The Mercury Coated Tubular Platinum Electrode T. 0. Oesterlingl and Carter L. Olson College of Pharmacy, The Ohio State University, Coluilabus, Ohio Preparation of a mercury film coated tubular platinum electrode to be used for analysis in flowing streams is reported along with the voltage range over which the electrode may be used in a variety of supporting electrolytes. The mercury film is suitably stable and can be used for the deterrnlnatlon of several rnstal ions with good sensitivity. Diffusion coefficients calculated from experimental results are in good agreement with reported literature values.
DEVELOPMENT of a tubular platinum electrode for use in flowing streams has been recently reported by Blaedel et ~ l . (I). The electrode has high sensitivity and appears to be well suited for use as an analytical sensing device; however, its application is restricted to substances axidiaable or reducible at a platinum surface. It was thought that the use of the electrode would be greatly increased if it could be coated with a mercury film taking advantage of the greater hydrogen overvoltage obtained on a mercury surface. Several procedures have been used ta place a mercury film on a platinum surface. Two common procedures are electrochemical deposition from mercury ion containing solutions (2-5) and direct physical contact with mercury (6-8). In this work a modification of the procedure by Ramaley, Brubaker, and Enke (7) similar to that used by Blaedel and Laessig (8) was chosen. The physical and electrochemical properties of a mercuryfilm electrode depend on coating conditions and electrode history. This study is concerned with developing a reproducible method for preparing a mercury coated tubular platinum electrode (MTPE), defining the useful voltage ranges over which the electrode may be used, and evaluating the stability of the electrode for electroanalytical measurements. EXPERIMENTAL Instrumentation. A polarograph similar to that described by Enke and Baxter (9) was used throughout the study. Philbrick chopper stabilized Model K2-XA amplifiers, Model SK2-B booster amplifier, and a model It-100B power supply were used in place of the Heath operational amplifier system. Recordings of current measurement were made on a Varian Model G-14 strip chart recorder. Solutions were pumped through the electrode by means of a Harvard Apparatus Model 600-1200 peristaltic pump. Tygon formulation B44-3 tubing (1Is2-inch i.d. by l/as-inch wall thickness) was used in the pump. The pump head was enclosed with a Plexiglas cover under which a positive 1
Present address, The Upjohn
q., Kalamazoo, Mich.
(1) W. J. Blaedel, C. L. Olson, and L. R.Sharma, ANAL.CHEM., 35,2100 (1963). ( 2 ) K . W. Gardiner and L. B. Rogers, Zbid.,25, 1393 (1953). (3) T. L. Marple and L. B. Rogers, Zbid.,25,1351 (1953). (4) S. Bruckenstein and T. Nagai. Ibid.,33, 1201 (1961). (5) F. L. Marsh and S. Bruckenstein, Zbid.,38, 1498 (1966). (6) S. A. Moros, Ibid.,34, 1584(1962). (7)L. Ramaley, R. L., Brubaker, and C.G. Enke, Zbid.,35, 1088 (1963). (8) W. J. Blaedel and R. H. Laessig, Zbid.,37, 333 (1965). (9) C. G . Enke and R. A. Baxter, J . Chem. Educ., 41,202 (1964).
SALT BR I D G E\
I
THERMOMETER
d
I
/
,
I \I I
?
-4
I
SOLUTION I N
Figure 1. Electrolysis cell pressure nitrogen atmosphere was maintained to prevent oxygen from penetrating the flexible tubing in the pump. All flow lines connecting the pump to stock solution and electrode were made from glass capillary tubing. Flow rates were measured by determining the time required to collect 10 ml of solution in a precalibrated 10-ml graduated cylinder. After fresh tubing had been placed in the pumping channels, the flow rates decreased a few per cent but after about 15 hours' use, they remained constant to within =t0.2%, Electrodes and Cell. Tubular platinum electrodes were constructed by sealing a piece of seamless platinum tubing (purchased from Engelhard Industries) 0.060-inch i.d. by 0.500 inch long, wall thickness 0.010 inch, into soft glass tubing. Care was taken to ensure a smooth transition of the solution from the glass to the platinum to prevent turbulence and ensure a parabolic velocity profile as the solution enters the electrode. The electrolysis cell, illustrated in Figure 1 , was constructed from a piece of 10-mm i d . glass tubing with a side arm connected to an aspirator through which waste solution flows. A saturated KC1-agar salt bridge from the saturated calomel reference electrode dips into the cell from above. A rubber septum placed on the bottom of the cell serves as a strain and shock resistance mount for the tubular platinum electrode. The electrode is mounted in an upright configuration to permit easy clearance of any gas bubbles which may enter or be evolved in the flow stream. The bottom end of the tubular platinum electrode is connected to the glass flow line by a short piece of Tygon tubing to provide a strain-free connection. VOL. 39, NO. 13, NOVEMBER 1967
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Table I. Mercury Film Data of Electrodes Volume Electrode Film Film of Hg, Hours used cc X lo4 thickness, p number number 1 2 3 1 2 3 4 5 7 8 9 IO 11 12 13 14 15 16 1 1 2 3 4 1 2 3 4 5 6 7 8 9 10
1
1 1 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 3 4 4 4 4 6 6 6 6 6 6 6 6 6 6 a
a 0
30 a
90 40 16 18 45 4 2 2 30 1 2 7 1 6 4
3 3 3 3 10 10 3 35 10 50 70 35 3 10
2.5 3.3 5.9 3.7 4.8 2.5 4.4 5.1 2.6 6.2 3.7 5.1 6.6 6.2 5.1 5.1 5.9 5.5 7.0 5.2 8.4 4.8 7.3 2.9 5.9 6.6 3.3 7.3 8.4 4.4 3.7 2.5 4.8
4.2 5.5 9.7 6.0 9.8 4.2 7.2 8.5 4.2 10.3 6.0 8.5 10.9 10.3 8.5 8.5 9.7 9.0 11.5 8.5 13.9 9.8 12.1 4.8 9.6 10.9 5.4 12.1 13.9 7.2 6.1 4.2 7.8
Data not recorded.
The procedure used to coat the electrode with mercury was similar to that used by Blaedel and Laessig (8). The electrode was cleaned with hot concentrated nitric acid, rinsed with distilled water, and placed into the cell assembly shown in Figure 1. One molar perchloric acid was passed slowly through the electrode at approximately 1 ml/min while a potential of -3 volt US. a platinum coil electrode was applied. The electrode was maintained under these conditions of cathodic hydrogen evolution for 15 minutes. Then about 30 ml of triple-distilled mercury was introduced into the flow system and pumped until it was in contact with the inside of the platinum tube. After 5 minutes of steady contact, the mercury was pumped forward and backward through the electrode for 5 cycles and then out of the flow system. The efficiency of the coating process was tested by running a current-potential curve while pumping a solution of oxygen free 0.1M K N 0 3 through the electrode at 3 ml/min. If the current remained less than 0.5 pA at a scan rate of 500 mV/ min in the potential range 0 to - 1 volt us. SCE, the coating was considered to be adequate. The weight of mercury present on the electrode was determined by the following procedure. The contents of the electrode were dissolved in 10 ml of hot 10N nitric acid. After cooling, this solution of mercuric nitrate was diluted to 100 ml with double-distilled deionized water. Five milliliters of this solution was then analyzed by a dithizone extraction assay (10). Reagents. All chemicals were reagent grade and were used without further purification. Solutions were prepared (10) “Treatise on Analytical Chemistry,” I. M. Kolthoff and P. J. Elving, Eds., Part 11, Vol. 3, Interscience, New York, 1961, p. 315.
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ANALYTICAL CHEMISTRY
/
G
T 0
-0,2
-0.4
-0.6
F -0.8
-1.0
-1.2
L
-1.4
VOLTS vs. SCE
Figure 2. Current-potential curves of background electrolytes 0.1MH2SOd 0.1M HC104 C. 0.1MHCl D. 0.1M HAC, 0.1M NaAc E. 0.1MKN08 F. 0.1MKCI G. 0.1MNaAc H. 0.1M Na2B40 A. B.
I.
0.1144&PO4
using double-distilled deionized water. Stock solutions of bismuth trinitrate, cadmium chloride, and copper sulfate were analyzed by complexometric titration with EDTA using xylenol orange as indicator (11). Hexamminocobaltic chloride stock solutions were analyzed by amperometric titration with potassium ferrocyanide (12). Thallium sulfate stock solutions were analyzed by titration with potassium iodate (13). Other stock solutions were prepared by weighing out appropriate quantities but were not further analyzed. RESULTS AND DISCUSSION
Studies were made to determine how reproducibly a quantity of mercury was placed on the electrode using the described coating procedure, whether the amount of mercury on the electrode could be correlated with time of usage, and if residual currents and diffusion currents were significantly affected by the quantity of the mercury coating. Data for 33 mercury films (considered adequate by residual current test) recovered from five different electrodes are presented in Table I. The weight of mercury determined by the dithizone assay divided by the density of mercury gives the volume of mercury on the electrode. The film thickness was then calculated by dividing the volume of mercury by the surface area of the electrode. It was assumed that the mercury coverage was uniform, which may not be true (I4,15) but no convenient way was found for testing it. Table I shows that a sizable variation in the calculated film thickness was obtained by the coating process. No correlation between the amount of mercury in the film and the period of time over which it had been used could be observed. In addition, the diffusion limited currents obtained for the re(11) J. Korbel and R. Pribil, Chemist-Analyst, 45, 102 (1956). (12) H. A. Laitinen and L. W. Burdette, ANAL.CHE~VI., 23, 1265 (1951). (13) I. M. Kolthoff and R. Belcher, “Volumetric Analysis,” Vol. 3, Interscience, New York, p. 449. (14) W. R. Matson, D. K. Roe, and D. E. Carritt, ANAL.CHEM., 37, 1594 (1965). (15) S. P. Perone and K. K. Davenport, J. Elecfroanal. Chem., 12,269 (1966).
Pb (II)
*Or
li
4
”
0.05
0.25
-0.15
Table 11. Calculated and Literature Diffusion Coefficients D,sq cm/second X Depolarizer Background Experimental a b Bi(I1I) 0 . 5 M HzS04 0.51 0.01 .,, 0.56 Cd(I1) 0.1MKNOa 0.68 i 0.01 0.69 0.85 Co(NHs)s(III) O . ~ M K N O I 0.80 i 0.03 ... 0.82 Cu(I1) O.lMKNO3 0.68 zk 0.02 .. . 0.79 0.85 zk 0.01 0.83 ... Pb(I1) O.IMKN03 Pb(I1) ~MKNOI 0.75 C O . 0 1 0.80 0.91 1.80 A 0.04 1.82 1.98 TKI) O.lMKNO3 1.65 ... Tl(I) 1MKNOa 1.61 zt 0.04 a Values determined by Cottrell method (17). b Values calculated by the Ilkovic equation (18).
/-
-0.35
Table 111. Concentration Study of Several Depolarizers at MTPE
-0.55 -0.75
E,VOLT vs. SCE Figure 3, Current-potential curve of 5 X 10-5M Cu(I1) and 5 X 10-5iMPb(II)in 0.1MKN03
Scan rate 100 mVimin duction of several metal ions were independent of the variations of film thickness. The best test for adequate mercury coverage on the electrode was a residual current curve. The electrodes behaved properly on metal ion reductions if the residual current for a 0.1M K N 0 3 solution fiowing at 3 ml per minute remained less than 0.5 p A out to - 1.0 volt cs. SCE. The usefulvoltage region over which the mercury film electrode can be used in several supporting electrolytes is best shown by means of the residual current-voltage curves in Figure 2. The useful cathodic voltage limits at the MTPE are greater than those at a bare platinum electrode, but not as great as at a mercury pool electrode. The need for very careful deaeration of solutions and prevention of oxygen being absorbed through the short length of Tygon tubing in the peristaltic pump is demonstrated by the following results. A typical residual current of 2 p A for carefully deaerated solutions was observed when the pump tubing was exposed to air. By introduction of nitrogen under the pump cover, the current dropped to 0.5 p A indicating that enough oxygen was absorbed during the brief five-second passage through the tubing to yield 1.5 p A of current. The equation for limiting current at a tubular electrode has been derived by Blaedel and Klatt ( I @ , ii
=
5.31 X 1 0 5 ~ C 3 D 2 ! 3 X 2 ’ 3 V ~ 1 ~ 3
(1)
where it is in p A when X the length of the electrode is in cm, V I the volume fiow rate is in cc/second, C” the bulk concentration is in mole/cc, and D the diffusion coefficient is in sq cm/second. This equation holds only for fully developed laminar flow in the axial direction and developing diffusional flow in the radial direction. Under the experimental conditions used, a smooth transition from the glass surface to the electrode surface is maintained, preventing turbulence and satisfying both above conditions. Several depolarizers were studied to determine the suitability of the MTPE for the reduction of metal ions. Current(16) W. J. Rlaedel and L. N. Klatt, ANAL.CHEM., 38,879 (1966).
Depolarizer and background Pb(I1) in 0.1M KNOB
Concentration, M 10-4 10-5 10-6
Cd(I1) in 0.1.W KNO 3
10-4 10-6 10-6 10-4 10-
Bi(II1) in 0.5144 &So4
Co(”3)6 (111) in 0.1MKNOa 3
TI(I) in 0.1M KN03
x
iilc”,
amp. cclmole 194 195 192 159 159
10-6
155 200 205 202
10-4 10-6 10-6
90 93 92
10-4 10-6
144
10-6
140
147
-
Table IV.
Analysis of Cu(IItPb(I1) Mixture in 0.1M KNOs iz/C”,amp. cc/mole Mixture Cu(S1) Pb(SI) 5 x ~O-~MCU(II) 166 188 5 x 10-6MPb(II) 5 x 10-6MCu(II) 168 184 1 x lO-jMPb(I1) 1 x 1 0 - 5 ~CU(II) 168 190 5 X lO-6iMPb(II)
voltage curves were run for each depolarizer to establish the potential region where the current is diffusion-controlled, The following studies were made to determine if the electrode was behaving as expected and would be useful for analytical purposes when operated in the diffusion-limited current region, Experimental values for the diffusion coefficients of the depolarizers used were calculated using Equation l . These are reported in Table I1 along with appropriate literature values obtained by other electrochemical methods. As can be seen there is good agreement between the values obtained at the MTPE and the values obtained by the Cottrell method (I?, whereas higher values for D were obtained by means of the Ilkovic equation using dropping mercury electrode data (18).
(17) M. von Stackelberg, M. Pilgram, and V. Toome, Z . Electrochem., 57, 342 (1953). (18) L. Meites, “Polarographic Techniques,” Interscience, New York, 1952. VOL. 39, NO. 13, NOVEMBER 1967
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The results of concentration studies run on several depolarizers are presented in Table 111. Current values at 10-4 and molar concentration were obtained from currentvoltage curves where the value of diffusion-limited current minus extrapolated residual current was used to calculate The values of il;, for 10-6M concentrations were obtained at a constant potential in the diffusion plateau region by determining the difference in current between the supporting electrolyte and 10-6M solution as they were alternately passed through the electrode. The relationship between current and concentration was found to be linear over the 100fold concentration range studied. Assays were run on mixtures of Pb+2 and Cu+2 to demonstrate that the electrode could be used to analyze mixtures of ions. Figure 3 shows a typical current-voltage curve obtained for a mixture of PbU2and CuL2. Table IV shows the
analytical results for different ratios of Pb+2to CutZ concentrations. The theoretical value of i l l c for Pb+2 using the diffusion coefficient reported by von Stackelberg et al. (17) was calculated to be 186 which is in agreement with the experimental results demonstrating that the earlier Cu+2 wave had no effect on the Pb+2diffusion current. It is concluded that a tubular platinum electrode can be conveniently coated with a film of mercury, thereby extending the useful cathodic voltage range, and that diffusion limited currents are independent of the quantity of mercury in the coating.
RECEIVED for review June 6, 1967. Accepted August 24, 1967. Thomas 0. Oesterling was a fellow of the American Foundation for Pharmaceutical Education.
Chronoamperometry at Tubular Mercury-Film Electrodes T,0.Oesterling’ and Carter L. Olson CoUege of Pharmacy, The Ohio State Unioersity, Columbus, Ohio The current-time equation for quiet solution electrolysis at a tubular electrode which is infinitely long or has closed ends is derived. At electrolysis times less than one second, currents calculated with the tubular electrode equation agree closely with currents calculated by the Cottrell equation. Currents of experimental chronoamperograms, collected at openended mercury-film tubular platinum electrodes (MPTE) of finite length, agree reasonably well with values calculated using both the tubular electrode and Cottrell equations for electrolysis times less than one second. However, at larger times, experimental currents resulting from the quiet solution electrolysis of Pb(ll), Cd(ll), TI(I), Bi(lll), Cu(ll), Co(NHs)~(lll), and KaFe(CN)6are approximately 10% higher than currents calculated with the tubular electrode equation because of increased mass transfer caused by end diffusion and density gradients.
CURRENT-TIME EQUATIONS have been derived and tested for quiet solution electrolyses at planar ( I , 2), cylindrical (2, 3), and spherical (2, 3) electrodes. In each case experimental data agreed with theoretical equations only during very short time intervals or under precisely controlled experimental conditions. In spite of these limitations many useful applications of current-time measurements at electrodes of various geometries have been utilized (4). The current-time equation for quite solution electrolysis at tubular mercury-film electrodes (MTPE) which are infinitely long or whose contents are physically isolated is presented in this article. Currents of experimental chronoamperograms of ion-ion and ion-amalgam reductions obtained at open1 Present address, Pharmacy Research Unit, The Upjohn Co., Kalamazoo, Mich.
(1) F. G. Cottrell, 2.Physik. Chem., 42, 385 (1902). (2) H. A. Laitinen and I. M. Kolthoff, J . Am. Chem. SOC.,61, 3344 (1939). (3) P. Delahay, “New Instrumental Methods in Electrochemistry,” Chap, 3 , Interscience, New York, 1954. (4) L. B. Anderson and C . M. Reilley, J . Chem. Educ., 44, 9 (1967).
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ended tubular mercury-film electrodes of finite length were compared with the derived expression, and the influence of end diffusion and convection was studied. THEORY
Derivation of Current-Time Equation. The differential equation for radial diffusion of an electroactive substance in an infinitely long tubular electrode containing a quiet solution is given by Barrer (5) and Jost (6),
dC(r, t) d2C(r, t ) =Dat ar2
1 X ( r , t)
+;dr
where C(r,f)is the concentration of electroactive substance at a distance r from the axis of the cylinder at time t, and D is the diffusion coefficient of the electroactive substance, To solve Equation 1, the following initial and boundary conditions are imposed :
C(r, f) = C”
C(r,t)=O
t =0 t > O
r
5R
r = R
(2)
(3)
C” is the bulk concentration of electroactive substance and R is the radius of the tube. Equation 2 is obeyed by maintaining a uniform concentration of electroactive substance in the electrode before electrolysis is begun. Condition 3 is attained by applying a potential well out in the limiting current region of the electroactive substance to the electrode so that the charge transfer process is very fast and electrolysis is limited by diffusion of electroactive species to the inside surface of the tube. The solution to Equation 1 for conditions 2 and 3 is given by Barrer (5) and Jost (6),
(5) R. M. Barrer, “Diffusion In and Through Solids,” Macmillan, New York, 1941, p. 34. (6) W. Jost, “Diffusion In Solids, Liquids, Gases,” Academic Press, New York, 1960, p. 52.
