SLM을 이용한 Cr(VI) 선택성 이온전극

최영우, 문승현

광주과학기술원 환경공학과 및 환경모니터링 신기술 연구센터, 광주광역시 북구 오룡동 1

ISE for determination of Cr(VI) by using SLM

Young-Woo Choi, Seung-Hyeon Moon

Department of Env. Sci. & Eng. and Advanced Environmental Monitoring Research Center

Kwang-ju Institute of Science & Technology, 1 Oryong-dong, Puk-gu, Kwang-ju, Korea

e-mail : shmoon@kjist.ac.kr

Introduction

Ion selective electrodes (ISEs) for determination of Cr(Ⅵ) have been studied using tetrakis(thiocyanato) chromate[1], diphenylguanidine[2] as electroactive substances.   However, those electrodes have some difficulties, such as inadequate selectivity, a short life-time, poor reproducibility of preparation, potential drift, and a long response time. In this study, a liquid membrane was employed as a sensor for specific metal ions. Supported liquid membrane (SLM) methods have advantages in terms of ease of the membrane preparation and controllability of the membrane selectivity.

The objective of this work is to develop a Cr(Ⅵ) selective electrode using SLMs for continuous environmental monitoring.

Theory of ISE

Most theoretical descriptions of the emf response of liquid membrane electrodes, including the Nicolskii-Eisenman equation, assume that Nernstian slopes are obtained for individual ions to be measured. However, it is known that inconsistent results are obtained for ions of different charge valency, while the equation gives reasonable prediction for ions with the same charge valency. Recently, a new model has been established to overcome the previous problem by determining Nernstian slopes experimentally[3]. Equation (1) is the modified emf expression for an activity region with low interference,

where E and E0 are the measured emf for the mixed ions and the standard potential, respectively, K1i is the selectivity coefficient, z1 and zi are the charges, a1 and ai are the activities of the primary and interference ions, respectively. R, T and F are the ideal gas constant, Kelvin temperature and the Faraday constant, respectively.

Additional assumptions were applied in this study; E0 has the constant value of 0.272 V. Since the concentration range of the primary and interfering ions are below 1E-3 M, the activity coefficients are asuumed to be unity and the predominant species of Cr(VI) in the test solution is HCrO4- (> 96%) at pH 4 ∼ 5.5 [4].

Experimental

The poly (vinyl chloride) based supported liquid membranes (SLMs) used in this study were prepared by modification of a previous study [5]. Trioctylphophine oxide (TOPO) was used as a carrier for the selective detection of Cr(Ⅵ) and o-Nitrophenyl octyl ether (NPOE) or kerosene were also used as plasticizers in order to provide mobility of the carrier. Cyclohexanone was used as a solvent for mixing the membrane materials.

Electromotive forces for prepared ISE membranes were measured by the following procedure. Prepared SLMs were cut into 1×1 cm2 and placed in a cell apparatus with 2 chambers, one filled with a standard solution and the other with a test solution. Each chamber was stirred at 600 rpm. Compositions of the standard solution were 10-5 to 10-3 M K2Cr2O7 and 2×10-5 to 2×10-3 M KCl M to examine the electromotive force as a function of the ion concentration. A chlorinated silver wire was the used as an electrode. Concentrations of the test solution were also varied from 0.1 to 10-8 M K2Cr2O7, while a Ag/AgCl electrode immersed in a 3.3 M KCl solution served as a reference electrode.

To observe the effects of interfering ions on the transport of the primary ion, K2SO4, KNO3, KNO2 and KCl were selected as interfering substances and their concentrations were fixed at 1E-4 M to examine the selectivity coefficient in a fixed interference method.[7] The electromotive forces were measured with an HP multi-meter (34401A) within ±0.1 mV during the detection time of 30 sec. Chemically pure reagents were used in this work. The following electrochemical cell was used to characterize the electrode.

Ag, AgCl	KClsat	Test solution	SLM	Inner filling KCl, K2Cr2O7 solution	Ag, AgCl

Results and Discussion

A stable response was obtained within 10 sec. and the potential drift was no greater than ±2 mV during the detection time.

Figure 1 shows the effects of TOPO and plasticizers.  Cr(Ⅵ) was not detected with the PVC membrane in the absence of TOPO. For the SLM containing TOPO without plasticizer, detection of Cr(Ⅵ) was not feasible. It is considered that the PVC polymer without a plasticizer did not form porous structure through which Cr(Ⅵ) are permeable. Kerosene and NPOE were tested as plasticizer in this study. The SLM with NPOE gave a greater emf value than the SLM with kerosene. In the following experiments, NPOE was used as plasticizer for the PVC membrane.

Figure 2 presents emf values for five ISE membranes with various TOPO and PVC concentrations as listed in Table 1. The responses of the ISE membranes gave Nernstian slopes in the wide concentration ranges, i.e., 1×10-3 to 1×10-6 M for the SLM No. 2, 1×10-3 to 5×10-5 M for No. 3, 1×10-3 to 1×10-4 M for No. 1. The responses of the other membranes (No. 4 and No. 5) were not acceptable for application as a Cr (Ⅵ) selective electrode due to very low sensitivity in the entire concentration range investigated in this study.

Table 1. SLM compositions used with NPOE 1 ml as a plasticizer

SLM No.	1	2	3	4	5
PVC(gr)	0.5	0.5	0.5	1	1
TOPO(gr)	0.096	0.193	0.289	0.193	0.289
Cyclohexanone(ml)	10	10	10	10	10
Thickness(mm)	0.166	0.176	0.168	0.39	0.392

It is known that Cr(Ⅵ) concentration ranges in industrial wastewater, such as plating wastewater, are between 1×10-4 and 1×10-6 M. In this regard, the SLM No. 2 was found to be the most suitable electrode material for determination of Cr(Ⅵ), of the five SLM prepared in this study.

The fraction of chromic species in a solution depends on pH. Equilibrium concentrations of HCrO4-, CrO42-, HCr2O7-, and Cr2O72- in terms of pH values were calculated by F. Brito, et al.[4, 6]. Since the pH value varied from 4.7 to 5.8 in absence of interfering ions, HCrO4- is the predominant species in the solutions tested in this study.

The slope of the electrode response depends on the predominant and potential determining species including the standard solutions. For the SLM No. 2, the slope of the response remained nearly unchanged, the average being 19.6 mV, even though the concentration of the standard solution varied from 10-3 to 10-5 M.  Based on E0 and Nernstian slopes calculated from the results, the detection limit for the best ISE membrane was 1×10-6 M Cr(Ⅵ) and the experimental results agree with the theoretical values calculated by the modified Nicolskii-Eisenman equation for the Cr(Ⅵ) concentrations between 10-3 and 10-6 M in the test solution as shown in Figure 3.

It is also important to investigate the effects of interfering ions on the detection limit and the selectivity of the primary ion in a test solution. Figure 4 shows the emf of the test solution containing interfering ions. The selectivity coefficients K were calculated for the No. 2 ISE membrane using the fixed interference method.  Table 2 summarizes the results for K and the detection limits in the presence of the interfering ions. All the tested interfering ions had K values of far below unity. Therefore, the ISE membrane was found to have a high selectivity to determine Cr(Ⅵ) in the mixed solutions.

Table 2. Detection limits and selectivity coefficients for interference ions

interference ion	theoretical K	experimental K	detection limit for Cr
SO42-	1.26E-3	1.00E-3	1E-5 M
NO3-	1.07E-1	1.23E-1	1.22E-5 M
NO2-	4.36E-2	4.57E-2	4.55E-6 M
Cl-	2.88E-1	4.60E-1	4.59E-5 M

Conclusion

An optimized ISE membrane by PVC based SLM was prepared using a specific composition of SLM (0.193 gr TOPO/1 ml NPOE/0.5 gr PVC) for detection of Cr(VI) in test solutions. The detection range of Cr(VI) was from 10-3 to 10-6 M and  the empirically modified Nicolskii-Eisenman equation was used for theoretical calculation. Selectivity coefficients and detection limits of Cr(VI) in the presence of interfering ions were determined experimentally and theoretically using the fixed interference method.

The developd SLM is to be used for preparation of ISE and monitoring of Cr(Ⅵ) in industrial wastewater.

References

1. Y. Masuda, E. Ishida, K. Hiraga, Nippon Kasaku Kaishi, No. 19, p. 1453, 1980

2. Kh. M. Yakubov, I. Ya. Kalontarov, et al., USSR Inventor's Certificate No. 1012118, Byull. Isobret., No. 14, 1983

3. E. Bakker, Electroanalysis, Vol. 9, No. 1, p. 8, 1997

4. R. K. Tandon, P. T. Crisp, J. Ellis, and R. Bakker, Talanta 31, 227, 1984

5. Y. W. Choi and S. H. Moon, et al., J. of KARP, Vol. 24, No. 1, p. 11, 1999

6. F. Brito, J. Ascanio, et al., Polyhedron, Vol. 16, No. 21, pp. 3835-3846, 1997

7. G. E. Baiulescu and V. V. Cosofret, Applications of ion-selective membrane electrodes in organic analysis, Ellis horwood Ltd., pp. 16-19, 1977