Pola rogrciphic of Te c hne ti um G.
B. S. SALARIA,
a nd Co uIome tric Determination
CHARLES L. RULFS, and PHILIP J. ELVING
The University o f Michigan, Ann Arbor, Mich.
b Since technetium in any oxidation state can b e readily converted to Tc(VII), the selective determination of the latter will allow technetium t o b e determined in most types of samples in which it may b e expected to occur. The polarographic and coulometric have condetermination of T:(VII) sequently been invest gated. Measurement of Tc(VII) wave I a t pH 2 generally results in optimum selectivity and sensitivity, with the measurement o f wave I or II a t pH 13 being used i f interfering polarogrciphic waves occur a t pH 2, or for equally cogent reasons. Measurement of catlilytic wave 111 at pH 1 3 affords maximum sensitivity for the esiimation of technetium at very low concentration levels. Technetium can also b e accurately determined b y controlled potential electrolysis and coulometric measurement. The feasibility o f determining technetium polarographically in complex mixtures is indicated b y the oploortunity of using different waves a t radically different pH values; the wave used would b e determined by the location o f the polarographic waves given b y other sample components. Technetium can b e readily determined in the presence of rhenium a t pH 2; technetium and rhenium in admixture can b e simultaneously determined by polarography in 2M hydrochloric acid. COMPLEX PATTERN Of pOlar0graphic behavior exhibited by technetium has been discussed in a systematic study (5) of the reduction of pertechnetate ion and succeeding osidation states over the p H range of -0.60 to 13.3. The polarography of technetium has also been investigated by Colton et al. ( f ) , Magee, Scott, and Wilson (S), and Miller, Kelley, and Thomason (4). LitiJe, however, has been rcported on the polarographic determination of technetium, especially in the presence of rhenium, and on possible sources of error. Miller et al. (4) reported that te2hnetium in concentrations as low as 0.2 p.p.m. can be determined in ])H 7 phosphate buffer. After the present paper had been submitted for publication, a paper by Terry and Zittel (6) was called to our attention. HE
The present investigation has established that the first polarographic wave, corresponding to a 4e reduction, obtained from pertechnetate solutions a t pH 2 and the first and second waves obtained a t p H 13, corresponding to 3e and l e reduction, respectively, can form the basis for the quantitative determination of technetium. The third wave observed a t p H 13, which is apparently catalytic in nature, can be used for qualitative assessment of very small amounts of technetium. The first wave a t p H 2 can be used to determine technetium in the presence of rhenium, which can itself also be determined (polarographically) together with technetium in 2M hydrochloric acid solution. Since technetium in any oxidation state can be readily converted to Tc(VII), the determination of Tc(VI1) is sufficient to cover most situations in which technetium is to be determined. EXPERIMENTAL
A solution of ammonium pertechnetate in water ( p H 4) containing 46.75 mg. of Tc99 per ml. and 3 X 10-6 mc. of Tc-95m per gram of Tcggwas obtained from the Oak Ridge National Laboratory; coulometric studies of this solution substantiate the stated concentration ( 5 ) . Technetium stock solution I was prepared by diluting 10 ml. of the pertechnetate solution in a 250-ml. volumetric flask; 10 ml. of solution I was further diluted to 100 ml. to give stock solution 11. Sitrogen (oil-pumped) was used without further purification for purging solutions of oxygen. All other chemicals used were of C.P. grade. The supporting electrolytes generally used were 0.5M KCl with HCl added to pH 2 and 0.25Jir K2SOawith KO13 added to p H 13. Apparatus. A Fisher Elecdropode and a Leeds and Xorthrup ElectroChemograph Model E were used for polarographic measurement. pH was measured with a Leeds and Northrup No. 7664 p H meter. The dropping mercury electrode essentially consisted of a capillary (Corning marine barometer, 8 cm. long) connected to a mercury reservoir with Tygon tubing. The capillary constants (at a corrected height of 42.4 cm. of mercury) a t potentials a t which the limiting currents mere measured, were Chemicals.
m = 3.47 mg. per second and t = 2.27 seconds a t p H 2, and 3.46 and 2.24 for wave I and 3.45 and 2.01 for wave I1 at p H 13. Polarographic measurements were made in an H-cell thermostatically controlled a t 25.0' + 0.1" C., one leg of which contained a saturated calomel electrode, S.C.E. All reported potentials are therefore vs. S.C.E. a t 25' C. A thin window Geiger counter (Nuclear Chicago Model 151A) and a flow transfer type scintillation well counter (Atomic Instrument Model 162) were used to measure the @-activity of technetium samples. Chemical
Reduction
Procedure.
Five milliliters of stock solution 11, mixed with a n equal volume of 2 M hydrochloric acid, was shaken with a n excess of mercury for 4 to 5 hours and left overnight. The clear solution was decanted; the mercury was thoroughly washed with distilled water and treated with ammonia and hydrogen peroxide to dissolve technetium. The ammonium pertechnetate solution thus obtained was evaporated under an infrared lamp onto an aluminum planchet, whose @-activity was counted. The results were compared with those for a known sample of technetium (Table I). Analogous experiments were performed by shaking 0.945M Tc(VI1) solutions (0.05 or 1.2111 HCl, or 1M HzS04) with mercury. The solution was then diluted to a known volume;
Table 1. Chemical Reduction of Pertechnetate" b y Mercury
Contact time, No. 1
[H+]
A4
1
2
1
3
1 0.1
4 5 6
7 8
10
0.1 0.1 0.1
Tc recover)-,*
min.
70
(4-5 hours; left
0.0511
overnight) (4-5 hours; left overnight) 10 10 10 1Od 20d
30d
0.0552 0.0012 0.0011
o. . .on1 ..- 3 .0.0009 0.0010 0.0011
Tc(VI1) concn. = 0.189mM. *Percentage of total Tc present recovered from mercury. c Present as H2S0r; HCl was used in the other experiments. d Time of contact with dropping-mercury electrode.
VOL. 35, NO. 8, JULY 1963
979
Table
II.
Coulometric Determination of the Faradaic n Value for Technetium(VI1) in Acidic Solution
No.
PHO
1
-0.3 -0.3 0.8 1 . 1 (1.3) 2 . 0 (7) 1.0 1.1 1 .75 1.78
2 3 4
5
6b 7b 8b
gb,c
5
b c
Acid 1M HISO' 1M H2SOd
+ KC1 KCl + + KCl HC1 + KCl HCl f KCl HCl + KC1 €IC1 + KC1 HCl HCl HCl
Potential applied, volts $0.12 -0.05 -0.03 -0.30 -0.16 -0.3 -0.3 -0.3 -0.5
n 3.05 4.09 3.05 3.81 3.08 3.92 3.94 3.05 3.74
Solution change brotvn ppt. black ppt. brown ppt. green soln. brown ppt. green soln. green soln. brown ppt. green s o h .
Numbers in parentheses indicate the pH at the end of electrolysis. Acid wm added during these runs to keep the pH constant. 9.35 mg. Tc was present in this run; 18.7 mg. was present in all others.
aliquots, after deoxygenation with nitrogen, were examined polarographically. Polarographic Procedure. Polarographic test solutions were usually prepared by diluting a n aliquot of stock solution I1 t o 50 ml. in a volumetric flask with t h e desired supporting electrolyte t o give a solution of the desired T c concentration. The p H of this solution was measured; about 15 ml. was transferred t o t h e H-cell, deoxygenated with nitrogen for 10 t o 15 minutes, and then polarographed over the desired potential range. Triton X-100 (concentration 0.0013%) was addoc! in a few cases to suppress ;llrzxlma. Eliz and id were determined graphically, utilizing the average of the recorder trace; in the case of the Elecdropode, the maximum deflections were used. Coulometric Procedure. For coulometric a n d macroscale electrolysis a t controlled potential, 50 ml. of buffer solution was added to t h e coulometer cell containing a large mercury pool cathode, deoxygenated for about 10 minutes, and electrolyzed at a potential several tenths of a volt more negative t h a n t h a t at which t h e electrolysis was t o be run, until t h e current fell to its minimum value, generally 1 to 2 ma. (Any hydrogen peroxide produced during the gas purging would be destroyed during the electrolysis.) A known volume of T c stock solution I was sewaratelv deoxvaenated and then added io the" coulometric cell; the coulometer was connected (the hydrogen-oxygen coulometer was presaturated with the gases); and the solution was electrolyzed at the desired potential as nitrogen was continuously passed through it. Initial currents of 35 to 40 ma, fell to 1 to 2 ma. a t the conclusion of a run. I n runs G to 9 of Table I1 hydrochloric acid was added from a buret to maintain the p H constant during the run. The volume of hydrogen-oxygen evolved was plotted against time to establish the optimum stopping point. From the curve thus obtained, the volume of the gas evolved during electrolytic reduction was determined and corrected for the residual volume due t o the background electrolysis, temperature, pressure, and vapor pressure. 980
ANALYTICAL CHEMISTRY
RESULTS A N D DISCUSSION
Sources of Error. Two sources of error observed during t h e present investigation are of general interest in connection with t h e chemistry of technetium as well as its polarography. Fifteen milliliters of pertechnetate solution [0.189miM Tc(VI1); 0.5-M KCI HCl to pH 1.001 in the polarographic cell at 25' C. was purged with nitrogen for 10 minutes. The nitrogen and entrained HTc04 escaping from the cell Lvere led into a tube containing ammonia solution, whose @-activity was equivalent to 0.84% of that of the Tc in the test solution, as calculated from a standard sample similarly counted. The loss would naturally be greater with longer bubbling or from solutions of higher acid concentration; it would be minimized by presaturating the nitrogen in an appropriately thermostatted bubbler containing a n identical solution. I n any event, it seems advisable that all effluent gases from acid media containing Tc(VI1) be trapped in alkali. The direct chemical reduction of Tc(VI1) by hydrochloric acid more dilute than 2 X is very slow at room temperature. However, in the presence of mercury some chemical reduction does occur even with more dilute acid. I n all cases a minute fraction of the technetium (perhaps 0.001% of the total present) appears as a grey film (Tc metal ?) on the surface of mercury during the time required to run a polarogram. Actually, the formation of the grey film at the surface of mercury drops was noticed. Since the solutions in experiments 1 to 5 (Table I) were shaken with a large excess of mercury, the low values of Tc recovered suggest that a thin film suffices to retard further reduction. Experiments G to 8 show that during the time normally required for the polarographic esamination of a pertechnetate solution, the evtent of chemical reduction of Tc(VI1) by mercury will be negligible.
+
The polarographic behavior of pertechnetate solutions in hydrochloric or sulfuric acid, which had been shaken with mercury, was quite informative. In 2.4M HCl, only the second of the two waves obtained for a Tc(VI1) solution was observed, indicating that Tc(VI1) is reduced almost quantitatively to Tc(1V) or (111). Evidently, there is some reduction to still lower states since 20 to 47% decreases in the height of wave I1 are found. I n 0 . l N HCl or 2.M H2S04, both Tc(V11) waves appear but wave I is reduced by 87 and 59%, respectively. Coulometry and Coulometric Analysis. The reproducibility of wave I diffusion currents in acidic mcdia is not always satisfactory ( 5 ) , because of the tendency for a stepwise separation of the (VI1)-(IV) and (1V)-(111) stages, which tends to be more prominent at higher acidities, and in sulfuric (as compared to hydrochloric) acid solution. It is possible to obtain an n-value of 3 by carefully electrolyzing just at the Eliz of wave I (cf. runs 1, 3, and 5, Table 11); this is readily done in sulfuric acid media. Continued electrolysis on the resulting brown T c 0 2 suspension at a more negative potential does not result in further reduction. However, initial electrolysis of Tc(VI1) at a more negative potential does give an n of 4; the resulting black suspension is apparently Tcz03. It is also possible to obtain an n of 3 in hydrochloric acid solution, resulting in a brown TcOz suspension, but one need only have the initial potential just on the plateau of the wave to obtain an n of 4 and a green solution (run 4), which still exhibits polarographic wave 11. Further investigation has shown that it is possible to obtain an n of 4 within 2%, if the coulometry in hydrochloric acid solution is made a t -0.30 us. S.C.E.-Le., on the plateau of the wave-and the pH is maintained constant within 0.1 unit by adding acid. Near p H 1.75 the stepwise separation of the (VI1)-(IV) and (1V)-(111) stages becomes more prominent (cf. runs 8 and 9) and an n of 3 can be obtained by electrolyzing on the plateau of the first wave. However, initial electrolysis of pertechnetate at more negative potential does give an n of 4. Determination of Technetium. Review of t h e polarographic behavior of solutions of pertechnetate ion (5) indicated that successful analytical procedures would most likely be developed from the wave patterns observed at p H 2 and 13. The chemical reduction of pertechnetate by mercury in the presence of hydrochloric or sulfuric acid make' i t difficult to obtain reliable and reproducible data for the first reduction
-
00
004 -c
(mi
008
012
Concenlrtition,
~ _ _ 016
020
At p H 2, the value of the diffusion current constant in the range of 0.01 to 0.2mM Tc(VI1) (Table 111; Figure 1) is fairly constant a t 11.9 i 0.1 for wave I, suggesting that technetium in concentrations as low as O.OlmM can be estimated with an accuracy of =t0.8y0. Waves I1 and I11 do not show any constancy in diffusion current constant, except in the low concentration range of 0.01 to 0.04mM Tc(VII), although their variation with drop-time (mercury height) shows that they are diffusioncontrolled ( 5 ) . At p H 13, Tc(VI1) also gives three waves of Eli2, -0.81. -1.02, and -1.60 volts. However, only wave I, v, hich corresponds to a 3e reduction process, is diffusion-controlled, Wave I1 is adsorption-controlled and apparently involves further reduction to Tc(II1). Kave I1 is followed by a large catalytic hydrogen discharge wave, which is probably due to the lowering of the high hydrogen overvoltage of mercury by the deposit of a thin film of technetium metal. The appearance of the dropping mercury electrode is similar to that of mercury shaken with acidic pertechnetate solution. Chemical extraction of the film gives a solution ehon-ing8-activity.
At p H 13, the diffusion current constant is constant a t 6.32 k0.13 for wave I (Table IV; Figure 1) and at 3.50 i 0.03 for wave 11. Both waves provide a means for accurately estimating very low technetium concentrations. Although it is possible to record wave I11 for low technetium concentrations, the large and variable values of il/C suggest that the wave is catalytic. Determination of Technetium in Presence of Rhenium. Polarography of a pH 2 solution, which was 0.01 to 0.189mM in Tc(VI1) and 0.1 t o 2 m M in Re(VII), gave a first wave corresponding to the 4e reduction of pertechnetate a t Eliz of -0.16 to -0.20 volt, which was 0.01 to 0.04 volt more negative than observed for solutions containing only Tc(VI1) (Table VI). The magnitude of the diffusion current was in agreement with the reduction of Tc(VI1) to (111). Measurements made on wave I gave a mean diffusion current constant of 11.2 j= 0.3 for an accuracy of 3y0, compared to 11.7 observed for Tc alone. Since perrhenate solutions do not show any polarographic wave a t pH 2, wave I can be used to estimate quantitatively pertechnetate in the presence of perrhenate even when
rn!
Figure 1 . Variatiori of wave height with pertechnetate concentration Table 111.
A.
Measured a t p H 2 with Electro-Chemograph 6. Measured a t p H 1 3 with Electro-Chemograph (lower two curves) and Elecdropode (upper curve) C. Measured a t p H 1 3 with Elecdropode
concn., mAf 0.0095 0.0189 0.0378 0.0756 0.1133 0.1511 0.1889
>d -Wave - I11 ~ I:>-
id
pa.
I*
id/C 55.8 12.0 11.9 55.6 11.8 55.8 48.4 11.7 38.0 11.9 11.9 36.9 11.8 34.0 11.86 098 IiCl HC1; tcmp.: 25' pa. 0.53 1.05 2.11 3.66 4.31 5.57 6.60
+
0.15 0.30 0.60 0.72 1.34 1.94 2.76
C.
16.8 15.9 15.9 9.53 11.8 12.8 14.6
71.6 71,5 71.7 57.9 49.8 49.7 49.5
Data obhined with
= id/C??22'3t116.
Table IV.
Variation of Current with Technetium Concenfration a t pH 13"
Tc No.
-Wave I1
id
id/C 1 0.30 31.6 2 0.59 31.2 3 1.17 31.0 4 2.33 30.8 3.54 31.2 5 31.2 6 4.71 31.1 7 5.88 Mean Std. dev. a Background elcctrolj te: 0.5M Electro-Chemograph.
*I
Waves
Wave I
Tc No.
wave of Tc(VI1) ii solutions more acidic than p H 1. On the other hand, the pH range of 3 to 51involves a change in the nature of the electrochemical reduction from a 4e to a 3e process. I n view of these difficulties, pertechnetate solutions of pH 2.0 were studied in the attempt bo devise a procedure for cstimating even very small amounts of technetium on the basis of diffusion current. At pH 2, Tc(VI1) gives three diffusion-controlled pola,rographic waves with half-wave potentials of -0.14, -0.91, and -1.12 volts. The magnitude of wave I1 (run:; 1 to 4, Table 111) is about 1.5 times that of wave I, evidently corresponding to a 6e reduction of Tc(III), which might involve the formation of a technetohydride, conceivably TcI-Is. In runs 5 to i , the magnitude of wave I1 is about the same 5s that of wave I and may involve the formation of 'I'cH. Attempted coulometry a t a potential appropriate t o wave I1 results in (electrolysis beyond an n equivalent t o seven, while the p H of the unbuffered solution rises continuously. Such behavior could result from cyclic process involving technetohydrides.
Variation of Current with Technetium Concentration a t pH 2"
concn., m;M
0.0095 0.0189 0.0378 0.0756 0.1133 0.1511 0.1889 1.889
1
2 3 4 5 6 7 8"
Wave I
K a v e I1
i d
pa. 0.160 0.320 0.620 1.23 1.86 2.,54 3.03 32.8
il
id/C
16.9 16.9 16.4 16.3 16.4 16.8 16.0 17.4 llem Std. dev.
pa.
I d
6.44 6.47 6.27 6.22 6.27 6.42 6.13 6.64 6 317 10.127
0.170 0.340 0.690 1.02 1.38 1.67 4.50
Wave IIIb -___
/C
Id
8.99
3.51 3.51 3.56 3.51 3.48 3.45 0.93 3.503 10.034
i l
8.99 9.13
9.01 S.93 5.84 2.38
il
pa. il/C 2 . 6 7 282 8 . 6 6 458 6 1 . 2 1619
+
Background electrolyte: 0.2511f K&04 ROH; temp.: 25" C Data obtained with FJlectro-Chemograph. * T t was not possible to record catalytic wave I11 for n'os. 4 to 8 because of the high values of the limiting current. e Values for this run were omitted in calculation of mean and standard deviation. d
I
=
id/CVL2/3t"6.
VOL. 35,
NO. 8, JULY
1963
981
Table V.
Variation
Tc
No. 1
2 3 4 5
6 7 8
concn., mM
pa.
0.0095 0,0189 0.0378 0.0756 0.1133 0.1511 0.1889 1.889
0.200 0.400 0.810 1.61 2.41 3.21 3.92 48.2
of Current with Technetium Concentration a t pH 13" Wave I Wave II* Wave I11
i d
il
il
id/C 21.1 21.2 21.4 21.3 21.3 21.2 20.8 25.5
pa.
Id
8.04 8.09 8.19 8.14 8.13 8.12 7.93 9.7P
10.40
il/C
pa.
il/C
5.50
2.43 7.11 18.4 94.9 282. 570. 654. 877.
255.8 376.2 486.8 1256. 2488. 3773. 3462. 464.2
Mean 8.091 Std. dev. 10.085 Background electrolyte: 0.25M K2S0, KOH; temp.: 25' C. Data obtained with Fisher Elecdropode. b Because of its ill-defined shape, it was not possible t o evaluate wave I1 for Nos. 1 t o 7. c This value was omitted in calculation of mean and standard deviation.
+
0
1 = id/Cm2/3t1/6.
d
Table VI.
Concentration-Current Relations for Solutions Containing Both Technetium and Rhenium"
Re Tc concn., concn., Run mM mM 1 2 3 4 5 6
0.0095 0.0095 0.0095 0.1889 0.1889 0.1889
0.100 0.200 2.00 0.100 0.200 2.00
Wave I
Wave I1
pa. 0.288 0.282 0.286 5.45 5.35 5.35
Wave 111
i d
id id/C
30.3 29.7 30.1 28.9 28.3 28.3
IC
pa.
11.6 11.4 11.5 11.1 10.9 10.9
0.97 0.97 0.99 5.55 5.55
il
i d/ C 102 102 104 29.4 29.4
IC
pa.
40.1 40.1 40.9 11.3 11.3
0.36 1.09 2.09 2.22 4.44 94.
Mean 11.23 Std. dev. 3 ~ 0 . 3 0 7 Background electrolyte: pH 2, 0.5M KCI HCI. Temp.: 25" C. Data obtained with Fisher Elecdropode. NH4Tc04 and KReO4 were used.
+
0
Q
c
1 = id/Cm2/St1/6,
the Tc(VI1) is present in very low concentration-e.g,, 0.OlmM. Although wave I1 appears at a n El,?of -0.84 to -0.93 volt compared to -0.93 volt observed for pertechnetate alone, the variation in the magnitude of the limiting currents does not permit any reliable generalization to be made. The magnitude of the limiting current for this wave (runs 1 to 3, Table VI) is approximately 1.5 times that for technetium alone. In runs 4 and 5 , the value of the limiting current (I = 11.3) is approximately the same as for technetium alone ( 6 ) . With higher concentrations of rhenium (run 6), wave I1 merges with wave I. Ellz of wave I11 is near to that observed for the perrhenate wave alone. The current of the composite wave for a mixture does not provide any feasible analytical tool, since the wave height is apparently influenced by other factors, such as mutual oxidation and reduction of the two species, catalytic reduction etc. The polarographic determination of rhenium in the presence of technetium was investigated, based on the fact that an Re(VI1) solution, which is 2 M in 982
ANALYTICAL CHEMISTRY
HCl, gives a well defined wave with an of -0.42 volt, whereas Tc wave I appears at -0.005 volt. Polarography of a solution, which was 0.189mM in Tc(VI1) and 2.0mM in Re(VI1) and 2iM HCl as contained 0.5M KCl background electrolyte, gave only a wave at -0.426 volt with a diffusion current of 25.8 ga., which is much less than the 38.2 pa. observed for Re(VI1) alone. The disappearance of the Tc(VI1) wave I may be due to a chemical reduction by Hg in acidic solution of Tc(VI1) to lower oxidation levels ( 5 ) , but not to T c " in this case, which are partially reoxidized with consumption of some Re(VI1) until an equilibrium obtains with most of the Tc in the (111) state. Most of the redox couples involved are irreversible in character. This is supported by the fact that when solutions 0.189mM in Tc(VII), 2mM in Re(VII), and 21M in HCl are shaken with metallic mercury, Tc(VI1) is not partially reduced chemically to metal as it is when Re(VI1) is absent. On the other hand, two well defined waves are observed for solutions 0.189mM in T c and only O.lmM in Re (O.5M KCI 2M HCl as background E112
+
+
electrolyte). The first wave (Eli2 = -0.005 volt) is due to technetium; the magnitude of its diffusion current supports the 4e reduction (6) of Tc(VI1) to (111)and indicates that the pertechnetate is not chemically reduced by mercury and hydrochloric acid with this ratio of perrhenate present. The second wave (Elia = -0.425 volt) represents the reduction of Re(VI1) to (IV) (2). If the Re(VI1) concentration in the polarographic test solution is kept low enough, these waves could provide a method for the estimation of pertechnetate and perrhenate when present together. Evaluation of Analytical Applicability. T h e polarographic determination of technetium, based on t h e measurement of wave I a t p H 2, generally provides for optimum selectivity and sensitivity. If measurement a t pH 2 offers difficulty because of interfering polarographic waves or equally cogent reasons, technetium can be determined by measurement of wave I or I1 at p H 13; measurement of wave I11 at p H 13 affords maximum sensitivity for the estimation of technetium at very low concentration levels. The flexibility afforded by the opportunity of using different Tc(VI1j reduction waves a t two radically different p H values indicates the feasibility of determining technetium polarographically in complex mixtures, where the choice of the analytical wave would be dictated by the location of the polarographic waves given by the other components of the sample. Technetium can also be accurately determined by controlled-potential electrolysis and coulometric measurement ; use of an electronic current integrating device rather than a hydrogen-oxygen gas coulometer would yield results of high precision and sensitivity. ACKNOWLEDGMENT
One of the authors (G.B.S.S.) thanks the National Academy of Sciences for an appointment supported by the International Cooperation Administration under the Visiting Research Scientist Program. LITERATURE CITED
(1) colt,on, R., Dalziel, J., Griffith, W. P., Wilkinson, G., J. Chem. SOC.1960, 71. (2) Lingane, J. J., J . Am. Chem. SOC.64, 1001 (1942). (3) Magee, R. J., Scott, I. A. P., Wilson, C. L., Talanta 2, 376 (1959). (4) Miller, H. H., Kelley, M. T., Thomason, P. F., "Advances in Polarography," I. S.Longmuir, ed., Vol. 2, pp. 716-26, Pergamon Press, London, 1960. (5) Salaria, G. B. S., Rulfs, C. L., Elving, P. J., J . Chem. SOC.1963, 2479. (6) Terry, A. A., Zittel, IT. E., AKAL. CHEM.35,614 (1963).
RECEIVED for rwiew January 21, 1963. Accepted April 22, 1963.
