When‘ standard conditions are used, Equation 1 reduces to 0x.h
=
- Zlog
i,j
(2)
The 0x.In. calculated for the oils tested as related to their stability is shown in Figure 7. This approach prows to be more satisfactory than the +1.0 volt correlation, since it allows a comparison of pure compound data with the data obtained for the oils. The 0s.h. values calculated for a number of the pure compounds are also presented in Figure 7: It would appear that the same structural factors in organic compounds which determine the ease of electron removal also determine to a very large extent the ease with which they are oxidized by osygen. The correlation shown applies only to the electrode, electrolyte, and concentrations used in this work. In order to be more universal, the osidizability index for a given electrode could be related to a pure hydrocarbon standard to compensate for variations in surface areas of the electrodes and concentration effects as indicated in Equation 1. The quantity of the petroleum fraction to
be tested depends on its hydrocarbontype, density, and molecular weight range. This approach should be‘valuable for studying- -the stability of other oil fractions such as fuel oils and lubricating oils. The data obtained suggest that the technique should be exploited fully as a qualitative and quantitative tool for hydrocarbon and nonhydrocarbon analysis. REFERENCES
(1) Adams, R. X., McClure, J. H., Morris, J. B., ANAL. CHEM.30, 471 (1958). (2) Am. SOC. Testing Materials, Phila-
delDhia. Pa.. Electrical Insulatine Liauids and Gases, 1st ed., Decmbe‘r 1959, Appendix 1, p. 307. (3) Baumann, F., Shain, I., ANAL.CHEJI. 29, 303 (1957). (4) Delahay, P.,
“New Instrumental Methods in Electrochemistry,” pp. 34, 223, Interscience, N& York, 1954. (5) Drushel, H. V., Miller, J. H., ANAL. CHEM.29, 1456 (1957). (6) Elving, P. J., Krivis, A. F., Ibid., 30, 1645 (1958). (7) Gaylor, V. F., Conrad, A. L., Landel, J . H., Ibid., 29, 228 (1957). (8)Ibid., p. 224.
(9) Gaylor, V. F., Elving, P. J., Conrad, A. L., Ibid., 25, 1078 (1953).
(10) Geske, I). H., J . Am. Chem. SOC. 81. 4145 (1959). (11) ’Geske,’ D. H., J. Phys. Chem. 63, 1062 (1959). (12) Hedenburg, J. F., Freiser, H., ANAL. CHEM.25, 1355 (1953). (13) Julian. D. B.. Rubv. W. R..’ J . Am. ‘ Chem. Soc. 72, 4719 (i950). (14) Kabasakalian, P., McGlotten, J., ANAL.CHEM.31, 431 (1959). (15) Kolthoff, I. M., Coetzee, J. F., J . Am. Chem. SOC.79, 1852 (1957). (16) Kolthoff, I. M Tanaka, X., ANAL. CHEM.26, 632 (19h4). (17) Lord, S. S., Jr., Rogers, L. B., Ibid., 26, 284 (1954). (18) Lund, H., Acta Chem. Scand. 11, 1323 (1957). (19) MacNevin, W. RI., Levitsky, M., ANAL.CHEM.24, 973 (1952). (20) Parker, R. E., Adams, R. N., Zbid., 28, 828 (1956). (21) Penketh, G. E., J. A p p l . Chem. 7, 512 (1957). (22) pice, W. C., Bralsford, R., Harris,
P. F ., Ridley, R. G., Preprint, Institute of Petroleum Hydrocarbon Research Group Conference on hlolecular Spectroscopy, Savoy Place, London, W. C. 2, Feb. 26, 28, 1958. (23) Voorhies, J. D., Adams, R. N., Ah-AI.. CHEM.30, 346 (1958). RECEIVED for review October 27, 1960. Accepted May 23, 1961. Division of Analytical Chemistry, Fisher Award Symposium Honoring Philip J. Elving, 137th Meeting, .4CS, Cleveland, Ohio, April 1960.
The Rotated, Mercury-Coated Platinum Electrode Preparation and Behavior of Continuously Deposited Mercury Coatings and Applications to Stripping Analysis STANLEY BRUCKENSTEIN and TOYOSHI NAGAI‘ School of Chemistry, University of Minnesota, Minneapolis, Minn.
b A reproducible, thin-mercury-film, rotated electrode can be prepared b y continuously plating at constant current a rotated platinum electrode with mercury from a Hg(ll) solution in 0.1M nitric acid. The use of a large, constant plating current permits high hydrogen overvoltages to be obtained. Mercury plating efficiencies of the order of 75% are attainable. The application of this electrode to the stripping analysis of thallium in the 1 0-7 to 8 concentration range 2 10-5M and lead in the concentration to 1 X 1O%I is range 2 X reported and the limitations of the technique are discussed. Both lead and thallium are deposited simultaneously with mercury and are stripped by chemical oxidation of the amalgam with Hg(ll). The chronopotentiogram Obtained during the chemical stripping process is used to determine the concentration of lead
x
x
and thallium in their mixtures. The rate of the reaction between thallium amalgam and Hg(ll) i s rapid compared to the rate of supply of Hg(ll), while the reaction of Pb with Hg(ll) is complicated by a slow chemical step.
I
recognized that a rotated, mercury-coated platinum electrode would be advantageous for anodic stripping analysis. Such an electrode would have all the advantages of a mercury electrode-Le. , high hydrogen overvoltage, reproducible surface, and suitability for multicomponent analysis without the principal disadvantage of a mercury pool electrode, the loss of deposited metals by diffusion into the bulk of the electrode. Previous attempts to prepare rotated, mercury-coated platinum electrodes have proved unsuccessful. Gardiner and Rogers ( 2 ) electrolytically deposited a 4-micron layer of mercury on a platT IS
inum electrode from a mercuric acetate solution and studied the properties of this electrode as applied to stripping analysis. They found that this electrode functioned satisfactorily, if it was kept stationary. However, rotation of the electrode produced lower hydrogen overvoltages, presumably because an unbroken film of mercury could not be maintained. There is no report in the literature of the preparation of a preplated, mercury-coated electrode suitable for -rotation. Marple and Rogers (9, f0) successfully applied a stationary, mercury-plated electrode to voltammetry. Gardiner and Rogers ( 2 ) also investigated the application of the rotated, amalgamated silver electrode and con-
1 Present address, Laboratory of Analytical Chemistry, Faculty of Science, University of Kyoto, Kyoto, Japan.
VOL. 3 3 , NO. 9, AUGUST 1961
1201
'
cluded that this electrode was of limited utility in stripping analysis because the electrode sensitivity varied erratically with age as a result of progressive amalgamation of the silver wire. Even if this problem could be overcome, intermetallic compound formation between silver and the deposited metal in the mercury film could limit the utility of this electrode. For example, Kemula and Galus (6) have found that silver forms intermetallic compounds with zinc and cadmium. Cooke (1) has reported the successful voltammetric application ,of the rotated, amalgamated silver electrode. This work is concerned specifically with the preparation and behavior of a rotated platinum electrode on which mercury is continuously deposited by electrolysis, and this electrode's application to the determination of Tl(1) and Pb(I1) in the micro- and submicromolar concentration range by stripping methods. Suppose that a solution 0.1M in nitric acid containing trace quantities of several metal ions also contains approximately 5 X lO-4M Hg(I1). If a rotated platinum electrode is introduced into this solution and a large cathodic current is passed through the rotated platinum electrode (and another platinum electrode in a separate compartment), mercury metal deposits a t the rotated platinum electrode, as do all other metals which are reduced. Large currents are used, in order to obtain the highest possible overvoltage for hydrogen evolution. The concentration of the various trace metals in the homogeneous amalgam can be determined from the anodic chronopotentiogram obtained on turning off the current-ie., by measuring the open circuit e.m.f. of the rotated platinum electrode us. a reference saturated calomel electrode as a function of time (see Figure 8). The electrochemical reactions occurring in a solution containing 0.1M nitric acid, 5 X 10-4M Hg(II), and 10"M Tl(1) a t the rotated platinum electrode on the cathodic cycle are Hg(1I) Hg
+ 2e
+ TUI) + e
-+ Hg +
(1)
T1~m.l
(2)
2 H + + 2e+H1f (3) When the current is interrupted, the following chemical reactions occur, successively, a t the rotated platinum electrode:
+
2Tl~ma1 Hg(1I) Hg(W
+ Hg
+
+
2T1(I)
+ Hg
[Hg(I)12
(4)
(5)
If, instead of merely turning off the current, the direction of current is reversed, in addition to Reactions 4 and 5 the following electrode reactions occur Tlr-1-
1202
0
TU)
+c
ANALYTICAL CHEMISTRY
(6)
2Hg
-
[HdI)Iz
+ 2e
(7)
Because of the numerous processes which occur during the. application of this method, i t was necessary to study the plating of mercury from solutions containing 0.1M nitric acid and 5 x 10-4M Hg(II), varying plating time and plating current, and also to study the solution of mercury in the same medium as a function of stripping current, using a rotated platinum electrode. Kemula and coworkers (7, 8) reported the amalgamation of platinum by mercury and the subsequent formation of an intermetallic compound with zinc when a stationary mercury-plated electrode of the type described by Marple and Rogers (9,IO) was employed for stripping analysis. The formation of intermetallic compounds might prove to be a limitation of the rotated, mercury-plated electrode. It has not been used for the determination of zinc, and it is not known if intermetallic formation occurs under our conditions. Since our electrode is coated with a new layer of mercury on each experiment, the rate of solution of platinum in mercury becomes an important consideration. EXPERIMENTAL
WATER. Conductivity water was used for all work. NITRIC ACID. Analytical reagent grade nitric acid was carefully fractionated and the middle third fraction used. MERCURICNITRATE. Mercuric nitrate solutions in 0.1M nitric acid were prepared by dissolving mercury in a threefold excess of purified concentrated nitric acid, evaporating excess acid, and dissolving in 0.1M nitric acid to make a 0.05M stock solution of mercuric nitrate. Triply distilled analytical reagent grade mercury metal contained sufficient lead (-0.001%) to interfere seriously with lead determinations. It was necessary to treat the analytical reagent grade mercury metal, in order to decrease the lead concentration to a negligible level. The conventional wet oxidation of base metals from mercury proved satisfactory. NITROGEN. Linde prepurified nitrogen was used to deoxygenate all solutions. Apparatus. A Sargent Synchronous Rotator, 600 r.p.m., was used in all experiments. A Sargent Model XXI Polarograph was used to obtain current-voltage curves. A platinum wire, 0.025 inch in diameter, was formed into a U shape and both ends were sealed into a 6-mm. soft glass tube a t right angles to the axis of rotation. Electrical contact was made to the platinum wire by filling the glass tube with mercury. The purpose of sealing both ends of the platinum wire was to ensure maximum rigidity of the wire and minimize the danger of bending it accidentally. The area of this elec-
trode was estimated to be 0.2 sq. cm. from its physical dimensions. A more precise area determination was made by chronopotentiometry of 10-91 potassium ferricyanide in 0.liM potassium nitrate. The solution was not stirred and all transition times obtained were under 1 second. The area of this electrode was calculated to be 0.16 sq. cm., using 8.9 X sq. cm. per second as the diffusion coefficient of ferrocyanide ion. The apparatus used for the plating of mercury on the rotated platinum electrode and the recording of the potentialtime curves consisted of a constant current source, electrolysis cell, d.c. amplifier, and appropriate recording equipment. An electronically regulated constant current source capable of regulating current to within d=O.l% from 10 @a.to 100 ma. was used during the plating cycle. Internal switches permitted control of the direction and magnitude of the current. The electrolysis vessel was a modified H cell, Its main compartment was fitted with a rubber stopper containing a hole to introduce the rotated electrode, a coarse sintered-glass dispersion tube for nitrogen bubbling, and one 12-mm. glass tube, open a t the top and terminating in a 10-mm. fine sintered-glass disk. This disk was submerged 1 cm. in the solution to be analyzed and filled by gravity. A salt bridge from a saturated calomel electrode was introduced into this tube. A piece of platinum wire (the auxiliary electrode) was placed in the other compartment of the H cell, which was separated from the main compartment by a fritted-glass disk. The constant plating current was passed through the rotating platinum electrode and the auxiliary electrode. The e.m.f. between the rotated platinum electrode and the reference saturated calomel electrode was fed to the input of a battery-operated d.c. amplifier with an input impedance of l o 9 ohms. The output of this amplifier was fed through suitable voltage dividers to a l/Ysecond, 1-mv., Bristol strip chart potentiometric recorder (Model lPH560-51-T46-T82), equipped with an Insco speed changer which permitted chart speeds between 7.5 and 120 inches per minute; a Sanborn single-channel recorder consisting of the Model 151lOOA recorder, the Model 150-1800 stabilized d.c. amplifier, and the Model 150-200 B/400 driver amplifier and power supply; or a Tektronix 502 oscilloscope equipped with a camera. The recording device used was determined by the length of the transition time. In all cases where overlapping measurements were possible, the same results were obtained independent of the recording device. All work was done a t 25" h 0.1" C. Pretreatment of Electrode. T o obtain reproducible results, the previous history of the rotated platinum electrode must be carefully controlled.
Usually it is satisfactory to place the electrode in a duplicate of the electrolysis cell filled with 0.1B4 nitric acid,
V O L T vs. S.C.E. Figure 1 .
Supporting electrolyte current voltage ‘curves
+
0.1M nitric acid, scanning voltage to 0.1M suffuric acid, scunning voltuge to C. 0 . l M sulfuric acid $us Q.IM potassium nitrak, scanning voltage
A. 8.
to-
+ -
+ VOLT vs. S.C.E.
pass an anodic current of 10 ma. through for 3 minutes, and reverse the current for 3 minutes. The potential of the rotated platinum electrode is then adjusted to +0.55 volt vs. the saturated calomel electrode. The current flowing in the rotated platinum electrode-saturated calomel electrode circuit normally drops to less than 1.5 wa. in about 15 minutes and the electrode is then suitable for use. On only two occasions in a one-year period did the above procedure €ail to yield a n electrode which reproduced previous results. On both occasions boiling in concentrated nitric acid for 1 hour, followed by the above treatment, yielded a satisfactory electrode. The above pretreatment procedure is partially empirical. The order of performing the various operations is to ensure that all oxidizable materials are removed from the electrode and that the final reduction reduces all platinum oxide which is formed upon oxidation of the electrode. Finally, setting the rotated platinum electrode e.m.f. a t +0.55 volt us. S.C.E. permits traces of adsorbed hydrogen or deposited materials arising from the final cathodic treatment to be oxidized without any platinum oxide formation. RESULTS AND DISCUSSION
Supporting Electrolyte CurrentVoltage Curves. Figure 1 presents the current-voltage curves obtained in sulfuric and nitric acid using a rotated platinum electrode. Curve A is obtained in 0.1M nitric acid on sweeping from positive to negative potentials, while curve B is obtained in 0.1M sulfuric acid under similar voltage sweep conditions. Curve C is obtained in a solution 0.1M in sulfuric
acid and 0.1M in potassium nitrate. I n all three experiments the platinum electrode was pretreated as described in the procedure, and the sweep started at +0.55 volt us. S.C.E. There is a distinct difference in the .results obtained in all three cases. The limiting reduction process in sulfuric acid must be the discharge of hydrogen. Adding nitrate to sulfuric acid produces a small limiting current of unknown origin, prior t o the evolution of hydrogen, but does not reproduce the result obtained in 0.1M nitric acid. In 0.1M nitric acid a large cathodic current is observed a t potentials more positive than those a t which hydrogen evolution is possible. As the potential of the electrode is swept to more negative values, this cathodic current decreases to a level consistent with the evolution of hydrogen. Initially some process other than hydrogen evolution can occur in 0.1M nitric acid solution, but a film which prevents this unknown reaction forms rapidly. If the e.m.f. of the rotated platinum electrode is held a t -0.3 volt, the observed cathodic current drops rapidly to a smaller value consistent with hydrogen evolution after 200 seconds. Figure 2 shows the current-voltage curves obtained on sweeping the potential of the rotated platinum electrode from a potential a t which oxygen is evolved to one a t which hydrogen is evolved, and vice versa. The current-voltage curve obtained in a 5 X 10-4M mercuric solution using a pretreated electrode is also shown. Curves A and B of Figure 2 show that in 0.1M nitric acid the oxide film on platinum is not removed unless an e.m.f. more negative than +0.55 volt is applied, nor does it form unless the
Figure 2. Oxide film formation and removal in 0.1 M nitric acid A. Scanning voltage from f to 8. Scanning voltage from to C. 0.1M nitric acid plus 5 X 10-4M Hg(ll).
- + volt to -.
Scanning from +0.60 pretreated as described in text
Electrode
rotated platinum electrode potential becomes more positive than $0.80 volt. From curve C it is seen that mercury does not deposit until the potential of a rotated platinum electrode is $0.60 volt, and that there is a slight minimum in the limiting current just before evolution of hydrogen, just as was noted for t h e supporting electrolyte alone. The limiting current is proportional to the concentration of mercuric ion and is 136 pa. in a solution of 5 X 10-4M Hg(I1) for the electrode used in the study. Plating of Mercury at Constant Current. LIMITING CATHODIC POTENTIAL. Figure 3 shows the cathodic chronopotentiogram obtained in a solution containing 5 X 10-4M mercuric ion and 2 X l O - 5 M Tl(1) in 0.1M nitric acid, using a current of 1.000 ma. The potential changes from +0.5 to -0.4 volt us. S.C.E. in less than 0.5 second and then rises over a period of 300 seconds to about - 1.05 volts. Continued electrolysis produces small additional changes of potential. A transition time for Hg(I1) is not observed, because the current density is so high and the rotated platinum electrode e.m.f. rises extremely rapidly to the potential a t which hydrogen is formed on the platinum surface. As the electrolysis continues, more of the platinum surface
-
VOL 33,
NO. 9,
AUGUST 1961
1203
becomes covcrctl 11ith mercury and the area of the platinurn electrode exposed to the solution decreases. U1timately all of the platinum surface is covered with mercury a d the limiting potential is governed by the overvoltage of hydrogen on mercury a t the current density employed. The current-voltage curve of the above solution was obtained with the same rotated platinum electrode. The voltage scanning rate was adjusted so that mercury plated on the electrode for approximately 5 minutes before hydrogen evolution occurred a t -0.9 volt us. S.C.E. Thus, the higher current density during the constant current plating increases the negative potential limit by about 0.15 volt. When a dropping mercury electrode is used, the residual current caused by hydrogen evolution from 0.1-21 nitric acid becomes excessively large at potentials more negative than -1.15 volts. The limiting current of 0.5mJf Hg(I1) solution in 0.1JI nitric acid, ( ~ J H ~ ( ~ Iis ) ) 136 pa. If a constant cathodic current of 1 ma. flows through the rotated platinum electrode, 0.864 ma. is available for other reduction processes. In the stripping technique described, only very dilute solutions of reducible ions are present and only a small fraction of the 0.864 ma. is required for processes other than hydrogen evolution. Therefore, the limiting cathodic potential will be virtually constant and independent of the presence of trace amounts of reducible materials, assuming the overvoltage for hydrogen evolution is not different on the deposited amalgam. Efficiency of Mercury Plating. When a constant current in excess of the limiting current for the reduction of mercuric ion flows, the quantity of mercury deposited may be calculated from T , and ( i J t l g ( l ~ ) . The amount of mercury recovered by oxidation a t constant current can readily be calculated from the transition time observed in an anodic chronopotentiogram and i,. Since only one half the number of coulombs is required to oxidize mercury metal to Hg(1) as is needed to reduce Hg(I1) to mercury metal, the plating efficiency is defined as
Table 1.
50
I
I
1
I
100
150
200
250
Plating Time ( S e d
x
Figure 3. Cathodic chronopotentiogram of 5.00 Hg(ll) and 2.00 1 OV5MTI(I) in 0.1 M nitric acid
x
Plating current 1 .OOO ma.
in Figure 4,curve C is a plot of the relative plating efficiency us. plating current for a 5 X 10-4N Hg(I1) solution in 0.1JP nitric acid when a plating time of 5 minutes is used. There is a maximum in the plating efficiency a t 1.O ma. The reasons for the change of plating efficiency with plating current are not clear, since all currents used are in large excess of ( i l ) H g ( l I ) . Constant potential plating experiments on the limiting current potential region of Hg(I1) for 5 minutes, followed by constant current oxidations, yielded plating efficiencies close to the maximum efficiency obtained at constant current. Probably some of the plated mercury is lost because this mercury is mechanically dislodged from the rapidly rotating electrode. Whatever the cause of loss of mercury metal, the optimum current efficiency for the particular electrode used in this work is 1.0 ma. The absolute value for the plating efficiency of mercury is 75y0,taking into account factors discussed in the section on chemical oxidation of plated mercury by Hg(I1). Effect of Hg(I1) Concentration and Plating Time on Stripping Time. The effect of Hg(I1) concentration on the anodic transition time mas studied by applying a plating current of 1.000 ma. for 5 minutes, and stripping the deposited mercury by oxidation a t 1.000 ma. The anodic transition times were determined from the anodic chronopotentiograms. The plot of 7 H g us. C R g ( I I ) is a straight line of slope 29.5 X lo3 seconds per Jf of Hg(I1) and passes through the origin.
Mercury Transition Times at Different Stripping Currents
T , = 5.00 minutes i, = 1.000 x 10-3 ampere C H ~ ( I I )= 5.00 x 1 0 - 4 ~ C"Os = 0.1M
i, X 10-3,ampere T H ~ seconds , ia7Eg x 103
1204
1.000
14.60 14.6
ANALYTICAL CHEMISTRY
0.700
20.28 14.2
1 O-*M
0.400
33.98 13.6
0,200 60.57
12.1
0,100
111.16 11.1
The Hg(I1) concentration range studied was 1 to 10 x 10-4X and 0.1M nitric acid was used as supporting electrolyte. In a second experiment, using a plating current of 1.000 ma., the plating time was varied between 1 and 10 minutes for a solution 5 x 10-~Jf in Hg(I1) and 0.1.11 nitric acid. Using a stripping current of 1.000 ma. the anodic transition time was determined. A plot of r H g us. T , yielded a straight line of slope 2.94 seconds per minute of plating and passed through the origin, as is shown in Figure 5 . Therefore, for the electrode used in this work
when i, = 1 x lo+ ampere. In Equation 8, T~~ is proportional to ~ ~ T,, ) because under the both C E ~ (and experimental conditions (using 50 ml. of solution) only O.llyo of the total Hg(I1) is deposited per minute of plating. The reproducibility of the plating of mercury a t the rotated platinum electrode over a %month period was good and Equation 8 continued to predict the anodic transition times to within 2% provided the pretreatment described in the procedure was followed. Effect of Stripping Current on T H ~ . A rotated platinum electrode in a 5.0 X 10-451 solution of Hg(I1) in 0.1Jf nitric acid was plated with mercury for 5 minutes, using a current of 1.000 ma. The mercury film was removed by electrolytic ovidation using currents in the range 0.1 to 1.O ma. The observed anodic transition times are given in Tahle I. The product, ia7Bg, is not constant, as would be espected for a simple system. This deviation is caused by the chemical oxidation of metallic mercury by Hg(I1) which occurs concurrently with the electrolytic oxidation of mercury. This reaction is considered in detail below. Chemical Oxidation of Plated Hg by Hg(I1). The equilibrium constant for Reaction 5 is about 100 ( 4 ) . If
of supply of Hg(1I) to the electrode and the rate of oxidation of mercury would be ( i l ) H g ( I 1 ) / 2 F moles per second. In a solution of 5 X 10-4M Hg(I1) and 0.1M nitric acid, the rate of chemical reaction of mercury with Hg(I1) is 3.3 X mole per second (Table 11), as compared to a value of 7.1 X mole per second calculated on the basis of zero Hg(I1) surface concentration. Therefore, the reaction of Hg(I1) with mercury on the platinum electrode is governed by a slow chemical step in Reaction 5. Under conditions similar to those used here, Haissinsky and Cottin (3) found that the reaction of mercury with Hg(I1) is measurably slow. An exact treatment of the kinetics of the above chemical oxidation is impossible because of the complicated mass transfer processes involved. However, an approximate treatment which
Table II.
l
-?I
l
Plating Current, rnilliampereres
Figure 4. Relative plating efficiency in 0.1M nitric acid or Hg(ll) and TI(I) as a function of plating current
rp. 5 minuter io.
1.000 ma.
+ +
A. 5.00 X 10-4M Hg(ll) 6.00 X 10-5M TI(I), ( 7 H g ) m a x = 16.78 seconds 6. 5.00 X 10-4M Hg(ll) 6.00 X IO-SM TI(I), ( 7 T l ) m a x = 22.20 seconds C. 5.00 X 10-4M Hg(ll), (7Hg)max = 15.13 seconds Maximum plating efficiency in all cases is 75% and occurs at i p = 1 .OO X 1 0-3 ampere. All in 0.1 M nitric acid supporting electrolyte
the rate of the oxidation of mercury by Hg(I1) is appreciable it should be possible to demonstrate this oxidation by baiting for varying times between the cessation of the plating of mercury and the start of electrochemical oxidation a t constant current, continuing the rotation of the electrode a t all times. Under these conditions, the observed anodic transition times will decrease as the time between cessation of plating and the start of electrochemical oxidation increases. Table I1 presents the anodic transition times observed in 0.1X nitric acid solution which contained 0.500, 1.00, and 2.00 X 10-3M Hg(II), using a plating current of 1.000 ma. for 5 minutes and a stripping current of 1.000 ma. after the indicated waiting period. The trend clearly demonstrates that chemical oxidation of mercury by Hg(I1) is occurring. The anodic chronopotentiogrnms obtained on oxidation of plated mercury show that the potential of the rotated mercury-coated platinum electrode is constant to within 2 mv. until about 50 to 70Oj, of the plated mercury is removed. Therefore, the surface concentration of Hg(I1) a t the electrode surface is constant during the waiting period and a significant portion of the electrochemical oxidation. The difference between the surface and bulk concentrations determines the flux of Hg(I1) to the rotated platinum electrode surface. Since the surface concentration of Hg(I1) is constant, the flux of this species must also be constant. If the rate of reaction of mercury with Hg(I1) were rapid compared to the rate of supply to the electrode surface, the surface concentration of Hg(I1) would be zero, and the rate
20
c
Tu on
T H ~
T , = '5.00 minutes i, = ip = 1.000 x ampere CHh.0, = 0.1M All data are triplicates. ' 7ng, Seconds, for C a g c I r ) = 1.00 X 2.00 X 5.00 X T,, lO-'M lO-3M Min. lO-3M 0 14.68 29.3' 57.3 25.6 50.9 1.50 11.82 3.00 8.88 17.8 48.2 3.92 12.0 6.00 . I
-
I
Effect of
k is the heterogeneous rate constant in centimeters per second for Reaction 5 as written. In obtaining Equation 9 the reverse of Reaction 5 has been neglected, since it would be negligible under the experimental conditions. The rate of supply of Hg(I1) to the rotated electrode is
where D is the diffusion coefficient of Hg(I1) and 6 is the Nernst diffusion layer thickness. Assuming that the rate of supply of Hg(I1) equals the rate of reaction of Hg(I1)-i.e., equating Equation 9 to Equation 10-Equation 11 is obtained.
Substituting Equation 11 in Equation 9 yields
-
d", dt
&. At
= -10-3kDA
6k
+
D
CAa(II)
Plating Time (Mln.) a t I mA.
(12)
Figure 5. Anodic transition times as a function of plating time 0,0. VIS
e. VI = = 1 .OOO X 1 0 - 3 ampere 0.1 M nitric acid supporting electrolyte
explains the main features of the experimental results is possible. The data of Table I can be adequately explained by assuming that the heterogeneous reaction of Hg(I1) with mercury metal produces a steady-state concentration of Hg(I1) a t the electrode surface. While it is unnecessary to assume any specific rate law to treat the data in Table I, it is convenient to assume that the reaction of Hg(I1) and mercury is first-order with respect to the surface concentration of Hg(I1) in discussing the data in Table 11. In Equation 9 &d
dt
-
d"g(ll)
dt
- -10-3 ka4COH,(11,
(9)
Because the bulk concentration of Hg(I1) is constant, the rate of oxidation of plated mercury is constant and is indicated in Equation 12 by the use of the ratio A N H g ( I 1 ) / A t . A constant rate of oxidation would be obtained
Stripping Current (rnAJ
Figure 6. Determination of N H ~ according to Equation 14 T,. ip.
5 minutes
1.000 X 10- ampere C H ~ ( I I ) . 5.00 X 1 O-'M 0.1 M nitric acid supporting electrolyte VOL. 33, NO. 9, AUGUST 1 9 6 1
1205
p
2.5
9
-$ 2.0
x
53 h'
1.5 0
>
-0.2-
I I I
Plating Current
1.0
!-off I
100 Tw
+
200
I
I
I
2.o
1.0
0
300
TM,, Seconds
I
'
*
3.0
TIME, Seconds
Figure 7. Test of first-order rote for Reaction 5 according to Equation 17
Figure 8. mixture
TP. 5.00 minutes i,, = i, = 1.000 X 10-3 ampere 0.1 M nitric acid supporting electrolyte C ~ ~ ( 1 1 ) M, . indicated in figure
CTI(1). CPb(l1).
Anodic chronopotentiogram of TI(I)-Pb( II)
5.0 x 10-'M 3.0 x iO-'M
CH.(II). 5.00 X 10-4M CHNO~. 0.1M 1.OOO X 1 O+ ampere i, T,. 5 minutes ia. 0
for any rate expression involving only
Chemical stripping by Hg(ll1
C'H~(II). If the plated mercury is removed by electrochemical oxidation,
where the first term to the right of the equality sign corresponds to electrochemical oxidation of mercury and the second to chemical oxidation. In the special case where T , = 0 and C H ~ ( I Iand ) N E s are constant, Equation 14 is obtained 10-akDA C H ~ ( I I ) _1 = THg
LgF
+
[m] "B
was given to the loss of mercury caused by Reaction 5. A comparison of Equations 8 and 15 shows that under the conditions of the plating efficiency experiments only 4% of the plated mercury is lost by Reaction 5 during the transition time interval. Using the value 136 m. for the limiting current of a 5 X 10-4M Hg(I1) solution, the maximum plating efficiency in Figure 4 is 75%. L , the thickness in Angstrom units of the mercury film deposited on the rotating platinum electrode, follows directly from Equation 15 and is given by Equation 16,
(14)
Therefore, a plot over ~ / T H US. ~ i, should be a straight line with the slope of lO-%DA CH~(II) F and intercept The data of Table I aTe plotted-acciFding to Equation 14 in Figure 6 and a straight line is obtained. The intercept is 0.0022 second-', and the slope is 65.8. Therefore N H g = 1.58 X 10-7 mole. The amount of mercury plated is proportional to CB~(II, and Tp; therefore Nag = 6.32 X lo-' CEI~(II) T p (15)
[
(16)
assuming uniform coating of the 0 . 1 6 sq. cm. electrode area. When T , = 5 minutes and CH~(II) = 5 X lO-'M, L = 1.5 x 1 0 3 ~ . A test of the assumed rate law (Equation 9) is obtained by substituting Equation 15 into Equation 13, and rearranging to obtain Equation 17 when i, = i, = 1.OOO X ampere and T, = 5 minutes.
1x.
when i, = 1.000 X ampere. Equation 15 is not dependent upon the particular rate expression (Equation 9) used. It is obtained from the intercept of Equation 14 and the propor) Tp tionality of r H g to C H ~ ( I Iand (Equation 8). In the discussion of the plating efficiency of mercury no conideration 1206
L = 5.9 X 106 C H ~ ( IT, I)
ANALYTICAL CHEMISTRY
CHI(II)
= 0.316F
-
Equation 17 predicts that a plot of TB./CH~(II) US. { T H ~ T w ) should be a straight line with an intercept of 3.05 X lo4. Figure 7 is a plot of the data in Table I1 according to Equation 17; it is apparent that Equation 17 does not adequately represent all
+
the data. If CBa,cII) = 5 X 10-4M the observed intercept has a value of 3.04 X lo4, while if CHg(ll) = 1 X l O - 3 M , the intercept is 3.12 X lo4, as compared to the predicted value of 3.05 X 104. However, there is a distinct deviation from linearity of large values of r a T,. This deviation is most apparent when C H s ( I I ) = 2.00 X 10-aM. The slopes of linear portions of the line a t all three concentrations are approximately the same, 65. The latter value is in good agreement with the value, 67, Calculated from the intercept in Equation 14. If CagcII)5 10-JM and ( T , T H ~ 5 ) 200 seconds, Equation 17 accurately describes the electrochemical and chemical oxidation of mercury at the platinum electrode. Therefore, k, the heterogeneous rate constant, can be calculated from the area of the electrode and the slope of Equation 17, provided the value of D/S is known. The value of this ratio is obtained from Equation 10 when C H g ( l ~ , is zeroLe., from the limiting current of Hg(I1). The calculated value of D/6 is 8.8 X 10" cm. per second and cm. per second. k = 8.1 x Plating and Stripping of Tl(1) and Pb(I1). The current-voltage curve of 5 x lO-'M Tl(1) in 0.lM nitric acid does not show any evidence for the reduction of Tl(1) prior to the evolution of hydrogen a t -0.4 volt us. S.C.E. The decomposition potential of a 5 X lO-4M Tl(1) solution is calculated to -0.83 volt us. S.C.E. and there be is no possibility of depositing Tl(1) on a platinum electrode from 0.1M nitric
+
+
N
acid. However, as was shown above, passing a current of 1.000 ma. through a supporting electrolyte consisting of 0.1M nitric acid and 5 X 10-4M Hg(I1) results in the attainment of a voltage of approximately - 1.05 volts us. S.C.E. Therefore, if a solution containing a trace of thallium and the above supporting electrolyte is electrolyzed with i, = 1.000 X loT3ampere, thallium will deposit simultaneously with the mercury on the platinum electrode, and the amount of thallium deposited can be determined from the anodic chronopotentiogram obtained on turning the current off [chemical oxidation by Hg(II)] or on reversing the current. Turning the current off yields the most sensitive analytical method, but experiments involving electrochemical oxidation are reported below in connection with establishing the stoichiometry of the chemical oxidation. The above discussion applies also to dilute solutions of Pb(I1) and solutions containing a mixture of Pb(I1) and Tl(1). Using the above described supporting electrolyte, the plating and stripping of Tl(1) and Pb(I1) have been studied in the concentration range 2.5 x 10-7 t o 8.00 X 10-6M (Table 111) and 2 X t o 5 X 10-6M (Table IV), respectively. A typical chronopotentiogram obtained on stripping a mixture of 2 X 10e6M Pb(I1) and 1 x 10-6M Tl(1) at zero current after plating for 5 minutes a t 1.000 ma. is shown in Figure 8. Reaction 4 results in the stripping of thallium from the amalgam, while the chemical stripping of lead is more complicated and appears t o result from two concurrent reactions PbAmal and PbArn.1
+ Hg(I1) = Pb(1I) + Hg
+ 2Hg(II)
=
Pb(I1)
+ [Hg(I)]2
aa discussed in the section on stoichiometry of chemical stripping. The use of a 1.000-ma. plating current yields the best plating efficiencies for thallium. In a series of experiments on a 6 X lO-5M Tl(1) solution various plating currents were used for a constant plating time of 5 minutes. The resulting anodic transition times, T T I , obtained on oxidation at 1.OOO ma. were determined and are plotted in Figure 4. In Figure 4 all transition times have been divided by the maximum transition time to permit comparison with the efficiency of mercury plating. A maximum in the plating efficiency occurs at a plating current of 1.00 ma. as was found for Hg(I1) in the absence of thallium. Figure 4 also contains the plating efficiency data for the mercury deposited from a solution containing 5 X lO-4M Hg(II), 0.1M nitric acid, and 6 X 10-5M Tl(1) stripped at 1.OOO ma. The maximum transition time also occurs a t a plating
Table 111.
Stripping of Tlamniby Hg(ll)
T P = 5 minutes Zp = 2"
=
1.OOO 0.0
x
10-8 ampere
Least squares equation for data is VI,
800 600 400 200 100 50.0
25.0 10.0 5.00 2.50
= 0.011
+ 3.96 X 106 CTI(II)
Seconds
29.20, 29.60 23.28, 22.52, 23.00 15.34, 15.30 7.75, 7.86 3.90, 3.87, 3.77, 3.96, 3.80 1 . 9 0 , 1.98 1.10. 1 . 0 4 0.400, 0.440;0.400, 0.415 0.210, 0.200 0.110, 0.110
Table IV.
3.68 f 0 . 0 3 3.82 f 0.04 3.83 f 0.01 3.91 f 0 . 0 2 3 . 8 4 f 0.06 3.86 f 0.08 4.28 f0.09 4 . 0 9 f 0.19 3 . 8 8 f 0.11 3.96 f0.00
Stripping of PbAml by Hg(ll)
T, = 5.00 minutes i *= 1.OOO X 10-8 ampere in
= 0.0
Cane,
= 0.1M C H ~ ~ I=I )5 X lO+M Least squares equation for data is r p b = -0.0084 rpb,
+ 5.267 X lo6
CPc(I1)
Seconds
50.0 20.0
2.54, 2.48 5 . 0 3 f 0.06 1.04, 1.06 5.29 f 0.05 10.0 0.504, 0.504,(0.500, 0.524, 0.510)" 5 . 1 6 f 0.07 8.00 0.396, 0.376, (0.424, 0 . 4 3 6 , 0.458)" 5.23 f 0.32 6.00 0.336, 0.316, (0.326, 0.340)" 5.63 f 0.14 4.00 0.200, 0.194, (0.200, 0.194)O 5 . 1 3 f 0.08 2.00 0.092, 0.090,(0.104, 0.102, 0.095)" 5.25 f 0.25 All data in parenthesis obtained in solutions which also contained 1.00 X 10-M TKI)
current of 1.OOO ma.; however, the observed transition time for mercury is 1.65 seconds larger than found in the absence of thallium. This difference arises from additional mercury deposited on the electrode because of Reaction 4, and is quantitatively predicted in the section on stoichiometry of chemical stripping. No experiments on the plating efficiency of lead were performed. The effect of plating time on the amount of thallium deposited was studied. The anodic transition time obtained on stripping 2 X 10-6M Tl(1) at zero current from a supporting electrolyte containing 5 X lO-'M Hg(I1) and 0.1M nitric acid was determined as a function of plating time, and the results are shown in Figure 5. A straight line is obtained only for plating time in excess of 1 minute. At shorter times nonlinear behavior is observed because the potential of the rotating platinum electrode is initially below the decomposition potential of the Tl(1) and becomes more negative only as the electrode becomes covered with mercury. The necessity of using plating times in excess of approximately 3 minutes for' the determination of'Tl(1) is no hindrance in analytical applications.
The thallium data (Table 111) were fitted to the straight line TTI = 0.011 X 3.958 X IO5 CTI(I) by the method of least squares, assuming that a constant relative error of 0.03 was present in the measurement of 7~1. The standard deviation in the intercept is 0.002 while it is 0.003 X 105 for the slope of the fitted line. A similar treatment of the lead data yielded 7 p b = -0.00&1 5.267 x lo6 CPb(I1). The standard deviation in the intercept is 0.00002 and is O.OOO4 x 105 for the slope. Four unknown mixtures of Pb(I1) and Tl(1) were prepared and analyzed by adding Hg(I1) and nitric acid 80 that the final concentration of the latter two species corresponded to 5 X lO-'M and 0.1M, respectively. The anodic transition times for Pb(I1) and Tl(1) were determined and the concentration of each species waa calculated from the least squares equations. The results are,given in Table V. The precision between pairs of results is never poorer than 576, and is substaptially better in most cases. The accuracy obtained varied from 2 to 8% for Tl(1) and from 0.5 to 7% for Pb(I1). Stoichiometry of Chemical Stripping. It is of some interest t o estab-
+
VOL 33, NO. 9, AUGUST 1961
1207
Table
V.
Chemical Stripping Analysis of TI(I)-Pb(ll) Mixtures
*Tf x 10' Taken %error
CTI(I)r
Sample In It?
Av. 2a 2h .4V. 3n 3b Av. 4a 4b AV.
TTI,
sec.
0.410 0.420 0.735 0,710 0.982 0.882 0.570 0.586
Found 10.0 10.4 10.2 f 0 . 2 18.3 17.7 18.0 f 0 . 3 24.3 22.0 23.2 f 1 . 2 14.1 14.5 14.3 f 0 . 2
9.45 9.45 9.45 18.7 18.7 18.7 23.7 23.7 23.7 13.5 13.5 13.5
++ l O5. .l 8 + 7.9 -
CPb(I1)
TPb, sec.
Found
0.504 0.480
9.73 9.26 9.50 f 0.24 5.00 4.94 4.97 f 0.03 10,45 10,15 10.30 f 0 . 1 5 13.05 13.15 13.10 f 0 . 5
++ +
lish the stoichiometry of the reaction of Hg(I1) with the deposited trace metals, lead and thallium. One method of accomplishing this is to strip the trace metal at a series of different oxidation currents. As in the chemical oxidation of mercury by Hg(II), an exact treatment is impossible, but the approximate treatment given below is successful in explaining some of the experimental results. If 31 represents the depoqited metal and .!!In+ the ion formed on oxidation, and the stripping of >I from mercury is considered to occur by chemical and electrochemical oxidation, then
where the first term on the left of Equation 18 represents moles of RZ stripped electrochemically and the second term gives the moles of M removed by the reaction of M with Hg(I1). In Equation 18, a = 2 if Hg(I1) is reduced to mercury metal. Assuming a steady state surface concentration of Hg(II), the rate of reaction of mercury with is RI a t the electrode surface, d"g(II), 7 constant during the transition time and has a maximum value defined by the limiting current of Hg(I1). On rearranging Equation 18, Equation 19 results.
9.85 9.85 9.85 5.35 5.35 5.35 10.35 10.35 10.35 13.95 13.95 13.95
-1.2 -6.0 -3.6 -6.5 -7,i -7.1 +1.0 -2.1 -0.5 -6.4
of mercury was deposited on the electrode by the chemical oxidation with Hg(I1) of thallium amalgam. Since the amount of thallium amalgam formed under constant plating conditions is ] amount of proportional to C T I ( ~the thallium amalgam formed in the 6 X -5.i lO-5Ji Tl(1) solution is three times the -6.1 amount found for a 2 x lO-'JI Tl(1) solution-Le., 3 X (1.13 X lo-* mole). Therefore, the amount of mercury produced by Reaction 4 is 1/2(3.39 X acts with thallium amalgam rapidly lo-*) = 1.70 x mole based on compared to the rate of supply of Equation 19, while determination of this Hg(I1) to form Tl(1) and Hg. amount from Figure 5 yields 1.71 X The situation with lead amalgam is mole. more complicated. The slope and inIn principle it should be possible to tercept are 3.32 X lo3 coulomb-' choose between two of the mechanisms respectively. and 0.372 second-', proposed for the chemical oxidation of Therefore h r P b = 1.59 X mole and lead amalgam on the basis of the inN H I~I ( a = 1.67 if A___ = 7.05 X lo-"'. creased transition time for the electroAt chemical oxidation of mercury, since This nonintegral value of a can be indifferent quantities of mercury would terpreted in several ways. For esbe produced, but in practice this proHg(I1) ample, if the reaction P b cedure is not possible because of the Pb(I1) Hg is slow compared to the AhrHg(r 11 rate of supply of Hg(II), then 7 limited solubility of lead in mercury. To deposit enough lead in mercury is actually 5.9 x mole per second, to produce a significant increase in because a nonzero steady-state conT H ~ ( I I ) , C P b ( I I ] would have to be so centration of Hg(I1) exists a t the eleclarge that the solubility of lead in trode surface. This hypothesis remercury would be exceeded severalquires that the reaction rate be zero fold. order with respect to the surface lead The solubility of the deposited species amalgam concentration in order to be in mercury places an upper limit on in agreement with all the experimental the maximum aqueous phase conresults. An alternative explanation is centration of that species which that if the above reaction is rapid, can be analyzed using the rotated, the reaction P b 2Hg(II) .-t Pb(I1) mercury-plated platinum electrode. [Hg(I)], also occurs, and some of the From Equation 15, the volume of mermercurous ion formed escapes into the cury deposited on the rotated platinum solution without reacting with lead CH~(II)T, cc. electrode is 9.36 X amalgam. The latter hypothesis is The number of moles of lead deposited, also indistinguishable from the rapid assuming a linear dependence with reaction of lead amalgam with Hg(I1) T , and C P b ( I I ) , is 6.16 X IOp5 C P ~ ( I I ) to form Hg(1) followed by a slow reT,, and the molar concentration of duction of Hg(1) to mercury by lead CPhlII) The lead amalgam is 65.7 X G. amalgam. solubility of lead in mercury is 0.96.V It is now possible to account quan-
2 . 1 0.255 - 5 . 4 0.252 - 3.7 3 . 5 0,550 - l . 2 0,526 - 2.1 4 . 4 0.680 7 . 4 0.690 5.9
+
M x io' Taken % difi.
titatively for the difference between the transition time noted for mercury in the presence and absence of 6 X 10-5.1f TlII) (Figure 4). These data indicate
+
+
+
-
+
1
In Figure 9, a plot of - us. i, for 5 X TM 10-6Ji Pb(I1) in 5 x 10-4.V Hg(I1) and 0.lM nitric acid and also 2 X lO-5M Tl(1) in the same supporting electrolyte is given. The lead data lie on a straight line and the straight line has been drawn through the tivo Tl(1) points. In the case of Tl(I), the slope and intercept are 0.92 X los coulomb-l and 0.128 second-'. respectively. These values yield NTl = 1.13 X and a = 2.07 using - AArHg(ll) - 7.05 X 1O-Io mole per
Figure
9.
Determination
of stoichiometry of stripping of T L a l and PbAmai according to Equation 19
At
second as calculated from the limiting current of Hg(I1). Thus, Hg(I1) re1208
ANALYTICAL CHEMISTRY
Ia milliamperes
(6); therefore, C P b ( l 1 ) I 0.0158 C H ~ ( I I ) if the solubility of lead in mercury is not to be exceeded. In these experiments C B ~ ( I=I ~5 X 10-4M and the maximum permissible value of C P b ( l 1 ) = 7.9 X 10-6M. Similar calculations for thallium, using the solubility of thallium in mercury as 27.4M (12), leads to CTI(Il I 1.1 X 10-4M when C H ~ ( I I=) 5 x lO-'JI. These conditions aie met by all of the experiments reported above. Efficiency of Plating of Pb(I1) and Tl(1). Equations 14 and 19 permit the calculation of the value N / C for Hg(II), Pb(II), and Tl(1) in 0.1M nitric acid. This ratio depends on the limiting currents for the various species, the time of plating, and the efficiency of plating. If the plating times are the same, the plating efficiencies may be calculated from the limiting currents a t the rotated platinum electrode. It is impossible to observe the limiting currents for Tl(1) and Pb(I1) in 0.1.11 nitric acid, and in order to obtain an estimate of the plating efficiency it is necessary to assume that the ratio of the limiting currents of Pb(I1) and Tl(1) at a rotated platinum electrode are not dependent upon the supporting electrolyte. Fmm the data Sightingale (11) obtained for the limiting currents of Pb(I1) and Tl(1) in neutral chloride and perchlorate media, using two different platinum electrodes rotated at 600 r.p.m., it is calculated that the ratio h T T I ( I ) / C T I ( I ) / l ~ P b , I I ] / ~ P b ( I l )is 1.76 0.02 From the intercepts of Figure I 1 this ratio is calculated to be 1.78. This close agreement between the limiting current data and the stripping data demonstrates that the efficiencies of
*
plating of Pb(I1) and Tl(1) are the same. It is of interest t o decide whether the absolute value of the plating efficiency of Hg(II), Pb(II), and Tl(1) are the same. The limiting current of Pb(I1) in 0.1M KNOs determined with the electrode used in this work, was found to be 141 pa. for a 5 XlO-*M Pb(I1) solution as compared t o 136 pa. for the same concentration of Hg(I1) in 0.2111 nitric acid. Thus, within the experimental error, the absolute plating efficiency of all three ions is the same. This result supports the conclusion reached earlier that mechanical dislodgement from the rotating electrode accounts for the loss of approximately 25% of all the mercury deposited.
at rotating platinum eleck L X,
= =
T,
=
T,
=
7,
=
=
trode in amperes heterogeneous rate constant thickness of mercury film, a number of moles of ith species time of plating of rotated platinum electrode (R.P.E.), minutes elapsed time between cessation of plating and start of stripping of R.P.E., seconds anodic transition time for ith species, seconds ACKNOWLEDGMENT
The authors are indebted to E. I. du Pont de Nemours & Co., Inc., for a grant-in-aid of research. LITERATURE CITED
(1) Cooke, W. D., ANAL. CHEM.25, 215 ( 1953). (2) Gardiner, K. W., Rogers, L. B., Ibid., a = apparent valence change of 25. 1393 (1953). Hg(I1) in reaction: ~ M . A , , , ~ I (3) Haissinsky, M . , Cottin, M., J . chim. nHg(I1) -+ aM3+fnHg(I1-a) phys. 46,476 (1959). A = area of rotated platinum elec(4) Hietanen, S., Sillh, L. G., Arkiv. f o r trode (0.16 sq. cm.) Kemi 10,103 (1956). C, = bulk analytical concentration of (5) Hoyt, C. S., Stegeman, G., J. Phys. ith species Chem. 38, 753 (1934). Co, = electrode surface concentration (6) Kemula,, W., Galus, Z., Bull. wad. of ith species polon., scz., sdr. sci. chim., gdol. et gdogr. 6,661 (1958). D = diffusion coefficient of Hg(I1) (7) Kemula, W., Galus, Z., Kublik, Z., 3 = instantaneous rate of change of Zbid., 10,723 (1959). at i t h species, moles per second f8) Kemula. W.. Kublik. Z.. Galus. 2.. a t electrode surface (Si Marple, T. L., Rogers, L. B., ANAL. ANi - - - constant rate of change of i t h CHEM.25,1351 (1953). At species, moles per second a t (10) hlarple, T. L., Rogers, L. B., Anal. Chim. Acta 11, 574 (1954). electrode surface (11) Nightingale, E. R., Jr., Ph. D. thesis, F = the faraday Universitv of Minnesota. 1955. i, = constant anodic current used in (12)- Teetec C. E., Jr., J . Am. Chem. SOC. stripping in amperes 53, 3917 (1931). i, = constant cathodic current used in plating in amperes RECEIVEDfor review January 23, 1961. Accepted May 17, 1961. (il)i = limiting current for i t h species NOMENCLATURE
+
Capillary Behavior in High Sensitivity Polarography W. D. COOKE Department o f Chemistry, Cornel1 University, Ithaca, N.
Y.
M. T. KELLEY and D. J. FISHER Oak Ridge National Laboratory, Oak Ridge, Tenn.
b A study has been undertaken to ascertain the factors that limit the sensitivity of polarography with the AIdropping mercury electrode. though many of the well known limitations can be removed by modern electronic instrumentation, another difficulty, caused by capillary "noise," remains important. Some sources of this noise have been discovered and a capillary has been designed which greatly reduces the erratic nature of polarographic backgrounds at high sensitivity and extends the scope of
polarographic methods to more dilute solutions. By using an ORNL Model Q-1988-ES controlled-potential and derivative polarograph with modified capillaries, it is possible to detect reducible species which would give diffusion currents as small as 0.0002 pa. by conventional methods. In the case of zinc, this corresponds to a concentration of 0.006 pg. per ml. The results of this investigation emphasize that the mechanical design of the dropping mercury capillary is an important polarographic parameter.
T
paper evaluates the factors which limit the reproducibility of current a t the dropping mercury electrode and which, therefore, establish the detection limit when it is used for the analysis of dilute solutions. Excluding those methods which depend upon catalytic effects, the lower limit of concentrations which can be readily determined by conventional polarographic methods, employing the dropping mercury electrode, is about 10-5M, which corresponds to a diffusion current of the order of 0.1 pa. Square wave HIS
VOL. 33, NO. 9, AUGUST 1961
1209
