Anal. Chem. 1988, 6 0 , 1235-1237
1235
CORRESPONDENCE Suppression of the Chloride Interference Effect on Solid-state Cupric Ion Selective Electrodes by Polymer Coating Sir: One of the major limitations to the use of a solid-state cupric ion selective electrode (Cu-ISE) in practical analysis is the chloride interference effect which causes surface deterioration of the copper-sensing membrane (1-4). All types of solid-state Cu-ISE's are to some extent susceptible to chloride interference, the major symptoms being variable, nonlinear and super-Nernstian calibration slopes, slow electrode response, unstable and drifting potentials, and tarnishing of the electrode surface ( I ) . Consequently, a Cu-ISE has so far been of limited value in media containing high concentrations of chloride, including seawater, and frequent regeneration of the electrode surface is necessary. Lewenstam et al. (5) have shown that addition of thiosulfate salts has a beneficial effect on the electrode performance in the presence of chloride. However, thiosulfate interacts with the coppersensing membrane in the same manner as chloride (6) and merely acts as an interference buffer. The calibration graphs obtained are curved and must be linearized by a complicated method. Moreover, the thiosulfate ions accelerate the corrosion of the membrane, leading to copper contamination of the sample (6). This paper describes how the chloride interference effect can be remedied to a large extent by coating the Cu-ISE with a thin cation-exchange membrane, which selectively hinders the permeation of anions by Donnan exclusion. The only drawback with this approach is an increased response time of the electrode. The coating material employed is a perfluorosulfonate resin named Ndion, which is chemically inert, hydrophilic, and insoluble in water and therefore possesses almost ideal properties for the preparation of modified electrodes. The permselective properties of Nafion have previously been exploited for elimination of ascorbic acid interference in the determination of dopamine (7),and it has been shown that coating of glassy carbon electrodes with thin Ndion membranes greatly enhances their resistance to fouling in anodic stripping voltammetry (8, 9). EXPERIMENTAL SECTION Apparatus. The Cu-ISE was a Radiometer F1112 Cu Selectrode which employs a single crystal of Cul.*Se as the sensing element. The crystal (5 mm diameter) is mounted in a Teflon stem (9 mm outer diameter). The reference electrode was a Radiometer K701 double-junction, saturated calomel electrode with 2 M KN03 as the bridging solution. Potential readings were taken with a Radiometer pHM 64 pH-meter (0.1 mV resolution). The cell was thermostated to 28 2 OC with a water jacket. Solutions were stirred with a Teflon-coated magnetic bar during measurement. Reagents. Analytical grade chemicals and triply distilled water were used throughout. A 5% solution of Ndion (1100 equiv wt) was obtained from Solution Technology, Inc. (Mendenhall, PA). Copper(I1) standards in the range to 10" M were made by successive 10-fold dilutions of a lo-' M copper(I1) standard prepared by dissolution of cupric nitrate. The standards were prepared in a 1.0 M NaCl medium as well as a 2.0 M KNO, medium. All standards were buffered to pH 4.7 by addition of a lo-, M acetic acid/lO-, M sodium acetate buffer and were kept in polyethylene bottles. Copper standards with lower electrolyte concentration were prepared by dilution from the two series of standards.
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0003-2700/88/0360-1235$01.50/0
Procedure. The Cu-ISE was maintained by regular polishing with 0.25-pm diamond paste and subsequent rinsing with ethanol. Between series of measurements (e.g., calibration runs), the uncoated electrode surface was wiped with lens paper wetted with ethanol, while the coated electrode was rinsed with distilled water. Electrode coating was carried out by inverting the Cu-ISE and applying 10 p L of ethanol and 5 p L of Ndion solution which was spread out to the outer edge of the Teflon stem. When the evaporating Nafion solution had retracted to the outer edge of the copper-sensing crystal, leaving a thin layer of Nafion on the Teflon, 20 p L of Ndion solution was applied to the crystal in order to thicken the central part of the coating. The modified electrode was left to dry overnight at room temperature before use and was stored in distilled water between measurements. After introduction of sample solution, the measuring cell was closed with plastic film,and oxygen was removed by continuous argon bubbling. In the calibration experiments, measurements were done in order of increasing copper concentration. The cell and the electrodes were carefully rinsed twice with 0.02 M nitric acid and once with water before a solution with a lower copper content than the previous was introduced into the cell.
RESULTS AND DISCUSSION Figure 1illustrates the distinct improvement in performance of the Cu-ISE in chloride-containing media obtained by coating the electrode with Nafion. The coating greatly suppresses the large, negative potential shifts caused by chloride at the uncoated Cu-ISE, especially at low cupric ion concentrations. For example, addition of 0.5 M NaCl to a lom6M copper(I1) solution caused a potential shift of respectively 112 and 11mV at the uncoated and coated Cu-ISE. It should be noted that complexation of cupric ion with chloride cannot account for the potential shifts seen in Figure 1. Assuming large excess of chloride over copper and a stability constant of loo.' for the CuC1' complex (IO), addition of 0.5 M chloride should only cause a potential shift of 6 mV by this mechanism. In chloride-containing media, the emf of the uncoated Cu-ISE was very sensitive to the convective conditions in the solution. With the normal measuring conditions, the potential underwent rapid fluctuations with an amplitude of up to 2 mV. Consequently, no reliable potential reading could be obtained. The fluctuations ceased when stirring was stopped and the argon tube was placed above the solution, but the emf slowly drifted in the positive direction. In a solution containing lo* M copper(I1) in 0.5 M NaC1, a new steady-state potential 20 mV more positive than the previous was reached after 10 min. When stirring and argon bubbling were resumed, the emf quickly attained its initial value. These findings strongly indicate that a corrosion process involving mass transport takes place a t the copper-sensing membrane in the presence of chloride. Coating the Cu-ISE with Nafion eliminated the potential fluctuations in stirred, chloride-containing solutions, and stable emf readings were obtained. In the lo4 M Cu(II)/0.5 M NaCl medium mentioned above, the potential drifted 8 mV in the negative direction during 15 min when stirring and argon bubbling were stopped. At both the bare and the coated Cu-ISE, the extent of the potential drift decreased with higher copper(I1) concentrations, and the new steady-state potential was reached more quickly. The positive potential shift observed a t the uncoated Cu-ISE in quiescent 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 11, JUNE 1, 1988
1236 E/mL
i,'m
A
Table 11. Comparison of Response Times for Bare and Nafion-Coated Cu-ISE"
I
4
+2r,c-
B
response time/min Nafion-coat-
medium 0.2 M 0.2 M 0.2 M 0.2 M 0.5 M 0.5 M 0.5 M 0.5 M
KN03 KNOB KNOB KN03 NaCl
NaCl NaCl NaCl
Ccu(II,/M
bare Cu-ISE
ed Cu-ISE
lo4 10-5 10-4 10-2 104 10-5 10-4 10-2
10 4 1
