Direct Simultaneous Determination of Trace Amounts (ppb) of Zinc(ll), Cadmium(ll), Lead(ll), and Copper(l1) in Ground and Spring Waters Using Anodic Stripping Voltammetry: The Analytical Method A. H. 1. Ben-Bassat,’ J.-M. Blindermann, and Avraham Salomon Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Israel
Eliyahu Wakshal Groundwater Research Center, The Hebrew University of Jerusalem, Israel
The estimation of trace amounts of metals is an important analytical task, and suitable methods are needed in water pollution research as well as in bioinorganic (oligoelements) and sea water or metallographic research. The characterization of ecologically important phenomena requires knowledge of the nature of nonpolluted, natural reference media. A survey of the occurrence of trace amounts of metals in ground water and springs, in Israel, is now being carried out in our laboratory. The following is an attempt to develop a sufficiently sensitive and accurate method for the direct simultaneous determination of traces of zinc, cadmium, lead, and copper ions in “natural” water, for the above survey. Anodic stripping voltammetry (ASV) using a hanging mercury drop stationary electrode (HMDE) was selected as the most appropriate technique ( I ) . No previous separation of the components being needed, and the high sensitivity of the method reaching reproducible results in the 10-6-10-7M range, rapid direct simultaneous determinations are assured. There is no need for laborious and timeconsuming “standard addition” techniques which make routine work almost impossible. A large amount of theoretical and experimental work has been done on stationary electrode processes, during the last two decades, and methods using ASV were proposed for the determination of trace amounts of metals in a large variety of materials such as steel, pure spectroscopic zinc, pure salts and acids, sea water, Dead Sea brine, biological materials, and drugs. The subject is fully reviewed in references (2-8).
EXPERIMENTAL A. Purification of Standard and Supporting Electrolyte Salts. Contamination by the ions to be determined, present in the reagents, is critical when working in the trace concentration range. Special care was therefore taken in the preparation and purification of reagents, distilled water, supporting electrolytes, and agaragar bridge components. 1. Distilled Water. Deionized water was distilled in two steps with a quartz built electrical distillator (Heraeus-Schott-Quartz Schmeltze Bi-Destillier Apparate) coupled with an ion-exchange column. The water thus obtained was stored in polyethylene containers and considered as triply distilled, TRIDEST water, with which all standard curve determinations, dilutions, and solution preparations were carried out. 2. Metallic Salts. Zn(NO&. 6H20, Cd(NO& 4H20, and C u ( N 0 3 ) ~3Hz0 . were Baker Analyzed; P b ( N 0 3 ) ~ was a Riedel-De Haen A.G. Seltze Hannover product. The analytical grade reagents were subsequently purified in two steps as follows. ELECTROLYSIS.A concentrated aqueous solution 2-4M was electrolyzed for 48 hr. in an electrolytic cell having a Pt-gauze anode ( S = 40 cm2) (Johnson Matthey Metal Ltd., London) and a mercury pool large cathode with magnetic stirring. With an apTo whom correspondence should be addressed. 534
plied potential of -1.50 V and with vigorous stirring of the solution, metallic ion impurities are practically removed and amalgamated into the mercury pool cathode. FRACTIONAL CRYSTALLIZATION. The electrolytically purified solution was concentrated by heating and submitted to fractional crystallization. The different fractions were dried and checked for impurities using ASV methods, with long preelectrolysis fiime. 3. T h e Supporting Electrolyte, N a N 0 3 , purified as described in 2. above, was a Riedel-De Haen product. Here, special ipecautions were needed because of the large relative concentrajtion of the NaN03 used (102-104 times greater than the determined cationic species). The highly purified NaN03 was also used for the preparation of the agar-agar salt bridge which separates the Fiaturated KC1 solution of the Saturated Calomel Electrode (SCE) wference electrode from the solutions in the working cell, thus preventing intercontamination. 4. T h e Agar-Agar Salt Bridge was prepared as Follows: 4 grams of agar-agar powder (CP) was introduced into 90 ml of TRIDEST water in a small beaker and gently heated on a water bath until dissolved; 30 grams of pure NaNO3 was then dissolved and the warm mixture transferred to the bridge arm of ‘,he SCE, provided with a fritted glass end. After 15-30 minutes, the gel solidified and was ready for use. The lifetime of such a bridge was 2-3 weeks. Effectiveness of contamination prevention was checked daily by adding Ag+ ions to the water container, which served to store the bridge during the night. 5. Commercial N 2 Gas (Merkaz Hakhamtzan, Nahariya) was used for deaeration without further purification, and proved completely satisfactory. Comparative experiments using the non-purified gaseous nitrogen alternately with a series of 3 traps (w8,shing bottles) containing alkaline pyrogalol in series with a Sargent Cu-containing oven, which absorbs any 0 2 possibly present, s.howed no difference a t all in the polarogram obtained. The total volume of electrolyzed solutions was usually 100 ml, and there was, no need for “scrubbing” or moistening the N2 gas. B. Electrolysis Cell and Accessories. An electrolysis cell was built in accordance with the “Sargent Instruction Manual” (9)and was preferred to other suggestions for its simplicity and versatility. Figure 1,a and b, shows a picture of the setup and the cell. C. The HDME Working Electrode. The HDME working electrode was prepared by plating with, mercury an especially designed Sargent S-29314-30 platinum electrode. The plating was carried out in an electrolysis bath contajning HgC104 as electrolyte, with the HDME serving as cathode and an Hg pool at the bottom as anode (9). The surface of the plated platinum substrate was ca. 1.0 mm2 and the volume of the suspended (HD) drops varied from 0.8-2.0 mm3. It is essential to carry out all determinations (of the same series) with the same HDME and H D drops of the same size. No interference from formation of intermetallic compounds, Pt-Hg-Me (Me = Zn, Cu, Pb, Cd) was detected as claimed in ( 2 ) , and the HDME of this type, prepared and used as described above, was greatly superior as compared with those of the Kemula type. D. Polarographs. Two instruments were used alternately, namely, the Sargent FS fast sweep recording polarograph, especially designed for ASV techniques, and the Electroscan-30 by Beckman. E. Working Procedures. ASV runs were carried out as usual, the preelectrolysis step being followed by the anodic run. Typical
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
Figure 1. Electrolysis cell and accessories (see text "Experimental" and also Sargent Manual for FS Polarograph( 9 ) .
( a )General ( b ) Details, electrodes, etc.
r-----
I
I
t ._
-200 -3w -4w -*ElmV) vls
-200 -400 -600 -600 -1000 -1200 -+E(rnVI ,... . , v..i + .5 C E
Figure 2. Anodic stripping voltammogram of a mixture of 7 X W 7 M Cd2+ and 7 X IO-'M PbZ+
Figure 3. Anodic stripping voltammogram of a mixture containing 9 X IO-'M 2n2+ and 3 X IO-'M Pb2+
(Supporting electrode lO-*Y NaNOs: 5-min preelectrolysisat -1300 mV. Y = 16.6 mV/sec. $ens = 0.004pA/mm) (lor measuring ip[h).A and A' = A 8: see text). Recorded With Sargent F. S. Polarograph
Supporting electrde 10-ZM NaNOI: 5-min Dreelectrolysis at -1200 mV lv = 20 mV/ sec.: sen$ = 2.8 MA.)Recorded with the Beckman 30 Eiectroscan
anodic dissolution (stripping) voltammograms are shown in Figure 2 and 3, together with the method we used for the measurement of the peak height and the areas under the curves. The advantages of using peak curves instead of the S shaped curves of classical polarography are obvious. Repeated determinations were widely used in this work. Calibration Curues are obtained by running the preelectrolysis and AS steps in identical experimental conditions with 4-5 separate solutions of different, known, concentrations, and plotting i , ( h ) or A (the area under the anodic curve) us. [c]. Calibration curves obtained for mixtures of cations at the concentrations indicated are given in Figures 2-6. The Medium Exchange Method (MEM), proposed hy Ariel and coworkers (10) to overcome problems arising in metal trace analy-
sis of De ad Sea Brine, was used by us when our samples were of ."On ll-> ..L . ..:LL --A:c--A:-^r A-:",," B high ' ' s ,:-:L..x a ~ n ~ y,L"AU p p ~IUI , I , LVUL W L L U IIIUUIIIL.UVII YL -LW technique. The preelectrolysis step was performed in the unmodified natural medium (the sample) and then the solution was exchanged for another deaerated solution of supporting electrolyte (NaN03). This technique proved indispensable with samples containing more than 200 ppm C1-, which made the determination of Cu2+ almost impossible owing to the shift of the Hg (of the HDME) dissolution peak to more negative values strongly overlapnine with the Cu Deak (Fieure 6). Soeeial care must be taken in
+
__I
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3. MARCH 1975
.
535
I
.-t
I
-
+300 +lo0 0 -100 -300 -500
c + E ( m V ) v / s S.C.E
+E(mV) v/s S.C.E.
Figure 6. Anodic stripping voltammogram of a solution containing 9 X 10-7M of Zn2+ Cd2+ -t Pb2+ Cu" and 2 X 10-3M NaN03
+
Figure 4. Anodic stripping voltammogram of an aqueous solution containing Pb2+ -tCu2+, both at a concentration of 5 X lO-'M Supporting electrode 10-2M NaN03; 5-min prlectrolysis time, mvlsec.; sens = 0.004 (Sargent F.S.)
Y
+
Preelectrolysistime = 5 min; v = 40 mV/sec.; sens = 2.8 FA (Beckman 30 Eiectroscan)
= 33.3
e. Stirring was stopped, the scanning voltage range set, and the electrode polarized from -1500 mV toward positive values at a scan rate of 40 mV/sec. The voltammogram i = f ( Swas ) recorded simultaneously. f. For repeated runs, the Me content of the amalgam drop was fully oxidized by applying a potential of +300 mV for 60 sec and stirring. g. Me concentrations were calculated by comparison with calibration curves. 2. Samples Containing from 200-720ppm C1-. a. Same as la-Id above. b. Step l e was carried out at a rate of u = 20 mVlsec. The anodic stripping stopped at -250 mV. The voltammograms of Zn, Cd, and Pb ions were thus obtained. c. The sample solution was quickly exchanged with a 100 ml aqueous solution of 2 X 10-2M NaN03 (supporting electrolyte only, previously deaerated). d. The solution was deaerated for another 60 seconds. e. An anodic run was carried out from -300 mV ( u = 40 mV/ sec.) Calibration curves were obtained by working as in 2(a-e). Special care must be taken to ensure non-contamination of the agar bridges (daily checks) and to work out all the determinations (calibration curve and sample measurements) in strictly identical experimental conditions by same-day successive runs, using reagents of high purity for calibration.
t
.-
Cu++ w it hou t CI -
1
t300 +lo0 -100 -300 -500 + E ( m V ) v i s S.C E Figure 5. Anodic stripping voltammograms of an aqueous solution of 10-7M Cu2+ in 2 X 10-2M NaN03 and in presence of 360 ppm CI-, or 720 pprn CI-
RESULTS AND DISCUSSION
Beckman 30 Electroscan, same experimentalconditions as for Figure 4
A series of six possible mixtures containing pairs of t h e four ions investigated-Zn2+, Cd2+, Pb2+,a n d Cu2+-were
trolyte components ( I O , 12). Our simple exchange technique was satisfactory, and we needed only to deaerate the supporting electrolytic solution for 60 additional seconds. Procedure. The simultaneous determination of Zn2+, Cd2+, Pb2+, and Cu2+ions was carried out as follows. 1. Samples Containing Less t h a n 200 p p m C1-. a. 100 ml of the sample containing 10-6-10-7M of the ions determined, and l ml of NaN03, 2M were introduced into the electrolysis cell (Figure 1). b. Deaeration was carried out by passing a stream of pure gaseous nitrogen through the sample for 10 minutes. c. A mercury drop was caught from the capillary supply (0.8-1.6 mm3) and hung on the mercurized platinum (central) working HMDE. d. The magnetic stirring of the solution was started and a potential difference of -1500 mV applied across the HDME and the reference SCE. This preelectrolysis step lasted exactly five minutes.
determined after characterization of t h e anodic peak potential of each isolated ion. T h e following parameters were systematically varied (one at a time): a. Absolute a n d relative concentrations (concentration ratios) of t h e M e n + determined. b. Concentration of t h e supporting electrolyte. c. T i m e of preelectrolysis a n d resting time. d. Range a n d r a t e of polarization of the anodic step. e. T o t a l C1- concentration. T h e same search for optimal conditions ensuring good resolution a n d reproducible results was repeated with mixtures containing three components, a n d finally with mixtures of t h e four cations. Our first experiments were devoted t o finding t h e optimal time of preelectrolysis, when working in t h e loW710-6M range. T h e results obtained show t h a t a preelectrolysis time of 300 seconds ( 5 minutes) is satisfactory.
536
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
Table 11. Results of AS Voltammograms Carried Out with Equimolar Mixtures of Zn2+ + Cd2+ + Pb2+ + Cu2+,in the Range 1.0-9.0 x 10-7Min 2 x 10-2M NaN03, at the Electroscan 30-Beckman Polarograph; 5-Minute Preelectrolysis at - 1300 mV
Table I. Results of AS Voltammograms Carried Out for ( 1 : 1) Mixtures of Zn2+ + Pb2+ in the Range 1.0-9.0 x 10-7M, in2 x 10-2MNaN03 at the Electroscan-30-Beckman Polarograph, 5-Minute Preelectrolysis at - 1300 mV ,IlP
ZnZT
concn, ml 10.'
:I
ip, mn,
Deviation,
A'
:.
cm'
Pb'l ,
Deviation,
Deviaip, m m tion,
;~
7.0 0 . 0 16 14.0 M . 1 30 19.6 -4.9 68 29.6 +3.0 98 3 5 . 6 M . 9 108 i,, = average height ( 5 determinations)
1 . 0 3 9 . 0 -2.50 3 . 0 71.0 +2.10 5 . 0 94.0 -4.00 7 . 0 130.0 +2.30 9 . 0 154.0 - 2 . 0 0
A'
8
cm2
Deviation,
$6
Z2+ A',
1 0 . 5 0 3 . 0 -0.05 ~ 3 . 0 0 1 0 . 1 +10.50 + 0 . 5 0 1 4 . 0 -0.05 +5.00 2 0 . 7 + 3 . 4 -10 24.0 - 4.6
of current peak, in mm. A ' = area under the voltammogram curve (A' = A B, see Figure
+
3).
Figures 2, 3, and 4 show the shape of curves obtained when the peaks occur at neighboring potential values. The "tailing" of one peak into the other interfered in the baseline determination. The figures also show the mode of measuring of the peak height, h = i p, and the determination of the surface under the curve ( A )and ( A ' ) which is the sum of the two triangular areas (A B). These determinations were repeated a t least five times with solutions of the same concentration, in addition to repeated runs of the same solution (see Experimental Section E). The percentage deviation found did not exceed 3% (calculated from data on 96 runs with the six possible couples between zinc, cadmium, lead, and copper ions). In addition, a series of runs was made with couples of ions at different concentration ratios, as in the case of Figure 3. One of the cations was kept at constant concentration ( i e . , 9 X 10-7M) while that of the other was varied from (1.0-9.0) X 10-7M. It is interesting to note that some cations and couples show better linearity than others, possibly owing to the differences in "solubility" and diffusion coefficients of different metals in Hg. Table I summarizes the results of the above described types of experiments, and shows the variation of ip(h)and A' in the case of the Zn2+-Pb2+ couple, as a function of concentration (calibration curves) for the ratio 1:l.Table I suggests that in this case, the plot of A' shows the best linearity, especially in the case of Pb2+. This is why we usually used this sort of plot especially when working with mixtures of the cations or with large excesses of C1-. Figure 5 shows the interference of high C1- concentration on the determination of the Cu2+ component. In these cases our medium exchange method (MEM) was used (see Experimental Section E). For each combination we sought optimal peak resolution and easy base-line determination, varying experimental conditions such as HD size, polarization rate, resting time between preelectrolytic and anodic steps, and sensitivity. The same kind of series was carried out also for triplets of ions. Once again we found that the sensitivity could be increased by the use of larger drops (HD), but this in turn has an opposite effect, providing for the metal amalgamated a greater volume to diffuse in, thus lowering the final concentration of the metal at the drop surface and
+
Metal ion concn, 10-7 W
1.0 3.0 5.0 7.0 9.0 A'
CIF'
C d2-
Deviation,
q
A'
I
cm2
1.7 4.0 +5.0 4.7 7.75 - 0 . 5 7.8 12.0 - 3 . 0 + 0 . 1 10.7 16.3 + 0 . 1 13.4 20.8
Deviation,
Pb2+ A'
I
%
cm2
-1.0
1.8 4.4 7.4 9.6 13.3
+0.1 +2.0
-0.2 -0.5
C u2-
Deviation, A' 9
.
Deviation,
cm2
+5.0 . -0.1 3.4 +0.2 6.2 -1.0 8.9 -0.111
-0.3 +0.5 +O.l -0.2
= see Table 11.
therefore decreasing sensitivity. We therefore concluded that the volume of the HD must remain in the range of 0.8-1.6 mm3 for our purposes. Increasing u, the rate of polarization, gives also an increase (times fief the i,, thus increasing sensitivity), but here again, this is achieved on account of a poorer resolution between peaks. We concluded here that a span rate of 40 mV/sec is optimal for chloride-less samples (20 mV/sec for C1--containing samples). All this information was finally checked on mixtures of the four cations. Figure 6 and Table I1 present typical results. The results thus far reached show that in our experimental conditions and concentration ranges, no interference was found from formation of intermetallics in the amalgams. Temperature control was not needed, provided each series was carried out a t room temperature (22 f 4 O C ) . The fact that no special purification of commercial nitrogen is required or replating of the central Pt-electrode after each determination as suggested by others ( 1 2 ) ,adds to the simplicity of our method.
LITERATURE CITED (1) Kh. H. Mancy, "Water Pollution Control," in "Analytical Chemistry-Key to Progress in National Problems," Nat. Bur. Stand. (U.S.), Spec. Pub/., No. 351, 1972. (2) L. Meites, "Polarographic Techniques," 2nd ed.. Interscience, New York, N.Y., 1965. (3) W. Kemula, Anal. Chem. Instrum.. 2, 123 (1963). (4) R . S. Nicholson and I. Shain, Anal. Chem., 36, 706 (1964). (5) I. Shain and J. Lewinson, Anal. Chem., 33, 187 (1961). (6) J. Buffle, Univ. Geneve, Arch. Sci. Geneve. 22, 393-496 (1969). (7) E. J. Maienthal and J. K. Taylor, "Polarographic Methods in Determination of Trace lnorganics in Water," in "Trace inorganics in Water," Advan. Chem. Ser., No. 73, American Chemical Society, Washington, D.C., 1968. (8) E. Barendrecht, Electroanal. Chem., 2, 43 (1967). (9) Instruction Manual S-293 13 Sargent Fast Sweep Polarograph, Sargent, Welsh & Co., 7300 Lynder Ave., Skokie, 111. (10) M. Ariel and U. Eisner, J. Nectroanal. Chem., 5, 362 (1963). (11) M. Ariel, U. Eisner, and S. Gottesfeid, J. Nectroanal. Chem., 7, 307 (1964). (12) G. Koster and M. Ariel, J. Electroanal. Chem., 33, 339 (1971).
RECEIVEDfor review May 20, 1974. Accepted August 29, 1974. The support granted by the National Council for Research and Development, Prime Minister's Office, Israel (Contr. No. 1545-1973) is kindly acknowledged.
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