standard amino acid solution, it is not necessary to know either the degree of isotope incorporation or the amount of each deuterated compound in the standard solution. The use of a separate deuterated internal standard for each amino acid being analyzed allows several errors inherent in commonly used methods of amino acid analysis to be eliminated. These include loss of material from nonquantitative transfer, derivatization and column (ion exchange and GLC) recovery; loss of the very volatile derivatives of alanine, valine, glycine, etc. during concentration of the derivatized sample prior to injection on the gas chromatograph (16); and loss of basic amino acids which are co-precipitated with protein during plasma work-up (27). Furthermore the chances of errors arising through co-elution of interfering compounds in the conventional GC or ion exchange methods of amino acid analysis are significantly reduced since the mass spectrometer detects only those ions known to be specific to the mass spectrum of the amino acid being analyzed. The fact that the quantitative result obtained for the amino acid composition or soil samples compared very fa(16) R. W. Zumwalt. K . Kuo. and C. W. Gehrke, J. Chromatogr.. 55. 267 (1971). (1 7) L. 2 . Bito and J. Dawson. Ana/. Biochem., 28,95 (1969)
vorably with results obtained from an amino acid analyzer (9) suggests quadrupole mass fragmentography will find wide application for the analysis of amino acids in the future. It will be particularly useful for determinations on neonatal plasma and amniotic fluid samples where low sample size or low amino acid content dictates that optimum sensitivity is an important consideration. The time taken for one complete analysis using this computer directed mass fragmentography system, exclusive of derivatization. is 30 minutes for data collection with an additional 10 minutes before the computer presents the final analytical result.
ACKNOWLEDGMENT The authors are grateful to Neil Buist, Department of Pediatrics, University of Oregon, for a sample of plasma from a patient with Maple Syrup Urine disease. Received for review June 11, 1973. Accepted November 12, 1973. This work was supported by the Xational Aeronautics and Space Administration (Grant No. YGR-05020-004) and the National Institutes of Health (Grant 60. RR00612).
Electrochemical Oxidation of Thin Palladium Films on Gold Steven H. Cadle' Chemistry Department, Vassar College, Poughkeepsie, N . Y. 12601
The results obtained in studying the anodic behavior of thin palladium deposits on gold are reported. The purpose of this work was to develop a technique for the quantitative determination of submonolayer deposits of palladium on gold in sulfuric acid media. Furthermore, it was hoped that a comparison between the results of this work and the work of other authors on bulk palladium would help characterize the surface oxidation state of bulk palladium electrodes. A knowledge of the surface oxidation state of palladium is needed to provide an electrochemical means of determining its roughness factor. The electrochemical behavior of palladium-gold alloys has been studied hy Woods (1) and Rand and Woods (2). They found that palladium and gold form a homogeneous alloy. The nature of the chemisorption of hydrogen and oxygen on these alloys is a composite of the properties of the individual metals. The potential of the adsorption and desorption peaks of a cyclic voltammogram of the alloy differs significantly from the potential of the corresponding peaks for the separate metals. The oxidation of palladium electrodes in aqueous acidic media has been investigated by several workers. However, there is disagreement about the potential at which mono-
'
Present address, Research Laboratories, G . M . Technical Center. Warren. Mich. 48090. (1) R . Woods. Electrochim. Acta, 14, 632 (1969) (2) D. A . J. Rand and R. Woods, J. EiectroanaL Chem., 36, 57 (1972).
layer oxygen coverage of the electrode occurs. Rand and Woods (3) investigated palladium oxide formation in 1M H z S 0 4 . A plot of the quantity of palladium oxide formed us electrode potential showed a step at -1.7 V LIS RHE. They concluded that a stoichiometry of 1 oxygen atom per surface palladium atom exists at this potential and that a t more positive potentials, a phase oxide forms. Burshtein e t al. ( 4 ) determined the surface area of palladium powders using the BET method. The surface area was then compared to the quantity of hydrogen and oxygen adsorbed on the electrode as a function of potential in 1N HzS04. It was concluded that a monolayer of oxygen is adsorbed on the electrode at +1.2 V us. RHE.
EXPERIMENTAL Electrochemical Equipment. Electrochemical experiments were performed using a Beckman Electroscan 30. An external voltage source was used to step the electrode potential when required. An all-glass 500-ml electrochemical cell was used. A Luggin capillary was used between the reference electrode and the rotating disk electrode. T h e reference electrode was a Leeds and Northrup 1199-31 saturated calomel electrode. T h e auxiliary electrode was a coil of platinum wire separated from the working electrode compartment by a medium-porosity glass frit. A beltdriven electrode rotator capable of speeds of 20 to 100 (rpm)' * in intervals of 10 (rpm)' was used. The rotating gold disk electrode a n d palladium disk electrode had a n area of 0.457 cm2 and 0.452 ( 3 ) D. A . J Rand and R . Woods. J . E/ectroana/. Chem.. 31, 29 (1971) (4) R. Kh. Burshteln. M . R. Tarasevich, and V . S. Vilinskaya. Eiectrokhimiya. 3. 349 ( 1967) A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 4 , A P R I L 1974
587
c
t
5'
I
300k
E,VO LTS vrSCE
Figure 1. Current-potential curves at a palladium disk electrode (-) and a gold disk electrode (- - - ) : 0.2M H2S04, 2500 r p m ,
potential scan rate 100 mV/sec
Initial quantity of deposited palladium, 500 pC. Electrode potential cycled continuously. 2500 rprn, potential scan rate 100 mV/sec. Curve 1 ) 3rd cycle, Curve 2 ) 6th cycle. Curve 3) 10th cycle, Curve 41 1 3 t h cycle. Curve 5) 16th cycle
i 200
IO0
0
1oc
I 12
1
I 08
E , VOLTS
I 0.4
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Figure 2. Current-potential curves at a gold electrode in 0.2M H2S04, 2.36 X IO-% P d ( l l ) ; 2500 r p m , potential scan rate 100 mV/sec
Quantity of palladium deposited Curve 1 ) 30 p C , Curve 2) 59 p C , Curve 3) 100 pC, Curve 4 ) 137 pC cm2, respectively. All potentials are reported us. the saturated calomel electrode (SCEj. Chemicals and Solutions. All solutions were prepared using triply distilled water. T h e 0.2M sulfuric acid supporting electrolyte was prepared from Fisher reagent grade sulfuric acid. Palladium does not dissolve in cold concentrated sulfuric acid. Therefore, a stock solution of 1.18 X lO-3M Pd(I1) was prepared by anodizing a 1.2 cm2 palladium electrode a t a constant current of 3.00 mA in concentrated sulfuric acid. T h e concentration of the Pd(I1) solution was calculated from the change in weight of the palladium electrode. Repetitive experiments showed that this process produced Pd(I1) a t 98.3 f 0.5% current efficiency. All solutions were deoxygenated by passing nitrogen through and over t h e solution. Electrode Pretreatment. In order to obtain reproducible current-potential curves in the supporting electrolyte, pretreatment 588
Figure 3. Current-potential curves at a gold electrode in 0 . 2 M H 2 S 0 4 , 2.36 X 10-5M P d ( l l )
ANALYTICAL CHEMISTRY, VOL. 46, NO. 4, APRIL 1974
of the gold electrode was required. First, the electrode was polished with Buehler 0.05-p gamma micropolish. The electrode was then introduced into solution and oxidized at +1.6 V for 10 minutes, followed by reduction at -0.4 V for 5 minutes. The electrode was then scanned repeatedly between -0.3 V and +1.6 V until a reproducible current-potential curve was obtained. A similar pretreatment sequence was used for the palladium electrode. Palladium Plating and Oxidation. Linear plots of w1 *, the square root of the rotation speed us. current were obtained a t -0.1 and -0.2 V in a 2.36 X 10-5M Pd(II), 0.2M H2S04 solution. T h e convective-diffusion controlled limiting current for t h e reduction of Pd(I1) a t 2500 rpm was 13.8 p A a t the rotating gold disk electrode. It was observed t h a t the current decreased slowly with time (-3%/min a t 2500 rpm) but returned to its original value if the electrode was oxidized at +1.3 V. For this reason, all plating experiments were limited to a maximum of 1 minute. In t h e experiments described below, palladium was deposited from solutions of various Pd(1Ij concentration a t 2500 rpm and -0.10 V. At the end of the plating time, the electrode potential was scanned a t 100 mV/sec to more positive potentials. In the experiments which required integration of the current-potential curves, the electrode potential was stepped from -0.10 V to +0.40 V at the end of the plating time, and then scanned at 100 mVjsec to more positive potentials. The resulting current-potential curve was integrated in the potential region +0.4 V 5 E 5 +1.0 V. In this region, the current is due solely to the residual current and the oxidation of palladium. In calculating the current due to palladium oxidation, it was assumed that the deposition of small quantities of palladium on gold does not change the residual current.
RESULTS AND DISCUSSION Current-Potential Curves of Palladium on Gold. Current-potential curves for bulk palladium and bulk gold electrodes are shown in Figure 1. This figure is presented to facilitate the comparison of the electrochemical processes occurring on the pure metals to those which occur on a gold electrode partially covered by palladium (Figures 2 and 3). Kote that the reduction peak associated with oxidized gold, ED = +0.82 V, is well separated from the reduction peak associated with oxidized palladium, ED = +0.29 V. Also, the oxidation of palladium at E D 5 +1.0 V occurs in a potential region in which no gold oxidation occurs.
Various amounts of palladium, Q p d , were plated on a gold disk electrode from a solution of known concentration and the current-potential curves were obtained by cycling the electrode potential between 0.0 and +1.4 V. Typical current-potential curves a t low coverage, Q p d < 150 IC, are shown in Figure 2 . Both the oxidation and reduction of palladium occur at more positive potentials than the corresponding processes a t pure palladium electrodes (see Figure 1). The high palladium coverage current-potential curves in Figure 3 were obtained in a different manner from those of Figure 2. Five hundred p C of palladium were deposited on the gold electrode from a 2.36 X 10-5M Pd(I1) solution. Then the electrode potential was cycled continuously between 0.0 and +1.4 V. This process results in the dissolution of palladium ( 2 ) , thereby decreasing the quantity of deposited palladium on each cycle. Under these conditions, a reduction peak occurs at approximately the same potential, +0.27 V, as the reduction peak of an oxidized bulk palladium electrode, +0.29 V (see Figure 1). Palladium reduction also occurs a t more positive potentials. This suggests that both thin and thick deposits of palladium exist on the gold electrode under these conditions, and that the thick palladium deposits are electrochemically identical to pure palladium. The thin palladium deposits interact with the gold and, when oxidized, form an oxide layer which is more easily reduced than the oxide layer formed on the thick deposits. Isopotential Points. Isopotential points ( 5 ) in Figure 2 are labeled A-E. The residual curve for the gold electrode passed through these points. Therefore, bulk gold must be one of the two independent areas, AI, on the electrode surface giving rise to these points. The other area, Az, must consist of the thin layer deposits of palladium on the gold electrode. A t isopotential point A, (IP-A), and IP-B the faradaic processes are the oxidation of gold on A1 and the oxidation of palladium on Az. IP-C occurs at i = -0.0 and must be due to the sum of the faradaic and charging current densities on both electrode areas. IP-D and IP-E are caused by the reduction of gold at A1 and the reduction of palladium at Az. IP-E is not as sharp as the other isopotential points. The presence of these isopotential points is important since they indicate that the electrochemical behavior of the thin palladium deposits is independent of surface coverage a t the potential of the IP’s under these conditions. Therefore, they support the assumption that the average oxidation state of thin palladium deposits on gold at a given potential-i.e., +1.0 V-will be independent of surface coverage. Isopotential points were also observed under conditions where both thin and thick layers of palladium were deposited on the gold electrode (Figure 3). These IP’s occurred at the same potentials as those in Figure 2 and are therefore assumed to be due to the same processes. This is somewhat surprising, since it implies that the current density on thin and thick palladium deposits must be the same a t the potent,ial of the IP’s. The presence of IP-E varied markedly with different surface coverages and frequently was not observed. Deviation from IP-B was observed a t high coverage (Figure 3, curve 1) due to the oxidation of HzO on palladium. At even higher coverage (not shown), the curves did not pass through IP-A and IP-D. Average Oxidation State of Thin Palladium Layers. Various amounts of palladium, Qpd, were deposited on a gold disk electrode. Q p d was determined from the plating time and the Pd(I1) convective-diffusion controlled cur( 5 ) D. F. Untereker and ( 1972).
s.
Bruckensteln, Ana/. Chem., 44, 1009
i
I /
I50
50
Qpd,
250
PCOUI
Figure 4. Relationship between the quantity of palladium, Q p d ,
deposited and the charge, Q p d O ( x ) . required to oxidize the palladium at ED I 4-1.0 V during a potential cycle of the electrode. Potential scan rate 100 mV/sec; electrode rotation speed, 0 rpm rent. The charge required to oxidize the palladium a t E I +1.0 v, Q p d O ( x i , during a potential scan at 100 mV/sec was calculated. The results are presented graphically in Figure 4. Each point is the average of a t least three separate measurements. Each measurement was obtained in a fresh solution. The data are represented by QpdOlxl = I 165 1.10 QPd where 0 IQPd I150 pC or 0 IQi’dO
&. Thus, the average oxidation state of palladium at +1.0 V is 2.20 under these conditions. I t should be noted that this data includes the oxidation state ( + 2 ) of soluble palladium which forms during the potential scan. The deviation from linearity a t Q p d > 150 p c suggests that thicker, incompletely oxidized deposits are formed. These results can be used to quantitatively analyze for small quantities of palladium in 0.2M HzS04 solution, and have been successfully used (6) to study the corrosion of palladium electrodes. Determination of the Depth of Coverage. The depth of coverage was determined using the following argument. The deposition of palladium on gold inhibits the subsequent oxidation of the gold electrode (Figures 2 and 3). One palladium atom deposited on the gold electrode surface will inhibit the oxidation of one gold atom if it deposits in a Pd/Au ratio of 1 : l or 0.91 gold atom if it deposits in a close packed plane ( 7 ) .If the palladium is deposited at depths greater than one monolayer, then less inhibition of the gold oxide will be observed-i.e., less than 0.91 gold atom inhibited. Current-potential curves of a gold electrode partially covered by palladium, Q p d < 150 pC, were recorded. The surface area of the gold was calculated from the gold oxide reduction peak (8). The existence of IP’s shows that it is valid to treat the oxidation of the gold as being independent of the palladium coverage. The quantity of palladium deposited on the electrode was calculated using the results of the previous section. The quantity of palladium must be determined on the first positive scan after the reduction of the gold oxide in order to minimize losses due to soluble palladium formation. The oxidation of 0.92 f 0.05 gold atom was inhibited for every palladium atom deposited, indicating that monolayer formation does occur. Comparison t o P u r e Palladium. Three points must be considered in the comparison of the average oxidation (6) S. H . Cadle, J . Eiecfrochem. Soc.. In press, 1974. (7) B. J. Bowles, Nature (London). 212, 1456 (1966) (8) S. H . Cadle and S. Bruckenstein, A n a / . Chem., 44, 2225 (1972)
ANALYTICAL CHEMISTRY, VOL. 46, NO. 4, A P R I L 1974
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state of monolayer palladium on gold obtained from Figure 4 to literature values for the average oxidation state of bulk palladium. First, complete surface oxidation of bulk palladium electrodes a t a given potential takes approximately 10 minutes. The data in Figure 4 were obtained from current-potential curves recorded a t 100 mV/sec. Longer oxidation times of palladium on gold are not practical because of the significant dissolution of the monolayer palladium. Second, both oxidation and reduction of monolayer palladium occur at more positive potentials than the corresponding processes on bulk palladium (Figure 2). Third, dissolution of Pd(I1) occurs during the potential cycle. Experiments showed that approximately 10% of the palladium deposit was dissolved during a potential cycle between 0.0 V and $1.0 V a t 0 rpm. All of these considerations indicate that the average oxidation state of the monolayer palladium deposits on gold under these experimental conditions may be less than the average oxidation state of bulk palladium a t f 1 . 0 V. To verify the above result, an experiment based on the following reasoning was performed. The deposition of copper a t underpotential has been studied on platinum (9, 10) and gold (11, 12) electrodes. One monolayer of the metal is deposited a t underpotential with a ratio of approximately one atom of copper per atom of platinum or (9) S.H . Cadle and S Bruckenstein. Anal. Chem., 43, 1858 (1971). (lo) M . W. Breiter, Trans. FaradaySoc., 65,2197 (1969). ( 1 1 ) E. Schmidt, P. Beulter, and W. J. Lorenz. Ber. Bunsenges. Phys. Chem., 75,71 (1971). (12) W. J. Lorenz, I Moumtzes. and E. Schmidt, Electroanal. Chem., 33,121 (1971).
gold. A similar phenomenon is found for several other metals (13, 14). If copper plates a t underpotential on palladium, it would be reasonable to expect that one monolayer of copper will be deposited. The charge required for this process can be compared to the charge required to oxidize the palladium electrode a t various potentials. The potential at which the two charges are equal should correspond to the potential a t which a monolayer of oxygen has been adsorbed-Le., an average oxidation state of 2.0. Copper was deposited on a palladium electrode from a 2 x 10-5M Cu(II), 0.2M H2S04 solution. Underpotential deposition of Cu(0) was observed. The maximum quantity of Cu(0) which could be deposited a t underpotential was 290 pC at +0.04 V. Comparison of this value to the oxidation of the electrode indicated that monolayer oxygen formation occurred a t +1.05 V. This result is in reasonable agreement with the above data on submonolayer palladium deposits on gold, although it does indicate a lower oxidation state than expected. The results of this work support the data of Burshtein et al. (4) who found that the average oxidation state of palladium is 2.0 a t +1.2 V us. RHE in 1N HzS04. I t is suggested that Burshtein’s results be used to estimate the roughness factor of palladium electrodes. Received for review July 30, 1973. Accepted November 16, 1973. (13) S. H. Cadle and S. Bruckenstetn, J. Electrochem. S O C , 119, 1166 (1972). (14) E Schmidt and N. Weithuck, J. Electroanal. Chem., 40, 400 (1972).
New Methods for the Preparation of Perchlorate Ion-Selective Electrodes T. J. Rohm and G. G. Guilbault Department of Chemistry, Louisiana State University in New Orleans, New Orleans, La. 70722
The increased interest in ion-selective electrodes has led to the development of new sensor materials which show selectivity for a variety of anions and cations and new methods for the construction of electrodes from these materials. Recently, Davies, Moody, and Thomas incorporated a commercially available liquid ion exchanger in a poly(viny1 chloride) matrix to prepare a nitrate selective electrode ( I ) . Griffiths, Moody, and Thomas have prepared calciumselective electrodes by mixing a liquid ion exchanger which is sensitive to calcium with poly(viny1 chloride) ( 2 ) . A potassium-selective electrode was reported by Davies, Moody, Price, and Thomas based on the same principle ( 3 ) .Kneebone and Freiser coated a platinum wire with a nitrate-selective liquid ion exchange in a poly(methy1 methacrylate) and used the electrode to determine nitrogen oxides in ambient air ( 4 ) . Ansaldi and Epstein prepared a calcium-selective electrode by coating a graphite rod with a calcium exchanger in poly(viny1 chloride) ( 5 ) . Davies, G . J. Moody, and J. D . R. Thomas, Analyst (London), 97,87 (1972) (2) G. H Grtffiths, G . J. Moody, and J. D . R . Thomas, Analyst, (London), 97,420(1972). (3) J E. W. Davies. G . J. Moody. W. M . Price, and J. D . R. Thomas, Lab. Pract.. 22,20 (1973) (4) E. M Kneebone and H . Freiser, Anal. Chem.. 45,449 (1973). (5) A . Ansaldi and S. I Epstein, Anal. Chem., 45,595 (1973). ( 1 ) J . E. W.
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These innovations greatly reduce the cost of ion-selective electrodes and provide insight for the study of charge transport through the membrane. Furthermore, these “solid” electrodes are reported to have longer lifetimes than the liquid electrodes ( I ) . In this study, we have prepared perchlorate-selective electrodes by mixing a commercially available (Orion) exchanger for perchlorate in PVC and used the material to construct an electrode in which the membrane is used with a reference solution and internal reference electrode, and an electrode in which the exchanger is coated on a platinum wire. The performance of these electrodes is compared to the commercial perchlorate electrode. EXPERIMENTAL The commercial electrode was prepared according to the manufacturer’s manual (Orion Perchlorate Ion Activity Electrode-9281) (6). The PVC perchlorate material was prepared by mixing 360 mg (20 drops) of the commercial liquid ion exchanger with 170 mg of PVC (Breon 119) dissolved in 5 ml of THF. When mixed, the solution was poured into a glass ring (32-mm i.d.) resting on a glass plate. The ring was then covered with a piece of filter paper and a watch glass. After 24 hours, the glass ring and membrane were turned over t o permit the solvent to evaporate from the underside of the membrane. Circles 1 m m in thickness (6) Orion Research Instruction Manual 92-17/92-81, Cambridge, Mass. 02139.
