number of enzymes and the metals reported as activators for each have been compiled (14). Many of these systems involve enzymes which are now commercially available, and many more are likely to become available in the future.
(c, Webb, ~ Academic Press, New York, 1964.
(14) M. ~i~~~ and E.
~
E 2nd~ed., ~p. 422, ~
RECEIVED for review August 22, 1966. Accepted October 10, 1966. First Great Lakes Regional Meeting, American Chemical Society, Chicago, Ill., June 1966. Work supported in part by the Research Committee of the Graduate School from funds supplied by the Wisconsin Alumni Research ~ Foundation, ~ ~ , and" in part by Public Health Service Research Grant G M 10978 from the National Institutes of Health.
Instrument for Fulnctional Readout of Chronocoulometric Data Application to Cadmium Adsorption from Iodide Medium George W. O'Dom and Royce W. Murray Department of Chemistry, University of North Carolina, Chapel Hill, N . C .
A functional readouit approach to automated data acquisition is descrlibed. In chronocoulometric adsorption studies, thiis approach involves an instrumental readout of charge on a square root of time axis, allowing immediate examination of the Q-t'/2 plot as a linear oscilloscope trace. Instrumentation for functional readout of Q-t'/z plots is described and evaluated by investigation of cadmium adsorption on mercury electrodes from iodide medium. The cadmium adsorption is measured as a function of cadmium and iodide concentrations and equilibration potential.
THE APPLICATION OF CHRONOCOULOMETRY (1) to adsorption measurements at electrode-solution interfaces has recently been established (1-5). This technique involves determination of the transient coulomb-time characteristic following application of a potential step to the working electrode to initiate the desired faradaic reaction. The relation appropriate for a single potential step (3) is
where Qads = nFr coulombs per sq cm, r is the surface excess of adsorbed reactant in moles per sq cm, Qdl is the current integral required to charge the double layer from Elnit to Erinnlof the potential step, and the other symbols have their usual electrochemical connotation. The chronocoulometric technique can provide adsorption data in copious quantil.ies, inasmuch as a single potential step experiment, after evaluation of Qdl, yields sufficient data for detection and measurement of the surface excess. The oscilloscopically rccorded Q-t data are subjected to analysis for surface excess by Equation 1 through preparation of a Q-t"' plot. The intercept of this plot with the correction for QdLprovides Qada. Experience with this tech(1) F. c. Anson, ANAL.CIIEM., 38, 54 (1966). (2) F. c. Anson, ANAL.CHEM., 36, 932 (1964). (3) J. H. Christie, G. Lauer, and R. A. Osteryoung, J. Electroanal. Chem.,7,60 (1964). (4) J. H. Christie, G. Lauer, R. A. Osteryoung, and F. C. Anson, ANAL.CHEM., 35, 1979 (L963). (5) R. W. Murray and D. J . Gross, ANAL.CHEM., 38, 392 (1966).
nique (5) has shown that manual preparation of the Q-t"' data analysis plot consumes a substantial portion of the total time required for a surface excess measurement. Several recent reports (6, 7, 8) have advocated the use of digital data collection and readout systems in electrochemical experiments. The digital approach has numerous virtues, notably in its flexibility of application to a variety of techniques, potentially greater precision than photographic readings from conventional oscilloscope traces allow, and possibilities for fruitful linkages to computation devices. The digital data collection system described by Lauer and Osteryoung (8) is directed principally toward an enlarged efficiency in chronocoulometric surface excess measurements. The transient coulomb-time signal from a chronocoulometric experiment is digitally stored in a multichannel analyzer for subsequent analysis by readout to suitable devices. The desirable features of digital readout of electrochemical data are purchased, in general, only at a cost of greater instrument complexity and expense. A simpler, nondigital approach toward more efficient data acquisition may be taken for transient techniques in which extraction of the desired parameter(s) from the experimental data involves plotting of the measured signal against some function of time. This approach (9) utilizes a direct recording of the signal during the experiment against an electronically generated time-base function identical to that required in the data analysis plot. It offers the convenience of direct reading of the desired parameter (slope, intercept, etc.) of the recorded curve without necessity for further data manipulation. In experiments at oscilloscopic times, preparation of permanent photographic records, if not desired, can be avoided by use of trace storage oscilloscopes. A further desirable feature is the relatively low cost for the generation of many types of time-base (or other) functions. The recording of data in the above functional readout fashion appears to be particularly attractive in the case of the (6) G. L. Booman, ANAL.CHEM., 38, 1141 (1966). (7) E. R. Brown, D. E. Smith, and D:D. DeFord, ANAL.CHEM., 38, 1130 (1966). (8) G. Lauer~andR. A. Osteryoung, ANAL.CHEM., 38, 1137 (1966). (9) C. H. Presby, Jr., Naval Research Lab., NRL Rept. 6355 (Jan. 5, 1966). VOL. 3 9 , NO. 1 , JANUARY 1967
51
chronocoulometric techniques. In adsorption measurements, the charge of Equation 1 would simply be recorded on a square root of time base and Qads Q ~read L immediately from the intercept of the recorded straight-line plot. The square root time function can be generated in a variety of ways; a simple electronic route lies in the use of the Type P, Model E, Quadratron (Douglas Aircraft Co.). A previous electrochemical study has successfully used this device for generation of a square-root function (10) at time scales approaching the requisite millisecond level. This report describes an instrument designed for direct recording of Q-t”’ curves in chronocoulometric experiments and compares adsorption data thus obtained with surface excess numbers from the conventional Q-r approach. The adsorption of cadmium on mercury electrodes from iodide medium is examined to illustrate the operation of the instrument, Evidence for adsorption of cadmium from iodide medium has been given by Barker (11) and more recently by Hamelin (12), but the surface excess values and their dependence on iodide concentration and the initial equilibration potential have not been assessed.
n
+
EXPERIMENTAL
Chemicals. Reagent-grade chemicals were used throughout with no further purification. Water was doubly distilled, the second distillation being from alkaline permanganate. KI solutions were prepared with oxygen-free water to minimize air oxidation of iodide with ensuing high residual currents. Cell. The jacketed cell, Pt auxiliary, SCE reference, and hanging mercury drop electrodes were of design essentially identical to that described in a previous report ( 5 ) . Solutions were deaerated with nitrogen which had been passed through a vanadous chloride tower and hot copper turnings. All experiments were conducted at 25” C. The cadmium iodide adsorption equilibration was rapid, and no variations in surface excess were observed with changes in stirring times. The calibrated area of the hanging mercury drop electrodes employed ranged from 0.05 to 0.06 sq cm. INSTRUMENT FOR FUNCTIONAL READOUT OF CHRONOCOULOMETRIC DATA
A diagram of the instrumental arrangement for direct readout of chronocoulometric Q-t’” curves is shown in Figure 1. Amplifier 1 is a Wenking fast-rise potentiostat (Model 61R, Brinkmann Instruments), The current-measuring amplifier, 2, and integrating amplifier, 3, are Philbrick Researches solid-state operational amplifiers Type P45AU and P65AU, respectively. All other operational amplifiers are provided by a Philbrick Model K7-Al0 manifold. The step from a potential producing no cadmium reduction (Einit) to that producing zero cadmium surface concentration is initiated with the opening of switch SI.The potential step at the output of the adding amplifier, 4, is applied to the working electrode through amplifier 1 and simultaneously fires the switching amplifier, 5, to initiate the square root time-base sweep. With Einitapplied to amplifier 5 the voltage, E,, is adjusted so that diode Dsjust conducts and the output of amplifier 5 is very small (and is ultimately
(10) R. W. Murray, ANAL.CHEM., 35, 1784 (1963). (11) G. C . Barker, “Transactions of the Symposium on Electrode Processes,” E. Yeager, ed., Philadelphia, May 1959, Chap. 18, Wiley, New York, 1961. (12) A. Hamelin, Compr. Rend., 262, 520 (1966).
52
ANALYTICAL CHEMISTRY
Y
I.
OX18
X o$r
0 IUF
Figure 1. Instrumental setup for functional readout of chronocoulometric Q-t’l’ plots D I= DilN485A P. Douglas Type P, Model E, Quadratron
cancelled by Ebalin amplifier 6). When Efinalis applied, the net input polarity to amplifier 5 is reversed, Dzopens, D1 conducts, and the switch output, E,, initiates a ramp voltage sweep output by amplifier 6. The square root of this ramp voltage is taken by amplifier 7 according to the relation E d - = - l o a , and E d - drives the X-axis of a Tektronix Model 564 storage oscilloscope equipped with a pair of Type 2A63 amplifiers. (Q-t and Q-t’/2 curves can be simultaneously displayed if desired by use of a dual trace amplifier Type 3A72 on the X-axis.) The response characteristics of the diode switch arrangement were adequate, 0.1 msec for E, outputs of 16.0 and 36.0 volts. Double-pole mercury-wetted relays proved unsatisfactory for the simultaneous application of the potential step and starting of the time base. The balance voltage added to amplifier 3 serves to compensate for trace residual currents at Einit. The capacitors of amplifiers 3 and 6 are shorted momentarily just prior to initiation of the experiment. The over-all instrument response time is about 0.2 msec for 1M KI solutions, satisfactory for measurements on appreciably absorbing systems. Use of current-measuring resistors (Rm) greater than the 1 KQ employed here causes some loss in response characteristics but no change, other than a lowered precision, in the intercepts of the Q-t‘l’ displays. Smaller current-measuring resistances, which can become necessary at high sample concentrations, were avoided at these low sample concentrations in order to circumvent the concurrent attenuation of output signal level. The dynamic accuracy of the square root time base was checked by observation o f t us. t 1 / 2traces, and a maximum relative error from a perfect square root of less than 3 z was observed at times comparable to the instrument response time. The excellent linearity of the Q-t’l’ displays obtained in measurements on the cadium iodide system is illustrated by the experimental curves shown in Figure 2 for several time scales. The coulombic intercepts and diffusion coefficients taken from the intercepts and slopes of these curves are independent of the time scale employed, No distortions in the recorded curves attributable to significant uncompensated resistance effects (13) were detected at these low cadmiun ion concentrations. The instrument was further evaluated by comparison of charge intercept and Q-t”’ slope data taken from Q-t and
(13) G. Lauer and R.A. Osteryoung,ANAL.CHEM., 38, 1106(1966).
Q-1‘” displays, as shown in Table I. The comparison shows a satisfactory agreement between the two modes of operation. The reproducibility of the intercept data is about 3 pcoul per sq cm for values taken over a substantial period of investigation. It is apparent from the above results that the functional approach to automated data acquisition in chronocoulometric adsorptior. ztudies can obviate much tedious data analysis involved in the conventional Q-t readout mode without sacrifice in precisim of the measured parameters. The additional equipment cost is nominal, involving only the Quadratron device and a few operational amplifiers in addition to the ones normally used in the chronocoulometric experiment. The cost in instrument-operating complexity is likewise nominal in comparison to the enhanced data acquisition efficiency. In addition, the square root of time-base instrument described has some versatility, in that it can be employed for chronocciulometric electron transfer rate studies (3) and also, with some switching modifications, for double potential step chronocoulometric adsorption measurements ( I ) . Thus, while the functional approach does not provide the enhancement of measurement accuracy available from digital systems, substantial gains in data acquisition efficiency can be acquired with relatively simple instrumentation. ADSORPTION OF CADMIUM AT MERCURY ELECTR0DE:S FROM IODIDE MEDIUM
The adsorption of cadmium ion at the hanging mercury drop electrode from K[ solutions was determined as a function of cadmium concentration, KI concentration, and the potential at which adsorption equilibrium was achieved, Einit. Equation 1 intercept values, Q a d s Qdl, were read directly from Q-tl” oscilloscope traces or photographs thereof; the intercept data, Qinter,obtained are given in Table 11. The boundaries of accessible E n i t values are determined by the faradaic mercury dissolution and cadmium waves. Evaluation of the double-layer charging correction is necessary for extraction of surface excess values from the data of Table 11. Entries at 0 m M cadmium in Table I1 give blank double-layer charging values, Q d l ( b l a n k ) , for a potential step from E,,,, to Efinalin the absence of added cadmium ion. In the presence of cadmiurn ion, the double-layer capacity is reasonably expected to be unchanged on the cadmium amalgam formed at Erin%\;however, alterations in the doublemay result from the presence of adsorbed layer charge at Einir
+
Table I.
t”2 Figure 2. Chronocoulometric Q-t‘bdisplays for 0.4 mM cadmium ion in 0.5M KI A . 0.017 sec1/2/scaledivision; E, = 36.0 volts B. 0.033 sec1/2/scaledivision; E5 = 36.0 volts C. 0.050 sec1/2/sca1edivision ; Es = 16.0 volts
ea,,.
cadmium, Correction of the blank double-layer charge values for this effect is necessary. Two methods for evaluation of Q d l in the presence of adsorption have appeared in the recent literature: the dropextrusion experiment ( I ) and the integrated DME experiment (5). In both methods, a measurement is made of the charge necessary to change, the electrode area at applied Einitin the presence of adsorbed reactant and at applied in the absence of reactant; Qdl results from the difference between these quantities. The area change in the former method is obtained by extrusion of a mercury drop from a micrometer buret into the solution; the latter method employs the growing drop of a DME. The drop-extrusion experiment proved unsatisfactory in the present study because of the presence of finite residual currents in the iodide medium and irreproducibility from breaks which tended to form in the micrometer buret capillary. The following abbreviated version of the integrated DME experiment proved convenient. A DME drop charge was measured at Einit, in both the presence and absence of cadmium ion; the difference between these charges represents the alteration in double layer charge at Elnitcaused by adsorption, Q A d l . A constancy of Qadi at different bulk cadmium concentrations was taken as indication of a satisfactory approach to adsorption equilibrium with the DME drop life. These Qadi values indicate a decrease in double-layer charge in the presence of adsorbed cadmium and are given in Table I1 as values c appropriate to be subtracted from Qdl(blank) to yield a Q ~value
Comparison of Chronocoulometric Data Obtained from -0.55 volt,
Einit
Efinsl=
Slope/[Cd+2] X 10-2, pcoul/sq crn2*secl‘*.mM [I-], M
[Cd+21,m M
1.00
0.4 0.6 0.8
1 .o 0.4
0.8
0.6 0.8 1. o
1.4 0.4
0.5
0.8
Q
- tb 5.0
5.3 5.1 5.3 5.4 5.2 5.3
5.4 5.1 5.1
= Qads
5.2 5.0
33.2 33.2 37.0 33.4 32.0 34.8 37.0 41 .O 38.8 38.0
5.4 5.2 =
Qintena ctcOul/Sq cm Q - t
5.0
QdI.
Q-t‘l’ Readouts
- t’/zb 5.2 5.2 5.2 4.9 4.7 4.9
5.1
+ Average diffusion coefficientcalculated from these values is D Qinter
Q
Q-r and
-1.00 volt
40.0
Q
- t’/z 31.6 36.4 36.4 36.4 30.0 32.0 34.6 37.2 35.6 36.0 38.0
5.6 x 10-6 sq cm/sec.
VOL. 39, NO. 1, JANUARY 1967
53
[KII, M 0.10
0.20
0.50
1.o
[KII, M
0.10 0.20
0.50
1.0
a
Table 11. Q-t’” Intercept and Double-Layer Charge Data for Cadmium Adsorption (Erinsl = - 1.00 volt for all potential step experiments) Qintsnm CtcOWSq cm at E i n i t [Cd+21,mM -0.40 -0.45 -Or50 -0.55 Ob 33.2-9.6 26.3-4.3 22.4-1 .0 53.8 0.4 50.6 46.8 56.6 0.8 53.5 50.7 50.2 1.2 49.5 46.7 Oh 35.1-9.4 27.4-3.6 22.4-2.2 18.5-0.9 0.4 49.2 48.2 45.8 44.5 53.2 51 .O 0.8 48.6 44.8 1.2 52.5 49.4 48.6 47.8 Ob 32.1-2.6 26.2-1 .3 21 , 6 2 , 1 0.4 41.2 40.7 39.6 43.2 0.8 43.2 40.0 45.3 1.2 45.3 44.5 Ob 33 ,5-2.9 26.8-3.6 21.9-2.1 36.3 0.4 34.0 34.0 39.6 0.8 39.6 39.6 1.2 39.6 41.2 40.4
Table 111. Surface Excess Values for Cadmium Adsorption from Iodide Medium (Eri-1 = -1.00 volt)” r X lolo, moles/sq cm, at ,Einit [Cd+21,mM -0.40 -0.45 -0.50 -0.55 0.4 1.56 1.48 1.31 0.8 1.71 1.63 1.52 1.2 1.38 1.41 1.31 0.4 1.22 1.26 1.32 1.40 0.8 1.42 1.41 1.47 1.41 1.2 1.39 1.32 1.47 1.56
17.9-0.8 36.7 38.1 41.2 18.2-1 0 33.0 37.9 40.4 I
-0.60
0.4 0.8 1.2
0.71 0.82
0.82 0.95 1.05
1.04 1.06 1.30
1.01 1.09 1.26
0.4 0.8 1.2
0.30 0.46 0.46
0.56 0.85 0.93
0.73 1.02 1.06
0.82 1.07 1.20
Surface excess data taken at
Efinal
0.60
0.46 =
-0.90 volt agree with values given in this table.
for correction of the chronocoulometric intercept data. Values of cadmium surface excess calculated from the Qdl and intercept data of Table I1 are given in Table 111. The mole per sq cm. precision of these data is 0.1 to 0.2 X A measurabIe adsorption of cadmium from the iodide medium was found under all conditions tested. Attempts to identify the coordination state of the adsorbed species, by examination of the marked trend in surface excess with iodide concentration, were not successful. If the adsorbed cadmium layer contains a predominance of one coordination state (with respect to iodide ligands), a correlation between the iodide concentration dependency of the concentration of that cadmium iodide complex in the bulk solution and the measured surface excess might be expected. No simple correlation of this type could be discerned for the several possible coordination states, however, Furthermore, estimates of monolayer saturation values for various cadmium iodide complex models yield surface excess values larger than any observed by factors of 2 and more. Either cadmium is capable of adsorbing in a variety of coordination states, or other factors significantly influencing the adsorption process are also iodide concentration-dependent. 54
-0.60
ANALYTICAL CHEMISTRY
Two other trends in the data of Tables I1 and I11 are also worthy of comments. The cadmium surface excess is dependent on potential, notably at high iodide concentrations. Although this trend is in the direction of increasing adsorption at more negative potentials, the magnitude of the effect of cadmium adsorption on the double-layer charge, QAdl, exhibits a trend in the opposite direction. A given quantity of cadmium adsorption appears to force a more substantial modification in the electrical double layer at the more positive potentials. The underlying reasons for these effects, which could be either competitional or electrical in nature, are not fully understood, and a detailed interpretation must await a more thorough understanding of the factors governing the adsorption of metal complexes containing surface-active ligands at a metal surface.
Received for review August 15, 1966. Accepted October 10, 1966. Work supported by the Directorate of Chemical Sciences, Air Force Office of Scientific Research, Grant AFAFOSR-584-64.
