rrninatisn of Faradaic and e at the Dropping George Lauer and R. A. Osteryoung North American Aviation Science Center, Thousand Oaks, Calif. 91360
THESEPARATION of double layer charging current and faradaic current at a dropping mercury electrode is of prime interest both in analysis and for studies of the electrical double layer. Butler and Meehan (1, 2) have reported some success in separation of capacitative and faradaic currents by analysis of the experimental i cs. t data obtained on a single drop. This paper presents a method which is suitable for separation of the currents, determination of the integral capacity curve, and trace determinations.
1 0.06
1
O / 12’3
vs I
”‘
IpF C d (11) 0.1 F Sulfate E:580 mV
THEORY
The current passed during the growth of an ideal, spherical mercury drop potentiostated at a potential where the reduction of any electroactive species is diffusion controlled is given by
where i(t) is the net current passing through the external circuit, qo is the double layer charge per unit area at E, and the other symbols have their usual meaning. Assuming a spherical drop, the area of the drop is given by A(t) = 0.8515 m2/3t2’3
(2)
and substituting this into Equation 1 and integrating Q(l) =
+
0.8515 n22/3t2/3qo 6.07 nm213D1/2Cot7/6 (3)
Dividing Equation 3 through by t Z l 3we obtain
Therefore, a plot of Q(t)/t2/3us. t 1 / 2will have an intercept of0.8515 7n2/3q0and a slope proportional to the concentration of electroactive material present in the solution. In essence, this assumes that the electrode reaction is polarographically reversible. Although current integration has been utilized to a limited extent to determine surface charge density (3, 4 ) , the separation proposed in this work has not been demonstrated, nor has the technique been applied with modern electrochemical apparatus. EXPERIMENTAL
All solutions were made up with triply distilled water. Chemicals were reagent grade and were used without further purification. The solutions were deaerated with prepurified argon which was scrubbed through V(I1) solutions to remove residual oxygen. The data were acquired using a RIDL 400 channel analyzer. The essential details of the experimental arrangement have ~
(1) J. N. Butler and M, L. Meehan, J . Phys. Chem., 70, 3582 (1966). (2) Ibid., 69,4051 (1965). (3) D. C . Grahame, Chem. Ren., 41,441 (1947). (4) J. S . L. Philpot,Phil. Mag., 13,175 (1932). 0
ANALYTICAL CHEMISTRY
,
I
been described previously ( 5 ) . Data were then obtained starting at a time just prior to drop fall and continuing through a complete drop life. The data acquired were then visually inspected and the t = 0 time was determined from the inflection of the q us. t curve. The data were punched on paper tape and then fitted to the equation on a digital computer using a standard nonweighted least square regression method. The best fit was obtained using data taken during the last half of the drop life. The deviations at short times indicate that the drop has not assumed spherical shape. The data were thus fitted to 120 to 180 points taken at the end of the drop life. RESULTS AND DISCUSSION
A typicalplot of Q j t 2 1 3 vs. t*l2in lpMCd(I1) in 0.1FNa2S04 al. -0.580 V us. SCE is shown in Figure 1. The solid line is the least square plot, the points are the actual data points. The fit is quite reasonable. As Cd(I1) is polarographically reversible the synthesis of a polarographic wave from the slopes of such plots, with simultaneous determination of qo at each potential, is possible. A composite curve is shown in Figure 2. The average faradaic current (curve A ) was calculated from the slope, assuming a 4-second drop time. The open circles (curve B) are the charge per square centimeter on the electrode obtained by this procedure. Each point represents the mean of three runs made at each potential. The fact that curve A does not go through zero current is probably due to a trace of 0,in the system; since we are on the first 0 2 diffusion plateau, the current is still diffusion limited. The differential double layer capacity of the DME in the supporting electrolyte alone was determined by classical means (6). The Cdt us. E curve ( 5 ) 6.Lauer and R. A. Osteryoung, ANAL.CHEM., 38, 1137 (1966). (6) D. C. Grahame,J. Am. Chem. Sac., 71,2975 (1949).
E
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-6-
25-
20-
$ z
15-
S
io-
I
,
5c 0
-200
I
I
-400
-600
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l
I
l
-800
-200
I
-400 -600 M i l l ~ v o l l svs S.C.E.
I
[ Cd”]
-800.
Figure 2. 4 p F Cd(I1) 0.1F Na2S04 A . Average faradaic current over 4 seconds as a function of E.
B. Specific charge (pc/cm2)on the DME as obtained in same solu-
tion Specific charge on DME obtained by integration of ac determined differential double layer capacity in 0.1F Na2S04 was then integrated (by counting squares and by weight) from -0.200 V us. SCE to each potential (the constant of integration was chosen as the charge determined by the present technique at - 200 mV). The closed circles in Figure 2 show these data thus obtained from the polarographic integration method. The agreement is quite good and would be better if centered about the point of zero charge. A series of solutions varying the Cd(I1) concentration from lpM to 10pM was run. The “polarographic” curve as well as the qo us. E curve was obtained in the potential region -0.200 to -0.850 V 6s. SCE for each solution. Three runs
(pM)
Figure 3. Wave height of faradaic current as a function of concentration of Cd(I1) were made at each potential in each solution and the mean value was used. The wave height as function of concentration of Cd(I1) is shown in Figure 3. The method described appears to have considerable merit as a tool to determine both the specific charge and the point of zero charge us. the DME. It is somewhat useful for trace analysis. Although quite tedious to perform manually, the method is simple and straightforward with automatic equipment. The use of an on line computer (7) simplifies and enhances the utility and accuracy of the method. RECEIVED for review July 24, 1967. Accepted October 2, 1967.
(7) G. Lauer and R. A. Osteryoung, Division of Analytical Chemistry, 154th National Meeting, ACS, Chicago, Ill., September 1967.
Separation of Gaseous Organic Fluoronitrogens by Liquid Column Chromatography R. L. Rebertus, K. R. Fiedler, and G . W. Kottong Central Research Laboratories, Minnesota Mining and Manufacturing Co., S t . Paul, Minn. 55101 ORGANICFLUORONITROGENS are frequently synthesized as complex mixtures. For example, the direct fluorination of guanylurea (1) produces unsaturated compounds containing the N,N,N’-trifluoroamidino moiety together with saturated perfluoroamines containing up to three difluoroamino groups bonded to the same carbon atom. Gas chromatography has been used most extensively as a method for separating these compounds (1, 2). The main limitation of this technique is the risk of explosion involved in trapping relatively large quantities of the neat fluoronitrogens. In this paper we describe the liquid column chromatographic separation of some N,N,N’-trifluoroamidinesand saturated perfluoroamines. The explosion hazard is reduced through the use of inert solvents, and separations have been carried out on both micro and macro scales. Qualitative tests which (1) R. J. Koshar, 4th International Symposium on Fluorine Chem-
istry, Estes Park, Colo., July 1967. (2) R. J. Koshar, D. R. Husted, and R. A. Meiklejohn, J. Org. Chem. 31,4232 (1966).
distinguish between the saturated perfluoroamines and the N,N,N’-trifluoroamidino compounds in the column effluent are also described. EXPERIMENTAL
Materials. Bis(difluoroamino)difluoromethane, tris(difluoroamino)fluoromethane, tetrafluoroformamidine, and pentafluoroguanidine were prepared by the methods described previously (1, 2). Solutions of the fluoronitrogens were prepared by condensing measured volumes of the gases into 3M brand inert fluorochemical liquid FC-75 (b. 216” C, d. 1.76) or heptane. Silica gel (100-200 mesh) was dried at 110” C and exposed to the atmosphere for a minimal period of time during the packing step. Procedure. Depending upon the sample size, safety equipment ranged from small l/a-inch laboratory shields to 14-inch concrete barricades with remote control facilities, and the columns ranged in size from 0.8 X 8 cm to 10 X 100 cm. The sample reservoir, chromatographic column, and receiver were cooled to about 0” C to avoid excessive loss of the gaseous fluoronitrogens. A 2 to 4% (w/w) solution of the VOL. 39, NO. 14, DECEMBER 1967
0
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