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Flow potentiometric and constant-current stripping analysis for silver(I) with carbon- and platinum-fibre electrodes
Analytica Chimica Acta, 207 (1988) 27-35 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
27
FLOW P O T E N T I O M E T R I C A N D C O N S T A N T - C U R R E N T S T R I P P I N G A N A L Y S I S FOR SILVER(I) WITH C A R B O N - A N D PLATINUM-FIBRE ELECTRODES
HUANG HUILIANGa, DANIEL JAGNER* and LARS RENMAN Department of Technical Analytical Chemistry, Chemical Center, University of Lund, P.O. Box 124, S-221 O0 Lund (Sweden)
(Received 15th October 1987)
SUMMARY At concentrations above 50 ]~g 1-1, silver(I) is determined in nitric acid medium by means of potentiostatic deposition onto a platinum-fibre electrode and subsequent constant-current stripping in the sample or potentiometric stripping in a potassium permanganate medium. Interference from copper (II) is reduced by a pulsed potential procedure whereby copper deposited onto the fibre electrode is reoxidized intermittently. At concentrations below 50 #g l-1, silver (I) is determined by using a mercury-coated carbon-fibre electrode and constant-current stripping in acetonitrile containing 0.20 M perchloric acid. Potentiostatic deposition for 30 min yielded a detection limit of 0.24 zg l- 1silver (I) at the 3a level. T h e r e are few r e p o r t s on t h e e l e c t r o a n a l y t i c a l d e t e r m i n a t i o n of silver (I) in the sub-l~M c o n c e n t r a t i o n range [ 1 - 4 ] . T h e m a i n r e a s o n for this is t h a t , in this c o n c e n t r a t i o n range, silver (I) has to be d e t e r m i n e d b y s t r i p p i n g techniques in which t h e p r e c o n c e n t r a t i o n step is e i t h e r d e p o s i t i o n of silver on a solid electrode or d e p o s i t i o n of silver a m a l g a m on a m e r c u r y drop or m e r c u r y film electrode. B e c a u s e of the slow rate o f t h e initial n u c l e a t i o n process, it is difficult to deposit silver r e p r o d u c i b l y o n t o a solid electrode. C o n s e q u e n t l y , silver is usually p r e c o n c e n t r a t e d b y m e a n s of s i m u l t a n e o u s r e d u c t i o n a n d a m a l g a m a t i o n , t h u s exploiting t h e high solubility of silver in mercury. Owing to the similarity in e l e c t r o c h e m i c a l p r o p e r t i e s of t h e two elements, however, it is difficult to s e p a r a t e t h e s t r i p p i n g p e a k of silver f r o m t h a t of m e r c u r y . E a r l y work b y K o l t h o f f a n d Coetzee [ 1 ] s h o w e d t h a t p e a k s e p a r a t i o n was n o t possible in aqueous solution, despite t h e use of various c o m p l e x i n g agents. Separ a t i o n was f o u n d to occur in acetonitrile, which was later exploited b y Glodowski a n d K u b l i k [4 ] for t h e d e t e r m i n a t i o n of t r a c e s o f silver in liquid m e r c u r y . T h e e l e c t r o c h e m i s t r y of silver (I) a n d m e r c u r y (II) in a c e t o n i t r i l e has b e e n studied e x t e n s i v e l y by H u b m a n n et al. [2 ]. In a b a t c h p r o c e d u r e , H u b m a n n et al. were aPermanent address: Scientific Instrumentation Department, Xiamen (Amoy) University, P.R. China.
0003-2670/88/$03.50
© 1988 Elsevier Science Publishers B.V.
28 able to detect silver (I) at a concentration of 50/~g l-1 in aqueous solution by means of anodic stripping voltammetry after dilution of the sample with acetonitrile. In order to obtain satisfactory separation between silver and mercury on the hanging mercury drop electrode, it was found that the water content in the acetonitrile solution should not exceed 10%, i.e., the aqueous sample must be diluted at least ten times with acetonitrile. Because the presence of acetonitrile is necessary only in the stripping step and not in the potentiostatic deposition step, sample dilution might be circumvented by using a flow system which permits medium exchange between the sample electrolysis and stripping steps [5 ]. This is investigated in the present study. The possibility of using potentiometric and constant-current stripping modes as alternatives to differential-pulse anodic stripping voltammetry, thereby facilitating a high degree of automation [5], is also examined. Here the optimum conditions for the flow determination of micromolar concentrations of silver (I) by using platinum- and carbon-fibre electrodes are reported. EXPERIMENTAL The computerized flow potentiometric and constant-current stripping analyzer used has been described elsewhere [5]. In this analyzer, six different solutions can be sucked, in random choice order, into the flow cell; all the flow and instrumental parameters are under computer control. During stripping, the potential vs. time transient is recorded with a real-time sampling rate of 25.6 kHz. Differentiation, digital smoothing with an averaging filter of 30 mV and a Savitzky-Golay filter of 15 mV (potential resolution 1 mV), location and integration of stripping peaks, and display of results are all done by the computer [5 ]. In this study, the flow system was modified in that the original perspex common entry point for solutions in the flow analyzer was replaced by a common entry point made of teflon, so as to permit the use of organic solvents. Carbon or platinum fibres with diameters of ca. 10 ~m were inserted, perpendicularly to the flow direction, in Viton tubes with an inner diameter of 0.8 mm, as described elsewhere [6]. A silver tube lined with silver chloride was connected downstream from the fibre electrode by means of a T-junction, as shown in Fig. 1. The position of the tubular platinum counter electrode is also shown in Fig. 1. All potentials given below are vs. the Ag/AgC1 in 1 M hydrochloric acid in 30% ethanol. All reagents were of analytical grade except the mineral acids which were of Suprapur (Merck) grade. All dilutions were made with Millipore-Q water. DETERMINATION OF SILVER(I) ON SOLID ELECTRODES In the stripping determination of silver (I) neither the sample nor the stripping medium should contain species capable of forming solid phases with sil-
29
Platinum tube counter electrode
Fibre working electrode
Suction
1 ml rain -1 Solutions
AgCl-lined silver tube reference electrode
entry point
1 M HCI in 30 % ethanol 0.05 ml min "1
Fig. 1. Schematic drawing of the flow electrode system.
a
b
c
d
V-1
.
0.1
.0
0:6
0:2
0:6
0:2
0:6
0:2
0:S
0:2
V vS Ag/AgCI
Fig. 2. Comparison between potentiometric and constant-current stripping analysis with a platinum-fibre electrode. Electrolysis for 4 min at - 0.60 V vs. Ag/AgC1 in 0.20 M nitric acid containing 30/~g l-~ silver (I). (a) Potentiometric stripping in the same solution after the addition of 0.30 mM potassium permanganate; (b) constant-current ( 1 ttA) stripping in the sample solution; (c,d) as for (a) and (b) except that the sample contained 90/lg 1-1 silver(I).
ver (I). For this reason, 0.20 M nitric acid was used both as sample and stripping medium in the studies below.
Comparison between potentiometric and constant-current stripping analysis and between platinum- and carbon-fibre electrodes Potentiometric and constant-current stripping analysis were compared for a platinum fibre electrode by means of electrolysis for 4 min at - 0 . 6 0 V vs. Ag/AgC1 in 0.20 M nitric acid containing 30 ~g 1-1 silver (I); potentiometric stripping was then conducted in 0.20 M nitric acid containing 0.30 mM potassium permanganate, or constant-current stripping in 0.20 M nitric acid with a
30
a
b
c V-I
.0.1
0.'6
~
o'.6
o:2
-o:2
V vs Ag/AgCI
Fig. 3. Constant-current (1 #A) stripping analysis for silver (I): (a,b) at a carbon-fibre electrode not coated with mercury under the experimental conditions used for Fig. 2 (b) and (d), respectively; (c) at a platinum-fibre electrode with electrolysis for 2 min at -0.50 V vs. Ag/AgC1 in 0.20 M nitric acid containing 100 #g 1-1 silver(I) and subsequent stripping in 0.50 M hydrochloric acid.
current of 1.0/IA. The differentiated and background-corrected potentiometric and constant-current stripping curves are shown in Fig. 2 (a,b). The experiments were repeated in a solution containing 90 ~g 1-1 silver (Fig. 2 c,d). From Fig. 2, it can be seen that the stripping peak potential is independent of the mode of oxidation and that the signal-to-background ratio is similar for potentiometric and constant-current stripping. Plantinum- and carbon-fibre electrodes were compared under the experimental conditions used for Fig. 2 (b) and (d). The background-corrected differentiated stripping curves obtained with the carbon fibre are shown in Fig. 3(a) and (b). As can be seen from comparison of Fig. 3(a) and 3(b) with Fig. 2 (b) and 2 (d), the signal-to-background ratio with the platinum fibre is approximately four times better than that with the carbon fibre.
Accuracy and precision The accuracy and precision of the stripping analysis for silver(I) on the platinum-fibre electrode were investigated by analyzing 0.20 M nitric acid containing 10, 20, 50 and 100 ~g 1-1 silver (I). Each sample was electrolyzed for 5 min at - 0.40 V vs. Ag/AgC1, before stripping with a constant current of i ~A. After each sample had been processed, an identical electrolysis/stripping cycle was repeated for a spiked sample in which the silver (I) concentration was four times that of the sample. Finally, the silver (I) concentration was evaluated with the computer program, by using the normal equations for standard addition. The standard addition procedure was repeated eight times at each concentration level. The results are summarized in Table 1.
31 TABLE 1 Accuracy and precision in the determination of silver (I) with platinum-fibre electrodes. Electrolysis at - 0.40 V vs. Ag/AgC1 for 5 rain prior to stripping in the sample with a constant current of 1/~A Silver (I) concentration (#g 1-1 ) In sample
In spiked sample
Found a
10 20 50 100
40 80 200 400
3.0 + 2.0 16.8 +_2.3 50.6 _+2.2 102.0 + 5.6
aMean and standard deviation (n = 8).
It is apparent that the value obtained for silver (I) is too low when the concentration of silver (I) in the sample is below 50/~g 1-1. This is due to the slow initial nucleation rate for silver on the platinum fibre in this concentration range. At higher concentrations, accurate and precise results are obtained. The precisions obtained at the 50 and 100 #g 1-1 levels are better than normal for a stripping technique evaluated with a single standard addition. This is because it is the pure, and not the amalgamated, metal that is stripped, so that the activity of the reduced form, silver(0) is unity throughout the stripping process. In addition, the silver (0)/silver (I) redox reaction is highly reversible. In the case of silver deposited onto a platinum electrode, the activity of both the reduced and the oxidized forms can be held equal to unity if stripping is done in chloride medium, because silver chloride is formed during stripping. This is illustrated in Fig. 3 (c), for which 0.20 M nitric acid containing 100 pg l-1 silver (I) was electrolyzed at - 0 . 5 0 V vs. Ag/AgC1 for 2 min prior to stripping in 0.50 M hydrochloric acid at a constant current of 1 #A. Although the peak width at half peak height is only 34 mV, compared with 54 mV in Fig. 3 (b), it is far greater than the theoretical value of 0 inV. This can be attributed partly to the fact that silver chloride is not formed immediately at the commencement of the stripping and partly to peak broadening caused by digital smoothing.
Interferences The most likely interferences in the determination of silver(I) are mercury (II), copper (II) and ions of the platinum group elements. The last do not interfere, however, because they are oxidized at more anodic potentials than silver(I). The stripping peak of mercury(II) coincides with that of silver(I) so that it is not possible to determine silver (I) in the presence of mercury (II) at a platinum working electrode. In such circumstances, it is necessary to use the medium-exchange procedure described below.
32
a
b
a
b
0.05
0.1
Ag
Ag
CU
.o
,0
o:~
V-1
~
o:5 V vs Ag/AgCI
o:3
~
0:3
V vs Ag/AgCI
Fig. 4. (a) Electrolysis for 4 rain at - 0.40 V vs. Ag/AgC1 in a sample containing 30 #g 1- ~silver (I) and 1 mg 1-1 copper (II) and stripping in the sample with a constant current of 1 #A. (b) Pulsed electrolysis 20 times for 12 s at - 0 . 4 0 V vs. Ag/AgC1, each 12-s interval being separated by electrolysis for 1 s at 0.10 V vs. Ag/AgC1; sample and stripping conditions as for (a), except that 10 mg l - 1 copper (II) was added. Fig. 5. Electrolysis for 20 min at - 0 . 8 0 V vs. Ag/AgC1 in a sample containing 0.20 M nitric acid, 10 mg 1-1 of mercury(II) and 1 #g 1-1 silver(I) (a) or 4/lg 1-1 silver(I) (b). Stripping in acetonitrile containing 0.20 M perchloric acid at a constant current of 0.50 pA.
The interference of copper (II) was investigated by electrolyzing 0.20 M nitric acid containing 30/lg 1-1 silver(I) and 1 mg 1-1 copper(II) for 4 min at - 0.40 V vs. Ag/AgC1, prior to stripping in the sample with a current of 1/IA. The differentiated background-corrected curve is shown in Fig. 4(a). The stripping peaks are seen to be well separated. Interference from even higher copper (II) concentrations can be avoided by using a pulsed-potential electrolysis procedure. This was illustrated by electrolyzing 0.20 M nitric acid containing 30/lg l - 1 silver (I) and 10 mg l - 1 copper (II) at - 0.40 V vs. SCE for 20 intervals of 12 s, each interval being separated by electrolysis at 0.10 V vs. Ag/AgC1 for 1 s. The constant-current stripping curve is shown in Fig. 4 (b). The copper peak can thus be eliminated by intermittently increasing the potential to 0.10 V vs. Ag/AgC1 at which potential copper is reoxidized. Further experiments showed that the pulsed-potential procedure permits the determination of silver(I) in the presence of at least a 1000-fold a m o u n t of copper (II). D E T E R M I N A T I O N OF SILVER (I) ON M E R C U R Y F I L M E L E C T R O D E S
Although it is possible to deposit a mercury film on a platinum surface, the solubility of mercury in platinum makes such films irreproducible. For this
33 reason, investigation of the determination of silver (I) with mercury film electrodes was restricted to mercury films on carbon-fibre electrodes. In a flow system, the mercury film can be deposited on the working carbon-fibre electrode either by using a separate mercury plating solution or by addition of mercury (II) ions to the sample solution. Both approaches were found to be equally successful in the present study. The former method is, however, simpler from a practical point of view, and was used. As mentioned above, it is not possible to obtain satisfactory resolution between the stripping peaks of silver and mercury in aqueous media. In agreement with the findings of Kolthoff and Coetzee [ 1 ], acetonitrile was found to be the only simple solvent in which the stripping peaks of the two elements could be resolved. If the water content in acetonitrile was above 5%, very poor sensitivity was obtained for silver. Moreover, the results were highly irreproducible and sometimes resulted in complete disappearance of the silver stripping peak. The concentration and ionic composition of the electrolyte added to acetonitrile in order to obtain electrical conductance was found to be of minor importance. This is, of course, mainly due to the small currents required to control electrolysis and stripping with the small surface area of the carbon-fibre electrode (0.03 mm2). Finally, 0.20 M perchloric acid was chosen as ionic medium. Removal of the water from the flow cell with acidified acetonitrile before stripping proved to be a very slow process. A rinsing solution containing 0.01 M perchloric acid in 60% (v/v) ethanol and 40% (v/v) acetonitrile was therefore sucked through the flow cell after electrolysis in the sample and before introduction of the stripping medium.
Recommended conditions Samples containing 0.2-50 ~tg 1-1 silver(I) were made 0.20 M in nitric acid and 10 mg l-1 mercury (II) (as its nitrate ) was added. The sample was electrolyzed at -0.80 V for 0.5-30 min depending on the concentration of silver(I) in the sample. The above acidic rinsing solution was then sucked through the flow cell for 30 s followed by the acetonitrile stripping solution (0.20 M perchloric acid in acetonitrile) for 1 min. Silver was then stripped by scanning the potential range - 0.20 to + 0.50 V vs. Ag/AgC1 with a constant current of 0.50 pA. All flow rates were 1 ml min-1.
Accuracy and precision The accuracy and precision were investigated by electrolyzing solutions containing 1 and 4/tg 1-1 silver(I) in 0.20 M nitric acid for 20 and 5 min, respectively. The silver(I) concentrations in the two samples were determined by means of standard additions; the spiked samples contained 4 and 16 zg 1-1 silver (I), respectively. The stripping curves obtained for the sample solution
34 containing 1 ttg 1-1 silver(I) is shown in Fig. 5(a) and that for the spiked sample in Fig. 5 (b). Ten consecutive standard-addition determinations in the sample containing 1/tg l - 1 silver (I) yielded a mean value of 0.96 #g l- 1 with a standard deviation of 0.11 ttg 1-1. The corresponding result for the sample containing 4/~g 1-1 of silver(I) was 3.74 +0.34 #g 1-1. The linear range was investigated by analyzing samples containing 5, 10, 20, 40 and 60 ~g 1-1, the electrolysis time being 2 min. The calibration curve was linear in the concentration range investigated, with a correlation coefficient of 0.994. The detection limit was estimated from 10 electrolysis/stripping cycles in a sample containing 1/lg 1-1 silver (I) with an electrolysis time of 30 min. The mean stripping time was 0.287 s with a standard deviation of 0.023 s. From this, the detection limit was estimated as 0.24/lg 1-1 at the 3a level.
Interferences The most likely interferences in the determination of silver (I) when acetonitrile is used as the stripping medium are copper, platinum, bismuth, antimony, arsenic, lead and tin. None of these elements formed intermetallic compounds with silver in the mercury film. When present in 100-fold excess, only bismuth interfered, the stripping peak being 0.090 V more positive than that of silver. Consequently, silver (I) cannot be determined in samples in which the bismuth (III) concentration is higher than that of silver (I). DISCUSSION When used in flow potentiometric or constant-current stripping modes, platinum- and carbon-fibre electrodes are simple and reliable sensors for the determination of micro- and nano-molar concentrations of silver(I). For silver (I) concentrations above 50 zg l - 1, potentiostatic deposition of elemental silver directly onto a platinum fibre provides the simplest analytical procedure. For lower silver (I) concentrations, the more time-consuming procedure based on mercury amalgamation and acetonitrile as stripping medium must be used. With this method, detection limits below 1 ~g l- 1 can be reached. Non-aqueous solvents are seldom used in connection with electroanalytical stripping techniques. One reason for this is that dilution of the sample, which is normally an aqueous solution, results in poor detection limits. This problem can be solved by using a flow system. Another problem associated with nonaqueous solvents is their poor conductivity and the limited solubility of electrolytes in them. In this respect, fibre electrodes offer an unusual advantage in that the internal resistance potential drop can be kept very small even in media with poor conductivity. Preliminary investigations indicate that fibre electrodes in flow streams can be used successfully in solvents far less polar than
35 acetonitrile. Organic solvents in c o m b i n a t i o n with such fibre electrodes m a y t h u s provide the possibility o f o b t a i n i n g i n c r e a s e d selectivity in e l e c t r o a n a l ytical stripping techniques. T h i s would be p a r t i c u l a r l y useful in cases for which it is impossible to o b t a i n sufficient p e a k r e s o l u t i o n with water-soluble complexing agents.
REFERENCES 1 2 3 4 5 6
I.M. Kolthoff and J.F. Coetzee, J. Am. Chem. Soc., 79 (1957) 870, 1852. J. Hubmann, J. Buffle and D. Monnier, Anal. Chim. Acta, 62 (1972) 393. D.C. Johnson and R.E. Allen, Talanta, 20 (1973) 305. S. Glodowski and Z. Kublik, Anal. Chim. Acta, 156 (1984) 61. L. Renman, D. Jagner and R. Berglund, Anal. Chim. Acta, 188 (1986) 137. H. Huiliang, C. Hua, D. Jagner and L. Renman, Anal. Chim. Acta, 193 (1987) 61.
27
FLOW P O T E N T I O M E T R I C A N D C O N S T A N T - C U R R E N T S T R I P P I N G A N A L Y S I S FOR SILVER(I) WITH C A R B O N - A N D PLATINUM-FIBRE ELECTRODES
HUANG HUILIANGa, DANIEL JAGNER* and LARS RENMAN Department of Technical Analytical Chemistry, Chemical Center, University of Lund, P.O. Box 124, S-221 O0 Lund (Sweden)
(Received 15th October 1987)
SUMMARY At concentrations above 50 ]~g 1-1, silver(I) is determined in nitric acid medium by means of potentiostatic deposition onto a platinum-fibre electrode and subsequent constant-current stripping in the sample or potentiometric stripping in a potassium permanganate medium. Interference from copper (II) is reduced by a pulsed potential procedure whereby copper deposited onto the fibre electrode is reoxidized intermittently. At concentrations below 50 #g l-1, silver (I) is determined by using a mercury-coated carbon-fibre electrode and constant-current stripping in acetonitrile containing 0.20 M perchloric acid. Potentiostatic deposition for 30 min yielded a detection limit of 0.24 zg l- 1silver (I) at the 3a level. T h e r e are few r e p o r t s on t h e e l e c t r o a n a l y t i c a l d e t e r m i n a t i o n of silver (I) in the sub-l~M c o n c e n t r a t i o n range [ 1 - 4 ] . T h e m a i n r e a s o n for this is t h a t , in this c o n c e n t r a t i o n range, silver (I) has to be d e t e r m i n e d b y s t r i p p i n g techniques in which t h e p r e c o n c e n t r a t i o n step is e i t h e r d e p o s i t i o n of silver on a solid electrode or d e p o s i t i o n of silver a m a l g a m on a m e r c u r y drop or m e r c u r y film electrode. B e c a u s e of the slow rate o f t h e initial n u c l e a t i o n process, it is difficult to deposit silver r e p r o d u c i b l y o n t o a solid electrode. C o n s e q u e n t l y , silver is usually p r e c o n c e n t r a t e d b y m e a n s of s i m u l t a n e o u s r e d u c t i o n a n d a m a l g a m a t i o n , t h u s exploiting t h e high solubility of silver in mercury. Owing to the similarity in e l e c t r o c h e m i c a l p r o p e r t i e s of t h e two elements, however, it is difficult to s e p a r a t e t h e s t r i p p i n g p e a k of silver f r o m t h a t of m e r c u r y . E a r l y work b y K o l t h o f f a n d Coetzee [ 1 ] s h o w e d t h a t p e a k s e p a r a t i o n was n o t possible in aqueous solution, despite t h e use of various c o m p l e x i n g agents. Separ a t i o n was f o u n d to occur in acetonitrile, which was later exploited b y Glodowski a n d K u b l i k [4 ] for t h e d e t e r m i n a t i o n of t r a c e s o f silver in liquid m e r c u r y . T h e e l e c t r o c h e m i s t r y of silver (I) a n d m e r c u r y (II) in a c e t o n i t r i l e has b e e n studied e x t e n s i v e l y by H u b m a n n et al. [2 ]. In a b a t c h p r o c e d u r e , H u b m a n n et al. were aPermanent address: Scientific Instrumentation Department, Xiamen (Amoy) University, P.R. China.
0003-2670/88/$03.50
© 1988 Elsevier Science Publishers B.V.
28 able to detect silver (I) at a concentration of 50/~g l-1 in aqueous solution by means of anodic stripping voltammetry after dilution of the sample with acetonitrile. In order to obtain satisfactory separation between silver and mercury on the hanging mercury drop electrode, it was found that the water content in the acetonitrile solution should not exceed 10%, i.e., the aqueous sample must be diluted at least ten times with acetonitrile. Because the presence of acetonitrile is necessary only in the stripping step and not in the potentiostatic deposition step, sample dilution might be circumvented by using a flow system which permits medium exchange between the sample electrolysis and stripping steps [5 ]. This is investigated in the present study. The possibility of using potentiometric and constant-current stripping modes as alternatives to differential-pulse anodic stripping voltammetry, thereby facilitating a high degree of automation [5], is also examined. Here the optimum conditions for the flow determination of micromolar concentrations of silver (I) by using platinum- and carbon-fibre electrodes are reported. EXPERIMENTAL The computerized flow potentiometric and constant-current stripping analyzer used has been described elsewhere [5]. In this analyzer, six different solutions can be sucked, in random choice order, into the flow cell; all the flow and instrumental parameters are under computer control. During stripping, the potential vs. time transient is recorded with a real-time sampling rate of 25.6 kHz. Differentiation, digital smoothing with an averaging filter of 30 mV and a Savitzky-Golay filter of 15 mV (potential resolution 1 mV), location and integration of stripping peaks, and display of results are all done by the computer [5 ]. In this study, the flow system was modified in that the original perspex common entry point for solutions in the flow analyzer was replaced by a common entry point made of teflon, so as to permit the use of organic solvents. Carbon or platinum fibres with diameters of ca. 10 ~m were inserted, perpendicularly to the flow direction, in Viton tubes with an inner diameter of 0.8 mm, as described elsewhere [6]. A silver tube lined with silver chloride was connected downstream from the fibre electrode by means of a T-junction, as shown in Fig. 1. The position of the tubular platinum counter electrode is also shown in Fig. 1. All potentials given below are vs. the Ag/AgC1 in 1 M hydrochloric acid in 30% ethanol. All reagents were of analytical grade except the mineral acids which were of Suprapur (Merck) grade. All dilutions were made with Millipore-Q water. DETERMINATION OF SILVER(I) ON SOLID ELECTRODES In the stripping determination of silver (I) neither the sample nor the stripping medium should contain species capable of forming solid phases with sil-
29
Platinum tube counter electrode
Fibre working electrode
Suction
1 ml rain -1 Solutions
AgCl-lined silver tube reference electrode
entry point
1 M HCI in 30 % ethanol 0.05 ml min "1
Fig. 1. Schematic drawing of the flow electrode system.
a
b
c
d
V-1
.
0.1
.0
0:6
0:2
0:6
0:2
0:6
0:2
0:S
0:2
V vS Ag/AgCI
Fig. 2. Comparison between potentiometric and constant-current stripping analysis with a platinum-fibre electrode. Electrolysis for 4 min at - 0.60 V vs. Ag/AgC1 in 0.20 M nitric acid containing 30/~g l-~ silver (I). (a) Potentiometric stripping in the same solution after the addition of 0.30 mM potassium permanganate; (b) constant-current ( 1 ttA) stripping in the sample solution; (c,d) as for (a) and (b) except that the sample contained 90/lg 1-1 silver(I).
ver (I). For this reason, 0.20 M nitric acid was used both as sample and stripping medium in the studies below.
Comparison between potentiometric and constant-current stripping analysis and between platinum- and carbon-fibre electrodes Potentiometric and constant-current stripping analysis were compared for a platinum fibre electrode by means of electrolysis for 4 min at - 0 . 6 0 V vs. Ag/AgC1 in 0.20 M nitric acid containing 30 ~g 1-1 silver (I); potentiometric stripping was then conducted in 0.20 M nitric acid containing 0.30 mM potassium permanganate, or constant-current stripping in 0.20 M nitric acid with a
30
a
b
c V-I
.0.1
0.'6
~
o'.6
o:2
-o:2
V vs Ag/AgCI
Fig. 3. Constant-current (1 #A) stripping analysis for silver (I): (a,b) at a carbon-fibre electrode not coated with mercury under the experimental conditions used for Fig. 2 (b) and (d), respectively; (c) at a platinum-fibre electrode with electrolysis for 2 min at -0.50 V vs. Ag/AgC1 in 0.20 M nitric acid containing 100 #g 1-1 silver(I) and subsequent stripping in 0.50 M hydrochloric acid.
current of 1.0/IA. The differentiated and background-corrected potentiometric and constant-current stripping curves are shown in Fig. 2 (a,b). The experiments were repeated in a solution containing 90 ~g 1-1 silver (Fig. 2 c,d). From Fig. 2, it can be seen that the stripping peak potential is independent of the mode of oxidation and that the signal-to-background ratio is similar for potentiometric and constant-current stripping. Plantinum- and carbon-fibre electrodes were compared under the experimental conditions used for Fig. 2 (b) and (d). The background-corrected differentiated stripping curves obtained with the carbon fibre are shown in Fig. 3(a) and (b). As can be seen from comparison of Fig. 3(a) and 3(b) with Fig. 2 (b) and 2 (d), the signal-to-background ratio with the platinum fibre is approximately four times better than that with the carbon fibre.
Accuracy and precision The accuracy and precision of the stripping analysis for silver(I) on the platinum-fibre electrode were investigated by analyzing 0.20 M nitric acid containing 10, 20, 50 and 100 ~g 1-1 silver (I). Each sample was electrolyzed for 5 min at - 0.40 V vs. Ag/AgC1, before stripping with a constant current of i ~A. After each sample had been processed, an identical electrolysis/stripping cycle was repeated for a spiked sample in which the silver (I) concentration was four times that of the sample. Finally, the silver (I) concentration was evaluated with the computer program, by using the normal equations for standard addition. The standard addition procedure was repeated eight times at each concentration level. The results are summarized in Table 1.
31 TABLE 1 Accuracy and precision in the determination of silver (I) with platinum-fibre electrodes. Electrolysis at - 0.40 V vs. Ag/AgC1 for 5 rain prior to stripping in the sample with a constant current of 1/~A Silver (I) concentration (#g 1-1 ) In sample
In spiked sample
Found a
10 20 50 100
40 80 200 400
3.0 + 2.0 16.8 +_2.3 50.6 _+2.2 102.0 + 5.6
aMean and standard deviation (n = 8).
It is apparent that the value obtained for silver (I) is too low when the concentration of silver (I) in the sample is below 50/~g 1-1. This is due to the slow initial nucleation rate for silver on the platinum fibre in this concentration range. At higher concentrations, accurate and precise results are obtained. The precisions obtained at the 50 and 100 #g 1-1 levels are better than normal for a stripping technique evaluated with a single standard addition. This is because it is the pure, and not the amalgamated, metal that is stripped, so that the activity of the reduced form, silver(0) is unity throughout the stripping process. In addition, the silver (0)/silver (I) redox reaction is highly reversible. In the case of silver deposited onto a platinum electrode, the activity of both the reduced and the oxidized forms can be held equal to unity if stripping is done in chloride medium, because silver chloride is formed during stripping. This is illustrated in Fig. 3 (c), for which 0.20 M nitric acid containing 100 pg l-1 silver (I) was electrolyzed at - 0 . 5 0 V vs. Ag/AgC1 for 2 min prior to stripping in 0.50 M hydrochloric acid at a constant current of 1 #A. Although the peak width at half peak height is only 34 mV, compared with 54 mV in Fig. 3 (b), it is far greater than the theoretical value of 0 inV. This can be attributed partly to the fact that silver chloride is not formed immediately at the commencement of the stripping and partly to peak broadening caused by digital smoothing.
Interferences The most likely interferences in the determination of silver(I) are mercury (II), copper (II) and ions of the platinum group elements. The last do not interfere, however, because they are oxidized at more anodic potentials than silver(I). The stripping peak of mercury(II) coincides with that of silver(I) so that it is not possible to determine silver (I) in the presence of mercury (II) at a platinum working electrode. In such circumstances, it is necessary to use the medium-exchange procedure described below.
32
a
b
a
b
0.05
0.1
Ag
Ag
CU
.o
,0
o:~
V-1
~
o:5 V vs Ag/AgCI
o:3
~
0:3
V vs Ag/AgCI
Fig. 4. (a) Electrolysis for 4 rain at - 0.40 V vs. Ag/AgC1 in a sample containing 30 #g 1- ~silver (I) and 1 mg 1-1 copper (II) and stripping in the sample with a constant current of 1 #A. (b) Pulsed electrolysis 20 times for 12 s at - 0 . 4 0 V vs. Ag/AgC1, each 12-s interval being separated by electrolysis for 1 s at 0.10 V vs. Ag/AgC1; sample and stripping conditions as for (a), except that 10 mg l - 1 copper (II) was added. Fig. 5. Electrolysis for 20 min at - 0 . 8 0 V vs. Ag/AgC1 in a sample containing 0.20 M nitric acid, 10 mg 1-1 of mercury(II) and 1 #g 1-1 silver(I) (a) or 4/lg 1-1 silver(I) (b). Stripping in acetonitrile containing 0.20 M perchloric acid at a constant current of 0.50 pA.
The interference of copper (II) was investigated by electrolyzing 0.20 M nitric acid containing 30/lg 1-1 silver(I) and 1 mg 1-1 copper(II) for 4 min at - 0.40 V vs. Ag/AgC1, prior to stripping in the sample with a current of 1/IA. The differentiated background-corrected curve is shown in Fig. 4(a). The stripping peaks are seen to be well separated. Interference from even higher copper (II) concentrations can be avoided by using a pulsed-potential electrolysis procedure. This was illustrated by electrolyzing 0.20 M nitric acid containing 30/lg l - 1 silver (I) and 10 mg l - 1 copper (II) at - 0.40 V vs. SCE for 20 intervals of 12 s, each interval being separated by electrolysis at 0.10 V vs. Ag/AgC1 for 1 s. The constant-current stripping curve is shown in Fig. 4 (b). The copper peak can thus be eliminated by intermittently increasing the potential to 0.10 V vs. Ag/AgC1 at which potential copper is reoxidized. Further experiments showed that the pulsed-potential procedure permits the determination of silver(I) in the presence of at least a 1000-fold a m o u n t of copper (II). D E T E R M I N A T I O N OF SILVER (I) ON M E R C U R Y F I L M E L E C T R O D E S
Although it is possible to deposit a mercury film on a platinum surface, the solubility of mercury in platinum makes such films irreproducible. For this
33 reason, investigation of the determination of silver (I) with mercury film electrodes was restricted to mercury films on carbon-fibre electrodes. In a flow system, the mercury film can be deposited on the working carbon-fibre electrode either by using a separate mercury plating solution or by addition of mercury (II) ions to the sample solution. Both approaches were found to be equally successful in the present study. The former method is, however, simpler from a practical point of view, and was used. As mentioned above, it is not possible to obtain satisfactory resolution between the stripping peaks of silver and mercury in aqueous media. In agreement with the findings of Kolthoff and Coetzee [ 1 ], acetonitrile was found to be the only simple solvent in which the stripping peaks of the two elements could be resolved. If the water content in acetonitrile was above 5%, very poor sensitivity was obtained for silver. Moreover, the results were highly irreproducible and sometimes resulted in complete disappearance of the silver stripping peak. The concentration and ionic composition of the electrolyte added to acetonitrile in order to obtain electrical conductance was found to be of minor importance. This is, of course, mainly due to the small currents required to control electrolysis and stripping with the small surface area of the carbon-fibre electrode (0.03 mm2). Finally, 0.20 M perchloric acid was chosen as ionic medium. Removal of the water from the flow cell with acidified acetonitrile before stripping proved to be a very slow process. A rinsing solution containing 0.01 M perchloric acid in 60% (v/v) ethanol and 40% (v/v) acetonitrile was therefore sucked through the flow cell after electrolysis in the sample and before introduction of the stripping medium.
Recommended conditions Samples containing 0.2-50 ~tg 1-1 silver(I) were made 0.20 M in nitric acid and 10 mg l-1 mercury (II) (as its nitrate ) was added. The sample was electrolyzed at -0.80 V for 0.5-30 min depending on the concentration of silver(I) in the sample. The above acidic rinsing solution was then sucked through the flow cell for 30 s followed by the acetonitrile stripping solution (0.20 M perchloric acid in acetonitrile) for 1 min. Silver was then stripped by scanning the potential range - 0.20 to + 0.50 V vs. Ag/AgC1 with a constant current of 0.50 pA. All flow rates were 1 ml min-1.
Accuracy and precision The accuracy and precision were investigated by electrolyzing solutions containing 1 and 4/tg 1-1 silver(I) in 0.20 M nitric acid for 20 and 5 min, respectively. The silver(I) concentrations in the two samples were determined by means of standard additions; the spiked samples contained 4 and 16 zg 1-1 silver (I), respectively. The stripping curves obtained for the sample solution
34 containing 1 ttg 1-1 silver(I) is shown in Fig. 5(a) and that for the spiked sample in Fig. 5 (b). Ten consecutive standard-addition determinations in the sample containing 1/tg l - 1 silver (I) yielded a mean value of 0.96 #g l- 1 with a standard deviation of 0.11 ttg 1-1. The corresponding result for the sample containing 4/~g 1-1 of silver(I) was 3.74 +0.34 #g 1-1. The linear range was investigated by analyzing samples containing 5, 10, 20, 40 and 60 ~g 1-1, the electrolysis time being 2 min. The calibration curve was linear in the concentration range investigated, with a correlation coefficient of 0.994. The detection limit was estimated from 10 electrolysis/stripping cycles in a sample containing 1/lg 1-1 silver (I) with an electrolysis time of 30 min. The mean stripping time was 0.287 s with a standard deviation of 0.023 s. From this, the detection limit was estimated as 0.24/lg 1-1 at the 3a level.
Interferences The most likely interferences in the determination of silver (I) when acetonitrile is used as the stripping medium are copper, platinum, bismuth, antimony, arsenic, lead and tin. None of these elements formed intermetallic compounds with silver in the mercury film. When present in 100-fold excess, only bismuth interfered, the stripping peak being 0.090 V more positive than that of silver. Consequently, silver (I) cannot be determined in samples in which the bismuth (III) concentration is higher than that of silver (I). DISCUSSION When used in flow potentiometric or constant-current stripping modes, platinum- and carbon-fibre electrodes are simple and reliable sensors for the determination of micro- and nano-molar concentrations of silver(I). For silver (I) concentrations above 50 zg l - 1, potentiostatic deposition of elemental silver directly onto a platinum fibre provides the simplest analytical procedure. For lower silver (I) concentrations, the more time-consuming procedure based on mercury amalgamation and acetonitrile as stripping medium must be used. With this method, detection limits below 1 ~g l- 1 can be reached. Non-aqueous solvents are seldom used in connection with electroanalytical stripping techniques. One reason for this is that dilution of the sample, which is normally an aqueous solution, results in poor detection limits. This problem can be solved by using a flow system. Another problem associated with nonaqueous solvents is their poor conductivity and the limited solubility of electrolytes in them. In this respect, fibre electrodes offer an unusual advantage in that the internal resistance potential drop can be kept very small even in media with poor conductivity. Preliminary investigations indicate that fibre electrodes in flow streams can be used successfully in solvents far less polar than
35 acetonitrile. Organic solvents in c o m b i n a t i o n with such fibre electrodes m a y t h u s provide the possibility o f o b t a i n i n g i n c r e a s e d selectivity in e l e c t r o a n a l ytical stripping techniques. T h i s would be p a r t i c u l a r l y useful in cases for which it is impossible to o b t a i n sufficient p e a k r e s o l u t i o n with water-soluble complexing agents.
REFERENCES 1 2 3 4 5 6
I.M. Kolthoff and J.F. Coetzee, J. Am. Chem. Soc., 79 (1957) 870, 1852. J. Hubmann, J. Buffle and D. Monnier, Anal. Chim. Acta, 62 (1972) 393. D.C. Johnson and R.E. Allen, Talanta, 20 (1973) 305. S. Glodowski and Z. Kublik, Anal. Chim. Acta, 156 (1984) 61. L. Renman, D. Jagner and R. Berglund, Anal. Chim. Acta, 188 (1986) 137. H. Huiliang, C. Hua, D. Jagner and L. Renman, Anal. Chim. Acta, 193 (1987) 61.