Table VI.
U235Determination," Comparative Results
Set I Sample Analyst A Analyst B Set IIb 0.959 0.0969 613 0 966 2.98 0.297 617 2 99 2.47 2 49 0 240 619 0.670 0,0663 622 0.673 All results are in pg./ml. (p.p.m.). * Sample solutions of Set I1 are 1:10 dilution of Set I. this method is useful for the relatively precise determination of submicro(see Table IV gram quantities of U235 and VI). The yield correction is independent of the quantity of uranium present. Thus, we are able to retain a precise yield measurement. Only the counting statistics, which, of course, are proportional to the quantity of uranium, will vary. Longer counting
times are necessary to attain maximum precision. An alternative choice is available in cases of extremely low levels of U235. The sensitivity of this method can be extended to subnanogram quantities by utilizing the isotope 1 1 3 4 ( t l i z = 53 minutes). The irradiation is carried out with a sample aliquot plus hydrazine (10%) and without iodide carrier. The gamma spectrum is shown in Figure 2. Preliminary work a t this level indicates that, with caution and recognition of background interference a t these levels, subnanogram quantities of L235 are readily determined. For a single sample, a double extraction of the iodide followed by precipitation as silver iodide and subsequent beta counting yielded a beta decay plot of the 53minute The gamma purity of this specimen was verified by gamma spectrometry.
ACKNOWLEDGMENT
The author expresses his appreciation to Jack Douglass for his assistance in performing the analyses reported here. LITERATURE CITED
(1) Audrieth, L. E., Ogg, B. A., "The Chemistry of Hydrazine," Chap. 6,
U'iley, New York, 1951.
( 2 ) Katcoff, S., Nucleonics 18, No. 11, 201
11960). (3) Ledicotte, G. W., Brooksbank, W. A,, USAEC Report TID-7531 (Pt. l ) , pp. 71-8, 1957. (4) Alaklman, H. A., Ledicotte, G. W., ANAL.CHEN.27,823 (1955). (5) Seyfang, A. P., Smales, A. A., Analyst 78.394 (1953). (6) -Stehn,' J.-F., Nucleonics 18, Xo. 11, 186 (1960). RECEIVEDfor review June 10, 1965. Accepted July 30, 1965. Research s u p ported in part by the U.S. Atomic Energy Commission under Contract AT(043)167, Project Agreement No. 17.
. .
Chronopotentiometric Study of Pheny mercuric Ion Adsorption on a Mercury Electrode R.
F. BROMANI
and ROYCE W. MURRAY
Department of Chemistry, University o f North Carolina, Chapel Hill, N. C . Both phenylmercuric ion and the produd of its one-electron reduction are strongly adsorbed on a mercury electrode surface. The adsorbed reactant gives rise to polarographic, potential-sweep chronoamperometric, and chronopotentiometric prewaves during which the adsorbed reactant is coulometrically electrolyzed before the diffusing solution reactant is reduced. Adsorption and desorption rates of both reactant and product are slow. A spike on the chronopotentiometric prewave has been interpreted as arising from a relatively slow reorientation process within the adsorbed layer leading to a transient reaction overpotential.
T
of adsorption in the electrochemical reduction of phenylmercuric ion was first studied by Benesch and Benesch (1, 2 ) and Vojir ( 2 4 ) . A prewave preceding the first diff usion-controlled, polarographic reduction wave was interpreted ( 2 ) as arising in the classical Brdicka manner (4) from adsorption of the reduction product (presumably the phenylmercury radical) on the D.M.E. surface. Because it has HE ROLE
1 Present address, Department of Chemistry, University of Nebraska, Lincoln, Neb. 68508
1408
ANALYTICAL CHEMISTRY
275J5
been noted (1, 2,9, 24) that both of the one-electron polarographic waves are irreversible and because the Brdicka (4) interpretation of polarographic prewaves and postwaves is strictly valid only for reversible electron transfers, it seemed appropriate to consider the electrochemical behavior of phenylmercuric ion more thoroughly. Adsorption of electroactive species a t electrode surfaces can be studied by chronopotentiometry. Various theoretical features of this technique with reference to adsorption have been described (8, 11, 15, 16, 21), and several applications have been reported (8, 10, 12-15, 17, 18, 25). Information on the order of reaction of the adsorbed and diffusing reactants, their kinetic interplay, and surface excess data can, in favorable cases, be derived from the morphology of the chronopotentiometric wave and the appropriate theoretical relationships (11, 15, 16, 21). The detection and measurement of product adsorption can be accomplished by using current reversal (8, 18). Application of the chronopotentiometric technique to the phenylmercuric system has produced evidence for the adsorption of both reactant and product and for the direct connection of the observed prewave to reactant adsorption rather than product adsorption. POtential-sweep chronoamperometry was
employed for the qualitative verification of several aspects of the chronopotentiometric results. EXPERIMENTAL
Reagents. Phenylmercuric hydroxide (Columbia Organic Chemicals Co., Inc.) was used either as purchased or after a single recrystallization from water. T h e two forms displayed identical electrochemical properties. A determination of total mercury was performed b y refluxing 500 mg. of the recrystallized substance in 8.11 nitric acid for 1 hour and then titrating the free mercuric ion potentiometrically with EDTA (20). The phenylmercuric hydroxide solutions were prepared by dissolving weighed nortions of the substance without further -standardization. ANALYSIS. Calculated for C6H5HgOH: C, 24.45%; H, 2.05%; Hg, 68.1%. Found. C, 24.92%; H, 1.87Q/,; Hg, 68.1%. The supporting electrolyte solution was 0.lM acetate buffer, p H 5.2, prepared from ordinary reagent grade chemicals and redistilled (from alkaline permanganate) water, Solutions were thoroughly deaerated with high purity nitrogen before use and a nitrogen atmosphere was maintained throughout each experimental series. Instrumentation. Experiments using t h e various techniques were all carried out with a modular instrument based on the DeFord design (6),utilizing
0.0
-0.5
- 1.0
- 1.5
E, v o l t s
Figure 2. Potential-sweep chronoamperogram in 0.446rnM phenylmercuric hydroxide Ei = $0.20 volt vs. S.C.E.; scan rate, -0.2 volt sec-1; A = 0.050 sq. cm. Curve A, 1-minute wait In unstirred solution before scan. Curve B , repeat of scan on same electrode drop Time
Figure 1 . Chronopotentiogram of 0.390mM phenylmercuric hydroxide in 0.1 M acetate buffer, p H 5.2 io = 1.99
X
amp. crn.-z;
1 -minute wait in unstirred solution before applying current
G. A. Philbrick Researches, Inc., K2-P,
-W', and -X operational amplifiers. Either a Sanborn Model 151 recorder with a 250-mv. d.c. coupling preamplifier (Model 150-13002) or a Tektronix Type 564 storage oscilloscope, with Type 2.163 differential amplifier and 21367 time base plug-in units and fitted with a Type C-1'2 Polaroid camera, was used to record experimental curves, depending on the duration of the experiment. Chronopotentiometric current-reversal experiments were carried out a t relatively low current densities using manual polarity switching. Cell and Electrodes. T h e cell was a 150-ml. capacity borosilicate glass beaker with integral water jacket, thermostated a t 25.0" =t 0.1" C. A machined Teflon cover, which had holes to accommodate the various electrode asemblies and a nitrogen disperser, was used. Stirring was provided magnetically. The counter electrode was a platinum wire sealed in glass and immersed directly in the cell solution. A 13eckman fiber t j pe calomel electrode (KO.39970), isolated in a separate compartment filled with supporting electrolyte and connected to the main cell through a fine po~ositysintered glass disk, served as the reference. The working electrode in all experiments (except as noted in the following paragraph) was :t hanging mercury drop electrode (HRIDE) prepared in the usual manner ( 7 ), with drops transferred from a conventional D J 1 . E . assembly with a Teflon scoop to an amalgamated plat,num wire tip electrode. The usual electrode area was about 0.05 sq. cm., the exact area being calculated from the D . N . E . characteristics. The D . N . E . drop time was monitored during leach experiment. X fresh hanging drop \\-as used for all experiments except those specifically noted. For experiments in which effects of the time of contact between the elec-
trode surface and the solution mere studied, the HNDE working electrode was a Kemula-type microburet with a micrometer head (Brinkmann Instruments, Yew York). This electrode was calibrated for drop area in terms of the micrometer divisions of the barrel. Reproducibility was within experimental error of transition time measurements as established by chronopotentiometry of Cd(I1) in 0 , l M KCl. Because the eauilibration time of the electrode surrace with the test solution is experimentally important, as will be shown below, this experimental parameter is listed with other data so that proper comparisons may be made among the various results. RESULTS A N D DISCUSSION
-1 polarogram of phenylmercuric hydroside in 0.1M acetate buffer, p H 5.2, exhibits two irreverqible one-electron reduction waves, with a prewave occurring before the first step a t solution concentrations greater than about 10-4.U (1, 2 , 2 4 ) . Current-time curves for individual drops in both the prewave region and in the early stages of the second diffusion wave (about -0.9 volt us. S.C.E.) are distorted (in a similar fashion). A complete chronopotentiogram in the same medium displays from four to f i e~breaks (Figure 1), depending on the concentration of the electroactive ion. The first two of these, identified as waves 1 and 2 , correspond in potential to the polarographic prewave and first diffusion wave. =i potential-sweep chronoamperogram (Figure 2, curve A ) also exhibits waves 1 and 2 . Our attention in this study has been focused on waves 1 and 2 . S o attempt will be made to interpret the complex
adsorption behavior of the system at. more negative potentials. Adsorption of Reactant. A t high recorder sensitivities, chronopotentiometric waves 1 and 2 are readily observed (Figure 3 ) . Esaminat'ion of transition times T~ and T~ as a function of current s h o w that the quantities ir1 and i ~ are ~ fairly ~ ' constant, ~ as shown in Table I. The trend in ir1 was not observed a t all concentrations and is probably caused by graphical uncertainty in the T~ measurement, accentuated by a change in the wave shape with current and concent,ration. The data of Table I indicate that chronopotentiomeiric waves 1 and 2 correspond to coulometric depletion of an adsorbed layer followed by diffusioncontrolled deplet,ion of t8he solutionphase reactant. This behavior implies that t,hese data should follow the equation representing the chronopotentiometric AR,SR adsorption model (10, 11, 16, 25)
where C i s the bulk concentration of electroactive species (moles em.?), r is
Table I.
Constancy of iorl and i O ~ L l ' a Values [+Hg+] = 1.07m.W; A = 0.050 sq. cm.; solution stirred for 1 minute, then allowed to stand unstirred for 1 minute io71 x 106, i0721'2 x 104 amp io X lo4, coulomb cm.? sec.1)2 cm.-Z amp. cm.-2 2.79 7.70 2.54 3.18 7.63 2.56 3,58 7.45 2.57 7.64 2.51 3.98 4.98 6.68 2.69 5.97 6.52 2.62 6.97 6.48 2.55 8.09 6.55 2.62 10.10 6.26 2.60
VOL. 37, NO. 11, OCTOBER 1 9 6 5
1409
-
from the i71 data of Table I and the intercept in Figure 4 produces values of 7.2 and 6.4 X 10+ moles cm.-2, respectively, indicating a substantial amount of adsorption. The numerical value of I' increases a t higher bulk concentrations. The quantitative significance of these numbers is dubious, however, as it will be shown below that the approach to adsorption equilibrium in this system is very slow. These data represent a nonequilibrium adsorption state which, by use of a controlled electrodesolution equilibration time, is reproducible. An experiment in which the H M D E was placed in a stirred solution of phenylmercuric ion and then transferred (after intermediate rinsing with supporting electrolyte) to a solution containing only the supporting electrolyte before application of the current is illustrated in Figure 6. The single transition time observed corresponds to the reduction only of adsorbed species (ifl is constant) and is similar in appearance to a curve recorded in a phenylmercuric hydroxide solution a t a much lower bulk concentration. For example, when the transfer w a made after 15 and 300 seconds of equilibration in a stirred solution, the transition times were 0.35 and 0.44 second, respectively, a t a current density of 1.0 x 10-4 amp. (Normal chronopotentiograms would yield comparable 71 values of 0.43 and 0.53 second, respectively.) When the electrode was allowed to stand with stirring in the supporting electrolyte solution for up to five minutes
-0.3
-0.2
-
-0.1
-
= In
>
J 0.0-
0.1
-
0.2
-
1
Time
Figure 3. Typical chronopotentiogram of phenylmercuric hydroxide [$"e+] = 1.07mM; i o = 3.97 X amp. 1 .minute wait in stirred roluHon, 1 minute in unstirred solution. TI taken as the rum of distances A6 f '/2(6C); 7 2 as the rum ' / * ( B C )
cm-2;
+ CD.
the surface excess of adsorbed reactant (moles cm.-2), and the other symbols have their usual significance. Figure 4 shows a plot of the data of Table I according t o Equation 1; an acceptable fit is evident. However, neither the experimental precision of transition times in this system nor the span of applied currents is sufficiently large to permit positive identification of the AR,SR model from the linearity of Figure 4 alone. The behavior of the individual values of irl and i7Z1lz plus the following stirring experiment constitute more convincing indication of AR,SR behavior. If the bulk solution is stirred during a chronopotentiogram, 71 is unaffected, whereas r2 is obliterated, as shown in Figure 5 , curve C. Application of the current for a sufficient length of time can produce a r2 through an apparent passivation of the electrode with formation of a visible film on the surface of the mercury drop. A similar experiment with potential-scan chronoamperometry shows the same effect in that the maximum current of wave 1is essentially unaltered while wave 2 is enormously increased by stirring. This confirms the nondiffusional character of wave 1. As expected for an AR,SR adsorption situation, the measured value of i ~ 2 * ' and the slope of a plot of Equation 1 are proportional to the bulk concentration of phenylmercuric ion. The average calculated diffusion coefficient was 0.78 (k0.05) X set.-' Another chronopotentiometric example of an AR,SR case has recently been reported (22). Calculation of the surface excess, r, 1410
ANALYTICAL CHEMISTRY
1.0
2.0
3.0
( I I I Jx i6: cm2 amp' Figure 4. AR,SR plot for phenylmercuric hydroxide
1.07mM
1 -minute wait in stirred solution, 1 minute in unstirred solution
after transfer, a decrease in the transition time of about 40% was observed, relative to application of the current immediately after the transfer. This behavior indicates that the desorption rate of the phenylmercuric species from the electrode is slow and not diffusioncontrolled (6). Adsorption of Product. The product obtained during both waves 1 and 2 is strongly retained a t the electrode surface. [Product as well as reactant adsorption is expected
-0.5
-1 ~
0.2
1-
OB see
Time
Figure 5.
Effekts of stirring and electrode history on chronopotentiograms
[$He+] = 0.752mM; io = 1.96 X lO-'amp. cm-2 Curve A, 1 -minute wait in stirred solution, 1 minute in unstirred solution. Curve 6, Current reapplied to same electrode drop after 1 -minute wait in unstirred solution. Curve C, Fresh electrode drop, 1 -minute wait in stirred solution, current applied with stirring
Table II. Current-Reversal Chronopotentiometric Behavior of 0.446mM Phenylmercuric Hydroxide 1 minute with stirring followed by 1
-0.3
minute without stirring before current is applied; io = 0.985 X lo-‘ amp. cm.-2; A = 0.0508 sq. cm. 71, sec. t, - T ~ sec. , T,, sec. 0.45 0.47 0.45 0.44
-0.2
1.60 0.91 0.71 0.22
1.59 0.86 0.72 0.26
-0.I
-B v)
c
li-
0.0
0.I
0.2 Time Figure 6. Chronopotentiogram on electrode drop hung in 0.488mM phenyl0.1 M acetate buffer, transferred immediately to 0.1 M mercuric hydroxide acetate buffer. Current applied immediately after transfer
+
io = 1.00
X 10-4
amp. crn.-*i
from t h e electrocapillary depression observed on both sides of t h e first polarographic wave, which had been reported earlier (2) and was confirmed in the present investigation.] A number of chronopotentiometric experiments were performed in mhich the current was interrupted prior to 71. Later reapplication of the same current, even after a short wait in an unstirred solution, produces only the unused balance of 71. If the first chronopotentiogram is carried beyond 71, subsequent experiments on the same electrode show no evidence of the first wave, even with 5 minutes of intermediate vigorous stirring, whereas 7 2 is unaffected, as shown in Figure 5, curve B. Thus, reduction of the adsorbed reactant pi*oduces a layer of strongly adsorbed (or insoluble) product Mhich blocks replenishment of the reactant adsorbed layer. A chronoamperogram taken on such a used electrode is shown in Figure 2, curve B. Here also, the adsorption prewave has disappeared. Current reversal experiments were used to further characterize the product adsorption (Figure 7). If the current , distinct reis reversed prior to T ~ no oxidation wave can be discerned, the potential falling to that of the anodic dissolution of mercury. However, re-
= 0.346 sec.
7
versal a t times after 71 u p to and including TZ reveals a reoxidation wave. Under conditions such that the quantity of product generated did not exceed several monolayers thickness, the value of the reverse transition time, ‘T?, was equal to t f - r1 (where t, is the total time of cathodic current application) (Table 11). Production of larger quan-
-
y)
-0.2
-
0.0
-
tities of product indicated, through a nonproportional increase of T ~ ,that there are limits to the total retention of electrolysis product. These results indicate that the product obtained during the first cathodic wave is in some manner different from that produced during the second. This difference might arise in three ways: (i) Both waves 1and 2 produce monomer (or dimer) phenylmercury radical-, and the overvoltage for the reoxidation of the primary (produced during T ~ )adsorbed product layer is greater than that for subsequent layers, falling a t or beyond the mercury dissolution potential. (ii) Wave 1 produces the monomer radical and wave 2 the dimer (or the reverse) , and the form initially produced has a larger anodic overvoltage. (iii) The phenylmercury radical produced during wave 1undergoes a rapid disproportionation to the nonelectroactive diphenylmercury species, whereas this does not occur a t so great a rate for the radicals produced during wave 2. Each of the above intermediates, the monomer and the dimer of the phenylmercury radical as well as the disproportionation product, diphenylmercury, has been proposed as a reaction product (2, 9, 24), but a clear subscription to any one of these possibilities does not seem warranted in the
0
-
w
0.2
0.4
I
I
Time
Figure 7. hydroxide
Current reversal chronopotentiogram in 0.892mM phenylmercuric io = 1.00
X
amp. cm.-a;
1-minute wait in unrtirred solution
VOL. 37, NO. 1 1 , OCTOBER 1965
141 1
2.0
1.6
1.2
0.8
0.8
(l/b)x16’,
I Time
Figure 8. solution
Effect of electrode equilibration time in stirred
Figure 9. hydroxide
AR,SR,
stirred solution (used in most of the above experiments), the value of r1 slowly increased by a small (but measurable and reproducible) amount (Figure 8). No significant concurrent increase in r2 could be detected. For example, the values of 0.52 and 0.58 second for r1 and 1.71 and 1.70 seconds for 7 2 were obtained for curves A and B , respectively, of Figure 8. A change in wave shape a t longer equilibration times prevented direct r1 and r2measurements, but the total transition time continued to increase. These results indicate that the adsorption rate of phenylmercuric ion is rather slow even at a fresh mercury surface. Because of this, it has not been possible to obtain accurate values of the saturated surface excess, r., of phenylmercuric ion. Some estimates have been obtained using a 15-minute stirred equilibration period. Data collected under this condition were analyzed using the AR,SR model and the SAR model [an approximate simultaneous (11 , 16) reduction of adsorbed and solution reactants]. The linear SAR equation is given by (11, 16, 23). nFA (7rD)1 4 7 ir = .+2 nFAr (2) 2
+
r is calculated from the i~ vs,
7112
intercept of an plot; D is calculated from the
Table 111. Surface Excess Results for Phenylmercuric Ion Adsorption Equilibration time of 15 minutes with stirring, followed by 1 minute without stirring
[+Hg+I x 103, M
AR, SR
D X lo6
rx
SAR 1010
D X lo6
1.20 (f.14) 1.90 (=!~.25)~ 0.421 0.96(&.12) 1.85 (f.25) 0.589 0.72 (f.09) 1.09(f.12) 0.842 a Figures in parentheses represent standard deviations of least-squares 7.62 (*.23) 7.84(*.16) 10.56 (f.90)
1412
ANALYTiCAL CHEMISTRY
rx
1010
6.30(*.27) 6.84(f.20) 8.64 (f.20)
plot.
0.2
z
and SAR plots for
0, 0.589; A, 0.842. 1 minute in unrtirred solution 0 , 0.421mM;
[+Hg+] = 0.537mM; io = 0.97 X lO-4amp.cm.-2 (Stirring followed by 1 minute in unstirred solution) Curve A, 30 sec.; Curve E, 1 min.; Curve C, 5 min.; Curve D, 30 min.
present case on the basis of the data obtained. T h a t the nonproportional increase in 7, noted when large amounts of product are generated occurs through diffusional mass transport away from the electrode rather than by deactivation through disproportionation is indicated by the fact that stirring the solution throughout the current reversal experiment shortens but does not eliminate T,. If a second current reversal is performed (Figure 7), only a small fraction of the original r1 reappears unless the second reversal is made a t or after rr, in which case a substantial portion of the original r1 can be observed. This cleansing of the mercury surface from the primary layer of adsorbed product (which was shown to be sufficient to block further reactant adsorption) could occur either through a faradaic reoxidation process (of adsorbed dimer or monomer radical) taking place just at the mercury dissolution potential, or through desorption of the primary layer, perhaps initiated by oxidation of the underlying layer of (electrode) mercury. Determination of Surface Excess of Adsorbed Reactant. As t h e equilibration time of t h e electrode with t h e solution was increased beyond t h e standard time of 1 minute in a
1.6 2.4 crn2 a m d l
0.4
,
0.6
tec1’2
phenylmercuric
15-minute wait in stirred solution
slope. Table I11 summarizes the leastsquares results of the linear plots of Equations 1 and 2 shown in Figure 9. The relative standard deviations are slightly smaller for the SAR data treatment, which would normally be expected from the appearance of the wave under these conditions (Figure 8, curve D ) . The experimental uncertainties in the measure of transition times are substantial, however, and a change from the previous AR,SR behavior to the SAR model cannot be positively identified. Indeed, the variation of the diffusion coefficients with concentration (Table 111) suggests imperfect adherence to either model. These data all indicate greater surface coverage than reported by Benesch and Benesch (2) a t similar bulk concentrations. Theoretical values for r, were estimated by considering the area of a phenylmercuric ion model for three possible orientations on the surface. Bond distances were taken from Pauling (fQ), and atomic radii were taken as equal to the van der Waals’ radii values of Bondi (3). For an orientation in which the plane of the phenyl ring is parallel with the mercury surface, a maximum monolayer surface coverage of 1.1 X 10-lO mole cm.+ is calculated. For the case in which the plane of the ring is perpendicular to the surface with the axis of symmetry of the ion parallel to the = 6.0 X mole surface, (rs)ea~o. cm.-2 When both the plane of the ring and the axis of symmetry are perpendicular to the surface (with the mercury atom either on the surface or solution side), (I’Jcalo. = 8.7 X mole cm.-2 The SAR data in Table I11 indicate that the last of these three orientations is probably most nearly correct while the AR,SR data indicate greater than monolayer coverage. A clear assignment, however, cannot be made. Orientation of Adsorbed Reactant. T h e time of electrode equilibration
Equlllbr.ation Time, aec 40 60 80
20
I
I
100
I
I
40
50
I
-0.3.
-0.2 '
-
u)
c
-0.1
'
0
> W"
0.0.
10
20
30 Current, yA
0.1 .
Figure 1 1. Chronopotentiometric spike height E,,, (vs. S.C.E.) as function of equilibration time and applied current
0.2.
Curve A, [$Hg+] = 1.1 lmM; io = 5.7 X amp. cm.-2; unstirred solutions Curve 8, 1-minute wait in stirred solution, 1 minute in unrtirred solution; 0 , 0.587mM $Hg+;
0,0.754mM; A, 1.07mM
Time Figure 10.
Effect of electrode equilibration time in unstirred solution
[$Hg+] = 1.1 1 mM; io = 5.7 X 1 0-4 amp. Curve A, 1 sec.; Curve 8, 1 2 0 sec.; Curve C, 10 min.; Curve D,
was noted in Figure 8 to exert a pronounced effect upon t h e shapes of waves 1 and 2. These shape changes are even more striking in unstirred equilibration experiments (Figure 10). [Some of the increase in r1 and all that in r2 shown in Figure 10 must be attributed to surface depletion of SR as AR is formed and consequent partial mass transport control of t'he amount of surface excess (@.] The applied current density and bulk solution concentration also affect the wave shape. At long transition times, low current densities, and low bulk concentrations, the two waves exhibit a normal irreversible appearance. As the equilibration time, current density, or bulk concentration is increased, a transient spike appears on t'he leading edge of the first wave. The potential minimum following the spike is about the same potent,ial level as a wave wit,hout a spike. The maximum (cathodic) potential of the spike increases as t'hese fact,ors are further increased, and eventually the minimum following the spike moves cathodically also. At sufficiently large equilibration times or currents, this ultimately produces a merging of iche two waves into a single smooth waye. The data analyzed in Table I11 had this appearance. This (merging) condition was approached but not achieved by the higher concentrations tested. Figure 11, curve
30 min.
A , shows the relationship between the spike potential and the equilibration time in a n unstirred solution. Stirring of the solution merely serves to effect merging a t shorter equilibration times (Figure 8). Figure 11, curve B , shows the relationship between the applied current and the spike potential. I n potential-scan chronoamperometric experiments, analogous behavior resulted in that an increase in equilibration time effected an eventual merging of the adsorption peak with that of the solution reactant (Figure 12). An increase in the rate of potential scan could effect a partial merging. These interesting alterations in wave shape can be explained in a rational (although admittedly speculative) manner on the basis of slow reorientation processes occurring within the adsorbed reactant layer. A basic presumption to be made is t h a t the reduction overpotential for the initially present orientation (adsorption-preferred) is greater than that for a different (reaction-preferred) state, and that the rate of conversion of the former to the latter can increase during the course of wave 1 . The anodic potential shift producing the spike of wave 1 then represents a shift from an initial predominance of reduction of the adsorption-preferred state to a predominance of reduction from the reaction-preferred state a t a point of
sufficiently rapid reorientation rate. Two mechanisms for the alterations in conversion rate can be proposed : the reaction-preferred state and its reduction product physically occupy less electrode surface area, or a specific chemical interaction of the adsorbed product occurs with the reaction-preferred state (and indeed may be responsible for the I
0.2
I
I
0.1
0.0
I
I
-0.1
1
-0.2
E, volts
Figure 12. Effect of equilibration time on potential-sweep chronoamperograms [$Hg+] = 0.892mM; Fi = +0.20 volt; Icon rote, -0.04 volt set.-'; A = 0.050 sq. cm. Curve A, 1-minute wait in unstirred solution; Curve 6, 1-minute wait in stirred solution, 1 minute in unstirred solution; Curve C, 5-minute wait in stirred solution, 1 minute in unstirred solution
VOL. 37, NO. 1 1 , OCTOBER 1965
1413
lower overvoltage of that state). I n each case, a lessening of surface crowding can increase the adsorption-preferred-. reaction-preferred conversion rate and, also, in the latter case the rate becomes accelerated as more product is generated. At short equilibration times, or low bulk concentrations, the smaller reactant surface excess provides less surface crowding, allowing a more rapid shift to a high conversion rate and producing a less prominent potential spike. Higher surface escewes, produced by increased equilibration times or bulk concentrations, prohibit attainment of a more rapid conversion rate and the spike is enhanced. Ultimately, in the smoothed wave, the reactant surface excess is completely consumed at the adsorptionpreferred potential. This physical picture, consistent with all the experimental results (including the current density effect), is supported by the fact that the adsorption-desorption kinetics in this system are apparently slow, making slow processes occurring within the adsorbed film more plausible. I n view of the above results, several cautions concerning polarographic adsorption waves seem warranted. Because the phenylmercuric ion prewave is shown to be governed by adsorption of the reactant rather than the product,
considerable care is indicated in interpretation of polarographic pre- and postwaves for irreversible systems. I n addition, while electrocapillary or polarographic prewave-height measurements might superficially appear to be well behaved and to give reliable r, data, i t is evident that in any adsorption system displaying the slow adsorption rates typical of phenylmercury ion, such approaches to surface excess data would be invalid. Last, the surface coveragedependent wave-shapes observed for phenylmercury ion indicate that the ultimate appearance of an adsorption wave in an irreversible polarographic (or chronopotentiometric) process can be largely a matter of adsorption or surface-orientation rate processes. LITERATURE CITED
(1) Benesch, R., Benesch, R. E., J . Am. Chem. Soc. 73, 3391 (1951). (2) Benesch, R. E., Benesch, R., J . Phys. Chem. 56, 648 (1952). (3) Bondi, A.,Zbid., 68, 441 (1964). (4) Brdicka, R., 2. Elektrochem. 48, 278 (1942). (5) DeFord, D. D., 133rd Meeting, ACS, San Francisco, Calif., April 1958. (6) Delahay, P., Trathtenberg, I., J . Am. Chem. SOC.79,2355 (1957). (7) DeMars, R. D., Shain, I., ANAL. CHEM.29, 1825 (1957). (8) Herman, H. B., Tatwawadi, S. V., Bard, A. J., Zbzd., 35, 2210 (1963).
(9) Hush, N. S., Oldham, K. B., J . Electroanal. Chem. 6, 34 (1963). (10) Laitinen, H. A., Chambers, L. M., ANAL.CHEW36, 5 (1964). (11) Lorenz, W.,2. Elektrochem. 59, 730 (1955). (12) Lorenz, W., RIuhlberg, H., Ibid., p. 736. (13) Lorenz, W., RIuhlberg, H., 2. Physik. Chem. (Frankfurt) 17, 129 (1958). (14) Rlunson, R. A., J . Electroanal. Chem. 5, 292 (1963). (15) RIunson, R. A,, J . Phys. Chem. 66, 727 i1962). (16) Rlurray, R. W., J . Electroanal. Chem. 7, 242 (1964). (17) RIurray, 1%.W., Gross, D. J., 149th Meeting, ACS, Detroit, Rlich., April 1965. (18) Osteryoung, R. A,, ANAL. CHEM. 35, 1100 (1963). (19) Pauling, L., "The Sature of the Chemical Bond," 3rd ed., Cornell Univ. Press, Ithaca, Y. Y., 1960. (20) Iteilley, C. &'., Schmid, R. W., Lamson, D. W., ANAL. CHEM. 30, 953 (1958). (21) Iieinmuth, W. H., Zbid., 33, 322 (1961). (22) Takemori, Y., Rev. Polarog. (Kyoto) 12, 63 (1964). (23) Tatwawadi, S. V., Bard, A. J., A N IL. CHEY.36, 2 (1964). (24) Vojir, V., Collection Czech. Chem. Commun. 16, 488 (1951). RECEIVEDfor review May 11, 1965. Accepted August 2, 1965. Work supported by Directorate of Chemical Sciences, Air Force Office of Scientific Research, Grant T o . AF-.4FOSR-584-64.
Use of Differential Scanning Calorimetry for the Analysis of Chloride-Bromide Mixtures SIR: The heat of fusion of an ideal solid solution of the type A,X,-B,X, or A,X,--Amy, is directly proportional to the concentration of solute ion. This property can be utilized for the determination of chloride-bromide mixtures in the complete concentration range of O-lOO%. I n the procedure described below, solutions containing both chloride and bromide are precipitated with silver nitrate, forming solid solutions of silver chloride-bromide. The heat of fusion of the mixed crystal is then determined, and the per cent chloride or bromide is obtained from a previously prepared standard curve.
tassium bromide. Prepare 0.1M solutions of each. Rocedure. Prepare standard binary solutions of KC1 and K13r in t h e range O-lOO%. Precipitate t h e halides from a slightly acidic medium with silver nitrate solution (10% excess), heat until t h e precipitate settles, filter and wash with very dilute nitric acid. D r y the solid a t 110" C. for several hours. (The precipitate
should be shielded from sunlight during the procedure.) Punch out disks from a thin sheet of mica with an appropriate size cork borer and use these disks in place of the aluminum sample pans supplied with the instrument. (This will prevent the reaction of molten silver halide with either the aluminum pan or the metal sample holder.) Place a mica disk in the sample and in the reference holder,
EXPERIMENTAL
Apparatus. A Perkin-Elmer DSC-1 differential scanning calorimeter, Perkin-Elmer Corp., Norwalk, Conn. Reagents. Reagent grade silver nitrate, potassium chloride, and po-
1414
ANALYTICAL CHEMISTRY
Figure 1. Differential scanning calorimeter traces fob A. AgBr, B. 24.6% AgCl in AgBr, C. 64.0% AgCl in AgBr, D. ASCI. Sample weights: A. 3.17 mg., B. 3.61 mg., C. 3.99 mg.,D. 3.23 mg.; heating rate 20" per minute
