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Determination of heavy metals in real samples by anodic stripping voltammetry with mercury microelectrodes
Analytica Chimica Acta, 219 (1989) 9-18 Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
DETERMINATION OF HEAVY METALS IN REAL SAMPLES BY ANODIC STRIPPING VOLTAMMETRY WITH MERCURY MICROELECTRODES Part 1. Application to Wine
SALVATORE DANIELE*, MARIA-ANTONIETTA ANTONIO MAZZOCCHIN
BALDO, PAOLO UGO and GIAN-
Department of Physical Chemistry, University of Venice, Calle Larga S. Marta 2137,30123 Venice (Italy) (Received 20th June 1988)
SUMMARY Differential-pulse anodic stripping voltammetry with a mercury microelectrode is used for the determination of zinc, cadmium, lead and copper in wine at its natural pH without pretreatment. The effects of the matrix on the stripping peaks are studied in detail by varying the concentration of the metals. Intermetallic (Cu-Zn) interferences and the effects of oxygen are described. The results obtained for the labile metal contents varied from 2 ,ug 1-l for cadmium to 148 w 1-i for zinc; standard addition plots were linear over about two orders of magnitude above these levels, demonstrating the negligible effect of organic matter. Acidification of the sample with hydrochloric acid to pH 1 allowed the total metal contents to be determined. The reliability of the method was tested by comparison with the results obtained with atomic absorption spectrometry; the differences were within lo-20%.
Anodic stripping voltammetry (ASV) has been widely used for measuring trace metals in numerous matrices [ 11. The excellent detection limits, reported to be in the 10-10-10-‘2 M range, result from preconcentration of the metal ion of interest into a mercury drop or film. Because this step is aided by mechanical stirring, its reproducibility is one of the source of errors in ASV techniques. Usually, a high concentration of the supporting electrolyte is needed in order to avoid undesirable phenomena such as shifting, broadening and lowering of the stripping peaks caused by ohmic drop effects [ 21. The deliberate addition of supporting electrolyte, however, can create complications when trace and ultratrace metals have to be determined, in that risks of contamination from heavy-metal impurities in the added chemicals are large. Moreover, addition of an electrolyte alters the kinetic and thermodynamic properties of samples, thereby invalidating data on, for example, speciation in natural liquid matrices which have low ionic strength. Voltammetric microelectrodes have received much attention in recent years, 0003-2670/89/$03.50
0 1989 Elsevier Science Publishers B.V.
10
thanks to their attractive features, including fast steady-state diffusional mass transport and low ohmic drop [ 31. Some papers have also demonstrated that sensitive ASV measurements for determining heavy metals can be obtained at mercury microelectrodes [ 4-121. These studies have emphasized, in particular, that the enhanced mass transfer characteristic of microelectrodes reduces the preconcentration times and obviates the need for convective hydrodynamics during the deposition step; moreover, because the ohmic losses are. negligible, the measurements can be conducted even in poorly conductive media without any deliberate addition of supporting electrolyte. All the reports have dealt with synthetic samples, however, while the features of microelectrodes just described suggest useful applications to untreated real samples which are seldom suitable for direct measurements by ASV with conventional electrodes. Because of its effective discrimination against the charging background current, differential-pulse anodic stripping voltammetry (DPASV) has been the most widely used stripping approach [l]. Thus, the combination of DPASV with ultramicroelectrodes should prove to be extremely powerful for applications involving ultratrace levels of heavy metals in difficult experimental conditions. The investigation reported in this paper was undertaken with the aim of determining labile heavy-metal contaminants in wine by using differentialpulse anodic stripping voltammetry at mercury microelectrodes. EXPERIMENTAL
Chemicals
Analytical-reagent grade chemicals, mercury (I) nitrate, lead nitrate, cadmium nitrate, copper (II) sulphate, sodium chloride and gallium chloride, were used to prepare stock solutions. The mineral acids used were of Suprapur grade (Merck). All the solutions were prepared with Millipore Milli-Q water. Nitrogen (99.99% ) was used to remove dissolved oxygen. The wine was a Tocai Grave from Friuli (Northern Italy) with the following main characteristics: alcohol content, about 13% (v/v); total acid content, expressed as tartaric acid, 7.7 mg 1-l; chloride, 18 mg 1-l; and pH, 3.21. These values were determined by using Italian standard methods for food [ 131. Apparatus and procedures
Voltammograms were obtained with an Amel Model 472 multipolarograph in conjunction with an Amel Model 863 X-Y recorder. A Princeton Applied Research (PAR) static mercury drop electrode (Model 303A) was used as the working electrode in some measurements made for comparison. All the experiments with microelectrodes were done in a two-electrode cell configuration; the multipolarograph was used only as a convenient data-acquisition interface and to generate the waveform for the stripping step. The
current flow was monitored by a picoammeter (Keithley Model 485)) the output from this amplifier being connected to the multipolarograph through a convenient resistor. The electrochemical cell was a laboratory-made 20-ml teflon cup. Working mercury microelectrodes were made by electrodeposition of mercury on to a platinum microdisc electrode fabricated by sealing l-pm diameter wire into glass [ 31. The platinum microelectrode was polished with alumina powder (0.05 pm) on a Metron polishing cloth (Banner Scientific Supplies). The mercury was deposited under potentiostatic conditions at large overpotential from 3.0 mM mercury (I) nitrate/l M KNO, at pH < 1 (acidified with nitric acid). The mercury deposit formed was found to be durable and reproducible in its behaviour. The amount of mercury deposited was controlled by the deposition time and was monitored by the current/time (i/t) plot (see Fig. la) recorded in the deposition step. The current increased linearly with the square root of time (Fig. lb), in agreement with a three-dimensional growth [ 141. This finding allows the theoretical relationship r= (2MDCt/p) ‘I2 [ 151 to be applied for estimating the radius of the deposit. For the experimental conditions chosen here, the radius of the deposit typically had the value 5.2 pm. Only deposits giving i/t plots which overlapped (within 2% errors) were used for ASV measurements. After the electrodes had been prepared, they were removed from the plating solution, rinsed carefully with ultrapure water and then transferred to the cell for DPASV measurements. In each experiment (unless stated otherwise), a freshly prepared deposit was used. The samples were kept quiescent during the preconcentration and, because of the unique behavior of microelectrodes associated with the steady-state diffusional flux [lo], no rest period was necessary before the anodic scan. All the DPASV measurements were conducted with a pulse height of 50 mV and a scan rate of 5 mV s-‘. The wine to be analysed was stored in a refrigerator at 4’ C until the day of ( b)
(a)
$A
%A
___1
5..
10.. 5
10..
‘.
0
100
200
300
5
10
15 t/h rll
%
Fig. 1. (a) Potentiostatic transient for the growth of the mercury deposit on a platinum disc (lflmdiameter); (b)plotofivs.t I’* for the transient shown in (a).
12
analysis; it was then equilibrated to room temperature, deaerated by passing purified nitrogen for 10 min and analysed (unless stated otherwise) as withdrawn from the bottle without further pretreatment. A mercury pool was used as reference/counter electrode for the electrodeposition of mercury; an aqueous saturated calomel electrode (SCE) was used for the DPASV measurements. Thelatomic absorption determinations were done with a Perkin-Elmer Model 5000 spectrometer. RESULTS AND DISCUSSION
Effect of the solution composition on the stripping processes Wine is a complex aqueous matrix in which many organic compounds are dissolved. It is known that organic matter may adsorb on the electrode surface, giving rise to unpredictable effects on the stripping peak current [ 11, so that removal or destruction of organic matter is usually necessary prior to classical stripping analysis of wines [ 161.For conventional electrodes, the effects of adsorption are more marked at a mercury drop than at a mercury film [ 171. Mercury microelectrodes behave similarly to mercury films [ 6, lo], hence similar effects are to be expected from organic matter. However, the experimental conditions, e.g., the deposition potential, cannot be chosen simply on the basis of those valid for conventional electrodes, in that the redox behaviour for the species may be different in principle. Thus, in order to verify the effects on the stripping peak current caused by the various substances of the sample, preliminary DPASV measurements were made by using the wine as solvent. Figure 2a shows a typical stripping voltammogram recorded at a mercury microelectrode on a quiescent sample spiked with known amounts of copper (II ) , lead (II ) , cadmium (II ) and zinc (II), after deposition for 1 min at - 1.4 V. These amounts were, in all cases, in excess of those present in the natural sample (see below). The peaks were sharp, well defined and located (except for copper) at potentials very close to those obtained at conventional mercury electrodes for synthetic samples containing ammonium citrate buffer. The peak potentials found are compared in Table 1. Stripping voltammograms recorded on wine spiked with different concentrations of the metals showed no change in the peak potential and shape for zinc, cadmium and lead, while a potential shift (Fig. 3a) and variation in the shape were obtained for the copper peak. Table 2 collects the results obtained for the peak potential and peak width at half-height ( wi12) of the four cations at some representative concentrations. It is noteworthy that the wij2 values of about 40 mV obtained for cadmium and lead, which are known to undergo a reversible two-electron process, are similar to those obtained at a mercury film electrode [ 181. The higher value of wl12 for zinc is a consequence of the less reversible electrochemical process involved
13
1
I
0.6
0.2
nA
nA
( b)
1
I -1.4
o
- 0.7
1.4
E/v
-07
0
E/V
Fig. 2. Differential-pulse anodic stripping voh.ammograms recorded at a mercury microelectrode: (a) wine spiked with 0.23 mg 1-l each of zinc, cadmium, lead and copper after deposition for 1 min; (b) unspiked wine after deposition for 2 min. Deposition at - 1.4 V in quiescent solution; pulse height 50 mV, scan rate 5 mV 8-l. See text for further details. TABLE 1 Comparison of peak potentials recorded on wine and on a synthetic sample prepared in 0.2 M ammonium citrate buffer Sample
Wine” Syntheticb
Peak potentials (V vs. SCE) cu
Pb
Cd
Zn
-0.100 - 0.060
- 0.500 - 0.480
- 0.650 - 0.630
- 1.010 - 1.050
“At the mercury microelectrode, deposition at - 1.4 V for 1 min, quiescent solution. bAt the HMDE, deposition at - 1.2 V for 2 min, stirring during the preconcentration, rest period 15 s. [ 191. The potential shifts and the dependence of wlj2 on the concentration shown for copper indicate the occurrence of a more complex pathway for its stripping. The calibration graphs for cadmium and lead added to the wine were linear over about two orders of magnitude (i.e., 0.002-0.20 mg 1-l Cd with a corre-
14
.
f
/
Fig. 3. Plots of (a) peak potential vs. log C and (b) i vs. C for copper. Deposition for 10 sat -0.800 V in quiescent solution. TABLE 2 Effect of the concentration on the DPASV responses Metal
Concentration bg
cu
Pb
1-‘1
0.088 0.260 0.635 0.104 0.261 0.518
E, WI
-0.120 -0.110 -0.085 -0.500 -0.500 -0.500
w/2
Metal
(mV)
85 70 55 38 40 40
Cd
Zn
Concentration
E,
w/2
his
W)
WV)
-0.650 -0.650 -0.650 -1.010 -1.010 -1.010
40 42 40 75 70 70
1-l)
0.011 0.056 0.112 0.163 0.245 0.327
lation coefficient, r, of 0.993, and 0.05-5.0 mg 1-l Pb with rz0.989); positive intercepts were observed because of the metals present in the wine. The good correlations found suggested that other substances in the wine exercised the same effects on the stripping peaks regardless of the metal concentration. Zinc did not provide linear i vs. concentration plots over the entire range of concentrations because of the formation of intermetallic Cu-Zn compounds [ 201. The occurrence of this interference was proved by plating/stripping measurements of zinc in the presence of gallium, which preferentially combines with copper [ 1,201; the calibration graph was then linear over the range 0.09-1.00 mg 1-l (r = 0.982).
15
The calibration graph for copper obtained by plating at -800 mV is characterized by two linear portions (Fig. 3b). This trend, together with the observed dependence of both wl12 and peak potential on the copper concentration, could be attributed either to hindered mass transport caused by organic matter adsorbed on the electrode surface, which is particularly effective with copper [ 11, or to a decreased number of electrons in the oxidation step, i.e., the formation of Cu+ at low concentrations of copper. The former hypothesis would imply broader and less resolved stripping peaks than those observed in the present experiments. The latter explanation fits better with both the higher wij2 values ( wl12for a one-electron process at a mercury film electrode is about 80 mV) and the decreasing peak current. The chloride ions present in the wine at concentrations as high as 5 x 10e4 M may stabilize copper (I) ions [ 211. If this is the case, the smaller the Cu/Cl- ratio, the greater would be the extent of such copper (I) stabilization; at higher Cu/Cl- ratios, Cu+ is no longer stabilized and the ECE process, via Cu+ disproportionation, would become more important, so giving rise to both higher currents and narrower peaks, in agreement with the present experimental findings. A similar observation was reported in a study set up to understand the effects of small amounts of chloride in the stripping of copper in acetic/acetate buffer solutions [ 221. Moreover, at microelectrodes, chemical reactions following charge transfer tend to be masked because of the rapid diffusion of the first electrogenerated species away from the electrode surface [3]; this substantiates further the formation of copper (I). The trend shown in Fig. 3a may also be explained by the Cu/Cl- ratios in the wine. As the concentration of spiked copper is increased, the transition from a one-electron to a two-electron process is observed with the corresponding change of the Elf2 involved. Experiments in which a set of stripping measurements were done on wine samples spiked with different amounts of sodium chloride demonstrated that the breaks shown by both plots in Fig. 3 occurred at higher copper concentrations as the chloride content was increased. However, a combined effect of chloride ions and other organic substances on the stripping peak of copper cannot be excluded. Effect of dissolved oxygen and exploitation of the same mercury deposit
Experiments were conducted to check the effect of oxygen in the sample and the possibility of using the same mercury deposit for more than one cycle. It was observed that only fully aerated samples caused low reproducibility of the data, particularly for copper which was almost obscured by the reduction peak of oxygen. Small amounts of oxygen remaining in the samples after inefficient deaeration did not cause appreciable effects on any of the stripping peaks studied, possibly because the differential-pulse waveform used is not very sensitive to irreversible processes. When the same mercury deposit was used for successive experiments, the
16
anodic scan was ended at 0.0 V (i.e., just after the copper had been stripped) in order to avoid stripping the plated mercury. In the second run, after preconcentration ,under the same experimental conditions, the stripping voltammograms always showed a copper peak enhanced by about 36% and a zinc peak diminished by about 30%, whereas the lead and cadmium peaks were substantially unchanged. This trend was even more marked for copper and zinc when further experiments were done with the same electrode, e.g., in the third run the copper peak increased by 150% and the zinc peak almost disappeared. This behavior can be explained on the basis of the formation of intermetallic CuZn compounds and by admitting that not all the copper plated in the first run is stripped in the subsequent anodic scan. In order to shed light on this matter, the procedure was repeated on freshly prepared mercury deposits and plating was done at a potential of -800 mV where zinc does not interfere. In this case, the copper peak was enhanced by only about 12% in the second run whereas the lead and cadmium peaks remained unchanged. These results show clearly that only the copper peak is enhanced, probably because its peak potential is so close to the end of the anodic scan. Moreover, when the differential pulse waveform is used, some of the metal stripped from the electrode during the pulse is replated on the electrode in the waiting period between pulses [ 11, an effect magnified here by the enhanced mass transport associated with the nonplanar diffusion at microelectrodes. This “memory effect” could also be ascribed to the formation of an insoluble film of copper salts or oxides on the electrode surface [ 23,241. The depression of the zinc height in the second run is a consequence of this memory effect; zinc would be plated in a mercury deposit containing larger amounts of copper so that formation of the intermetallic compound would be accentuated. Blank levels, precision and accuracy
Figure 2b shows the stripping voltammogram obtained on the wine sample taken from the bottle; deposition was done for 2 min at - 1.4 V in the quiescent sample. At the sensitivity used (0.01 ,uA full scale), four peaks (a-d) were detected. On the basis of the previous results and of the peak potentials recorded, these peaks can be assigned to zinc, cadmium, lead and copper, respectively. The metals were quantified by using the standard addition method. The potentials for the preconcentrations after standard additions were chosen so that the interferences arising from intermetallic Cu-Zn compounds were excluded. Thus copper was determined by plating at - 800 mV where zinc is not reduced. Zinc was determined after addition of gallium chloride solution to give a final concentration of 1 x lo-* M gallium. Table 3 gives the results obtained with this procedure. The proposed method, of course, allows only the labile metal to be determined. If the total content of each metal is required, the measurements must usually be made at low pH [ 251. Accordingly, the wine was adjusted with con-
17 TABLE 3 Results obtained in analysis of the wine Metal
cu Pb Cd Zn
Deposition potential (V)
Deposition time (min)
Concentration” (M 1-l) Labile
Total
-
1 1 5 1
61 52 2 148
(1.7) (2.5) (4.0) (1.9)
95 (1.9) 99 (2.1) 3 (4.7) -
_
_
174 (2.4)
0.800 0.800 0.800 1.400b
- 1.200
“Mean with relative standard deviation (%) in parentheses, for five separate determinations. natural pH. “After addition of HCl to give 0.1 M acid.
bAt
centrated hydrochloric acid to an acidity of 0.1 M for further tests. The results obtained (Table 3) are in agreement with the average amounts found in Italian wines [26, 271. The accuracy of these total metal contents was checked by comparison with atomic absorption spectrometric measurements. The discrepancies between the two methods never exceeded 20% even for the lowest concentration determined (i.e., cadmium). The precision of the method was tested by applying the procedure to five separate aliquots of wine. The relative standard deviations reported in Table 3 were, on average, lower than those obtained with conventional drop or film mercury electrodes (6-12% ) [ 281. The marked difference in the deviations over the range of metal concentrations may be due to the increased reproducibility of mass transport. Conclusions The combination of a mercury microelectrode with differential-pulse anodic stripping voltammetry provides excellent sensitivity and gives reliable results for the determination of heavy metals in wine. The measurements can be made directly on the sample without any pretreatment, so that only the labile metals are determined. Some information on speciation can be obtained by determining the total metal concentrations after minor pretreatment (addition of strong acid). Finally, it should be noted that the behaviour of mercury microelectrodes is similar to that of mercury film electrodes, in that intermetallic interferences remain high, whereas the effects of organic matter are less important. The authors thank Mrs. M. Boschian Pegoraro and Mr. D. Rude110for skilful experimental assistance. Financial aid from the Italian National Research Council (CNR) and the Ministry of Public Eduction is gratefully acknowledged.
18 REFERENCES 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
J. Wang,.Stripping Analysis: Principles, Instrumentation and Applications, VCH, Deerfield Beach, FL, 1985. T.R. Copeland, H.H. Christie, R.K. Skogerboe and R.A. Osteryoung, Anal. Chem., 45 (1973) 995. See, e.g., M. Fleischmann, S. Pons, D. Rolison and P.P. Schimdt, Ultramicroelectrodes, Datatech Science, Morganton, NC, 1987. M.R. Cushman, B.G. Bennett and C.W. Anderson, Anal. Chim. Acta, 130 (1981) 323. G. Schulze and W. Frenzel, Anal. Chim. Acta, 159 (1984) 95. K.R. Wehmeyer and R.M. Wightman, Anal. Chem., 57 (1985) 1989. A.S. Baranski and H. Quon, Anal. Chem., 58 (1986) 407. J. Golas and J. Osteryoung, Anal. Chim. Acta, 186 (1986) 1. A.S. Baranski, Anal. Chem., 59 (1987) 662. J. Wang, P. Tuzhi and J. Zadeii, Anal. Chem., 59 (1987) 2119. J. Wang and P. Tuzhi, Anal. Chim. Acta, 197 (1987) 367. L.J. Li, M. Fleischmann and L.M. Peter, Electrochim. Acta, 32 (1987) 1585. Ministero dell’Agricoltura e Fore&e, Metodi Ufficiali di Analisi per i Mosti, i Vini e gli Aceti, Istituto Poligrafico dell0 State, Rome, 1965. B. Sharifker and G. Hills, J. Electroanal. Chem., 130 (1981) 81. J.A. Gunawardena, G.J. Hills and I. Montenegro, Electrochim. Acta, 23 (1978) 693. M. Oehme and W. Lund, Fresenius’ Z. Anal. Chem., 294 (1979) 391. J.E. Batley and T.M. Florence, J. Electroanal. Chem., 72 (1976) 121. C. Brihaye and G. Duyckaerta, Anal. Chim. Acta, 146 (1983) 37. J. Heyrovsky and J. K&x, Principles of Polarography, Academic, New York, 1966. T.R. Copeland, R.A. Osteryoung and R.K. Skogerboe, Anal. Chem., 46 (1974) 2093. T.E. Edmonds and J. Guoliang, Anal. Chim. Acta, 151 (1983) 99. T.M. Florence and G.E. Batley, J. Electroanal. Chem., 75 (1977) 791. G. Piccardi and R. Udisti, Anal. Chim. Acta, 202 (1987) 151. M. Fleischmann, personal communication. D. Jagner and S. Westerlund, Anal. Chim. Acta, 117 (1980) 159. K. Harju and P. Ronkainen, Z. Lebensm.-Unters.-Forsch., 170 (1980) 445. S. Mannino, Riv. Vitic. Enol. Conegliano, 6 (1982) 297. T.M. Florence, J. Electroanal. Chem., 27 (1970) 273.
Printed in The Netherlands
DETERMINATION OF HEAVY METALS IN REAL SAMPLES BY ANODIC STRIPPING VOLTAMMETRY WITH MERCURY MICROELECTRODES Part 1. Application to Wine
SALVATORE DANIELE*, MARIA-ANTONIETTA ANTONIO MAZZOCCHIN
BALDO, PAOLO UGO and GIAN-
Department of Physical Chemistry, University of Venice, Calle Larga S. Marta 2137,30123 Venice (Italy) (Received 20th June 1988)
SUMMARY Differential-pulse anodic stripping voltammetry with a mercury microelectrode is used for the determination of zinc, cadmium, lead and copper in wine at its natural pH without pretreatment. The effects of the matrix on the stripping peaks are studied in detail by varying the concentration of the metals. Intermetallic (Cu-Zn) interferences and the effects of oxygen are described. The results obtained for the labile metal contents varied from 2 ,ug 1-l for cadmium to 148 w 1-i for zinc; standard addition plots were linear over about two orders of magnitude above these levels, demonstrating the negligible effect of organic matter. Acidification of the sample with hydrochloric acid to pH 1 allowed the total metal contents to be determined. The reliability of the method was tested by comparison with the results obtained with atomic absorption spectrometry; the differences were within lo-20%.
Anodic stripping voltammetry (ASV) has been widely used for measuring trace metals in numerous matrices [ 11. The excellent detection limits, reported to be in the 10-10-10-‘2 M range, result from preconcentration of the metal ion of interest into a mercury drop or film. Because this step is aided by mechanical stirring, its reproducibility is one of the source of errors in ASV techniques. Usually, a high concentration of the supporting electrolyte is needed in order to avoid undesirable phenomena such as shifting, broadening and lowering of the stripping peaks caused by ohmic drop effects [ 21. The deliberate addition of supporting electrolyte, however, can create complications when trace and ultratrace metals have to be determined, in that risks of contamination from heavy-metal impurities in the added chemicals are large. Moreover, addition of an electrolyte alters the kinetic and thermodynamic properties of samples, thereby invalidating data on, for example, speciation in natural liquid matrices which have low ionic strength. Voltammetric microelectrodes have received much attention in recent years, 0003-2670/89/$03.50
0 1989 Elsevier Science Publishers B.V.
10
thanks to their attractive features, including fast steady-state diffusional mass transport and low ohmic drop [ 31. Some papers have also demonstrated that sensitive ASV measurements for determining heavy metals can be obtained at mercury microelectrodes [ 4-121. These studies have emphasized, in particular, that the enhanced mass transfer characteristic of microelectrodes reduces the preconcentration times and obviates the need for convective hydrodynamics during the deposition step; moreover, because the ohmic losses are. negligible, the measurements can be conducted even in poorly conductive media without any deliberate addition of supporting electrolyte. All the reports have dealt with synthetic samples, however, while the features of microelectrodes just described suggest useful applications to untreated real samples which are seldom suitable for direct measurements by ASV with conventional electrodes. Because of its effective discrimination against the charging background current, differential-pulse anodic stripping voltammetry (DPASV) has been the most widely used stripping approach [l]. Thus, the combination of DPASV with ultramicroelectrodes should prove to be extremely powerful for applications involving ultratrace levels of heavy metals in difficult experimental conditions. The investigation reported in this paper was undertaken with the aim of determining labile heavy-metal contaminants in wine by using differentialpulse anodic stripping voltammetry at mercury microelectrodes. EXPERIMENTAL
Chemicals
Analytical-reagent grade chemicals, mercury (I) nitrate, lead nitrate, cadmium nitrate, copper (II) sulphate, sodium chloride and gallium chloride, were used to prepare stock solutions. The mineral acids used were of Suprapur grade (Merck). All the solutions were prepared with Millipore Milli-Q water. Nitrogen (99.99% ) was used to remove dissolved oxygen. The wine was a Tocai Grave from Friuli (Northern Italy) with the following main characteristics: alcohol content, about 13% (v/v); total acid content, expressed as tartaric acid, 7.7 mg 1-l; chloride, 18 mg 1-l; and pH, 3.21. These values were determined by using Italian standard methods for food [ 131. Apparatus and procedures
Voltammograms were obtained with an Amel Model 472 multipolarograph in conjunction with an Amel Model 863 X-Y recorder. A Princeton Applied Research (PAR) static mercury drop electrode (Model 303A) was used as the working electrode in some measurements made for comparison. All the experiments with microelectrodes were done in a two-electrode cell configuration; the multipolarograph was used only as a convenient data-acquisition interface and to generate the waveform for the stripping step. The
current flow was monitored by a picoammeter (Keithley Model 485)) the output from this amplifier being connected to the multipolarograph through a convenient resistor. The electrochemical cell was a laboratory-made 20-ml teflon cup. Working mercury microelectrodes were made by electrodeposition of mercury on to a platinum microdisc electrode fabricated by sealing l-pm diameter wire into glass [ 31. The platinum microelectrode was polished with alumina powder (0.05 pm) on a Metron polishing cloth (Banner Scientific Supplies). The mercury was deposited under potentiostatic conditions at large overpotential from 3.0 mM mercury (I) nitrate/l M KNO, at pH < 1 (acidified with nitric acid). The mercury deposit formed was found to be durable and reproducible in its behaviour. The amount of mercury deposited was controlled by the deposition time and was monitored by the current/time (i/t) plot (see Fig. la) recorded in the deposition step. The current increased linearly with the square root of time (Fig. lb), in agreement with a three-dimensional growth [ 141. This finding allows the theoretical relationship r= (2MDCt/p) ‘I2 [ 151 to be applied for estimating the radius of the deposit. For the experimental conditions chosen here, the radius of the deposit typically had the value 5.2 pm. Only deposits giving i/t plots which overlapped (within 2% errors) were used for ASV measurements. After the electrodes had been prepared, they were removed from the plating solution, rinsed carefully with ultrapure water and then transferred to the cell for DPASV measurements. In each experiment (unless stated otherwise), a freshly prepared deposit was used. The samples were kept quiescent during the preconcentration and, because of the unique behavior of microelectrodes associated with the steady-state diffusional flux [lo], no rest period was necessary before the anodic scan. All the DPASV measurements were conducted with a pulse height of 50 mV and a scan rate of 5 mV s-‘. The wine to be analysed was stored in a refrigerator at 4’ C until the day of ( b)
(a)
$A
%A
___1
5..
10.. 5
10..
‘.
0
100
200
300
5
10
15 t/h rll
%
Fig. 1. (a) Potentiostatic transient for the growth of the mercury deposit on a platinum disc (lflmdiameter); (b)plotofivs.t I’* for the transient shown in (a).
12
analysis; it was then equilibrated to room temperature, deaerated by passing purified nitrogen for 10 min and analysed (unless stated otherwise) as withdrawn from the bottle without further pretreatment. A mercury pool was used as reference/counter electrode for the electrodeposition of mercury; an aqueous saturated calomel electrode (SCE) was used for the DPASV measurements. Thelatomic absorption determinations were done with a Perkin-Elmer Model 5000 spectrometer. RESULTS AND DISCUSSION
Effect of the solution composition on the stripping processes Wine is a complex aqueous matrix in which many organic compounds are dissolved. It is known that organic matter may adsorb on the electrode surface, giving rise to unpredictable effects on the stripping peak current [ 11, so that removal or destruction of organic matter is usually necessary prior to classical stripping analysis of wines [ 161.For conventional electrodes, the effects of adsorption are more marked at a mercury drop than at a mercury film [ 171. Mercury microelectrodes behave similarly to mercury films [ 6, lo], hence similar effects are to be expected from organic matter. However, the experimental conditions, e.g., the deposition potential, cannot be chosen simply on the basis of those valid for conventional electrodes, in that the redox behaviour for the species may be different in principle. Thus, in order to verify the effects on the stripping peak current caused by the various substances of the sample, preliminary DPASV measurements were made by using the wine as solvent. Figure 2a shows a typical stripping voltammogram recorded at a mercury microelectrode on a quiescent sample spiked with known amounts of copper (II ) , lead (II ) , cadmium (II ) and zinc (II), after deposition for 1 min at - 1.4 V. These amounts were, in all cases, in excess of those present in the natural sample (see below). The peaks were sharp, well defined and located (except for copper) at potentials very close to those obtained at conventional mercury electrodes for synthetic samples containing ammonium citrate buffer. The peak potentials found are compared in Table 1. Stripping voltammograms recorded on wine spiked with different concentrations of the metals showed no change in the peak potential and shape for zinc, cadmium and lead, while a potential shift (Fig. 3a) and variation in the shape were obtained for the copper peak. Table 2 collects the results obtained for the peak potential and peak width at half-height ( wi12) of the four cations at some representative concentrations. It is noteworthy that the wij2 values of about 40 mV obtained for cadmium and lead, which are known to undergo a reversible two-electron process, are similar to those obtained at a mercury film electrode [ 181. The higher value of wl12 for zinc is a consequence of the less reversible electrochemical process involved
13
1
I
0.6
0.2
nA
nA
( b)
1
I -1.4
o
- 0.7
1.4
E/v
-07
0
E/V
Fig. 2. Differential-pulse anodic stripping voh.ammograms recorded at a mercury microelectrode: (a) wine spiked with 0.23 mg 1-l each of zinc, cadmium, lead and copper after deposition for 1 min; (b) unspiked wine after deposition for 2 min. Deposition at - 1.4 V in quiescent solution; pulse height 50 mV, scan rate 5 mV 8-l. See text for further details. TABLE 1 Comparison of peak potentials recorded on wine and on a synthetic sample prepared in 0.2 M ammonium citrate buffer Sample
Wine” Syntheticb
Peak potentials (V vs. SCE) cu
Pb
Cd
Zn
-0.100 - 0.060
- 0.500 - 0.480
- 0.650 - 0.630
- 1.010 - 1.050
“At the mercury microelectrode, deposition at - 1.4 V for 1 min, quiescent solution. bAt the HMDE, deposition at - 1.2 V for 2 min, stirring during the preconcentration, rest period 15 s. [ 191. The potential shifts and the dependence of wlj2 on the concentration shown for copper indicate the occurrence of a more complex pathway for its stripping. The calibration graphs for cadmium and lead added to the wine were linear over about two orders of magnitude (i.e., 0.002-0.20 mg 1-l Cd with a corre-
14
.
f
/
Fig. 3. Plots of (a) peak potential vs. log C and (b) i vs. C for copper. Deposition for 10 sat -0.800 V in quiescent solution. TABLE 2 Effect of the concentration on the DPASV responses Metal
Concentration bg
cu
Pb
1-‘1
0.088 0.260 0.635 0.104 0.261 0.518
E, WI
-0.120 -0.110 -0.085 -0.500 -0.500 -0.500
w/2
Metal
(mV)
85 70 55 38 40 40
Cd
Zn
Concentration
E,
w/2
his
W)
WV)
-0.650 -0.650 -0.650 -1.010 -1.010 -1.010
40 42 40 75 70 70
1-l)
0.011 0.056 0.112 0.163 0.245 0.327
lation coefficient, r, of 0.993, and 0.05-5.0 mg 1-l Pb with rz0.989); positive intercepts were observed because of the metals present in the wine. The good correlations found suggested that other substances in the wine exercised the same effects on the stripping peaks regardless of the metal concentration. Zinc did not provide linear i vs. concentration plots over the entire range of concentrations because of the formation of intermetallic Cu-Zn compounds [ 201. The occurrence of this interference was proved by plating/stripping measurements of zinc in the presence of gallium, which preferentially combines with copper [ 1,201; the calibration graph was then linear over the range 0.09-1.00 mg 1-l (r = 0.982).
15
The calibration graph for copper obtained by plating at -800 mV is characterized by two linear portions (Fig. 3b). This trend, together with the observed dependence of both wl12 and peak potential on the copper concentration, could be attributed either to hindered mass transport caused by organic matter adsorbed on the electrode surface, which is particularly effective with copper [ 11, or to a decreased number of electrons in the oxidation step, i.e., the formation of Cu+ at low concentrations of copper. The former hypothesis would imply broader and less resolved stripping peaks than those observed in the present experiments. The latter explanation fits better with both the higher wij2 values ( wl12for a one-electron process at a mercury film electrode is about 80 mV) and the decreasing peak current. The chloride ions present in the wine at concentrations as high as 5 x 10e4 M may stabilize copper (I) ions [ 211. If this is the case, the smaller the Cu/Cl- ratio, the greater would be the extent of such copper (I) stabilization; at higher Cu/Cl- ratios, Cu+ is no longer stabilized and the ECE process, via Cu+ disproportionation, would become more important, so giving rise to both higher currents and narrower peaks, in agreement with the present experimental findings. A similar observation was reported in a study set up to understand the effects of small amounts of chloride in the stripping of copper in acetic/acetate buffer solutions [ 221. Moreover, at microelectrodes, chemical reactions following charge transfer tend to be masked because of the rapid diffusion of the first electrogenerated species away from the electrode surface [3]; this substantiates further the formation of copper (I). The trend shown in Fig. 3a may also be explained by the Cu/Cl- ratios in the wine. As the concentration of spiked copper is increased, the transition from a one-electron to a two-electron process is observed with the corresponding change of the Elf2 involved. Experiments in which a set of stripping measurements were done on wine samples spiked with different amounts of sodium chloride demonstrated that the breaks shown by both plots in Fig. 3 occurred at higher copper concentrations as the chloride content was increased. However, a combined effect of chloride ions and other organic substances on the stripping peak of copper cannot be excluded. Effect of dissolved oxygen and exploitation of the same mercury deposit
Experiments were conducted to check the effect of oxygen in the sample and the possibility of using the same mercury deposit for more than one cycle. It was observed that only fully aerated samples caused low reproducibility of the data, particularly for copper which was almost obscured by the reduction peak of oxygen. Small amounts of oxygen remaining in the samples after inefficient deaeration did not cause appreciable effects on any of the stripping peaks studied, possibly because the differential-pulse waveform used is not very sensitive to irreversible processes. When the same mercury deposit was used for successive experiments, the
16
anodic scan was ended at 0.0 V (i.e., just after the copper had been stripped) in order to avoid stripping the plated mercury. In the second run, after preconcentration ,under the same experimental conditions, the stripping voltammograms always showed a copper peak enhanced by about 36% and a zinc peak diminished by about 30%, whereas the lead and cadmium peaks were substantially unchanged. This trend was even more marked for copper and zinc when further experiments were done with the same electrode, e.g., in the third run the copper peak increased by 150% and the zinc peak almost disappeared. This behavior can be explained on the basis of the formation of intermetallic CuZn compounds and by admitting that not all the copper plated in the first run is stripped in the subsequent anodic scan. In order to shed light on this matter, the procedure was repeated on freshly prepared mercury deposits and plating was done at a potential of -800 mV where zinc does not interfere. In this case, the copper peak was enhanced by only about 12% in the second run whereas the lead and cadmium peaks remained unchanged. These results show clearly that only the copper peak is enhanced, probably because its peak potential is so close to the end of the anodic scan. Moreover, when the differential pulse waveform is used, some of the metal stripped from the electrode during the pulse is replated on the electrode in the waiting period between pulses [ 11, an effect magnified here by the enhanced mass transport associated with the nonplanar diffusion at microelectrodes. This “memory effect” could also be ascribed to the formation of an insoluble film of copper salts or oxides on the electrode surface [ 23,241. The depression of the zinc height in the second run is a consequence of this memory effect; zinc would be plated in a mercury deposit containing larger amounts of copper so that formation of the intermetallic compound would be accentuated. Blank levels, precision and accuracy
Figure 2b shows the stripping voltammogram obtained on the wine sample taken from the bottle; deposition was done for 2 min at - 1.4 V in the quiescent sample. At the sensitivity used (0.01 ,uA full scale), four peaks (a-d) were detected. On the basis of the previous results and of the peak potentials recorded, these peaks can be assigned to zinc, cadmium, lead and copper, respectively. The metals were quantified by using the standard addition method. The potentials for the preconcentrations after standard additions were chosen so that the interferences arising from intermetallic Cu-Zn compounds were excluded. Thus copper was determined by plating at - 800 mV where zinc is not reduced. Zinc was determined after addition of gallium chloride solution to give a final concentration of 1 x lo-* M gallium. Table 3 gives the results obtained with this procedure. The proposed method, of course, allows only the labile metal to be determined. If the total content of each metal is required, the measurements must usually be made at low pH [ 251. Accordingly, the wine was adjusted with con-
17 TABLE 3 Results obtained in analysis of the wine Metal
cu Pb Cd Zn
Deposition potential (V)
Deposition time (min)
Concentration” (M 1-l) Labile
Total
-
1 1 5 1
61 52 2 148
(1.7) (2.5) (4.0) (1.9)
95 (1.9) 99 (2.1) 3 (4.7) -
_
_
174 (2.4)
0.800 0.800 0.800 1.400b
- 1.200
“Mean with relative standard deviation (%) in parentheses, for five separate determinations. natural pH. “After addition of HCl to give 0.1 M acid.
bAt
centrated hydrochloric acid to an acidity of 0.1 M for further tests. The results obtained (Table 3) are in agreement with the average amounts found in Italian wines [26, 271. The accuracy of these total metal contents was checked by comparison with atomic absorption spectrometric measurements. The discrepancies between the two methods never exceeded 20% even for the lowest concentration determined (i.e., cadmium). The precision of the method was tested by applying the procedure to five separate aliquots of wine. The relative standard deviations reported in Table 3 were, on average, lower than those obtained with conventional drop or film mercury electrodes (6-12% ) [ 281. The marked difference in the deviations over the range of metal concentrations may be due to the increased reproducibility of mass transport. Conclusions The combination of a mercury microelectrode with differential-pulse anodic stripping voltammetry provides excellent sensitivity and gives reliable results for the determination of heavy metals in wine. The measurements can be made directly on the sample without any pretreatment, so that only the labile metals are determined. Some information on speciation can be obtained by determining the total metal concentrations after minor pretreatment (addition of strong acid). Finally, it should be noted that the behaviour of mercury microelectrodes is similar to that of mercury film electrodes, in that intermetallic interferences remain high, whereas the effects of organic matter are less important. The authors thank Mrs. M. Boschian Pegoraro and Mr. D. Rude110for skilful experimental assistance. Financial aid from the Italian National Research Council (CNR) and the Ministry of Public Eduction is gratefully acknowledged.
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