Comparison of Pyrolytic Carbon Film Electrode with Wax-Impregnated Graphite Electrode ALVIN L. BEILBY, WALTER BROOKS, Jr., and GARY L. LAWRENCE Department of Chemistry, Pomona College, Claremont, Calif.
b
Pyrolytic carbon film electrodes prepared in the laboratory by the decomposition of natural gas at 1025' C. have been compared with waximpregnated graphite electrodes and platinum electrodes using the techniques chronopotentiometry and integral chronoamperometry. With the ferrocyanide-ferricyanide system the results obtained with pyrolytic carbon film electrodes compare favorably with the results obtained with platinum electrodes for both El/d and E, values and for log plot slopes. Reproducibility for both E values and for transition times and diffusion charges is within the limits of the measuring systems. At the wax-impregnated graphite electrodes, the reaction appears to be more irreversible, which is in disagreement with the results of other workers.
C
as an electrode material for voltammetric studies was first suggested in 1952 by Rogers and Lord (24). The main dficulty associated with the use of carbon is the presence of pores in the usual forms of carbon which can contribute to nonreproducible results and large residuals. Two approaches, both attempting to fill the pores with some sort of inert material, have been used in trying to eliminate these difficulties. The first approach has evolved from the initial work of Rogers and Lord using solid graphite rods. In this approach the pores are filled by impregnating the graphite with molten wax. Elving and Smith (10) discuss the latest techniques for preparing these wax-impregnated graphite electrodes based upon their findings and the procedures of previous workers. Good results have been obtained, especially with the oxidation of some organic compounds, and this type of electrode is continuing to be used (27). The other approach is that of Adams (1) who suggested the carbon paste electrode. Here the pores are filled with an organic solvent by using a technique of making a paste of graphite powder and an organic solvent that is immiscible with the electrolysis solution. Further studies by Olson and Adams (22) showed that this electrode was use22
ARBON
ANALYTICAL CHEMISTRY
ful for many oxidations, again, especially for organic compounds. Neither of these two types of carbon electrodes can be called ideal because of the necessity for introducing substances to fill the pores of the carbon which could affect the electrode reactions or the nature of the carbon as an electrode material. A more ideal carbon electrode would be one whose surface is hard, smooth, and without the porous nature of most forms of carbon. I n recent news releases (6,26) a form of carbon, pyrolytic carbon or graphite, which has existed primarily as a laboratory curiosity for many years, was announced as a new material for space and missile technology. The description given for it as a "hard, impermeable layered material that looks like black porcelain" (26) fits some of the requirements for an ideal type of carbon electrode. Laitinen and Rhodes (14) were the first to use pyrolytic carbon as an electrode material in their electrochemical studies of VzO6 in a LiC1-KCl eutectic melt. The use of pyrolytic carbon as an indicator electrode in aqueous solutions was introduced simultaneously by Beilby and Brooks (3) and Miller and Zittel (18). Miller (16) has described its use as an indicator electrode for potentiometric titrations and Miller and Zittel (17) have discussed its use in voltammetry. Pyrolytic carbon is prepared by the thermal decomposition of a carbonaceous gas and the deposition of the carbon formed on a substrate. The basic property of pyrolytic carbon is that the graphite crystallites forming the bulk structure of the pyrolytic carbon are well aligned relative to each other, with the layer planes of the crystallites all closely parallel to the substrate. Because of this high degree of crystallite alignment, pyrolytic carbon is anisotropic. High thermal anisotropy is one of the properties of interest to space technology. The temperature and pressure of the gas used in the preparation of pyrolytic carbon are important in determining the nature of the deposit and the properties of pyrolytic carbon. Thin films of pyrolytic carbon produced a t temperatures about 1000" C. and a t atmospheric pressures have been made for some time
(11). However, Brown and Watt (6) showed that massive coherent deposits can be produced when the temperature is about 2000" C. and the pressure is on the order of a few millimeters. It was this advancement of being able to obtain thick deposits that led to the new, strong interest in pyrolytic carbon. Under certain conditions there can be a drop in the density of pyrolytic carbon between deposition temperatures of about 1300' and 2000" C., but outside of this range on both the upper and lower ends of the temperature range the density approaches the theoretical density of graphite (9). The maximum degree of hardness of deposits has been obtained between 1000" and 1025" C., with some deposits reported as having a hardness on Moh's scale of 9.8 which is practically equivalent to diamond hardness (11). The hardness is probably related to the graphite crystallite size which increases with increase in the temperature of deposition (11). Metallographic examination of pyrolytic carbon has shown the presence of neither closed nor opened pores (23). Other properties of pyrolytic carbon are reviewed and discussed by Grisdale, B s t e r , and van Roosbroeck ( I I ) , Pappis and Blum (E?),Klein ( l a ) , and Walker (26). Although bulk samples of pyrolytic carbon can be obtained commercially, it is possible to prepare films of pyrolytic carbon in the laboratory without undue dficulty. Except for the work of Beilby and Brooks (3) who used pyrolytic carbon films prepared in the laboratory a t 1000" C., the other authors (1.4, 16-18) have all used commercial pyrolytic graphite prepared a t about 2000" C. In this paper, an extension of the work of Beilhy and Brooks, pyrolytic carbon film electrodes are compared to the wax-impregnated graphite electrodes and to platinum electrodes. No attempt was made to run all the various compounds determined with the wax-impregnated graphite electrode. Rather, a detailed study of the response of the two types of carbon electrodes to the often used ferrocyanide-ferricyanide system was made, using the techniques of chronopotentiometry and integral chronoamperometry .
:MENTAL
iration. The pyrodectrodes were prelly the manner as sdale, Pfister, and ~ ! l )A. 1 to 3 mixture of natural gas and nitrogen a t atmospheric pressure with a flow rate of 450 ml. per minute was passed over a 0.15-inch diameter 99% ceramic rod substrate (AD-99 alumina, Coors Porcelain Co., Golden, Colo.) in a rotating combustion tube in a furnace a t 1025" C. The natural gas was purified hy passing it through an activated charcoal trap a t dry ice-acetone temperature. The combustion tube was constricted in the end portions to allow for greater ease in making gas fittings to the tube and to Drevent the rods from rolling out of (he tube. In order to get a< even deposit a layer of sand was put in the bottom of the center section of the tube ( 1 1 ) . The rods were cleaned with eoap and water before heating and were not touched with the hands after cleauing. While the furnace was coming up to temperature, oxygen was passed through the tuhe to oxidize anything remaining on the surface. Nitrogen was then passed through the tuhe for awhile to purge the oxygen before introducing the natural gas. The gas was passed through the tube for 24 hours after which only nitrogen was passed until the furnace had cooled to room temnerabme. The surface of the electrodes prepared in this manner appeared hard and metallic and could not he scratched by moderate pressure from a pocket knife. The thickness of the film measured approximately 5 X 10-4 cm. which is in fair agreement with the results of Grisdale, Pfister, and-yan R.nnahroe.nlr f.. n. r R. rlennsition ._____._.... - 24-hour ~. ~ ~ . . ~ time. A 2-inch length had a resistance of 20 ohms. The electrodes were mounted by force-fitting them into short piece of machined Teflon rod with the point of tightest fit a t the end where the electrode projected into the solution. The Teflon rod was then force-fitted into a glass tube into which mercury was placed to make electrical contact with the electrode. In some cases the deposit on the end of the rod was removed and in other cases the deposit was left on the end of the rod. The completed electrode assembly is shown in Figure 1. The wax-impregnated graphite electrodes were prepared in the manner described by Elving and Smith (10). Graphite spectroscopic electrodes (No. L3809, National Carbon Co.) were impregnated with ceresin wax (Fisher Scientific Co.) and coated with an insulating layer of either Seal-All (Allen Products Corp.) or with an epoxy resin mixture used by Miller and Zittel (17) for preparing their pyrolytic graphite electrodes. When using the epoxy resin it was necessary to clean thoroughly the surface of the electrode of all w&x before applying the resin. Fresh ends were prepared by lathing. Before use the electrodes were immersed in a 0.003% Triton X-100 solution. The platinurn electrode consisted of
Figure 1. Pyro.-. . film ...... lytic cnrbon electrode assem-
~
~~~~~~
~~~~~
1~
~~
~~
~
-~-.~~~~~~~~
bly
a short piece of platinum wire sealed into a soft class tube in the usual manner. Chemical and Reagents. Solntions were prepared from laboratory distilled water redistilled from an alkaline permanganate solution. All ohemicals were reagent grade and were used without further purification Chronopotentiometric Apparatus. Chronopotentiometric measurements were made in the conventional manner as described by Lmgane (16). Five 45-volt batteries in series provided the source of current with the current level determined by measuring the I R drop across precision resistors with a Rubicon Model 2746 potentiometer. The potential of the working electrode in the earlier experiments was measured against a Beckman saturated calomel electrode with a Beckman "Zeromatic" pH meter. To get a faster response with the "Zeromatic," the capacitor across the meter was changed from 500 pf. to 250 pf. In the later experiments the ~ potential was measured against a Leeds and Northrup saturated calomel electrode with a Leeds and Northrup Model 7401 p H meter. The outputs of the pH meters were fed to a Brown IO-mv. recorder with a 1-second pen response speed. A chart speed of 6 inches Der minute was used. Potentials on t,hekeeorder were calibrated with the ~
electrodes and the reference electrode were placed in one side of the cell a t the same time. with care taken to locate each working electrode the same distance from the reference electrode. The auxiliary electrode consisting of a platinum plate with an area of 2 sq. cm. was placed in the other side of the cell. All measurements were made with the cell placed in a water bath held a t 25.0 0.1' C. Chronopotentiometric Procedure. Solutions were huhbled with nitrogen, which had heen passed through an ...., alkaline pyrogallol solution, water. and a portion of the test solut ion, before use and between runs. W hen the solution was quiet a chri sual potentiogram was run in the u,nomanner. Any prepolarisations 1were done in a separate cell containin'g a portion of the indiflerent electrolyte
*
against a large size calomel electrode. The pyrolytic carbon film electrodes were stored in distilled water between runs. Since the shapes of the difFerent electrodes varied, the current levels were adjusted to give only approximately the same transition times for the different electrodes. Transition times and EINvalues were determined from the graphs in the manner described by Elving and Smith (IO). values for the pyrolytic carbon The film electrodes were corrected for the I R drop introduced by the 10-ohm resistance of the electrodes. Slope values were obtained from the usual plot of log [ ( & * / t " 2 ) - 11 US. E (7). Integral Chronoamperometric Apparatus. Manual integral chronoamperometric measurements were made using a previously described apparatus (4). Integration times were measured with a Model S-1 Standard precision timer, with a listed accuracy of &0.01 second. All measurements were made in a water bath held a t 25.0
*
n i o r Y . l
V.
Integral Chronoamperometric Procedure. Calibration and determination of charge-voltage curves were made in the previously described manner (4). The solution was buhbled with nitrogen, which had heen passed through a portion of the test solution before use. In most cases the only pretreatment to the electrodes was to make several measurements a t the starting potential or to polarize the electrode for several minutes a t the starting potential. Slope and E, values were ..determjned by the---.._ logarithmic 1r equaaons 01 morgan, narrnr, -it^i d Crittenden (19) for integral chrona0amperometry. ~
~~
~~~~
RESULTS AND DISCUSSION
Ferrocyanide-Femcyanide System. I n attempting to choose a redox system t o evaluate and compare solid electrodes, one would prefer a system composed of two soluble species, with a 1-electron change and a fast electron transfer step, and with no breaking or formation of chemical bonds other than in the degree of hydration. Although there is no really ideal system, ferrocyanideferricyanide comes fairly close to meeting the above requirements. Hence, it has been used for the evaluation of many types of electrodes and techniques, including the wax-imuremated graphite electrode. For thii reason, it was also chosen for these studies. Although under most conditions this system appears to be reversible, in some instances a small degree of irreversibility has been indicated depending upon the method and --..>:L:--"
li""UlU""U
'9,U).
' 0, 1 . IO
Another species which has heen used for calibration Iurposes is the oxidation of iodide (4, 1I1). However, as might be expected with carbon surfaces, adsorption of iodine could take place. - .. . k'rehmnary studies with both chronoVOL. 36, NO. I , JANUARY 1964
e
23
potentiometry and integral chronoamperometry gave results indicating that something of this nature was taking place at the pyrolytic carbon film electrode. No further studies were made at that time to ascertain the reason for the results observed. Since the reproducibility of the waximpregnated graphite electrode has already been demonstrated (10, do), and since, as will be shown later, the reproducibility of the pyrolytic carbon film electrode is at least as good as the wax-impregnated graphite electrode, the most logical comparisons to use between the two electrodes are E values and log plot slope values. The formal potential of the ferrocyanideferricyanide system is dependent upon
Table 1.
the concentration of the electrolyte present, or, more exactly, upon the ionic strength of the solution. For 0.5M KC1 solutions, Kolthoff and Tomsicek (18) reported a formal potential of +0.4581 volt us. NHE (or +0.216 volt us. SCE) obtained potentiometrically for an equimolar mixture. Morris and Schempf (20) reported a potentiometric value of +0.220 volt. A potentiometric value of $0.223 3Z0.002 volt was obtained in these studies with the L&N p H meter and Brown recorder. The equimolar concentrations of ferrocyanide-ferricyanide used in obtaining these three values were 0.4, 1, and 10 mM, respectively. From the data of Kolthoff and Tomsicek one would espect only about a 1-mv. change in the
Chronopotentiometric Results with Pyrolytic Carbon Film Electrode
Transition Current Area, Slope time, sec. pa. sq. em. Reduction of Ferricyanide-Beckman "Zeromatic" pH Meter Pyrolytic carbon 0.205 3Z 0,002 0.073 19 3 0.6" 206 0.36 film I* Platinum 0.210 f 0.002 0.072 22.1 0.5n 168 0.29 Reduction of Ferricyanide-L$N pH Meter Pyrolytic carbon 0.219 f 0.000 0.062 19.0 + 0 . 2d 400 0.68 film 118 Platinum 0.220 0.001 0.061 21.7 zk 0.1/ 168 0.29 Oxidation of Ferrocyanide-LbS pH Meter Pyrolytic carbon 0.226 =t0,000 0.065 17.6 It O . O d 400 0.68 film 111 Platinum 0.228 i 0.000 0.059 18.6 f 0.01 168 0.29 Electrolyte, 0.5M KCl; ferrocyanide and ferricyanide concentrations, 10.0 mM. Reproducibilities are shown by 957, confidence limits, except as noted. Transition time accuracy: f 0 . 2 second. E l i d accuracy: f 4 mv., the accuracy of the L&N pH meter and recorder. a Averages of 7 rum on 2 solutions. Film on the end of the electrode was removed. Averages of 8 runs on 4 solutions. Averages of 12 runs on 3 solutions. * Film was left on the end of the electrode. f Averages of 2 runs on single solutions, reproducibilities shown by range Electrode
Ellc vs. SCE, volt
*
*
Table II. Manual Integral Chronoamperometric Results with the Pyrolytic Carbon Film Electrode
Electrode Pyrolytic carbon film I o Platinum Pyrolytic carbon film I" Platinum
E, us. SCE, volt
Slope Reduction of Ferricyanide 0.203 f 0.001 0,072
0.068 Oxidation of Ferrocyanide 0.220 f 0.001 0.065
Diffusion charge Anodic Cathodic 148 zt IC
0.211
119 137 f l b
0.216 0.063 112 Ferrocyanide-Ferricyanide Mixture 0.214 f 0 000 0.072 137 f 1
Pyrolytic carbon 149 f lo film Io Platinum 0.215 109 113 0.067 Electrolyte, 0.5144 KCl; ferrocyanide and ferricyanide concentrations, 1.00mM. Electrode areas: Platinum = 0.29 sq. cm., pyrolytic carbon film I = 0.36 sq. em. Integration time: ti = 1.58 sec., tz = 5.87 see. Film on the end of electrode was removed. Triplicate determination, reproducibilities given by range. Duplicate determination, reproducibilities given by range. 24
ANALYTICAL CHEMISTRY
formal potential in a solution of 0.531 KC1 due to the change in ionic strength caused by the above differences in the ferrocyanide-ferricyanide concentrations. The rest of the difference is probably due to variations in the experimental conditions. Wax-Impregnated Graphite Electrode. With manual voltammetry Morris and Schempf (20) obtained EIi2 values of $0.217, 0.216, and 0.215 volt for ferricyanide, ferricyanide-ferrocyanide mixture, and ferrocyanide, respectively, at a wax-impregnated graphite electrode. Log plots gave slope values of 0.060. These results definitely indicate reversibility with values that are in good agreement with the formal potential. Elving and Smith (10) report chronopotentiometric E I 4 values of f0.186 and 0.184 for ferricyanide and ferrocyanide, respectively, with log plot slope values of 0.074 and 0.069. However, they also state that when different graphite surfaces were employed El,c values Tvere generally reproducible only to M ithin 1 0 . 0 1 volt. Both Morris and Shempf and Elving and Smith also made automatic voltage scan roltammetric measurements. The two sets of data for E,lz values do not agree too well. For either set of data the differences betn een EplP for the reduction process and E P z for the oGdation process do not agree with the theoretical value of 0.056 volt for a reversible process (21). These data were obtained, horever, Kith voltage scan rates that may be slow enough to have convection interfering with the diffusion so that exact agreement nith the value of 0.056 volt would not be expected. Thus, it is qeen that only one set of data has given both E values that are close to the formal potential of the ferrocyanide-ferricyanide system and log plot slopes that indicate reversibility at the wax-impregnated graphite electrode Since there is not good agreement among the previous results with the wax-impregnated graphite electrode, it seemed necessary to compare the waximpregnated graphite electrode with the pyrolytic carbon film electrode under the same experimental conditions rather than just to compare the results of the pyrolytic carbon film electrode Kith the previous findings. Qualitatively, it was found that for both chronopotentiometric runs and integral chronoamperometric runs, the curves were always somewhat drawn out or rounded a t the top indicating a certain amount of irreversibility. In agreement with the findings of Elving and Smith (10) for a given qolution and electrode surface, the reproducibility was n-ithin the preciqion of the measuring apparatus, but 11hen using different electrodes and
surfaces the variance was approximately 1 0 . 0 1 volt for E 114 values. In the course of this work the Seal--ill coating presented at times some difficulties such as obtaining a satisfactory lathe cut or solution penetrating between the coating and the electrode, the latter of which was noticed by whitening of the coating. An epoxy resin mixture used by Miller and Zittel ( 1 7 ) was tried as a coating material and seemed to eliminate these difficulties. In one set of chrctnopotentiometric runs using a Beckman “Zeromatic” pH meter, an average value of +0.186 volt for E14 was observed for the reduction of ferricyanije a t an electrode, coated ~ i t Seal-All h which agrees with the value given by Elvi ig and Smith (10), but a log plot slope of 0.092 was obtained which disagree: with their value. In a set of chronopotentiometric runs using the L&N p H meter and with the epoxy resin coating on the electrode average values of +0.236 and 0.214 volt were oblained for ferrocyanide and ferricyanide, respectively, with log plot slope values of 0.067 and 0.073. I n this case the slope values agree fairly well with the values of Elving and Smith. I ; is probable that some of the difference in values between these two sets of data may be due to a faster re.ponse of the L&N p H meter. The values obtained a t a platinum electrode under sim lar experimental conditions are given ir Table I. Ihief evperiments with manual integral chronoamperometry also gave similar results. E , T7alues of +0.228 and 0.198 volt and log plot slope values of 0.081 and 0.079 were obtained for ferrocyanide and Ferricyanide, respectively. Values obtained a t a platinum electrode are given in Table 11. From these data it must be concluded ferrocymide-ferricyanide that the system is somewhat irreversible a t the n-ax-impregnated grttphite electrodes used in this study and that E values can vary by a fair aiiount as different wax-impregnated grliphite electrodes are used. Pyrolytic Carbon Film Electrode. I n Table I are jummarized the chronopotentiometric: results for the ferrocyanide-ferricyanide system using the pyrolytic carbon film electrode. Table I1 contains cata obtained by manual integral ch-onoamperometry. Results using platinum electrodes are included for comparis ,n purposes. The platinum electrodes a w e polarized before use in the manner discussed by Anson (6) for the ferrous-ferric system. For both methods the re-ic ualq for the region of the ferrocyanide and ferricyanide curves nere negligih e. The effective range of the pyrolrtic carbon film electrode in 0.5Jf E X 1 was approximately from +1.3 volt to -1.6 volt.
The first thing to notice is that the chronopotentiometric results do vary between the two different pH meters used for measuring the potential for both the platinum electrode and the pyrolytic carbon film electrode with the variance about the same for both. It is felt that the results obtained with the L&N pH meter are more accurate since, as was mentioned above, its response appears to be faster. The slightly high log plot slope values from the manual integral chronoamperometric data are probably due to the measuring system since slightly high values for the oxidation of iodide a t a platinum electrode have also been obtained (4). Integral chronoamperometric data for both the oxidation of iodide and the ferrocyanide-ferricyanide system obtained with electronic type apparatus have given log plot slope values somewhat closer to the expected theoretical values (19). As is noted on Table I the difference between electrode I and electrode I1 is that the film on the end of the ceramic substrate was removed from electrode I before use. Obviously, this technique would leave a ring of pyrolytic carbon with the edges of the graphite crystallites exposed. With the thick commercial, high-temperature pyrolytic graphite, problems such as seepage of mercury betiveen the layers and permeation of aqueous solutions and molten salts have been observed when the edges have been exposed which could give rise to nonreproducible results (16, 17). However, the reproducibility of the results as shown in both Tables I and I1 for both E values and transition times and diffusion charges obtained with electrodes where the end film n a s removed, would lead one to the conclusion that, with the thin-film pyrolytic carbon electrodes, the exposure of the very thin edge a t the end of the electrode has a negligible effect. This difference b e h e e n the edge effects of the thick, high-temperature pyrolytic graphite and the low-temperature pyrolytic carbon films is probably due to the facts that the graphite crystallites in the lowtemperature films are smaller, a condition which can lead to hardened deposits, and that the crystallites are randomly stacked with no definite layer planes in the bulk structure as is observed with the thick, high-temperature pyrolytic graphite (11,12). It is seen from the results, then, that the ferrocyanide-ferricyanide system appears to act almost as reversibly a t the pyrolytic carbon film electrode as at the platinum electrode, with E values for the two types of electrodes about the same and in good agreement with the formal potential. It should be noted, however, that chronopotentiometric curves of ferrocyanide or ferricyanide which were drawn out or
rounded a t the top more than for a reversible curve were obtained when the pyrolytic carbon film electrode had been exposed to oxidizing conditions such as cleaning with a H2S04-HN03mixture or a high positive potential. Similar effects were also noted, although to a lesser degree, when the electrode had not been used for a period of time. These effects were noted both when the film had been removed from the end of the electrode and when the film had been left on. Empirically, it was found that by polarizing the electrode a t +1.5 volt for 15 minutes, followed by polarizing a t -0.3 volt for 5 minutes, reversible and reproducible curves could be obtained again. A41thoughin some instances polarization a t - 0.3 volt would suffice, in other cases only the two polarizations appeared to have the proper effect. Hence, it became a routine treatment to carry out the two polarizations occasionally to ensure reversible and reproducible results. Similar treatment to the wax-impregnated graphite electrodes did not improve their results and, as was mentioned above, the curves of the wax-impregnated graphite electrodes always indicated a certain degree of irreversibility as shown qualitatively by the shape of the curves or quantitatively by the log plot slopes. The reasons for the effects of the polarizations are not known a t the present but are the object of continuing investigation. CONCLUSIONS
Elving and Smith (10) state that nonideal cell geometry probably accounts for most of the deviation of their slope value from the theoretical value for a reversible process although such things as roughness of the electrode surface, recorder response, and transition time measurement error may also contribute. These reasons may also perhaps be used to explain some of the deviations of the results reported here. However, these reasons cannot be used to explain the difference between the results for the wax-impregnated graphite electrode and the pyrolytic carbon film electrode-i.e., the fact that the reaction of ferrocyanide-ferricyanide appears to be more reversible under the conditions used in these experiments at the pyrolytic carbon film electrode than a t the wax-impregnated graphite electrode. The only difference between the two sets of measurements was the nature of the carbon electrode used. The problem of the nature of carbon surfaces will not be so simple as the problem for a metal since the properties of carbon and the nature of the surface can vary with the mode of preparation. It cannot be concluded from the present information whether the differences between the reVOL. 36, NO. 1 , JANUARY 1964
* 25
sults of the two types of carbon electrodes are due only to presence of foreign material at the electrode surface or are due to a basic difference in the nature of the carbon surface. In fact, results with low-temperature pyrolytic carbon film electrodes may differ from results obtained under similar conditions with high temperature bulk pyrolytic graphite electrodes because of differences in their nature, ACKNOWLEDGMENT
The initial work on the preparation of the pyrolytic carbon film electrodes was carried out by Allan L. Budd, and his work is gratefully acknowledged. LITERATURE CITED
(1) Adams, R. N., ANAL.CHEM.30, 1576 (1958). (2) Anson, F. C., Ibid., 33, 934 (1913ll (3) Beilby, A. L., BrookR. 7
Meeting, ACS,
CHEM.34,493 ( i
(5) Brown, A. R. G., Watt! W., “Industrial Carbon and Graphite,” p. 86, Society of Chemical Industry, London, 1958. (6) Chem. Eng. News. 37, 56 (Nov. 30, 1959). (7) Delahay, P., “New Instrumental Methods in Electrochemistry,” p. 182, Interscience, New York, 1954. (8) Delahay, P., Mattax, C. C., J . Am. Chem. SOC.76,874 (1954). (9) Diefendorf, R. J., J . Chim. Phys. 57, 815 (1960). (IO) Elving, P. J., Smith, D. L., ANAL. CHEM.32, 1849 (1960). (11) Grisdale, R. O., Pfister, A. C., van Roosbroeck, W., Bell System Tech. J . 30, 271 (1951). (12) Klein, C. A., Reo. Modern Phys. 34, 56 (1962). (13) Kolthoff, I. M., Tomsicek, W. J., J . Phys. Chem. 39,945 (1935). (14) Laitinen, H. A., Rhodes, D. R., J . Electrochem. SOC.109, 413 (1962). (15) Lingane:, J. J., “Electroanalytical Chemistry, 2nd ed., Chap. XXII, Interscience, New York, 1958. (16) Miller, F. J., ANAL.CHEM.35, 929 (1963). (17) Miller, F. J., Zittel, H. E., Zbid., p., 1866.
(18) Miller, F. J., Zittel, H. E., 144th
Meeting, ACS, Los Angeles, Calif., April 1963. (19) Morgan, E., Harrar, J. E., Crittenden, A. L., ANAL.C m x 32, 756 (1960). (20) Morris, J. B., Schempf, J. M., Zbid.,
31, 286 (1959). (21) Mueller, T. R., Adams, R. N., Anal. Chim. Acta 25, 482 (1961). (22) Olson, C., Adams, R. N., Zbid., 22, 582 (1960). (23) Pappis; J., Blum, S. L., J . Am. Ceram. SOC.44, 592 (1961). (24) Rogers, L. B., Lord, S. S., Jr., Pittsbirgh Conference on Analytical Chemistry, March 1952. (25) Time 74, 84 (Xov. 23, 1959). (26) Walker, P. L., Jr., Am. Scientist 50, 259 (1962). (27) Ward, G. A,, Talanta 10, 261 (1963).
RECEIVEDfor review April 2, 1963. Accepted November 6, 1963. Acknowledgment, is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society for partial support of this research and to the Undergraduate Science Education Program of the National Science Foundation for partial support of this research. Presented in part at the Division of Analytical Chemistry, 144th Meeting, ACS, LOBAngeles, Calif., April 1963.
Cathodic Action of the Uranyl-Itaconate Complex at the Dropping M erc ury Electrode TSAI-TEH
LA1 and BI-CHENG WANG’
Chemical Engineering Department, Cheng Kung University, Tainan, Taiwan, China
b The uranyl-itaconate complex has been studied polarographically. From the potential and current data the existence of a mixture of monomer and polymer, having the chelate species of (UOzAz-*),, (n = 1 3) and (UO2A-’),, (n’ = 1 2), were identified. The electrode reactions in three ranges of pH value were given and the influence of pH and uranyl ion concentration on the character of waves was discussed.
- -
T
CONSTANTS for the complex ions formed between Caf2 or Srf2 with itaconic acid have been determined by an ion exchange method (IO). Andrews and Keefer (1) calculated the solubility of cuprous chloride in itaconic acid based on the formation of cuprous chloride complexes, and determined their formation constants. Although the polarographic behavior of itaconic acid has been studied by Markman and Zinkova (8) and Nerheim and Estee (9), little is known concerning the polarography of metalitaconate complexes. As a part of polarographic investigation of a series of complexes of uranium
26
HE FORMATION
ANALYTICAL CHEMISTRY
with various organic ligands (4-7) , this work was undertaken to determine the polarographic behavior of the uranylitaconate system. EXPERIMENTAL
A Fisher Elecdropode was used for the polarographic measurements. Applied potentials were determined with a Leeds and Northrup student potentiometer. The samples were placed in a modified H-cell kept at 30” + 0.1” C. and deoxygenated with purified nitrogen. A saturated calomel reference electrode (S.C.E.) was used in coniunction with a notassium chloridc salt bridge. The characteristics of capillary were: drop time, 4.866 seconds: rate of flow, 1.445 mg. per second in 0.2.11 ?u’aC104 at -0.200 volt us. S.C.E. The p1-I value of the solution was adjusted with perchloric acid or potassium hydroxide and determined using a Beckman Model H-2 pH meter. Stock uranyl perchlorate solution was prepared and standardized with the same method described in a previous paper ( 5 ) . A 0.4M stock solution of itaconic acid n-as prepared and the exact concentration was determined potentiometrically
with carbonate free standard sodium hydroxide solution. Since no maxima were observed, a maximum suppressor was not used. RESULTS AND DISCUSSION
The temperature coefficients of the half-wave potential and the diffusion current were measured in a solution of O.lmM uranyl perchlorate in 40mM itaconic acid and 0.2M sodium perchlorate at pH 4.2. The obtained values of -0.2 mv. per degree centigrade and 1.36% per degree in Figure 1 show that the reduction is reversible and diffusion controlled. The diffusion current constant is calculated to be 2.5. Effect of pK, and pK2 on HalfWave Potential. Itaconic acid is a n unsatursted dicsrhouylic acid with pK1 = 3.8 and pK2 = .i.7 (2). If a chelate of uranyl-itaconate has been formed with the formula U02A,(2-2r), then its electrode reaction must be UO2A,@-*) e = UOaA,(’-”)
+
+
(r
- s)A-’
1 Present address, Department of Chemistry, University of Arkansas, Fayetteville, Ark
