crown ether film electrode and its application in the anodic stripping voltammetric determination of traces of silver

Analytica Chimica Acta, 212 (1988) 341-347 Elsevier Science Publishers B.V., Amsterdam -

341 Printed in The Netherlands

Short Communication

A NAFION/CROWN ETHER FILM ELECTRODE AND ITS APPLICATION IN THE ANODIC STRIPPING VOLTAMMETRIC DETERMINATION OF TRACES OF SILVER

SHAOJUN DONG* and YUDONG WANG Changchun Znstctute of Applied Chemistry, Academia Smica, Changchun, Jilin 130021 (China) (Received 12th January 1988)

Summary. Glassy carbon electrodes are modified by coating with dicyclohexyl-l&crown-6 in Nafion-117. The electrode is used for a very sensitive anodic stripping voltammetric determination of silver. High sensitivity is obtained owing to the release of crown molecules from the silvercrown complex during the deposition. The detection limit is 2 X lo-” M after electrodeposition for 30 min. The recommended supporting electrolyte is 4 x 10m3-7 X low3 M potassium chloride in 0.01 M nitric acid with a deposition potential of -0.30 V vs. SCE and a linear potential scan. Three typical calibration graphs were linear over the range 2 X 10-“-l x 10-s M for deposition times of 30, 20 and 8 min, respectively. The silver content of reagent-grade ammonium nitrate was found to be 0.48~ 10e4% with a relative standard deviation of 3.7% (n=7) for parallel determinations.

Chemically modified electrodes have received considerable attention in recent years [l-3]. Their combination with sensitive techniques such as cyclic voltammetry [ 4-71, pulse voltammetry [ 8,9], flow-injection [lo-131 and stripping voltammetry [ 9,14,15] offers analytical methods with improved sensitivity and selectivity. One example is the determination of trace silver with a carbon-paste CME [4]. Guadalupe and Abrufia [5] suggested that a film consisting of a redox centre and a metal coordination site could be used for analysis and for indicating saturation of the electrodes. Price and Baldwin [ 81 reported on the use of a platinum electrode with adsorbed aromatic amines for the determination of ferrocene carboxaldehyde. Studies on the determinations of copper [ 141, potassium [ 61, sulfhydryl compounds [lo] and trace oxygen [ 71 have also been described. Modification of a glassy carbon electrode with a Nation polymer film incorporating a crown ether [ (DClX!G/Nafion)/GC;DCUJC6 is the abbreviation of dicyclohexyl-18crown-61 and its application to the anodic stripping voltammetric determination of trace thallium have been reported [ 151. In the present communication, it is shown that trace silver can be determined by anodic stripping voltammetry with the (DClXJG/Nafion)/GC electrode. A detection limit of 2 x lo-l2 M can be reached after electrodeposition at - 0.30 V vs. SCE for 30 min.

0003-2670/88/$03.50

0 1988 Elsevier Science Publishers B.V.

342

Experimental Apparatus. As BAS Model CV-47 electrochemical system (Bioanalytical Systems, U.S.A.) was used with a Model ATA-1A rotating disk electrode (Jiangsu Electroanalysis Instruments Manufacturer, China). Data were recorded on a Model LZ3-204 X-Y recorder (Dahua Meters Manufacturer, China). The three-electrode configuration used consisted of a saturated calome1 electrode (SCE) with double salt bridges as reference, a platinum wire auxiliary electrode and the CME as working electrode. All electrode potentials were measured and reported with respect to the SCE. Reagents. Dicyclohexyl-l&crown-6 (purum) was obtained from Fluka. A 5% (w/w) Nafion-117 solution in lower aliphatic alcohols and 10% water (Aldrich Chemical Co.) was diluted to 1% (w/w) with ethanol. Other reagents were of analytical grade or GR grade, made in China, and were used as received. All solutions were prepared with doubly distilled water. Modification procedure. A portion (5 ~1) of 0.1 M DC18C6 in acetonitrile was added to 500 ~1of the diluted solution of Nafion to get the DCHCG/Nafion modifier solution. Glassy carbon disk electrodes (diameter 3 mm or 4 mm) were polished with magnesium oxide to a mirror-like finish and rinsed with doubly distilled water; then the electrodes were cleaned by several ultrasonic treatments in a water bath. The clean electrodes obtained were dried and 4 or 7 ~1 of the DC18CG/Nafion solution (for an electrode diameter of 3 or 4 mm, respectively) was placed on the electrode surface from a microsyringe. The electrodes were then allowed to dry in a dust-free area for about an hour. Analytical procedure. The electrode was first immersed in the supporting electrolyte solution, and background voltammograms were recorded to test its performance. Then the electrode was placed in a cell containing the silver (I) solution (2 x 1O-‘1-1 x lo-’ M) in 0.01 M nitric acid/4x 10m3-7 x 10e3 M KCl. A deposition potential of - 0.30 V was applied to the electrode for the selected time, while the solution was stirred by the rotation of the electrode at 1100 rpm. The rotation was stopped, and after 30 s the voltammogram was recorded by applying a positive-going potential scan to + 0.5 V at a scan rate of 100 mV S -.‘. After this, the potential was switched to +0.80 V for 2 min while the electrode rotated, the system was then ready for the next deposition/stripping cycle. The performance of the (DC18CG/Nafion) /GC electrode was found to deteriorate with use; after 60 determinations, the sensitivity of the electrode was so low that it was no better than bare GC or Nafion/GC electrode. Then the DC18CG/Nafion film was removed by wiping the electrode surface with a soft tissue soaked with ethanol and the modification procedure was repeated. Results and discussion Previously [ 151, it was found that the anodic stripping peak current increased with increasing concentration of DC18C6 in the DC18CG/Nafion film

343

on the electrode surface. But when the concentration of DC18C6 was too high, the film often split, so that the best concentration of DC18C6 in the film was selected at 0.5 mol 1-l. (The concentration of DC18C6 in the film was estimated by using the density of dry commercial Nafion-117 membrane, about 2 g cme3. ) Effects of supporting electrolyte and of deposition potential. Five nitrates [ LiN03, NaN03, KNOB, NH,NO, and Mg( NO,),] and nitric acid were tested as the supporting electrolyte by monitoring the anodic stripping peak current of silver at the (DC18CG/Nafion) /GC electrode. The results showed that ammonium nitrate and nitric acid were better than the others for the silver determination. To avoid the interference of traces of silver in the reagents, nitric acid was selected because it was easily purified. When the deposition potential was varied from -0.10 V to -0.60 V, the anodic stripping peak current of silver increased as shown in Fig. 1. The current increased more rapidly in the

-1.60

f 1.40 ;I : : * 1.20 s

/

0

I

1 i% 1.00

0.80

,/. -C 1.10 -0.20

-0.30

.

,

-0.40

-0.50

E,V vs. SCE

-0.60

.50

0.30

0.10

-0.10

-0.30

E,V vs. SCE

Fig. 1. Effect of varying the deposition potential on the anodic stripping peak current of silver (2 X lo-sM Ag+ ) in 0.1 M HNOs. Deposition time, 3 min; scan rate, 100 mV s-l. Fig. 2. Effect of chloride adding to the supporting electrolyte solution on the anodic stripping peak current for 3 x 1Om7M Ag+ in 0.01 M HNOs: ( a ) no chloride; (b) 7 X 10e3 M KCI. Deposition potential, -0.30 V vs. SCE; deposition time, 1.5 min; scan rate, 100 mV s-l.

344

potential range -0.10 to -0.30 V. Thus the deposition potential was controlled at - 0.30 V for determination of silver. Effect of chloride. The anodic stripping peak current of silver on the (DCl%G/Nafion) /GC electrode was found to be greatly increased by adding potassium chloride to the nitric acid electrolyte. Figure 2 shows the anodic stripping voltammograms of 3 x 10e7 M Ag+ with and without chloride. The anodic stripping peak is higher and sharper after the addition. The reason may be that, in the presence of chloride, the number of silver ions accumulated near the electrode surface during anodic stripping can be decreased rapidly, resulting in enhancement of the concentration gradient at the electrode/solution interface. Thus the anodic stripping peak current was greatly increased by increasing the stripping rate of Ag+ ions. The use of potassium bromide or iodide instead of chloride decreased the anodic stripping current for silver. Table 1 shows the effect of the chloride and nitric acid concentrations on the stripping peak for silver. The highest peaks are obtained with 0.005 M HN03 and 7~ 10e3 M KCI. The better results obtained with low concentrations of nitric acid may be due to minimization of the loss of DC18C6 from the film at low acidities. When the solution contained a small amount of nitric acid but a large amount of chloride, the reproducibility for silver at the (DC18CG/Nafion)/GC electrode was very poor. The recommended conditions, therefore, are 0.01 M nitric acid and 4 x 10m3-7 x 10m3 M potassium chloride, depending on silver concentration. Effects of other variables. The stripping peak current for silver increased linearly with the deposition time in the range O-30 min. A linear relationship TABLE 1 Effect of supporting electrolyte and chloride concentration”

cKC,

Anodic stripping peak current (PA)

(MI 0 1 x 10-4

3x10-4 7x10-4 1x10-3 2x10-3 4x10-3 7x10-3 1x10-2 2x10-2 4x 10-z

0.005 M HNO,

0.01 M HNO,

0.02 M HNO,

0.05 M HN03

1.30 1.53 1.85 2.45 3.75 6.93 9.30 10.40 10.20 9.30 6.30

1.35 1.45 1.90 2.70 3.70 5.72 7.60 9.25 9.80 9.55 7.00

1.30

1.45 1.35

2.20 2.60 5.30 6.00 5.85 4.90

2.00 2.60 3.20 3.95 3.70 2.92

“Concentration of silver, 3 x 10e7 M; deposition potential, -0.30 V vs. SCE; deposition time, 1.5 min; scan rate, 100 mV s-l.

345

between peak current and the square root of the scan rate was observed in the range 36-256 mV s-l. The relationship between the peak current and the square root of the rotation speed was not linear, illustrating that the deposition of silver is not a diffusion-controlled process. For quantitative work, the potential scan rate was held at 100 mV s-l and the electrode rotation speed at 1100 rpm; the peak currents were then high and stable. Interferences. There was no interference from Pb(II), Sb(III), Bi(II1) or Se (IV). A lo-fold amount of copper (II) increased the stripping peak of silver but the interference was not very serious (510% ). The presence of &fold Hg (II) or Au (III) caused severe interference. The presence of large amounts of metal ions which can form complex cations easily with DC18C6, such as K+, Na+, decreased the peak current for silver at the (DC18CG/Nafion)/GC electrode slightly. Cyclic voltummetry. Figure 3 shows the cyclic voltammograms for 1 x 10T4 M Ag+ at the (DC18CG/Nafion)/GC electrode, Nafion/GC electrode and bare GC electrode. The electrodes were immersed and rotated in the solution for 1 min before the potential scan started. With the (DC18CG/Nafion)/GC electrode (curve a), both the anodic and the cathodic peaks are higher than those at the other electrodes. The anodic peak potentials of silver at the (DC18C6/ Nafion) /GC and Nafion/GC electrodes shift in the negative direction slightly. The cathodic peak current with Nafion/GC is higher than that on the bare GC, but there is no notable difference in the anodic peak currents. These results shows that incorporation of DC18C6 in the Nafion film gives the electrode a strong affinity for silver cation. This is because the DC18C6 on the electrode surface can form complex cations with the silver ions in solution, and the resulting large complex cation is strongly attracted by the sulfonate groups of the Nafion film. At the deposition potential, silver metal is deposited on the electrode surface, releasing the DC18C6 molecules for further preconcentration of silver ions from solution. There is no problem of saturation effect [5] because of this releasing of DC18C6 and the efficiency of preconcentration is increased as is the sensitivity of the determination of silver. Sensitivity and limit of detection for silver. Under the recommended conditions, the detection limit was 2x lo-l2 M (0.2 ng 1-l) after preconcentration for 30 min. Figure 4 shows the voltammogram at the detection limit. Three good linear relationships were obtained between the stripping peak current and concentration of silver in the range 2~ 10-“-l x lOma M (correlation coefficients > 0.999) for deposition times of 30,20 and 8 min, respectively, and the calibration graphs passed through the origin. Stability of the modified electrode. To study the electrode stability, stripping voltammograms were recorded in solutions containing 4 x 10e8 and 3 x 10e7 M silver. The repeatability of the peak currents was examined with the same (DC18CG/Nafion)/GC electrode in the same solution. For 3~ 10m7 M Ag+ with a deposition time of 1 min at - 0.30 V in 0.01 M HNOJ6 x 10e3 M KCl,

346

0.60

0.40

0.20

0

-0.20

0.30

0.10

-0.10

-0.30

E,V VS. SCE E,V VS. SCE

Fig. 3. Cyclic voltammograms for silver on different electrodes: (a) (DClWG/Nafion)/GC; bare GC; (c) Nafion/GC. (1X 10e4 M Ag+ in 0.01 M HNO,; scan rate, 100 mV s-l.)

(b)

Fig. 4. Detection limit curve for the anodic stripping voltammetric determination of silver with the (DC18CG/Nafion)/GC electrode: (a) blank solution; (b) after adding 2~ lo-” M Ag+. Solution contains 0.01 M HNOs and 7 x low3 M KC1;deposition potential, -0.30 V vs. SCE; deposition time, 30 min; scan rate, 100 mV s-r.

the average peak current was 4.67 PA with a relative standard deviation of 3.3% (n= 7). For 4 x lOmEM Ag+ with a deposition time of 3 min at - 0.30 V in 0.11M HNOB, the, average peak current was 1.50 fi with a relative standard deviation of 0.9% (n = 7). Though the repeatability is worse in the presence of chloride, it is quite satisfactory for the determination of ultratrace silver. The silver contained in analytical-grade ammonium nitrate reagent (Xincheng Chemical Co., China, production number 721210) was determined by single standard addition method with the (DClSCG/Nafion) /GC electrode. A solution containing 5 x 10e3 M ammonium nitrate, 0.01 M HNOB and 6 x 10m3

341

M KC1 was prepared as the sample solution and 1.00 x 10e5 M Ag+ was used for standard additions. The relative standard deviation for parallel determinations of seven samples was 3.7% and the amount of silver found in the ammonium nitrate was 0.000048%. This project was supported by the National The aid is gratefully acknowledged.

Sciences Foundation

of China.

REFERENCES 1 2 3 4 5 6 I 8 9 10 11 12 13 14 15 16

R.W. Murray, in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, M. Dekker, New York, 1983, pp. 191-368. R.W. Murray, A.G. Ewing and R.A. Durst, Anal. Chem., 59 (1987) 379A. S. Dong, Fenxi Huaxue, 13 (1985) 870. G.T. Cheek and R.F. Nelson, Anal. Lett., 11 (1978) 393. A.R. Guadalupe and H.D. Abruiia, Anal. Chem., 57 (1985) 142. J.A. Cox and B.K. Das, Anal. Chem., 57 (1985) 2739. L. Xu and S. Dong, Fenxi Huaxue, 16 (1988) 511. J.F. Price and R.P. Baldwin, Anal. Chem., 52 (1980) 1940. J. Wang and L.D. Hutchins-Kumar, Anal. Chem., 58 (1985) 402. M.K. Halbert and R.P. Baldwin, Anal. Chem., 57 (1985) 591. M.K. Halbert and R.P. Baldwin, Anal. Chim. Acta, 187 (1986) 89. J. Wang and L.D. Hutchins-Kumar, Anal. Chem., 57 (1985) 1536. J.A. Polta and D.C. Johnson, Anal. Chem., 57 (1985) 1373. T. Miwa, L. Jin and A. Mizuike, Anal. Chim. Acta, 160 (1984) 135. Y. Wang and S. Dong, Fenxi Huaxue, 16 (1988) 216. CR. Martin, T.A. Rhoades and J.A. Ferguson, Anal. Chem., 54 (1982) 1639.