Anal. Chem. 1995,67,4484-4486
A CLHybrid99 Mercury Film Electrode for the Voltammetric Analysis of Copper and Lead in Acidified Seawater and Other Media Rosa Garcia-Mom6 Carra,t Antonio Mnchez=Misiego,tand Albert0 Zirino*v* Code 521, RDT & E Division, Naval Command, Control and Ocean Sutveillance Center, San Diego, California 92152
Plating conditions for preparing a consistentlyreproducible Hg film electrode on a glassy carbon substrate in acid media are evaluated. It is found that a “hybrid electrode”, Le., preplated with Hg and then replated with Hg in situ with the sample, gives very reproducible results in the analysis of copper (and lead) in seawater. Consistently reproducible electrode performance allows for the calculation of a cell constant and prediction of the slopes of standard addition plots, useful parameters in the study of copper speciation in seawater. For a number of years we have been interested in studying the speciation of copper in seawater, primarily by electrochemical means and, if possible, in situ.’ In stripping voltammetry, one method of doing this is to study the slopes and intercepts of standard addition plots under various experimental To carry out these studies, it is essential that the electroactive surface of the electrode is independent of the experimental conditions, e.g., pH, presence of organic complexants, etc. These considerations lead us to Hg electrodes, either as liquid Hg or as a Hg film on an inert substrate. By far, most studies and practical applications have been made with the HMDE or a glassy (vitreous) carbon-mercury film electrode (GC-MFE). Because of the sensitivity and desirable physical characteristics for field applications of GC-MFEs, a great deal of work, both practical and theoretical, has been directed toward studying those parameters which affect its performance. These parameters include supporting electrolyte, solution pH, temperature, Hg concentration, stimng rate or rotational velocity, film thickness, deposition potential, and sweep v e l ~ c i t y . ~ - ’ ~ GC-MFEs may be preformed, e.g., the film is prepared in a pure HgQI) solution, under conditions usually different from those for the analysis of the sample, or they may be formed in situ in Permanent address: Departamento de Quimica Analitica y Electroquimica, Universidad de Extremadura, Badajoz, Ex., Spain. * Also at Instituto de Investigaciones Oceanologicas, Universidad Autonoma de Baja California, Ensenada, B.C., Mexico. (1)Tercier, M. L.; Buffle, J.; Zirino, A; DeKtre, R. R Anal. Chim. Acta 1990, 237,429-437. (2) Scarano, G.;Morelli. E.; Seritti, A,; Zirino, A Anal. Chem. 1990,62,943948. 177(3) Scarano, G.; Romei, C.; Seritti,A,; Zirino, A Anal. Chem. 1991,245, 181. (4) Sanchez-Misiego,A; Garcia-Monco Carra, R; Zirino, A. Electroanalysis, in press. (5) Nurnberg, H. W. Electrochim. Acta 1977,22,935-949. (6) Florence, T. M. Analyst 1986,111,489-505. (7) DeVries, W. T. J. Electroanal. Chem. 1965,9,448-456. (8) DeVries, W. T.; Van Dalen, E. J. Electroanal. Chem. 1967,14,315-327. (9) Florence, T. M.]. Electroanal. Chem. 1970,27,273-281. (10) Ellis, W. H. J. Chem. Educ. 1973,50, A131-A147. +
4484 Analytical Chemistry, Vol. 67, No. 24, December 15, 1995
the medium to be analyzed. ’This is done by adding HgQI) to the sample matrix and depositing the film simultaneously with the analyte. In the latter case, the conditions used for the deposition of the film are usually chosen to suit the analyte. Despite its frequent use, there is still a great deal of empiricism associated with the GC-MFE. For instance, some workers advocate tossing out the first experimental result,11J2while others recommend stripping the electrode a number of times before initiating the electrolysis of the analyte.13814Also, with an in situ plated electrode, where the deposition potential of the Hg film is, in fact, determined by the plating of the analyte, it is unclear whether the plating potential leads to optimum electrode performance. This may indeed be the cause of the variability in performance often associated with the GC-MFE. This problem generally does not interfere with quantitative analyses done by standard addition, where the added standard compensates for variability in active electrode area or performance. It does, however, seriously interfere in studies of chemical speciation in natural media, where analyte flux in and out of the electrode is taken as a measure of physicochemical f ~ r m Indeed, . ~ ~ ~while ASV on the GC-MFE is known for the excellent linearity of its calibration plots, to our knowledge, the slopes of these curves are never used as a measure of analyte flux. Seawater and other natural waters samples are usually acidified during routine voltammetric analyses. Also, the increased signal obtained after acidification is often taken as evidence of a change in chemical speciation. In the latter studies, it is essential that the mercury film coverage be independent of pH.’j However, the preparation of a Hg film in acidiiied media may produce less than optimum film coverage and an electrode which is unsuitable for speciation studies in which the pH of the medium is changed. In acidic media, the presence of bubbles may be an important indicator of film condition. Under certain experimental conditions, we have observed the formation of bubbles of different sizes that adhere strongly to the electrode’s surface. In some cases, they appear to be entrapped in the Hg. These bubbles, which are formed during preparation of the film, cannot be easily removed by agitating either the electrode or the solution vigorously. Because the degree of bubble formation increases with the acidity of the sample as well as the applied overvoltage, we attributed the bubbles to the reduction of hydrogen ion on the glassy carbon substrate. (11) (12) (13) (14) (15)
Batley, G. E.; Florence, T. M. J. Electroanal. Chem. 1974,55,23-43. Lund, W.; Salberg, M. Anal. Chim. Acta 1975,76,131-141. Lund, W.; Onshus, D. Anal. Chim. Acta 1976,86,109-122. Florence, T. M. J. Electroanal. Chem. 1974,49,255-264. Zirino, A In Marine Electrochemistry; Whitiield, M., Jagner, D., Eds.; Wiley: New York, 1981; Chapter 10.
0003-2700/95/0367-4484$9.00/0 0 1995 American Chemical Society
In spite of the potential importance of bubble formation on analytical results, most published works have said little on this subject. Only Stulikova16determined that a glassy carbon surface contains sites of special activity that easily lead to adsorption or electrolytic phenomena and suggested that simultaneous hydrogen evolution (bubbles) during Hg deposition produced more uniform films. In this work, we have studied the process of film formation in order to optimize the conditions under which a consistently reproducible Hg film can be deposited on glassy carbon in acidified seawater media. EXPERIMENTAL SECTION Pb(II), Cu(II), and Hg(II) stock solutions were prepared from atomic absorption standard solutions (Aldnch Chemical Co.) that also contained small quantities of HN03. Other materials (sodium citrate, HCl, ethylenediamine, etc.) were of reagent grade quality. Deionized water (Milli-Q) with a resistivity of greater than 18 MQ was used for making the appropriate dilutions. Seawater samples were collected in Janurary 1992 from the pumped seawater system at the Scripps Institution of Oceanography (San Diego, CA) and analyzed without further purification or filtration. The artificial seawater was prepared according to the recipe of Kester et al.” ASV experiments were carried out with a BAS100 (Bioanalytical Systems) polarographic analyzer equipped with an Autocell stand. pH measurements were made with an Orion Model SA720 pH meter. The working electrode was a vitreous carbon disk, obtained from EG&G Princeton Applied Research, with an approximate area of 0.5 cm2. Potentials were measured against a Ag/AgCl reference electrode, and a platinum wire was used as the auxiliary electrode. GC-MFEs with different Hg coverage were prepared by depositing a mercury film on the vitreous carbon disk at different potentials for various periods up to 15 min in a stirred solution. This solution was the seawater sample to be analyzed, made 1 x M in HguI), acidified with HC1 (to pH 2.3), and previously purged with NPfor 10 min. At the end of the film preformation period and regardless of the initial plating potential chosen, the stripping of the film was carried out by sweeping the potential linearly from -800 to 0.0 mV, at 100 mV/ s. This procedure removed any plated metals from the film. Cu and Pb determinations were performed by plating at -1.000 V for 240 s in the same seawater solution used to prepare the electrode. During the deposition of mercury film and metals, the solution was stirred at constant velocity. A 10 s “rest” period followed deposition. By preplating Hg on the electrode for a prolonged time and then plating it again under conditions best suited for the analyte, we have essentially created a “hybridelectrode” combining the properties of both prefilm formation and in situ film preparation. Determinationsof Cu and Pb concentrationswere carried out by standard addition, with three additions for each determination. RESULTS AND DISCUSSION First Tests. The initial studies that were conducted to examine the effect of pH on Hg coverage (1 x M Hgz*) also determined that the presence of large amounts of chloride, citrate, (16) Stulikova, M. J. Electroanal. Chem. 1973,48,33-45. (17) Kester, D. R; Duedall, I. W.; Connors, D.N.; Pytkowicz, R M. Limnol. Oceanog. 1967,12, 176-178.
sulfate, and nitrate in the medium had no effect. Mercury coverage was determined by observing the Hg oxidation peak. The redissolution signal of the Hg film itself can be an indicator of the degree of film formation. In general, when the Hg oxidation signal was quantiiied as low, the analyte signal was also deemed unsatisfactory. This indicator was used to establish the minimum deposition time in which the film is considered totally formed. In this way, the study of applied potentials could be made with the guarantee that in all cases the film was sufficiently formed. An electrolysis time of 15 min always produced a fully functioning film. However, with shorter electrolysis times, it was often observed that optimum coverage of the electrode was only obtained during the second stage of the electrodeposition, that is, after the first analysis cycle. This may possibly explain why some authors recommended that the results of the first analysis cycle be systematically discarded. Influence of pH and the Deposition Potential of the Fib. Once the observations above were confirmed, we studied the influence of solution pH and deposition potential on Hg film coverage by observing (1) the reproducibility of the peak heights of Cu and Pb in the same solution, after cleaning the electrode (wiping off the film with a paper tissue) after each measurement; (2) the linearity of the Cu peak current as a function of standard added; (3 ) the presence or absence of small bubbles on the electrode surface; and (4) the degree of adhesion of the Hg film to the vitreous carbon substrate. Degree of adhesion refers to the observation that occasionally,when the electrode is removed from solution after a determination,the mercury film can be seen to separate and coalesce on a small part of the vitreous carbon. This phenomenon is also related to solution pH and deposition potential. We worked at several pH values between 1.4 and 3.0, because it is at these values that the formation of bubbles and degradation of the film can be readily observed and because many ASV analyses of trace metals in seawater require acidification of the sample to a pH of about 2. At pH < 1.4, the reduction of H+ makes it difficult to avoid bubbles at any reasonable overvoltage; at pH > 3.0, the effect cannot be readily observed. For similar reasons, we chose mercury deposition potentials between -100 and -1100 mV, these being the usual limits in the ASV analysis of Cu and Pb in seawater. Table 1 contains a summary of the results obtained at pH = 2.3. It can be seen from the data that Cu and Pb peak heights approach a maximum plateau when the film is first prepared at between -300 and -500 mV. The reason for this is that insufficient mercury is deposited at potentials more positive than -300 mV, and that at potentials more negative than -500, the production of hydrogen gas bubbles interferes with the deposition of the film. This also explains the observations noted in the last two columns, which suggest that something has interposed between the glassy carbon surface and Hg which prevents better cohesion between them. At pH < 2.3, this phenomenon is more pronounced, while at pH > 2.3, the phenomenon mentioned disappears gradually with increasing pH. The reproducibility of the peak heights was studied in artiiicial seawater, where the possible effects of dissolved organic matter are minimized. The results for Cu and Pb are presented in Table 2, which shows that precision improves when the film is preplated at less negative potentials; similarly, peak heights were higher under these conditions. However, when the film was preplated Analytical Chemistry, Vol. 67, No. 24, December 75, 7995
4485
Table I. Effect of Potential on Film Formation (Seawater HCI, pH = 2.3)'
+
12
stripping signal Pb
I
cu
deposition potential (mv)
-E (m\S
I, -E GA) (m$
- 100 -200 -300 -400 - 500 -600 -700 -800 -900 - 1000 - 1100
419 414 421 421 420 426 420 420 419 418 420
1.33 1.36 2.35 2.25 2.57 2.88 2.16 2.48 1.72 2.00 1.93
216 211 188 183 186 204 202 207 202 202
I, presence adherence @A) ofbubbles offilmC 1.79 5.82 5.06 6.39 2.89 4.12 3.84 2.68 5.06 2.46
yesb no no no no no no no/yes Yes yes no/yes
d e
*** ** ** * * *
a
1
Q
- 6 4 2
*
**
(I
Table 2. Reproducibility of Results (Artificial Seawater HCI, pH = 2.0)'
+
RSD (%)
T
***
Deposition potential is for preparation of Hg film. Deposition time is 900 s. Cu and Pb are plated at -1000 mV in same solution for 240 s. Appears during stripping cycle. Degree of adherence: ***, film is strongly attached, does not break within electrode is removed from solution; **, film breaks when electrode is out of solution and tapped with finger; *, film breaks when electrode is removed from solution. No film observed. e Film is poor.
deposition potential (mv)
Pb
cu
no. of measmnts
- 1000 -800 -300
5.3 4.7 3.9
8.3 10.6 4.7
6 6 5
Same experimental conditions as for Table 1.
at potentials more positive than -300 mV, peaks were irreproducible. It may be inferred from the Cu and Pb peak signals that, in acid media, Hg deposition at -300 mV is more uniform. After preparing the electrode film at -0.300 V, we studied the linearity of standard additions of Cu(II) ion to natural seawater under two very different conditions: natural seawater made pH = 2 with HCl and natural seawater to which ethylenediamine (en) was added (PH x 9). In one case, we first carried out the standard additions in acidfied media and then brought the sample to pH = 9 with en. The results are presented in Figure 1. It can be seen from the figure that the standard addition curves possess a very high degree of linearity and reproducibility, even though they were produced with three individually deposited Hg films (essentially three different electrodes). Also, the slopes were maintained irrespective of whether the deposition of analyte was first made in basic or acid media. This indicates that the initial mercury deposition at -0.300 V yields deposits of equal area and similar morphology. It may be noted that the ratio of the slopes in the two media (en/HCl) approaches 2.5. In fact, in all our work with Cu, the
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/
10
Analytical Chemistry, Vo/. 67, No. 24, December 15, 1995
0
1
2
3 4 5 ppb Cu2+ ADDED
6
7
8
Figure 1. Linearity of peak height (6) against concentration of added copper in seawater media. Film preplated at -300 mV for 900 s; Cu plated at -1000 mV for 240 s.
slope ratio was always between 2 and 3 in these two media. Theory and experiment indicate that the ideal slope ratio in these media is 2 for spherical electrodes and 4 for planar electrode^.^^^ Therefore, it may be inferred that the mercury coverage of the vitreous carbon lies between the two geometries or, alternatively, may be composed of numerous microscopic drops. CONCLUSION In acidic media, it is necessary to be especially careful during the first moments of mercury plating, because at this time one is working with bare glassy carbon substrate. The precautions consist of depositing Hg at potentials which do not significantly reduce H+, thereby avoiding the formation of bubbles, but which are sufticientlynegative to deposit Hg effectively. After this stage, it is possible to perform the electrolysis at more negative potentials, thereby improving the quality of the film, as suggested by Stulikova.16 Today, the possibility of programming the deposition potential of the film at a potential different from the plating potential of the analyte offers no technical daculties and offers the possibility of uniting the advantages of both preforming the film and producing the film in situ, without having to change solutions and thus risk contamination of the sample. This should make it possible to obtain better results with in situ instrumentation and to conduct in situ studies of speciation. ACKNOWLEDGMENT RG.-M.C. and AS.-M. acknowledge support via a fellowship from the Ministerio de Educacion y Ciencia (Spain). A.Z. was supported by-the US. Office of Naval Research. Received for review July 31, 1995. Accepted September 29, 1995.@ AC950758E Abstract published in Adoonce ACS Absfructs, November 1, 1995.
