Alternating Current Polarographic Determination of Electroactive Species More Negatively Reduced than the Major Component A. M. Bond and J. H. Canterford’ Department of Inorganic Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia
The influence of an electrode process of a species in high concentration, on another more negative electrode process of a species at relatively low concentration, has been examined with respect to the use of ac polarography for the determination of a species more negatively reduced than the major component. The major form of interference encountered ik from the ohmic /R drop produced by the high currents associated with high concentrations of the more positive electrode process. However, chemical interference from the product of the preceding process could be encountered in some cases. The influence of the electrode Cu(ll)
+ 2e
--+
Cu(amalgam)
process on the more negatively reduced cadmium(l1) and zinc (11) species in perchlorate and fluoride media, as well as the determination of cadmium(l1) in the presence of iron(ll1) and uranium(V1) are examined to show the extent of the interference and its minimization.
WITHDC POLAROGRAPHY, it is extremely difficult to determine a trace of one species in the presence of large amounts of a second species, which is reduced at a more positive dc potential. The problem arises from the large current produced by the major and more positively reduced species, and this leads to difficulties in the measurement of the small current produced by the relatively low concentration of the more negatively reduced species. This difficulty of actual measurement is overcome in ac polarography because the current returns to the common base line after each peak. That is, the resolvability of ac polarography is better than that of dc polarography because the waves in sequence do not need to be measured with reference to each other as they do in the latter technique ( I , 2). The analysis of systems with possible polarographic behavior of this type occurs commonly when the determination of trace components of alloys, metals, ores, etc. is required. The major component can usually be determined by any number of techniques. Trace analysis of minor constituents, however, may be amenable to only a few approaches, polarography usually being one of them. If the major component and the minor constituent are both electroactive at the dropping mercury electrode (DME), then, in principle, it should be possible to determine the minor constituent by ac polarography, without interference ( I , 2). Despite this alleged advantage of ac polarography, no quantitative information verifying this is, available, and such claims appearing in rePresent address, Division of Mineral Chemistry, CSIRO, P.O. Box 124, Port Melbourne, Victoria 3207, Australia (1) B. Breyer and H. H. Bauer, “Alternating Current Polarography and Tensammetry,” Interscience, New YorklLondon, 1963. (2) H. Schmidt and H. von Stackelberg, “Modern Polarographic Methods,” Academic Press, New York/London, 1963. 1658
views ( I , 2 ) and in advertising literature would appear to be unsubstantiated in any way by experimtntal evidence. Previous work by the present authors (3) however, has in fact shown that the influence of preceding (more positive) electrode reactions may not generally be negligible. Iron (111), for instance, was shown to interfere with the reduction of tin(I1) (3). Oxygen is reduced more positively than cadmium(II), indium(II), lead(II), and thallium(1) and certainly would interfere with the determination of these latter species (3-6) if normal ac polarographic procedures are used. That is, the peak height, id-, of the ac wave in the presence and in the absence of oxygen are not the same. Interference from the preceding (more positive) electrode reaction could conceivably arise from several sources : Chemical, where the product(s) of the first reaction change(s) the electrode process of the second species being determined ; physical, where the electrode reaction alters the physical nature of the DME, for example, amalgam formation. [This type of interference is important in inverse or anodic stripping polarography (7)]; instrumental, where the high currents produced by high concentration of the more positive depolarizer cause high ohmic ZR drops and this could conceivably cause distortion of any subsequent wave. Available evidence (3, 4, 8) in fact indicates that with low concentrations of the more positively reduced species, no interference arises in many cases. However, no experimental data appear to be available for the case where ac polarography should be extremely advantageous, that is, for a vast concentration excess of the more positively reduced species where dc polarography cannot be used a t all. In this work we will consider first, two examples where the more positive electrode process produces a soluble chemical product. The two examples chosen in this category were traces of cadmium(I1) in the presence of iron(II1) in 5M HC1, and traces of cadmium(I1) in the presence of uranium (VI) in 0.5MNaCIO,. Iron(II1) and uranium(V1) in their respective electrolytes of HCI and NaCIOl are both reduced at considerably more positive dc potentials than cadmium (11) and no interference due to overlapping waves occurs. Any measured changes in the cadmium(I1) electrode process must therefore be due to the preceding electrode reactions. We will also consider the reduction of cadmium(I1) and zinc(I1) in the presence of copper(II), in both perchlorate and (3) A. M. Bond and J. H. Canterford, ANAL.CHEM.,43, 228 (1971). (4) B. Breyer, F. Gutmann and S. Hacobian, Aust. J. Sci. Res., Ser. A , 3, 558, 567 (1950). (5) K. Itsuki and F. Suzuki, Jap. Anal., 8,89 (1959). (6) W. F. Head, Anal. Chim. Acta, 23,297 (1960). (7) E. Barendrecht in “Electroanalytical Chemistry,” A. J. Bard, Ed., Marcel Dekker, New York, N.Y., Volume 2, Chap. 2, 1967. (8) A. M. Bond and J. H. Canterford, ANAL.CHEM.,43, 393 (1971).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
4
Table I. Ac Polarographic Characteristics of 2.00 X lO-4M Cd(I1) in 5M HCI in the Presence of Varying Amounts of Iron(1II)
- E,
[Fe(WI, M
(volt us. Ag/AgCl)
0 4 x 10-6 8 x lo-' 2 x 10-4 4 x 10-4 6 X lO-' 8 x 10-4 2 x 10-3 4 x 10-3 8 X
0.686 0.694 0.694 0.700 0.716 0.722 0.740 0.826 0.962 1.162
Halfwidth,
id-,
PA 4.47 4.46 4.46 4.48 4.48 4.38 4.34 4.26 3.88 3.78
mV 50 52 52 52 53 53 55 58 68 70
Exptl [Cd(II)], M 2.00 x 2.00 x 2.00 x 2.00 x 2.00 x 1.97 X 1.94 x 1.89 x 1.73 x 1.68 X
10-4 10-4 10-4 10-4
4pA-
lo-' lW4 10-4
Figure 1. Ac polarogram of 2 x lO-'M Cd(I1) in 5MHCl
10-4
10-4 10-4
fluoride media. Copper is known t o form a n amalgam when reduced at the D M E in these media. Both cadmium(I1) and zinc(I1) are reduced at significantly more negative potentials than copper(II), so high concentrations of copper(I1) can be added without the formation of overlapping ac waves. For this reason copper(I1) is a n ideal system to investigate the effect of a preceding electrode reaction in which amalgam formation provides the potential interference. EXPERIMENTAL
Reagents. All solutions were prepared from reagent grade chemicals. The copper(II), cadmium(II), zinc(II), and uranium(V1) solutions were prepared from the nitrate salts. Iron(II1) was added as iron trichloride hexahydrate, and iron (11) as anhydrous iron dichloride. Sufficient nitric acid was added t o the copper(I1) solution in 0.5M sodium fluoride to prevent precipitation of basic copper salts. Apparatus. Polarograms were recorded with a Metrohm Polarecord E261. Ac polarography was carried out using the Metrohm ac Modulator E393 with a n applied ac voltage of 10 mV RMS at 50 Hz. To minimize cell impedance, a three-electrode system was used, the modulated ac voltage being applied through an auxiliary tungsten electrode. Maximum damping was applied and the polarograms were recorded at a dc potential scan rate of 1 volt/6 min. All solutions were degassed with argon and thermostated at 25.0 f 0.1 "C. All potentials were measured relative to Ag/AgCl(5M NaC1). RESULTS
Cadmium(II1) Determination in the Presence of Iron(II1). I n 5M HCI, the iron(II1) is reduced according to the equation Fe(1II)
+e
+ Fe(I1)
+ 2e
+
-
-0.8 -0.6 volt vs. A g / A g C l
in 5M HCI has a summit potential, E,, of -0.686 volt us. Ag/AgCl. Figure 1 gives a typical ac polarogram of cadmium(I1) in 5M HCl. The ac wave was shown to be reversible with a half-width of 50 =t 2 mV (uncompensated for ohmic IR drop). A plot of peak height, id -, us. concentration was very close to linear up to 4 x 10-4M cadmium(I1). Table I shows the apparent E,, id-, and half-width for a series of solutions with a constant concentration of cadmium (11) and varying amounts of iron(II1). The apparent concentration of cadmium(II), calculated by reference t o the calibration curve, is also given. As can bc seen from Table I, the cadmium(I1) wave suffers considerable interference from the preceding iron(II1) electrode process. With increasing iron(II1) concentration, the wave becomes broader, shifts to more negative potentials, and id- decreases. Figure 2 shows the cadmium(I1) wave as a function of iron(II1) concentration. Cadmium(I1) Determination in the Presence of Uranium(V1). I n 0.5M NaCI,, cadmium(I1) and uranium(V1) were both found to be reversibly reduced in the ac sense, according to the electrode reactions Cd(I1)
+ 2e
+ Cd(ama1gam)
and
a t open circuit, that is, it is reduced by the mercury itself (9). The product of the reaction, iron(I1) must be at a significant concentration at the electrode at all usable potentials of the mercury electrode and reduction of other species must proceed in the presence of this region of iron(I1) species. Effectively, the second species has to diffuse through a n iron(I1) zone before it itself is reduced at the D M E and this iron(I1) zone could cause interference. The cadmium(I1) electrode process Cd(I1)
I
Cd(ama1gam)
(9) J. Heyrovsky and J. Kuta in "Principles of Polarography," Academic Press, New YorkiLondon, 1966, p 537.
U(V1)
+e
+
U(V)
with E, values of -0.558 and -0.162 volt us. Ag/AgCl, respectively. The half-widths were 50 + 2 mV and 94 i 3 mV, respectively (uncompensated for ohmic IR drop). The us. concentration cadmium(I1) calibration curve of idis strictly linear up to 2 X lO-'M with slight curvature a t higher concentrations. A plot of id- L'S. concentration for uranium(V1)gives a linear plot for concentrations up to ~ o - ~ M . Table I1 gives E,, i d - , and half-widths of the cadrnium(I1) wave as a function of uranium(V1) concentration. The experimental cadmium(I1) concentration was deduced from the calibration curve prepared in the absence of uranium (VI). From Table 11, it can be seen that the presence of
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
1659
Table 11. Ac Polarographic Characteristics of 2.00 X lO-4M Cd(I1) in 0.5M NaC104 in the Presence of Varying Amounts of Uranium(V1) - E8 Halfid-, width, Exptl [Cd(II)], (volt us. [U(VI)], MA rnV M M Ag/AgCl) 0 4
x 8 x 4 x 6 x 8 X 2 x 4 x
0.558 0.560 0.560 0.564 0.572 0.584 0.652 0.748
10-6 lo-' 10-4
lo-'
lo-' 10-3
10-3
4.61 4.58 4.54 4.08 4.04 3.96 3.92 3.00
2.00 x 1.99 x 1.97 x 1.77 x 1.75 x 1.72 x 1.70 x 1.30 x
50 48 49 48 49 50 42 24
lo-' 10-4 10-4 10-4
10-4 10-4
10-4 10-4
Cadmium(I1) Determination in the Presence of Copper(I1). IN0.5M NaC104. In this medium the electrode reactions
--1.3
-1.1 - 1 . 1
-0.9 volt vs. Ag/AgCL
Cu(I1)
+ 2e --., Cu(ama1gam)
Cd(1I)
+ 2e
and I
-0.8
I
-0.6
Figure 2. Same solution as in Figure 1 but with (a) 8 X 10-4M Fe(II1) (b) 4 X 10-3MFe(III) (c. 8 X 10-2A4 Fe(II1)
-.
Cd(ama1gam)
were observed t o have summit potentials of +0.054 and -0.558 volt us. Ag/AgCl, respectively. The copper(I1) wave is very close to symmetrical about Es at low concentrations, whereas a t high concentrations, there is a distinct tailing-off o n the more negative side, as shown in Figure 4. The half-width of the copper(I1) wave was 56 2 mV a t low concentrations but increased significantly a t high concentrations, presumably because of the increasing importance of the ohmic ZR drop. Thus it would appear that the copper(I1) wave possesses a high but not complete degree of reversibility in the ac sense. Cadmium(I1) is reversibly reduced at the DME in 0.5M NaC104, as indicated by a half-width of 50 f 2 mV, independence of E, o n concentration, and the symmetrical nature of the wave for all cadmiurn(I1) concentrations.
*
uranium(V1) causes a marked interference to the characteristics of the cadmium(I1) wave. E, becomes more negative, id- decreases, and the half-width of the cadmium(I1) wave decreases at high uranium(V1) concentrations. At very high uranium(V1) concentrations, there is some overlap of the two waves. Figure 3 shows two polarograms of constant cadmium(I1) with different amounts of uranium(V1) and clearly illustrates the phenomenon described above.
Cd
I Figure 3. Ac polarogram of 2 X 10-4M Cd(II1) in 0.5M NaC104 with (a) 6 X 10-4MU(VI) (b) 4 X lO+M U(V1)
I 4uA-
I
1 -0% volt
1660
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
a
-0.4 -0.2 A~/A~CI
VS.
0.0
-0.0
-0.6
volt vs. Ag /Ag CL
h
Table 111. Ac Characteristics of 2 X 10-4M Cadmium(I1) in 0.5M NaC104 as a Function of Copper(I1) Concentration - E8 Half[Cu(III)], (volt us. idN, width, Exptl [Cd(II)], M Ag/AgCl) fiA mV M 0.558 4.25 50 2.00 x 10-4 0 4 x 10-6 0.558 4.24 50 1.99 x 10-4 8 x 10-6 0.558 4.22 50 1.98 x 10-4 2 x 10-4 0.558 4.21 50 1.98 X lo-' 4 x 10-4 4.30 46 2.02 x 10-4 0.574 2 x 10-3 0.590 4.48 42 2.11 x 10-4 4 x 10-3 0.604 4.52 38 2.12 x 10-4 8 x 10-3 0.619 4.46 38 2.10 x 10-4 Table IV. Ac Characteristics of in 0.5M NaF as a Function of -E, [CU(WI, (volt 11s. M Ag/AgCI) 0 0.568 4 x 10-5 0.569 8 X loW5 0.572 2 x 10-4 0.574 0.580 4 x 10-4 0.582 2 x 10-4 0.602 4 x 10-3
id-,
Exptl [Cd(II)],
PA
M
3.70 3.70 3.72 3.60 3.54 3.52 3.46
2.00 x 2.00 x 2.01 x 1.95 x 1.91 x 1.90 x 1.87 x
10-4 10-4 10-4 10-4 10-4 10-4 lo-'
-E,, M 0
4 x 10-5 8 X 2 x 10-4
4 x 10-4 2 x 10-3 4 x 10-3 8 x 10-3 2 x 10-2
(volt DS. Ag/AgCl) 0.966 0.984
1.006 1.023 1.029 1.056 1.068 1.086 1,159
b
2 X 10-4M Cadmium(I1) Copper(I1) Concentration
Table V. Ac Characteristics of 2 X 10-4M Zinc(1I) in 0.5M NaC104 as a Function of Copper(1I) Concentration [CU(II)I,
a
id-, PA
1.22 1.23 1.24 1.22 1.21 1.24 1.24 1.26 1.19
Exptl [Zn(II)], M
2.00 x 2.02 x 2.04 x 2.00 x 1.98 x 2.04 x 2.04 x 2.06 x 1.95 x
10-4 10-4 10-4 10-4 10-4 10-4 10-4 10-4
Table I11 gives some ac polarographic parameters for cadmium(I1) in the presence of varying amounts of copper (11). The experimental cadmium(I1) concentration is that calculated by reference to a calibration curve prepared in the absence of copper(I1). Above a copper(I1) concentration of about 10-*M, no cadmium(I1) wave was observable because of the tailing-off of the preceding copper(I1) wave. At low levels of copper(II), there is no interference to the cadmium(I1) wave but at high concentrations there is a small degree (
