Anal. Chem. 1986, 58, 2101-2103
in defining time of the experiment impede quantitative analysis of the results. Previous attempts to emulate the conditions of polarographic techniques in applications of pulse methods a t solid electrodes involved slow continuous rotation of the working electrode (18-20) or stirring of the solution (21). These methods can be used only in forward potential scan experiments, because they cause the convection-disturbing concentration gradients during the relatively long generation step of reverse pulse experiments. On the other hand, the proposed method of restoring the initial conditions a t the working electrode surface between successive current measurements, tested in normal and reverse pulse experiments, can be used also in applications of other pulse techniques at solid electrodes. A current response during short pulses is generally less affected by the convection than a dc current in RPV.
ACKNOWLEDGMENT I acknowledge the helpful comments of Zenon Kublik and the assistance of Janusz J. Borodzinski. Registry No. DOPA, 59-92-7; catechol, 120-80-9; carbon, 7440-44-0; dopaquinone, 25520-73-4.
LITERATURE CITED (1) Osteryoung. J.; Kirowa-Eisner, E. Anal. Chem. 1080, 5 2 , 62-66. (2) Osteryoung. J.; Talmor, D.; Hermolin, J.; Klrowa-Eisner, E. J. f h y s . Chem. 1981, 8 5 , 285-289.
2101
KashtCKaplan, S.;Hermolin, J.; Kirowa-Eisner, E. J. Electrochem. SOC. 1981, 128, 802-810. Wojciechowskl, M.; Osteryoung. J. Anal. Chem. 1085, 5 7 , 927-933. Kulkarni, C. L.; Scheer, E. J.; Rustling, J. J. Electroanel. Chem. 1082, 140. 57-74. Karpinski, 2. J.; Osteryoung, R. A. J. flectroanal. Chem. 1984, 164, 28 1-298. Karpinski, 2. J.; Osteryoung, R. A. J. ffectroanal. Chem. 1084, 178, 281-294. Karpinski, 2. J.; Nanjundiah, Ch.; Osteryoung, R. A. Inorg. Chem. 1084, 2 3 , 3358-3364. Barker, G. C.; Bolzan, J. A. 2. Anal. Chem. 1066, 216, 215-238. Lovric, M. J. Electroanal. Chem. 1084, 170, 143-173. Lovric, M. J. flectroanal. Chem. 1084, 187, 35-49. Schwarz, W. M.; Shain, I , J. Phys. Chem. 1985, 6 9 , 30-40. Raper, H. S. Blochem. J. 1927, 2 1 , 89-96. Mason. H. S. J. BIOI. Chem. 1048. 172. 83-99. Brun, A.;Rossett, R. J . Electroanai. Chem. 1974, 49, 287-296. Young, T. E.; Griswold, J. R.; Hulbert, M. H. J. Org. Chem. 1974, 3 9 , 1980- 1982. Britz, D. Digital Simulation In Electrochemistry; Springer-Verlag: Berlin, Heidelberg, New York, 1981; Chapter 5. Myers, D. J.; Osteryoung, . - R. A.; Osteryoung, . - J. Anal. Chem. 1074, 46, 2089-2092. Stojek, 2.; Llnga, H.; Osteryoung, R. A. J. flectroanal. Chem. 1081, 1. 1 .-9 ,. 365-370. - - - - . -. Thornton, D. C.; Corby, K, T.; Spendel, V. A,; Jordan, J.; Robbat, A., Jr.; Rutstom, D. J.; Gross, M.; Ritzel, C. Anal. Chem. 1985, 5 7 , 150-1 55. Brumleve, T. R.; Osteryoung, R. A.; Osteryoung, J. Anal. Chem. 1982, 5 4 . 782-787.
RECEIVED for review December 23,1985. Accepted March 20, 1986. This work was supported by the MR 1-11Research Project.
Carbon Paste Coated Wire Selective Electrode for Nitrate Ion Yong-Keun Lee,* Jung-Tae Park, and Chang-Kue Kim Department of Chemistry, College of Science, Yonsei University, Seoul 120,Korea
Kyu-Ja Whang Department of Manufacturing Pharmacy, College of Pharmacy, Sookmyung Women’s University, Seoul 140,Korea An increase in the use of ion-selective electrodes as an analytical tool has been observed in recent years. The increased interests in ion-selective electrodes has led to the development of new sensor materials that show selectivity for a variety of anions and cations and new methods for the construction of electrodes from these materials. Davies, Moody, and Thomas incorporated a commercially available liquid ion exchanger in a poly(viny1 chloride) (PVC) matrix to prepare a nitrate ion selective electrode (1). Coetzee and Freiser (2) have prepared PVC-CWE (CWE, coated were electrodes) to determine various anions by mixing a liquid anion selective ion exchanger with PVC. Kneebone and Freiser coated a platinum wire with a nitrate ion selective liquid ion exchanger in a poly(methy1 metacrylate) (PMMA) and used the electrode to determine nitrogen oxides in ambient air (3). Suzuki et al. constructed nitrate-CWE utilizing epoxy resin as a supporting material (4).These CWE’s have limited relative response characteristics, selectivity, and pH range. In order to compensate for the above limitations, this study deals with the optimum condition for the construction of improved nitrate-CWE in which plasticizer was added. But the result was far from satisfactory. Ansaldi and Epstein prepared a calcium selective electrode b y coating a graphite rod with a calcium exchanger in PVC (5). Mesaric and Dahmen constructed carbon paste electrodes for halides and silver(1) ions (6). 0003-2700/86/0358-2101$01.50/0
In this paper the behavior of a carbon paste coated wire nitrate ion selective electrode (carbon nitrate-CWE), which was prepared by coating a copper wire with the nitrate ion selective electroactive paste, was described. The electroactive paste was made from graphite powder, epoxy resin, liquid ion exchanger, and plasticizer. The advantages of using the carbon nitrate-CWE instead of the commercial nitrate ion selective electrode were ease of construction, increased portability, sturdiness, and economy.
EXPERIMENTAL SECTION Apparatus. All electrode potentials and pH measurements were made with an Orion microprocessor ionanalyzer (Model 901) using an Orion double junction electrode (Model 90-02) as the reference electrode. Resistance measurements of polymer membrane were measured with a Hewlett-Packard high resistance meter (Model 4392 A). The glass cell thermostat was maintained at 25 f 0.5 OC by means of Haake constant temperature circurator (Model T 31). Materials and Reagents. Aliquat 336s (tricaprylmethylammonium chloride), triethylenetetramine (TETA), tetrahydrofuran (THF), and dioctyl phthalate (DOP) were obtained from Tokyo Kasei Kogyo Co., Ltd. Epoxy resin (Bisphenol A, Shell Co.) and amorphous graphite powder (200 mesh, Pyungtaick Mining) were also used. Unless otherwise noted, all reagents used in this study were of analytical reagent grade. Ion Exchanger. A 0.4-g portion of Aliquat 336s was dissolved in 0.6 g of 1-decanol and was shaken with the same ratio of 1.0 M KNOBto affect the exchange of NO3- for C1- in Aliquat 336s 0 1986 American Chemical Society
2102
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
Table I. Response Characteristics in the Carbon Coated Wire Nitrate Ion Selective Electrode composition of membranea (in ratio of A:B:C:D)
slopeb (mV/log C)
2:l:O:O 2:1:0.4:0 2:1:0.4:0.1 2:1:0.4:1 2:1:0.4:2 2:1:0.4:3 2:1:0.4:4 2:1:0.4:5
53.6 f 1.0 56.3 f 0.6 56.8 f 0.9 57.2 f 1.0 57.9 f 0.6 59.4 f 0.5 58.3 i 0.8 56.9 f 1.1
region of linear response, M low activity limit upper activity limit 5x 6X 6X 6X 4x 3x 3x 8x
4x 5x 5x 5x 6X 7x 6X 5x
10-5 10-j 10-5 10-5 10-5 10-5
resistivity," R cm
7.7 x 5.0 x 5.0 x 3.6 x 3.2x 2.2 x 1.9 x 9.9 x
10-2
10-1
lo-' 10-1 lo-' 10-1 10-I 10-1
*
108 107 107 107 107 107 107 107
"A, epoxy resin; B, ion exchanger; C, plasticizer; D, graphite powder. bThe means standard deviation in slopes obtained for more than 10 electrodes. eThe resistivities were measured at 20 O C and 60% relative humidity. The resistivities of the polymer membrane without ion exchanger, plasticizer, and graphite powder were 3.0 X R cm. Those of the polymer membrane in which epoxy resin and ion exchanger were incorporated in the ratio of 5:l and 1O:l were 1.8x 10'" and 6.3 X 10" R cm. resoectivelv.
solution. After each shaking, the mixed solution was centrifuged to separate the aqueous layer and organic layer. The final aqueous layer was tested for the presence of C1- with AgN03. The absence of C1- indicated complete exchange. Again the organic layer was centrifuged to remove traces of water. To prepare 0.1 M nitrate standard, 1.0110 g of potassium nitrate (E. Merck) was dissolved in distilled water. All nitrate solutions were prepared by using successive dilution of this standard solution with distilled water. Nitrate solutions over the range of l0-'-lO4 M were prepared at the time of measurement. Deionized and distilled water was used throughout. Construction of Carbon Nitrate-CWE. The epoxy resin was incorporated with a liquid ion exchanger, plasticizer, graphite powder, and TETA as hardener in THF solvent. After mixing, the solution was allowed to stand about 2-3 h in an oven at 40 "C. When an appropriate viscosity was attained, a copper wire approximately 0.5 mm in diameter and 10 mm in length was dipped in the solution several times for uniform coating, and it was allowed to stand overnight in the oven at 50 "C. The utility, composition of polymer membrane, response characteristics, and selectivity of carbon nitrate-CWE were investigated. The carbon nitrate-CWE was preconditioned in 0.1 M KNO, solution at least 1 h prior to use. The electrode was soaked in deionized water for about 20 min prior to each measurement. The electrode may be stored dry when it is not in use but should be preconditioned again prior to reuse. The electrode potential measurement was made under constant conditions by taking 20 mL of solution for each measurement in a cell thermostated at 25 f 0.5 "C, immersing the electrode to a constant depth in the solution, and stirring at a constant rate by means of a magnetic stirring bar. Electrode potential measurement was carried out from low concentration to high concentration. The carbon nitrate-CWE tip was rinsed with deionized water and then immersed in one of the standard solutions.
RESULTS A N D DISCUSSION Response Characteristics. First of all the polymer membrane of the nitrate-CWE was composed of epoxy resin incorporated with a liquid ion exchanger and plasticizer. I t was found that the best composition of the polymer membrane was obtained by mixing epoxy resin, ion exchanger, and plasticizer in the ratio of 2:1:0.4. In most cases with the nitrate-CWE, there was 56 mV change for every decade change in activity. To increase the conductivity of membrane, graphite powder was added to the membrane of the best composition (2:1:0.4). As a result, the ratio of the composition of the membrane was 2:1:0.4:3 (epoxy resin:ion exchanger:plasticizer:graphite powder), of which the best potential response was a 59.4 mV change for every decade change in activity, which is reasonably close to the 59.2 mV change expected for ideal behavior as shown in Table I. Calibration curves of the carbon nitrateCWE made with DOP as plasticizer and an Orion nitrate ion selective electrode (Model 93-07) (Orion-ISE) are shown in Figure 1. Under above composition, the linear response range was more extensive than any other conditions. The addition
5
3
1
- Log[NOj] Flgure 1. Calibration curve of carbon nitrate-CWE (0), Orlon-ISE (O), and theoretical curve ( 0 ) .
of graphite powder seems to increase a conductivity of the polymer membrane. The effect of thickness (0.1-1.0 mm) of the polymer membrane to the responses of the carbon nitrate-CWE is shown in Figure 2. It was true in this experiment that when the thickness of the electrode was over 0.25 mm, the output decreased, but the thickness was 0.15-0.25 mm and no potential change occurred. However, when the thickness became less than 0.1 mm, the resistivity became bad because the membrane was impaired. The response time of the carbon nitrate-CWE at each concentration level from 10-l to lo4 M KNOBwas between 20 and 60 s. The potential variation of the carbon nitrate-CWE was within f 0.2 mV. For reliable measurements with these electrodes, it was necessary to restandardize for each test. When the potential was measured with several identical electrodes, the identical response was obtained for up to 6 weeks. Resistivity of Polymer Membrane. The resistivities on the composition of the polymer membrane of the carbon nitrate-CWE are shown in Table I. As the amount of epoxy resin increased, the resistivity of the membrane increased. But when the amount of epoxy resin was kept constant and more
Anal. Chem. 1986, 58,2103-2105
2103
Table 11. Selectivity Coefficients Ki, for the Coated Wire Nitrate Ion Selective Electrode CWE interferants
c1-
carbon paste'
c10; CH8COOH2P04-
0.035 2.331 0.195 0.00064 0.347 1.628 0.026 0.028
useful pH range
2.5-10.3
1NO2-
so42c10,
PMMAb 0.0398 0.158 0.00079 1.82
epoxy
PVC'
Orion-ISEb
Orion-ISEc
0.097 3.1 0.29 0.00075
0.063 7.7 0.15 0.012
0.00631
0.006 20 0.06 0.0006
>10
2.3
3-8.5
2.7-10.0
0.0631 0.00063 2
1000
2.5-10.7
2-12
2-12
'Present work. bData from ref 3. 'Data from ref 4. Reference solution, 5 X lo-, M KNO,; interfering solution, 0.09 M. Electrode response in series of pure test solution, 59.4 mV/lO-fold change in concentration. nitrate-CW in which that the plasticizer and graphite powder were not incorporated. Effect of Interfering Anion. The effect of foreign ions on the response of the electrodes was studied by making potentiometric measurements of the solution of 5 X M KNOBcontaining 0.09 M interfering anion of interest. Selectivity coefficients were calculated from the following Eisenman equation:
-200
> E
-2c
-100
c
AE = (slope) log
L
0 a
c
v
?
1
+ Ki-
1
where ai and z are the activity and the charge of the interfering anion, respectively. Selectivity coefficients are given in Table
L
-Y W
I
o
11.
100
1
I 5
3
1
-Log[NO;]
Figure 2. Effect of thickness of polymer membrane on carbon nitrate-CWE: 1.0 mm (0),0.6 mm (O),0.4 mm (e), 0.2 mm (O),0.1
mm (e).
plasticizer was used,the resistivity was less. When epoxy resin and graphite powder were mixed a t ratios of 2:O.l-3, more graphite powder caused less resistivity. When the ratio of added graphite powder to epoxy resin was over 3 to 2, both the resistivity and the potential of the polymer membrane were reduced, because the surface of the polymer membrane was not uniform. As the result, the response of the carbon nitrate-CWE was considerably better compared to the epoxy
Sulfated and chloride did not interfere even a t concentrations 1500 times and 28 times that of nitrate, respectively, and the selectivity of iodide and perchlorate for the carbon nitrate-CWE was improved much more than the selectivity of the other liquid membrane nitrate ion selective electrodes for the above ions. Registry No. Biphenol A, 80-05-7; graphite, 7782-42-5;nitrate, 14797-55-8. LITERATURE CITED (1) Davies, J. E. W.; Moody, G. J.; Thomas, J . D. R. Analyst (London) 1972, 97. 87-94. (2) Coetzee, C. J.; Freiser, H. Anal. Chem. 1969, 4 1 , 1128-1130, (3) Kneebone, Barbara M.; Freiser, H. Anal. Chem. 1973, 45, 449-452. (4) Suzukl, K.; Wada, H.; Shirai, T.; Yanagisawa, S. Jpn. Anal. 1980, 29, 816-820. (5) Ansaidi. A.; Epstein, S. I. Anal. Chern. 1973, 4 5 , 595-596. (6) Mesaric S.; Dahmen, E. A. M. F. Anal. Cbim. Acta 1973, 6 4 , 431-438.
RECEIVED for review October 1,1985. Accepted April 10,1986. This work was supported by the Korea Research Foundation under grant in 1984.
Laser Two-Photon Excited FluoreScence Detector for Microbore Liquid Chromatography William D. Pfeffer and Edward S. Yeung*
Ames Laboratory-USDOE and Department of Chemistry, Iowa State University, Arnes, Iowa 50011 The fluorometric detector in liquid chromatography (LC)
has the special features of high selectivity and high sensitivity compared to the absorption detector and the refractive index detector. With the addition of a laser as the excitation source, impressive results have been reported (1-4). In particular,
the laser allowed better stray light reduction (due to better beam quality), and better focusing into small volumes (due to better spatial coherence). Coupling with microbore LC, where detector volumes less than 1 pL are required, is then relatively straightforward, and mass detedability is enhanced.
0003-2700/86/0358-2 103$01.50/0 0 1986 American Chemical Society
