All qualitative features of the predicted effects of spherical diffusion Beem to be substantiated by experimental data on appropriate (diffusion-controlled) model systems. However, preliminary quantitative comparisons of theory and experiment show noticeable disparities. For example, the observed relative heights of the two second harmonic peaks do not agree satisfactorily with theory. This may be seen by comparing Figures 3A and 3B. The polarogram for t = 5 seconds in Figure 3A was calculated on the basis of paramDE,ro, etc.) values which are eter (Do, appropriate for the system employed in obtaining Figure 3B. Theory predicts a ratio of second harmonic peak heights (ratio of cathodic peak magnitude to anodic peak magnitude) of 1.28 while experiment yields a value of 1.15. Such quantitative disagreement between theory based on the stationary sphere electrode model and experiment is not surprising. It probably illustrates that this level of theory is of sufficient rigor to demonstrate the existence of a spherical diffusion contribution, but it fails to provide a precise quantitative model of the dropping mercury electrode. Effects neglected in the theoretical calculation such as drop growth, shielding [which is known to influence the spherical correction ( I 6)], depletion ( I S , 1 4 , and streaming ( 1 , 2 4 ) may play a role in determining the
nature of the experimental observations.
CHEM. 38, 1 (12) Imai, H Chem. 66, liua (IYW). (13) Kuta, J., Smoler, I., in “Advances in Polarography,” I. S. Longmuir, ed., Vol. 1. nD. 350-8, Pergamon, New York,
Detailed comparisons of theory and experiment directed toward gaining additional insight into the perturbations on the mass transfer process which influence a.c. polarographic data of amalgam systems are in progress. Likewise, efforts are being made to improve the rigor of the theory for such systems and to extend the theory to more complex mechanisms.
(147 Kuta, J., Smoler, I., in “Progress in
Polarography,” P. Zuman, ed., with the collaboration of I. M. Kolthoff, Vol. 1, Cha . 3, Interscience, New York, 1962. (15) atsuda, H.,Bull. Chem. SOC. Japan 26, 342 (1953). (16) Matsuda, H., 2.Elektrochem. 61, 489
3
(19.57). ,_
LITERATURE CITED
\ _ _ _ _
(17) Matsuda, H.,Oka, S., Delahay, P., J . Am. Chem. SOC. 81, 5077 (1959). (18) Paynter, J., Doctoral Thesis, Columbia University, New York, 1964. (19) Paynter, J., Reinmuth, W. H., ANAL. CHEM.34, 1335 (1962). (20) Reinmuth, W. H.,Zbid., 36, 200R
(1) Aylward, G.H., Hayes, J. W., J . Electroanal. Chem. 8 , 442 (1964)) (2) Barker, G. C., Anal. Chzm. Acta 18, 118 (1958). (3) Barker, G.C., in “Transactions of the S posium on Electrode Processes, PI?iladelphia, May, 1959,” E. Yeager, ed., Wiley, New York, 1961. (4) Barker, G.C.,Faircloth, R. L., GardNature 181, 247 (1958). ner, A. w., (5) Bauer, H. H., Elving, P. J., ANAL. CHEM.30, 341 (1958). (6) Biegler, T., Laitinen, H. A., Zbid., 37, 572 (1965). (7) Breyer, B.,,Bauer, H.H., in “Chemical Analysis P. J. Elving and I. M. Kolthoff. eds.. Vol. 13. Interscience, New York, 1963. (8) Brown, E.R., McCord, T. G., Smith, D. E.. DeFord. D. D., ANAL. CHEM. 38, 1119 (1966):’ (9) Delahay, P., in “Advances in Electrochemistry and Electrochemical Engineering,” P. Delahay, and C. W. Tobias, eds.. Vol. 1. ChaD. - 5,. Interscience, New York, 1961.’ (10) Delmastro, J. R., Smith, D. E., J . E l e c t ~ o a ~Chem. ~ l . 9, 192 (1965).
(1964). \ - - - - I .
(21) Senda, M.,Tachi, I., Bull. Chem. SOC. Japan 28, 632 (1955).
(22) Smith, D. E., in “Electroanalytical Chemistry,” A. J. Bard, ed., Vol. 1, Chap. 1, M. Dekker, Inc., New York, in press. (23) Smith, D. E., Reinmuth, W. H., ANAL.CKEM.33, 482 (1961). (24) Strehlow, H., Stackelberg, M. von, Z . Elektrochem. 54, 51 (1950). THOMAS G. MCCORD ERICR. BROWN DONALD E. SMITH
Department of Chemistry Northwestern University Evanston, Ill. RESEARCH supported by the National Science Foundation.
Effect of Desorption on Chronopotentiometric Adsorption Studies SIR: I n this communication we demonstrate the desorption of adsorbed reactant in the “double-layer-charging” region of a chronopotentiogram. The reduction of Zn(I1) in thiocyanate solution is ideal for illustrating this “desorption effect” because of the pronounced potential dependence of the adsorption of Zn(I1). Figure 1 shows the variation of 67 with the initial bias potential in a set of chronopotentiometric experiments for 1.0 m F Zn(I1) in 0.5F NaSCN - 0.5F NaNOs. The line indicated by “diffusion” corresponds to the value of calculated from the Sand equation (using D = 7.5 x 10-8 cm.Z/sec.). These results indicate that there is little adsorption a t -900 mv. us. SCE and that the amount of adsorption increases m the bias potential is made more anodic. [The Zn(I1) reduction wave is at about -1.0 volt us. SCE.] The potential dependence of the adsorption has been investigated quantitatively (8) using double potential step chronocoulometry ( 2 ) ; the results are in accord with Figure 1. To demonstrate the existence of the “desorption effect” we have used a 1620
ANALYTICAL CHEMISTRY
the steeply-rising double-layer-charging region of a chronopotentiogram. A charge-time response to the potential sweeppotential step function is shown in Figure 2. If there is no desorption of material
technique which combines linear potential sweep and potential step chronocoulometry ( 1 , S ) . The potential s w e e p potential step program is shown in Figure 2. The potential sweep, a t rates of 10 to 100 volts/second, approximates
75
-
N
d 3a 60.e c’
45
I
-300
I
I
-500
&, mv.
VI.
-700 SCE
I
-900
I
Figure 1. Chronopotentiometric io7 vs. bias potential for 1.0 mF Zn(l1) in 0.5F NaSCN 0.5F NaNOa at a hanging mercury drop electrode
+
io = 1 5 0 pa./0.032 cm.‘
the initial and the final potentials and nFr is the charge consumed by the reaction of material adsorbed a t the initial potential. Figure 3 shows Q tl/* plots for potential steps from -300 to -1400 and from -900 to -1400 mv. The intermediate curves shown in Figure 3 are Q - t 1 / * behavior for sweepstep combinations. The double-layer charging contribution from -300 to -900 mv. has been r e moved by taking as the origin the point of step application, and the double-layer charging contribution from -900 to -1400 mv. by shifting the &-axis so that the Q - t l / l plot for the -900 to -1400 mv. step passes through the origin. This is justified by the absence of adsorption a t -900 mv. The difference between the &-axis intercepts of the upper and lower lines is therefore nFr, the amount of Zn(I1) adsorbed at -300 mv. It is apparent that the slower the sweep, the greater is the loss due to desorption during the sweep interval. It is clear that desorption must occur during the double-layer-charging region of a chronopotentiogram for the reduction of Zn(I1) in thiocyanate. If material which was initially adsorbed at the electrode rest potential or bias potential desorbs in the doublelayer-charging region before the chronopotentiometric wave proper, an in-
1500potential
1300
-
-
1100-
uii
TIME
Figure 2.
Potential-time function and charge-time response
I20 during the potential sweep, the chargetime behavior after the potential step is applied should be the same as that observed for a direct potential step from -300 to -1400 mv. On the other hand, if the material initially adsorbed a t -300 mv. desorbs as the potential is swept cathodic and begins to diffuse away from the electrode surface, some of the desorbed material will be iilost,”-i.e., it will not diffuse back to the electrode under the influence of the concentration gradient resulting from application of the potential step from -900 mv. The observed values of the charge will a t all times be less than if there were no desorption. According to the theory of potential step chronocoulometry (a), a plot of charge (Q) us. t1/* is linear and has an intercept on the Q axis equal to Qdl nFr, where Q d l is the difference in electronic charge on the electrode a t
+
-300-
-1400m
IO0
00 . 1
d 4
\
60
d
40
b
-
Figure 3. Q in 0.5F NaSCN
4-
plots. 0.5 rnFZn(l1) 0.5F NaNOa
Upper and lower lines are for direct potential steps between the potentials shown. Intermediate curves are for potential sweeppotentlal step combinations. Sweep time from -300 to -900 rnv. is shown on each curve. Zero tlme is taken as the instont of application of the step. All points are corrected for double Ioyer charglng
20
0 /
f 0
/
I
I
I
0.05
0.1 0
0.1 5
I 0.20
1 0.25
tva, (sei#/:
VOL 38, NO. 11, OCTOBER 1966
1621
crease in the &r1l2product with increasing current density will still be obtained; but attempts to correlate the data with various models (4, 6, 7, 9) will lead to erroneous conclusions. The existence of this effect is further evidence of the unreliability of chronopotentiometry for the study of adsorp tion (6). It should also be recognized that adsorption can occur during the double-layer-charging region and that this would also invalidate the application of the usual models. A detailed treatment of the desorp-
tion effect for chronopotentiometric and chronocoulometric conditions is in progress and will be published later, LITERATURE CITED
(1) Anson, F. C., ANAL. CHEM.38, 54
( 1966). (2) Christie, J. H., Anson, F. C., Osteryoung, R. A., J . Electroanal. Chem., in press (1966). (3) Christie, J. H., Lauer, G., Osteryoung, R. A., Ibid., 7, 60 (1964). (4) Laitinen, H. A., Chambers, L. M., ANAL.CHEM.36, 5 (1964). (5) Lingane, P. J., unpublished data, 1966.
(6) Lorena, W., Z. Elektrochem. 59, 730 (19.55). - ~, (7) Murray, R. W., Gross, D. J., ANAL. CHEM.38, 392 (1066). (8) Osteryoung, It., unpublished data, 1966. (9) Tatwawadi, S. V., Bard, A. J., ANAL. CHEM.36, 2 (1964). JOSEPH H. CHRISTIE ROBERTA. OSTERYOCNG North American Aviation Science Center Thousand Oaks, Calif. 91360 and Gates and Crellin Laboratories of Chemistry California Institute of Technology Pasadena, Calif. 91109 \ - -
Application of Photoactivation to the Determination of Germanium in Titanium SIR: Techniques with photonuclear reactions are used for the determination of several elements (2-4). Lukens, Otvos, and Wagner ( 5 ) , in commenting on photoactivation with the 3-m.e.v. van de Graaff accelerator, suggest that photoactivation appears to be a useful supplement to neutron activation analysis. I n this case the (y,y') reaction is of predominant. Photoproduction neutron-deficient isotopes by the (y ,n) reaction is considered to be more effective with the use of a high-power linear accelerator. I n this paper, photoactivation of germanium and titanium is investigated with a 20-m.e.v. linear electron accelerator, Its application to the nondestructive determination of germanium in titanium is presented. Radioactivation analysis of germanium is generally based on the formation of G d 5 or Gen by irradiating samples with reactor neutrons. The ( y , n ) reaction of germanium yields Gees, which is a neutrondeficient isotope and has a half life of 40.4 hours. The production of Gees is of particular interest, because GeB9 has favorable nuclear properties as a tracer of germanium. The present work is based on counting the Gee9 photopeak activity. The available photon flux intensity is monitored by measuring the induced activity of Sc47,
Table 1. WGe/WTi
(=
1.992 x 9.851 X 4.982 X 2.456 x 1.284 X 0.648 X
1622
Rw)
10-1 lo-'
lo-'
IO-' lo-*
EXPERIMENTAL
Germanium and titanium oxides of high punty were used. Synthetic samples were prepared by mixing known weights of germanium oxides and titanium oxides. Samples of 50 mg. each were weighed and wrapped doubly in thin aluminum foil of approximately 5 mg./cm.2 These samples were irradia.ted for 1 hour with a 20-m.e.v bremsstrahlung generated from the electron linear accelerator at the Japan Atomic Energy Research Institute. The high energy electron beam from the accelerator was converted to bremsstrahlung by a platinum converter. In the forward direction, the photon flux intensity was estimated to be about 5 X 106 roentgens per minute with an average beam current of 40 pa. During irradiat,ion, the target assembly containing the samples was cooled by circulating water. After irradiation, the samples were transferred into polyethylene capsules and activity measurements were made using a Technical Measurements Corp. 256-channel pulse height analyzer with a 3-@ X 3-inch NaI-TI activated crystal.
RESULTS AND DISCUSSION
Gamma-Ray Spectra of Germanium and Titanium. Gamma-ray spectra of irradiated germanium and titanium are shown in Figures l and 2, respectively. The spectrum of germanium, taken 96 hours after irradiation, exhibits prominent photopeaks a t 0.51, 0.88, 1.12, and 1.34 m.e.v. These photopeaks decay with 40.4 hours half life, indicating the presence of Ge69. The spectrum (Figure 1) has a shoulder on the high side of the 0.51m.e.v. photopeak and the activity has the same half life. As a result, of the inability of the spectrometer to resolve the two peaks, the combined area is used for the activity measurements. I n the photoactivated germanium samples, large amounts of the 82-minute G d 5 were also produced by the (7,n) reaction. It was identified by the short-lived contribution to the peak a t 0.26 m.e.v. and disappeared after about 10 hours. No other photopeaks were observed in the spectrum of irradiated germanium. The y-ray spectrum of photoactivated titanium shows intense photopeaks at 0.16 m.e.v. and 0.51 m.e.v. nnd weak ones at 1.02 m.e.v. and 1.32 m.e.v. The peaks at 0.51 m.e.v. and 0.16 m.e.v. decayed with a half life of 3.07
Relationship between WG,/WT~and AG,/AB, for GeOrTiO:, Mixtures
AG., c.p.m. (8.51, f 0.05,) x (4.698 f 0.08,) X (2.04, f 0 . 0 ~ x) (1.251 f 0.038) X (3.930 i 0.186) X (3.641 f 0.010) X
ANALYTICAL CHEMISTRY
which is produced by the ( y , p ) reaction of titanium. This internal standard method has been applied to the analysis of zirconium in hafnium (6) and tantalum in niobium ( I ) .
104
lo4 104 lo4 lo3 lo3
Am, c.p.m. (1.203 f 0.020) X (1.362 f 0.006) X (1.18, i 0.01s) X (1.558 i 0.00s) X (8.578 f 0.16,) x (1.608 f 0.011) X
106 lo6
106
106
10'
106
Ao./Am ( = R A ) 7.07 X 10-1
3.44 x 10-1 1.72 x 10-1 8.05 x 10-2 4.58 X 10-2 2.27 x 10-2
RA/Rw 3.55 3.50 3.47 3.28 3.56 3.53 3.49 f 0.09,
