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Electrochemical Masking with an Adsorbed Determination of Silver-Mercury Mixtures Royce W. Murray and R. L. McNeely Department of Chemistry, Unieersity of North Carolina, Chapel Hill, N . C . 21514
MANYEXAMPLES of the inhibition of electrode reactions by adsorbed substances can be found in the polarographic literature. In 1956, Reilley et ul. ( I ) observed that gelatin surfactant caused the essential obliteration of the reduction waves of certain metal chelates without producing inhibiting effects on the corresponding free metal ions. Such selective inhibition, or electrochemical masking, can be analytically useful and several applications of this principle, using organic surfactants, have been subsequently published. Shetty et al. (2, 3) have used camphor surfactant to inhibit more easily reduced metal ions in the polarographic determination of thallium(1) and silver(1). Frei and Miller ( 4 ) have employed cationic surfactant polymers t o achieve selective permission of electrode reactions of anions. Adsorbed, optically active brucine produces a slight discrimination between enantiomers of certain metal complexes ( 5 ); broad application of this variation of masking to enantiomeric analysis appears, from more recent studies ( 6 ) ,not to be highly promising. Electrochemical masking can also be applied to analysis of the surfactant concentration using an indicator depolarizer, as shown by Phillips (7). Reports of reduction inhibitions by adsorbed metal complexes are comparatively rare. Kolthoff and Woods (8) have found that persulfate reduction is strongly inhibited by adsorbed copper(1) chloride. Adsorbed lead(I1) has been demonstrated by polarography and chronopotentiometry to block the reduction of mercury(I1) from bromide and iodide media (9). Further polarographic investigation of the inhibiting effects of adsorbed lead has revealed, for species reduced at potentials positive of mercury dissolution, that silver(1) (Br- and Imedia), gold(II1) (Br- medium), and iron(I1I) (Br- medium) reductions are totally uninhibited, whereas platinum(I1) (Brmedium) and iodine (I- medium) reductions are, like mercury(II), inhibited by adsorbed lead. The adsorbed lead layer is, thus, selectively penetrable by different ions. This report presents an example of application of this selectivity for analytical purposes : a determination of silver-mercury(I1) mixtures based on an appropriate analysis of the morphology of a single polarographic current-time curve. EXPERIMENTAL Equipment. The operational amplifier instrument and cell assembly employed were of conventional design. Currenttime curves for solutions of mercury:silver in ratios < 2 : 1 ( I ) C. N. Reilley, W. G. Scribner, and C. Temple, ANAL.CHEhf.,28,
450 (1956). (2) P. S. Shetty, P. R. Subbaraman, and J. Gupta, Anal. Chim. .4cta, 27,429 (1962). (3) P. S . Shetty and P. R. Subbaraman; Indian J . Chem., 2, 397 (1964). (4) Y . F. Frei and I. R. Miller, J. Phys. Clzem., 69, 3018 (1965). (5) R. W. Murray and M. Kodama, ANAL.CHEM., 37, 1759 (1965). (6) R. W. Murray and W. R. Heineman, University of North Carolina, unpublished results, 1966. (7) S. L. Phillips, ANAL.CHEW,38, 343 (1966). (8) I. M. Kolthoff and R. Woods, J . Am. Clzem. Soc., 88, 1371 (1966). (9) D. J. Gross and R. W. Murray, ANAL.CHEW,38,405 (1966).
were recorded on a Sargent Model SR recorder; larger ratios required the faster response of a Sanborn Model 151 recorder or an oscilloscope. Procedure. All solutions were 0.01M in perchloric acid to prevent lead hydrolysis. Bromide electrolyte was chosen rather than iodide to avoid the inconvenience of working with easily oxidizable acidic iodide solutions. Use of NaBr, however, restricts the silver concentration to an upper (solubility) limit’of 0.4mM; the higher solubility of silver in NaI extends this limit to 1.OmM and provides an equally effective blocking system. Analytical studies of the silver-mercury system were conducted at -0.27 volt cs. SCE, an arbitrarily selected value in the potential window between electrode dissolction and lead reduction. The analysis involves recording the polarographic current-time curve at this potential for the blank electrolyte, the electrolyte plus unknown mixture (it is possible to omit this step), and the electrolyte plus unknown mixture with added lead solution. Current measurements reported here have been corrected for the blank. RESULTS AND DISCUSSION Mercury-Lead Mixtures. In the potential interval where mercury(1I) is reduced but lead is not (-0.11 to -0.40 volt cs. SCE in 1M bromide), polarographic current-time curves of the form of curve B in Figure 1 are obtained in mercury(1P)lead mixtures. Coverage of the growing drop by a Pb-Br layer, when completed by lead diffusion, suppresses the mercury(I1) reduction current to negligible values at small mercury(I1) concentrations. The magnitude of the resulting peak current for mercury(I1) reduction is dependent on both the mercury and lead concentrations, but the time 0 (obtained by extrapolating the descending portion of the current-time curve), necessary for complete inhibition is governed only by lead diffusion, approximately according to the equation (10)
0
=
1.82 X l o 6
(g2)
where C, r, and D a r e the bulk lead concentration ( M ) ,surface concentration (molestcm 2). and diffusion coefficient, respectively. Although 0 from this equation is not precise for mercury-lead inhibition in either bromide or iodide media (P), it is useful for estimating lead concentrations practical for analysis. A lead concentration is desired that produces a 0 which is a compromise between the small value of 0 desirable for silver analysis (oide infra) and a large \ d u e for mercury analysis. For 0 to be one fourth of a 4.8 second drop time, for example, Equation 1 suggests a 0.6mM lead concentration; a 1.OmM value was employed here. The mercury(I1) peak current from curve B is proportional to mercurS (11) concentration: i,
=
kl[Hgl
For 1.00mM lead in ¶ . O M bromide, kr
(2) =
2.04
* 0.04 (PA/ ~
(10) J. Koryta; Collection Czech. Chem. Commim., 18,206 (1953). VOL. 39, NO. 13, NOVEMBER 1967
1661
Table I. Results for Mercury(I1) in the Presence of Silver 7 varies from 0.20-0.35 see) PA Z errors0 i,-J[Hgl LHgl in, PA 0.46 1.80 0.25 0.98 -8.0 1.02 2.04 0.50 1.52 0.0 1.99 0.75 2.02 1.49 -2.1 2.05 1.75 4.12 3.59 0.6 2.04 4.08 2.00 4.61 0.0 2.02 2.25 5.06 4.54 -1.3 2.11 2.50 5.77 5.27 3.2 2.08 5.71 2.75 6.23 1.8 6.29 2.10 3.00 6.81 2.7 * As compared to kl, Equation 2, in absence of silver.
I
( E = -0.27 volt, [Ag+] = 0.200mM, i d = 5.2 sec,
0.8
0.6
-5 I-’0.4
3U 3
0.2
I
I
L
0
i 2 a TIME, SECONDS
I
4
5
Figure I. Polarographic current-time curves at -0.27 V cs. SCE in 1.OM NaBr Curve A. NaBr alone Curve B. 0.250mM Hg(II), 1.00 mM Pb(1I) Curve C . 0.200 mM Ag(I), 1.00 mM Pb(I1) Curve D. 0.250mMHg(II), 0.200mM Ag(I), 1.00mMPb(1I) The points represent the graphical sum of curves B and C
m M ) up to the highest mercury concentration tested, 3.OOmM. Equation 2 holds exactly only when the time at which i, occurs is independent of mercury concentration, which was experimentally verified. Silver-Lead Mixtures. The polarographic current-time curve for silver(1) reduction, curve C in Figure 1, is the same as in the absence of lead, and silver reduction is not inhibited by the adsorbed lead layer. It is imperative that the silver solution be isolated from the DME mercury until measurements are about to be made, as direct reaction of silver with the mercury pool converts silver ion into mercury(II), a species whose reduction is inhibited by the adsorbed lead layer. A 1-hour contact with mercury metal in stirred solution produces about a 10% loss in silver reduction current, for example. Silver-Mercury-Eead Mixtures. A mixture of these three ions in bromide medium produces the current-time curve D in Figure 1, No silver-mercury interaction occurs, as this curve is a precise addition of the individual silver-lead and mercury-lead curves. Determination of silver from curve D is based on the precise proportionality of the diffusion-controlled current at the end of drop life to silver concentration: id =
kdAgl
(3)
The value of k 2 = 4.32 + 0.04 (pA/mM) over a silver concentration range of 0.025 to 0.30mM is independent of the lead concentration and, for [Hg]/[Ag] < 5 , is also independent of D
ANALYTICAL CHEMISTRY
the mercury concentration. At higher [Hg]/[Ag] ratios, a trace of mercury(I1) “penetration” current flowing at the end of drop life interferes with the silver determination at 1.OmM lead concentration. This current can be suppressed further by use of higher lead concentrations; 5mM lead permits analysis of silver at [Hg]/[Ag]ratios up to about 75 :1with less than 1 interference from mercury. This ratio represents the practical upper limit for a selective, accurate silver concentration measurement in silver-mercury mixtures. Analysis of the mercury concentration from the same current-time curve D is possible by application of Equation 2 and an appropriate correction for the silver reduction current flowing at the time, T , corresponding to i,. This correction is based on assumption of an Ilkovic t1’6 relation for the silver current-time curve and application of the expression ip-c = i,
- (r/td)1/6id
(4)
where t d is the drop time and i, is the measured composite of the mercury peak current (ip of Equation 2) and the silver current flowing at time T . Table I gives results for mercury analysis from curve D in this manner, which are seen to be quite satisfactory for [Hg],/[Ag]> 2. At smaller concentration ratios, the correction for silver current is not sufficiently exact for accurate mercury analysis, owing to the inexactness of the tlie relation, and an alternate route to the mercury analysis becomes preferable. This route depends on an additional experiment with lead absent in which the current measured at the end of drop life is the sum of uninhibited mercury and silver currents. Subsequent selective measurement of the silver id in the presence of lead yields the mercury current by difference and with reasonable accuracy for [Hg]/[Ag] > 0.2. This latter procedure, not requiring accurate measurement of the mercury peak current of curve D,can be carried out on a conventional, undamped polarograph. The above approach can obviously be extended to other metal pairs where selectivity of the adsorbed lead layer is appropriate; for instance, gold(II1)-mercury(1I) mixtures could be determined by a similar procedure. A better understanding of the reasons for the selective penetration by various ions of the Pb-Br and Pb-I adsorbed layers, which have been described as two-dimensional crystalline films (9), would, of course, be desirable, and investigations to this end are in progress. RECEIVED for review June 28, 1967. Accepted August 25, 1967. Work supported by the Directorate of Chemical Sciences, Air Force Office of Scientific Research, Grant AF-AFOSR-584-64. R.L.M. also acknowledges fellowship support from Chemstrand Corp. and National Science Foundation (traineeship).
