Electrochemical Detector Based on Nafion Membrane for Online ...
Electrochemical Detector Based on Nafion Membrane for Online ...
Electrochemical Detector Based on Nafion Membrane for Online ...
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CHINESE JOURNAL OF ANALYTICAL CHEMISTRY<br />
Volume 40, Issue 5, May 2012<br />
<strong>Online</strong> English editi<strong>on</strong> of the Chinese language journal<br />
Cite this article as: Chin J Anal Chem, 2012, 40(5), 675–680.<br />
RESEARCH PAPER<br />
<str<strong>on</strong>g>Electrochemical</str<strong>on</strong>g> <str<strong>on</strong>g>Detector</str<strong>on</strong>g> <str<strong>on</strong>g>Based</str<strong>on</strong>g> <strong>on</strong> Nafi<strong>on</strong> <strong>Membrane</strong> <strong>for</strong><br />
<strong>Online</strong> M<strong>on</strong>itoring of Column Chromatography Purificati<strong>on</strong><br />
Process<br />
TANG Yu-Hang 1 , LIU Wen-Han 1, *, TENG Yuan-Jie 1 , QIAN Jun-Qing 2 , MA Chun-An 1<br />
1<br />
College of Chemical Engineering and Material Science, Zhejiang University of Technology, Hangzhou 310014, China<br />
2<br />
College of Pharmaceutical Science, Zhejiang University of Technology, Hangzhou 310014, China<br />
Abstract: <str<strong>on</strong>g>Electrochemical</str<strong>on</strong>g> detecti<strong>on</strong> of eluent of column chromatography is c<strong>on</strong>siderably simple <strong>for</strong> m<strong>on</strong>itoring the purificati<strong>on</strong><br />
process, In this detecti<strong>on</strong> system, electrolyte is needed to decrease the resistance of soluti<strong>on</strong> during the electrochemical detecti<strong>on</strong>.<br />
However, in the case of natural medicine purificati<strong>on</strong>, it is highly challenging to added electrolyte into the purificati<strong>on</strong> process<br />
without influencing the purificati<strong>on</strong> efficiency. Herein, <strong>on</strong> the basis of Paraffin-graphite Powder electrode, an <strong>on</strong>line electrochemical<br />
detector was developed without any additi<strong>on</strong>al electrolyte. The cyclic voltammograms of myricetin in ethanol-water (80%, V/V) as<br />
mobile phase is obtained directly by using the solid acid characteristic of Nafi<strong>on</strong> N-324 membrane in the micro-gap flow cell.<br />
Peristaltic pump stimulates the chromatographic process, and then it realizes the <strong>on</strong>line detecti<strong>on</strong> of eletroactive substances in the<br />
eluent at 0.53 V. Peak area is linear with the myricetin c<strong>on</strong>centrati<strong>on</strong> in the range of 2.0 × 10 –5 –1.0 × 10 –3 M, and the linear equati<strong>on</strong> is<br />
A = 2.7227c + 0.4341, r = 0.9976. Furthermore, the detector achieves the <strong>on</strong>line detecti<strong>on</strong> of 30% AOB (antioxidant of bamboo leaves)<br />
purified by macro porous resin without any supporting electrolyte in 80% (V/V) ethanol-water at 0.62 V, and the amperometric<br />
time-current (t-i) curve during the purificati<strong>on</strong> process was presented. The result indicated that the detector is feasible <strong>for</strong> applicati<strong>on</strong>.<br />
Key Words: Natural medicine; Nafi<strong>on</strong> membrane; Antioxidant of bamboo leaves; Cyclic Voltammetry; Micro-gap flow Cell;<br />
Paraffin-graphite Powder electrode<br />
1 Introducti<strong>on</strong><br />
There is practical significance to separate and purify the<br />
active ingredients of herbal medicine. At present, the<br />
spectroscopy technology is generally selected <strong>for</strong> <strong>on</strong>line<br />
detecting liquid chromatography in most of pharmaceuticals<br />
companies. However, because of high sensitivity and simple<br />
instrument, also electrochemical detecti<strong>on</strong> can be used to<br />
detect the insensitive matter, which cannot be distinguished by<br />
spectroscopy.<br />
Flav<strong>on</strong>oids bel<strong>on</strong>g to comm<strong>on</strong> active ingredients of herbal<br />
medicine which can clear up the free radicals [13] , when the<br />
reducti<strong>on</strong> potential of free radicals is lower, the ability of<br />
antioxidant and electrochemical activity are str<strong>on</strong>ger [4,5] , thus<br />
it is suitable <strong>for</strong> the electrochemical detecti<strong>on</strong>. The crude<br />
flav<strong>on</strong>oids extracted from bamboo leaves are so complex that<br />
it needs macroporous absorpti<strong>on</strong> resin to purify. Because the<br />
carb<strong>on</strong> pasted electrode is sensitive <strong>for</strong> flav<strong>on</strong>oids [6–9] .<br />
However, the traditi<strong>on</strong>al Cyclic Voltammetry (CV) method<br />
does not fit <strong>for</strong> the <strong>on</strong>line detecti<strong>on</strong> due to large amounts of<br />
electrolytes. New electrochemical detecti<strong>on</strong> method should be<br />
developed without electrolytes or less.<br />
Nafi<strong>on</strong> is a kind of polymer which c<strong>on</strong>sists of<br />
tetrafluoroethylene and perfluoro-2-(sulf<strong>on</strong>ic acid ethoxylate)<br />
Received 1 August 2011; accepted 9 November 2011<br />
* Corresp<strong>on</strong>ding author. Email: liuwh@zjut.edu.cn<br />
This work was supported by the Science and Technology Project of Zhejiang Province/Analysis Test Foundati<strong>on</strong> of China (No. 2009F70001), and Zhejiang Province<br />
Key Scientific and Technological Innovati<strong>on</strong> Team of China (No. 2011R09002-12).<br />
Copyright © 2012, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.<br />
DOI: 10.1016/S1872-2040(11)60546-X
TANG Yu-Hang et al. / Chinese Journal of Analytical Chemistry, 2012, 40(5): 675–680<br />
propylvinyl ether. Tefl<strong>on</strong> is used as framework and SO 3– M +<br />
is attached as the side chain, while M + usually is prot<strong>on</strong> or<br />
alkali metal i<strong>on</strong>, the center of acidity is single and has str<strong>on</strong>g<br />
acidity. The side chain present the surface acidity in water or<br />
organic soluti<strong>on</strong> [10] . Other research groups have designed a<br />
kind of flow cell that the noble metal is deposited <strong>on</strong> the film<br />
of Nafi<strong>on</strong> which serves as working electrode using the solid<br />
acidity character. The organic c<strong>on</strong>tents of hydroquin<strong>on</strong>e,<br />
4-butyl ferrocene, cysteine have been detected without<br />
electrolytes [11–13] .<br />
In this paper, a new flow electrochemical detector was<br />
designed using the solid acidity character of the Nafi<strong>on</strong> film to<br />
directly <strong>on</strong>line detect the flav<strong>on</strong>oids without electrolytes, the<br />
carb<strong>on</strong> pasted electrode serving as working electrode. And this<br />
method possesses sensitive dectecti<strong>on</strong> capability, simple<br />
experimental setup and reproducible results. Furthermore, it is<br />
cost-effective <strong>for</strong> <strong>on</strong>line detecting the active ingredients of<br />
herbal medicine during the separati<strong>on</strong> and purificati<strong>on</strong><br />
processi<strong>on</strong>.<br />
2 Experimental<br />
2.1 Apparatus, materials and reagents<br />
CHI660B electrochemical workstati<strong>on</strong> (Shanghai CHI<br />
Instruments Inc.), VC9808+3 1/2 figure multimeter (Shenzhen<br />
Victor Hi-tech Co., Ltd); KQ-100DB ultras<strong>on</strong>ic cleaner (Kun<br />
Shan Ultras<strong>on</strong>ic Instruments Co., Ltd), PHSJ-4A PH meter<br />
and DDS-307 c<strong>on</strong>ductivity meter (Shanghai REX instruments<br />
factory), HL-2 Peristaltic pump with fixed flow rate (Shanghai<br />
Qipuluxi instruments factory) are used in this experiment.<br />
AgCl/Ag is as reference electrode and platinum is as counter<br />
electrode (Changshu noble metal Co. Ltd.). Nafi<strong>on</strong> N-324<br />
cati<strong>on</strong> exchange film was purchased from DuP<strong>on</strong>t Company,<br />
USA, H103 macroporous absorpti<strong>on</strong> resin and graphite were<br />
purchased from Sinopharm Chemical Reagent Co., Ltd. The<br />
pureness of myricetin was 98% (Ningbo Dekang biological<br />
product Co., Ltd), and the pureness of Antioxidant of bamboo<br />
leaves was 30% (Zhejiang Anji Shengshi Biological<br />
Technology Co., Ltd). All other reagents were of analytical<br />
grade and all aqueous soluti<strong>on</strong>s were prepared with ultra pure<br />
water after deoxygenated by nitrogen.<br />
2.2 Experimental method<br />
As shown in Fig.1, the flow detecti<strong>on</strong> cell is <strong>for</strong>med by two<br />
PVC pieces (about 5.0 cm × 5.0 cm), both the reference<br />
electrode and the counter electrode are in the same<br />
compartment called counter-reference room c<strong>on</strong>taining<br />
electrolyte, which is separated by Nafi<strong>on</strong> film from the<br />
working electrode. Nafi<strong>on</strong> film needs to be dipped in H 2 SO 4<br />
soluti<strong>on</strong> (pH 2) <strong>for</strong> 8 h and washed by ultra pure water be<strong>for</strong>e<br />
using.<br />
Fig.1 Schematic view of the cell assembly<br />
W, the carb<strong>on</strong> paste working electrode; R, the reference electrode and C: the<br />
counter electrode<br />
Graphite and liquid paraffin were mixed (1 g mL –1 ) in<br />
proper proporti<strong>on</strong> to keep plasticity. The carb<strong>on</strong> paste was<br />
filled into the pore of working electrode, pressed and in close<br />
c<strong>on</strong>tact with the copper down-lead. And then the surface of<br />
electrode was grinded by filter paper until the surface of<br />
carb<strong>on</strong> paste becomes shiny. The new made electrode was<br />
dipped in anhydrous ethanol and ultras<strong>on</strong>icated <strong>for</strong> 1 min and<br />
washed by ultra pure water. All parts of instrument were fitted<br />
together by screw. 1.0 mM HCl soluti<strong>on</strong> was filled into<br />
counter-reference room, and then CV measurements were<br />
implemented by scaning the voltages over potential range<br />
from 0.2 V to 1.0 V successively until the curve was stable.<br />
Finally, the electrode was washed repeatedly with anhydrous<br />
ethanol and ultra pure water after every using. The carb<strong>on</strong><br />
paste electrode needed to be filled and treated again after <strong>on</strong>e<br />
week using.<br />
The sample liquid flowed in the inlet and out from the<br />
outlet which passed by the carb<strong>on</strong> paste electrode surface. The<br />
distance of working electrode and Nafi<strong>on</strong> film could be<br />
adjusted by the thickness of gasket. The CVs of different<br />
c<strong>on</strong>centrati<strong>on</strong> of myricetin scanned from –0.2 V to 1.2 V at a<br />
scan rate of 100 mV s –1 when the sample liquid was still. The<br />
amperometric Time-Current (t-i) curves were tested at the<br />
potential of 0.53 V with the 0.1 s sampling interval during the<br />
flow detecti<strong>on</strong>. All the electrochemical tests were per<strong>for</strong>med<br />
at the room temperature.<br />
3 Results and discussi<strong>on</strong><br />
3.1 Proporti<strong>on</strong> of liquid paraffin and graphite<br />
Liquid paraffin serves as adhesive and does not<br />
participate in c<strong>on</strong>ducting, so less in carb<strong>on</strong> paste electrode<br />
will be better. However, many organic compounds not <strong>on</strong>ly<br />
absorb <strong>on</strong> the surface of electrode, but also extracted into<br />
the electrode by liquid paraffin, which has enrichment effect,<br />
thus the proper proporti<strong>on</strong> of liquid paraffin and graphite<br />
should be selected.<br />
A series of 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4 and 1.6 mL liquid
TANG Yu-Hang et al. / Chinese Journal of Analytical Chemistry, 2012, 40(5): 675–680<br />
paraffin were added into 1.0 g graphite respectively. After<br />
mixing well, the mixture was made as working electrode. The<br />
CV was tested in 2 × 10 –5 M myricetin soluti<strong>on</strong> (pH 2.01) at a<br />
scan rate of 100 mV s –1 . The relati<strong>on</strong>ship of current of<br />
oxidati<strong>on</strong> peak and the volume of liquid paraffin was recorded<br />
at 0.43 V. The electric resistance across the two sides of<br />
electrode was recorded at the same time (seen Fig.2).<br />
The results show that the resp<strong>on</strong>se current of carb<strong>on</strong> paste<br />
electrode was str<strong>on</strong>ger when more liquid paraffin was added,<br />
because the extract effect may play a major role in enhancing<br />
current. The curvature became smaller when the volume was<br />
up to 0.8 mL, which illustrated that the electr<strong>on</strong> transfer<br />
resistance was str<strong>on</strong>ger than extract effect of liquid paraffin in<br />
electrode until the resp<strong>on</strong>se reached to the str<strong>on</strong>gest at 1.0 mL.<br />
And then, the resp<strong>on</strong>se decreased and the electric resistance<br />
increased quickly until the resp<strong>on</strong>se decreased to 0 at 1.6 mL,<br />
illustrating that the liquid paraffin became a sticky film <strong>on</strong> the<br />
surface of graphite to hinder electr<strong>on</strong> transfer. There<strong>for</strong>e, it is<br />
best to mix 1.0 g graphite with 1.0 mL liquid paraffin to <strong>for</strong>m<br />
carb<strong>on</strong> paste electrode.<br />
3.2 Direct electrochemical behavior of myricetin <strong>on</strong><br />
carb<strong>on</strong> pasted electrode<br />
The CVs were scanned in ethanol soluti<strong>on</strong> (pH 2.0)<br />
c<strong>on</strong>taining 2 × 10 –5 M myricetin from –0.2 V to 1.0 V at a<br />
scan rate of 100 mV s –1 . As shown in Fig.3, there were three<br />
oxidati<strong>on</strong> peaks at 0.41 V (peak 1), 0.69 V (peak 2), 0.85 V<br />
(peak 3) respectively in the first cycle and two reducti<strong>on</strong> peaks<br />
appeared at 0.36 V (peak 4) and –0.11 V (peak 5) in the<br />
sec<strong>on</strong>d cycle.<br />
The reducti<strong>on</strong> peak 4 and the small peak 5 appeared when<br />
the CV reverse scaned after passing by peak 1. Am<strong>on</strong>g this,<br />
peak 1 relateed to the hydroxyl oxidati<strong>on</strong> of Bring [8,9] and the<br />
peak 4 and peak 5 corresp<strong>on</strong>d to the reducti<strong>on</strong> of hydroxyl of<br />
B ring. The CV of myricetin is similar with rutin, but rutin has<br />
no peak 2 [8] . Comparing with the structure of myricetin and<br />
rutin, peak 2 may corresp<strong>on</strong>d to the third positi<strong>on</strong> of hydroxy<br />
oxidati<strong>on</strong> of C ring. According to the literatures [14–16] , the free<br />
radicals are produced during the reacti<strong>on</strong> of myricetin <strong>on</strong><br />
carb<strong>on</strong> paste electrode. The mechanism shows below:<br />
The relati<strong>on</strong>ship of different terminal potential and the ratio<br />
of peak 4 oxidati<strong>on</strong> current to peak 1 reducti<strong>on</strong> current (i op /i rp )<br />
in order to prove that the step 1 is a reversible process and the<br />
step 2 is not at the beginning potential of –0.2 V. With the<br />
increasing of terminal potential from 0.6 V to 0.8 V, the i op /i rp<br />
decreased from 0.934 to 0.694 gradually. However, when the<br />
terminal potential exceeded to 0.8 V which is the positi<strong>on</strong> of<br />
peak 3, the i op /i rp decreased to 0.368 quickly. If the terminal<br />
potential further increased, the i op /i rp decreased to 0.1.<br />
There<strong>for</strong>e, peak 3 should be the sec<strong>on</strong>d process of the third<br />
mechanism, which refers to the disproporti<strong>on</strong>ati<strong>on</strong> reacti<strong>on</strong><br />
process that the free radical obtained <strong>on</strong>e electr<strong>on</strong> transfer to<br />
the structure of p-quin<strong>on</strong>e.<br />
Volume of liquid paraffin (mL)<br />
Fig.2 Effect of added liquid paraffin <strong>on</strong> the electric resistance of<br />
working electrode and the current of main oxidati<strong>on</strong> peak<br />
3.3 Effect of prot<strong>on</strong> from Nafi<strong>on</strong> to background soluti<strong>on</strong><br />
The outside three electrodes system was used to detect the<br />
CVs of liquid sample be<strong>for</strong>e and after flowing by the detecti<strong>on</strong><br />
instrument in order to observe the effect of prot<strong>on</strong> diffusing<br />
from Nafi<strong>on</strong> to background soluti<strong>on</strong>. The result showed that<br />
there was no distinct change. The c<strong>on</strong>ductivities of the liquid<br />
sample cycled in the cell at different times were tested to<br />
further c<strong>on</strong>firmati<strong>on</strong>. The c<strong>on</strong>ductivity of water used in this<br />
experiment was 1.2 S cm –1 . 10.0 mL pure water and 80%<br />
ethanol served as solvent. The results were presented in Table 1.<br />
The results indicated that the c<strong>on</strong>ductivity increased <strong>on</strong>ly<br />
1.2 S cm –1 when the 80% ethanol soluti<strong>on</strong> circulati<strong>on</strong> flowed<br />
Fig.3 Cyclic voltammograms of 2 × 10 –5 M myricetin in pH 2.0 B-R<br />
buffer (1) the first scan, the sec<strong>on</strong>d scan, scan rate 100 mV s –1
TANG Yu-Hang et al. / Chinese Journal of Analytical Chemistry, 2012, 40(5): 675–680<br />
Table 1 C<strong>on</strong>ductivity of the solvent flowing in the cell <strong>for</strong> different<br />
time<br />
Time (min) 1 2 5 10 15 30 60<br />
Water<br />
(S cm –1 )<br />
80% Ethanol<br />
(S cm –1 )<br />
1.2 1.2 1.2 1.3 1.5 1.6 2.2<br />
1.2 1.2 1.3 1.4 1.8 2.0 2.4<br />
<strong>for</strong> 1 h. Because the detecti<strong>on</strong> time of flowing is so short that<br />
the number of hydrogen i<strong>on</strong> from electrolyte room to flow<br />
room can be ignored, because it almost has no effect to<br />
background soluti<strong>on</strong>.<br />
calculated after baseline correcti<strong>on</strong> (as shown in Table 3).<br />
The relati<strong>on</strong>ship of c<strong>on</strong>centrati<strong>on</strong> and peak area is A =<br />
2.723c + 0.4341 (r = 0.9976), where A is peak area, c is<br />
c<strong>on</strong>centrati<strong>on</strong>. And the linear range is 2.0 × 10 –5 –1.0 × 10 –3 M.<br />
3.7 <str<strong>on</strong>g>Electrochemical</str<strong>on</strong>g> <strong>on</strong>line detecti<strong>on</strong> of AOB extracted by<br />
macroporous absorpti<strong>on</strong> resin<br />
70% AOB (2.0 × 10 –5 M) was filled into flow cell directly<br />
without electrolyte and the CV was detected to select the<br />
detecti<strong>on</strong> potential. There were two weak peaks at 0.37 V and<br />
0.80 V respectively and a str<strong>on</strong>g peak appeared at 0.62 V, also<br />
3.4 CVs of myricetin under the still mobile phase<br />
0.5 mL 80% ethanol soluti<strong>on</strong> c<strong>on</strong>taining myricetin (0.4 mM)<br />
was filled into the flow room of the detecti<strong>on</strong> instrument. The<br />
CV of myricetin was tested when the mobile phase was still<br />
(solid curve, in Fig.4), which was compared with the CV<br />
obtained after adding supporting electrolyte (Fig.3). The<br />
background current increased and the shape of the peak was<br />
retained. The positi<strong>on</strong> of peaks shifted. For example, the peak<br />
1 transferred from 0.43 V to 0.53 V and the peak 5<br />
disappeared.<br />
The different distances (0.5, 1.0 and 1.5 mm) between<br />
carb<strong>on</strong> paste electrode and Nafi<strong>on</strong> film were adopted to<br />
validate the acidity of Nafi<strong>on</strong> film. The highness of peak<br />
decreased quickly with the distance increased. When the<br />
distance was 1.0 mm, there was no distinct difference between<br />
the sample and background signal (dashed line, Fig.4). It<br />
illustrates that the Nafi<strong>on</strong> film has acidity character and the<br />
diffusi<strong>on</strong> of prot<strong>on</strong> can be ignored in short time.<br />
Fig.4 CVs of myricetin without electrolyte while the mobile phase<br />
is still<br />
Full line: The distant between WE and Nafi<strong>on</strong>: 0.5 mm; Dashed line: 1 mm<br />
3.5 Effect of flow rate to amperometric time-current (t-i)<br />
curves<br />
As shown in Fig.5, the amperometric t-i curves were tested<br />
at different flow rate (1.0, 1.8 and 2.6 mL min –1 ) at 0.53 V,<br />
while the c<strong>on</strong>stant flow pump simulated the situati<strong>on</strong> of liquid<br />
chromatography.<br />
The baseline was corrected and the peak area was<br />
calculated by Origin 8.1. The results are listed in Table 2.<br />
As the decreasing of flow rate of mobile phase, the shape of<br />
peak became wider, seeming like chromatography. Because<br />
the liquid sample is in the state of flowing, the curve should be<br />
affected by peristaltic pump, and the noises and relative<br />
standard deviati<strong>on</strong> increase corresp<strong>on</strong>dingly at low flow rate.<br />
3.6 Relati<strong>on</strong>ship of c<strong>on</strong>centrati<strong>on</strong> and peak area<br />
The amperometric t-i curves of different c<strong>on</strong>centrati<strong>on</strong>s of<br />
myricetin in 0.5 mL 80% ethanol were tested at 0.53 V with<br />
the flow rate of 1.8 mL s –1 . The integral peak areas were<br />
Fig.5 Time-Current resp<strong>on</strong>se up<strong>on</strong> injecti<strong>on</strong> of 0.5 mL of 0.4 mM<br />
myricetin in 80% (V/V) ethanol-water, under the volume flow<br />
rate of 1.0, 1.8, 2.6 mL min –1 respectively<br />
Table 2 Peak area of time-current curve at different flow rate<br />
Flow rate<br />
(mL s –1 )<br />
Appearance time<br />
(s)<br />
Peak area<br />
RSD<br />
(%)<br />
2.6 65, 60, 64 3.28, 3.45, 3.32 2.6<br />
1.8 93, 89, 90 3.44, 3.20, 3.32 3.6<br />
1.0 116, 117, 113 3.18, 3.38, 3.60 6.2<br />
Table 3 Integral peak area of the different c<strong>on</strong>centrati<strong>on</strong>s of<br />
myricetin<br />
C<strong>on</strong>centrati<strong>on</strong><br />
(10 –4 M )<br />
0.2 0.4 0.8 1.0 2.0 4.0 8.0 10<br />
Peak area 0.55 1.23 2.79 3.48 5.90 11.54 23.56 26.48
TANG Yu-Hang et al. / Chinese Journal of Analytical Chemistry, 2012, 40(5): 675–680<br />
two reducti<strong>on</strong> peaks overlapped. Thus the final detecti<strong>on</strong><br />
potential should be 0.62 V.<br />
This electrochemical chromatography detector was used <strong>for</strong><br />
the detecti<strong>on</strong> of AOB during the purificati<strong>on</strong> process by<br />
macroporous absorpti<strong>on</strong> resin. The sample c<strong>on</strong>tinuously<br />
flowed into the detector by peristaltic pump at 0.62 V at the<br />
flow rate of 1 mL min –1 , and the sampling interval was 0.5 s<br />
(as shown in Fig.7). The peak appearing time, maximum time<br />
and the tendency are similar with the eluti<strong>on</strong> curve of<br />
nowadays detecti<strong>on</strong> method.<br />
that of other solid electrodes.<br />
3.8 Stability and repeatability<br />
The working electrode was dipped in anhydrous ethanol,<br />
0.1 M HCl and 0.1 M NaOH successively in <strong>on</strong>e week. The<br />
peak potential and the current of myricetin soluti<strong>on</strong> (2.0 × 10 –5<br />
M, pH 2.01) were tested <strong>for</strong> three times. The mean values are<br />
listed in Table 4. The data cannot be obtained after the<br />
electrode dipping in ethanol and NaOH <strong>for</strong> 10 d, because the<br />
surface of electrode has chapped. Combining with the results<br />
in Table 4, it shows that the peak potential, current and shape<br />
have not changed in different time, because the carb<strong>on</strong> paste<br />
electrode has good stability.<br />
However, the most stable potentials of carb<strong>on</strong> paste<br />
electrode are changed at different pH. The increasing of<br />
positive potential would make the carb<strong>on</strong> <strong>on</strong> the surface of<br />
electrode loosen and detached under the c<strong>on</strong>diti<strong>on</strong>s of pH 2.01,<br />
3.13, 4.01, 5.02, 6.30 and 7.00, so the most stable potential is<br />
1.37, 1.45, 1.50, 1.55, 1.65 and 1.78 V corresp<strong>on</strong>dingly. The<br />
positive potential range of carb<strong>on</strong> paste electrode is wider than<br />
Fig.6 CV of AOB in the cell<br />
Fig.7<br />
Time-Current resp<strong>on</strong>se of spent regenerant during the<br />
absorpti<strong>on</strong> and purificati<strong>on</strong> process of AOB<br />
Table 4 Stability of carb<strong>on</strong> paste electrode in different media<br />
1 d 2 d 4 d 8 d 10 d<br />
Pp Pc Pp Pc Pp Pc Pp Pc Pp Pc<br />
Alcohol (V, A) 0.43 6.5 0.45 6.3 0.45 5.9 0.46 5.3 - -<br />
Acid (V, A) 0.43 5.8 0.43 5.7 0.43 5.4 0.44 5.0 0.43 5.3<br />
Alkali (V, A) 0.43 5.9 0.43 5.4 0.45 5.2 0.50 4.3 - -<br />
Notice: Pp, Peak potentia ; Pc, Peak current.<br />
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