Acta 312 (1995) 329-335
by: 12.5
M TBACl (W’)]O.Ol M
(NB or DCE)()O.Ol M buffer + xM
vitamin B, (W) (AgCl IAg (RE, )
0
With regard to a liquid-liquid interface supported at the tip of a micropipette, Stewart et al. [22] have defined the movement of ions towards the interface and from inside the pipette as ingress and egress transfer. Voltammogram for vitamin Bf+ at 6 mV s-’ [16] showed that the ingress transfer of vitamin Bf+ from the W to the NB phase displayed a sigmoidal shaped current curve as observed at a solid microelectrode. While the peak current Zr agreed with the egress transfer of vitamin B:+ from the NB to the W phase. The limiting current I,, was smaller than Zr due to the concentrating effect (or pre-concentration) of vitamin B,2+ from W into the little volumetric NB in the capillary during this slow forward scan. The half-wave potential of 0.43 V coincided with the results observed in the polarogram of vitamin B,2+. Thus the formal transfer potential EzsW + ” and the Gibbs energy G,c;),w+ NB were calculated as 234 mV and -45.2 kJ mol-’ for E,‘IvW+ DCE vitamin BF+, respectively. Accordingly, and GzsW + DCE were calculated as 207 mV and - 39.9 kJ mol-‘. The diffusion coefficient D” was also obtained for vitamin Bi+ ion (in W) as 6.7 X lop6 cm2 s- ’ based on the steady current (4.2 nA). Logarithmic analysis of this S-curve gave a line with a slope of 30.6 mV decade-‘. This result was not fully equivalent to the expected value for a reversible two-charge transfer according to the following equation: E=E,,,+(RZ’/nF)ln[(Z,-Z)/Z]
(4)
The slight deviation of the slope from what we had expected could be explained by the residual iR drop effect of the highly resistive microelectrode. This iR drawback could be eliminated by offering a proper compensating potential electrically. However, we usually chose a little smaller compensation than the complete one in order to avoid the oscillation of system. The Z-E curves of vitamin Bff exhibited a pair of peaks as the potential sweep rates increased, currents for both of the forward and the reverse scan
12.5 H
-25.0
-37.5
0.2
0.3
0.4
EN Fig. 2. Asymmetric sweep rate cyclic voltammogram of vitamin B:+ (DCE/W). @ = 64 pm COME); concentration of vitamin B, =1X10-” mol I-‘; initial potential, Ei =0.2.5 V; negative limit potential, E, = 0.18 V; positive limit potential, E, = 0.47 V; constant sweep rate: 10 mV s-‘; various sweep rate (a --t f): 20, 50, 80, 100, 150, 200 mV s- ’
tended to be identical. This phenomenon can be considered as the concentrating effect of vitamin Bf+ ion is not important during the forward scan rate u)40 mV SK’ m . this experiment. We found that the dependence of current Zr, on the sweep rate did not increase as in the classical manner with u’/~ and did not coincide with the ordinary behaviour of the peak height in linear potential sweep voltammetry at an electrode of conventional size. This phenomenon was also observed on the micropipette interface and was explained in [22]. In this case, asymmetric sweep rate cyclic voltammetry [22,23] was introduced in our study. This technique provided a constant potential sweep rate in one direction and various sweep rates in another, therefore keeping the same amount of ions inside the capillary. It was performed by an asymmetric waveform software in our experiment [23]. Fig. 2 showed the asymmetric sweep rate voltammogram for vitamin By+ ion transfer. It could be seen that the S-shaped curves in each scan cycle coincided well
333
B. Huang et al. /Analytica Chimica Acta 312 (1995) .129-335
(4
1
1.25 nA
1
. cl
40
80
(-3
(4 t
b)
Fig. 3. Chronoamperometric curve of vitamin B, (DCE/W). Cell, (~~,)~g/Agci lo.01M TBACI(W') lo.01M TBATPB~DCE) IIO.01M LiCl+ 0.01 M citric acid-Na,HPO, (pH 2.2) + 2X 10eJ M vitamin B, (W) 1AgCl/Ag (RE,); curve (a), forward scan current: curve (b), reverse scan current; curve (cl, cyclic voltammetric curve (v = 30 mV SK’ ).
and the peak height for reverse scan obeyed the dependence on the square root of sweep rate which could be exploited to demonstrate that linear diffusion controlled the egress transfer of vitamin B:+ ion across the DCE/W microinterface. The relevant data (peak current I,> are summarized in Table 2. Thus the asymmetric sweep rate cyclic voltammetry provided its advantage in this system over the classical cyclic voltammetry.
linear diffusion controlled transfer (F,) of vitamin B*+ back from DCE to the W phase. When the 1 sampling time t(40 s, curve (a) decreased sharply as time elapsed, while at t)40 s, the decrease tendency was very smooth. This suggested that the concentration of vitamin Bf+ on the interface inside the microelectrode was nearly zero. Based on the above experiment, we found that it took 15 s to reach the steady state (following the spherical diffusion mode F,) for the voltammetric study of this microelectrode, thus the pre-electrolysis time 7; needs to be at least 15 s for stripping voltammetry. 3.4. Stripping
rroltammetry
The voltammetric curve of vitamin Bff at 6 mV s-l showed that the cathodic current was rather larger compared with the limiting current of the anodic current. This interesting fact was explained by the elevated-concentration effect [19] which was similar to the ordinary stripping voltammetry effect.
60
40 a F 0 T
3.3. Chronoamperometry 20 Fig. 3 shows current-potential curves when applying a potential step E,-E, (or &-Et) to the cell. Curve (b) corresponds to the S-shaped voltammetric curve (c), whose current stemmed from spherical diffusion controlled transfer (F,) of vitamin B:+ from the bulk of W into the cylinder of DCE in the capillary. For about 15 s, curve (b) gradually tended to be parallel to the time curve. When applying E,-E, to the cell, the reverse chronoamperometric curve (a), however, corresponded to peak-shaped voltammetric curve (c) whose current resulted from
4 Cl1 iS4 M Fig. 4. Dependence of stripping current on the concentration of vitamin Bf+ in aqueous phase. v = 20 mV s-’ : preconcentration potential: 0.46 V: preconcentration time T, (I + 4) = 30, 60. 300. 600 s.
B. Huang et al./Analyticn Chimica Acta 312 (1995) 329-335
334
0.1
0.2
0.3
0.4
0.5
EN Fig. 5. Stripping voltammogram of vitamin Bf+ (DCE/W). potential: 0.43 V; preconcenv = 20 mV s-l, pre-concentration tration time Ti (a + f) = 0, 30, 60, 120, 300, 600 s.
During the ingress transfer [22], a higher concentration area formed around the inside of an interface in contrast to the bulk concentration outside. When applying an appropriate potential step which corresponded to the limiting current (forward scan> for a period of pre-concentration time and followed by the reverse linear potential scan, a large peak current was obtained. Thus trace vitamin B, was detected down to 4.6 X 10e6 mol 1-l with a linear range of 5 X lop5 to 1 X 10m3 mol 1-l by this microinterface electrode (Fig. 4). The detection limit for vitamin B, was lower than (within 1 order of magnitude of) continuous flow chemiluminescence 1241 and HPLC-UV determination [25], equivalent to HPLCatmospheric pressure chemical ionization-mass spectrometry [26] and derivative spectrophotometry [27]. Fig. 5 displays the stripping voltammogram of 1 X 10m3 mol 1-l vitamin B,. It gave good linearity between peak current Zr, and the square root of pre-concentration time Ti. In the potential range concerned, the stripping peak current increased with Ei as well as the scan rate (v= 20-200 mV s-r>. The selectivity of this microelectrode was evaluated. Measurement of 1 X 10m4 mol 1-l vitamin B, was not affected by addition of 5 X 1O-3 mol 1-l vitamin B,, 1 X lop3 mol 1-l vitamin B,, 0.05 mol 1-l ascorbic acid (vitamin c) and 0.01 mol 1-l Mg’+, Ca*+ or K+. Such compounds did not show
apparent transfer peak currents under these conditions, but higher concentrations (higher than the concentration of the supporting electrolyte) might lead to distortion of the vitamin B, peak. Aqueous solution (pH 2.2) containing 1 X lop4 mol 1-r vitamin B, was electrolyzed during 5 min at 0.46 V. The standard deviation of the stripping currents was estimated as 0.2 nA when successively determined six times. The average relative standard deviation was 3.8%. Recovery experiments were performed by adding 1.5 X 10m4 mol 1-l vitamin B,, the average recovery was 103%. Based on the above experiments, it can be concluded that this vitamin B, ion-selective microelectrode based on a liquid-liquid interface supported at the tip of a micropipette could be a useful tool for studying the ion transfer mechanism and for analytical applications. The half-wave potential of vitamin B, in voltammetry corresponds to the one in current scan polarography. Asymmetric sweep rate cyclic voltammetry proves the occurrence of linear diffusion controlled-egress transfer of vitamin B, across the microinterface. Trace vitamin B, can be determined satisfactorily by stripping voltammetry.
Acknowledgements The authors gratefully acknowledge the support for this research by a doctoral special grant and the Trans-Century Training Programme Foundation for the Talents from the State Commission of Education of the People’s Republic of China.
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