ACKNOWLEDGMENT
The authors are indebted to William H. Perry and Catherine Golden of the Air Quality Section, Division of Air Pollution, U. S. Public Health Service, and Helen A. Huitt of the National Center for Atmospheric Research for assistance in some phases of the experimental work. Additionally, the assisb ance and furnishing of dye samples and data by William J. Schafer of Fisher
Scientific Co., are gratefully acknowledged. LITERATURE CITED
(1) Emery, Jr., A. J., Statz, E., Stain Tech. 28,235 (1953). (2) Fisher Scientific Co., Mimeo Bull., PararosanilinHydrochloride, 2 pp., May, 1961. (3) Hormann, H., Grossman, W., Fries, G., Ann. 616,125 (1958). (4) Nauman, R. V., West, P. W., Tron, F., Gaeke, Jr., G. C., ANAL.CHEM.32,1307 (1960). ( 5 ) Schiff, H., Ann., 140,92 (1866).
(6) West, P. W., Gaeke, Jr., G. C.. ASAL. CHEM. 28,1816 (1955). JOHN B. PATE JAMES P. LODGE, JR.
ARTHUR F. WARDURQ National Center for Atmospheric Research Boulder, Colo. RECEIVED for review July 9, 1962. Accepted September 21, 1962. Presented at the 33rd Annual Meeting of the Colorado-Wyoming Academy of Science, May 4, 1962, Greeley, Colo.
Coulostatic Anodic Stripping with a Mercury Electrode SIR: The possibility of combining anodic stripping with a mercury electrode (Cooke, Kemula, Shain, and others) nith the recently developed coulostatic method was previously pointed out (2, 3). Basic equations are derived here and experimental conditions are discussed for the combination of these two methods. Mass transfer of the metal in mercury is supposed to be controlled by semi-infinite linear diffusion to simplify matters, and the essential features of the method are deduced. A hanging mercury drop is preferable in practice, but the mathematical treatment, though similar to that for the plane electrode, is more cumbersome. We consider two cases: plating of metal on mercury with mms transfer of the metal ion in solution controlled by semi-infinite linear diffusion; and plating a t constant current from a stirred solution. The metal concentration, C M , in mercury, in the first case, is at time T after the beginning of electrolysis and for the diffusion current range
AE = -
:Lt
-
Zdt
(3)
[(t
+
.)1/2
-
.I/,,!
The shift of potential is negative, since an anodic process is involved and is composed of two terms: a term in t U z corresponding to decay for a uniform concentration C M = CO (2, 3); and a T ) ~ / Z - T 1 ’ * ] resulting term in [(t from nonuniformity of C,u in mercury and causing IAEI to be smaller than the shift for a uniform concentration (Figure 1). The contribution of the latter term becomes smaller when T is increased, and the AE us. t l / * plot departs only slightly from linearity when T/ t> 10. The sensitivity of coulostatic anodic stripping, under these conditions, is practically the same as that of direct coulostatic analysis by deposition of the metal ion on mercury-i.e., concentrations in 10-6 to IO-’ mole per liter can
+
t
0
02
04
06
08
I
t % ( sec”21
ANALYTICAL CHEMISTRY
+ t ) sin-’ (St)] (5)
One easily verifies that AE = 0 for t = 0 and any T and that AE = 0 for T = 0 and any 1 (no metal deposition). Further, one deduces from Equation 5 that
1
1662
The shift of potential during coulostatic decay is for c independent of potential (cf. the first Equation 3)
H (7
(sec.1
Under coulostatic condit,ions, the potential is brought by a charge pulse from the cathodic diffusion current range to the anodic diffusion current range and the subsequent decay of potential is followed a t open circuit. By application of the method of images ( I ) , one obtains for the stripping diffusion current density
The shift of potential during coulostatic decay is for a double layer capacity independent of potential
conveniently be determined (8-4). Coulostatic anodic stripping, however, has the advantage that the decay of potential can be observed in a range of potentials in which oxygen is not reduced and mercury is hardly oxidized. No strenuous oxygen removal is therefore necessary, and time saving should be appreciable, since oxygen removal is the time-consuming operation in coulostatic analysis. We now consider the second method, in which the solution is stirred and the plating current is constant. The stripping diffusion current density is (6)
Figure 1 . Influence of plating time on coulostatic decay of potential for different times 7 (in seconds)
The same value of (dI3ldt)t-o iS deduced by calculating AE on the assumption that C M is uniform and equal to its value a t t = 0. The stripping diffusion current density is then given by the IlkoviE equation, as written for the plane electrode (6), and AE is directly obtained by integration as for the first Equation 3. Thus,
NOMENCLATURE
integral capacity of double layer per unit area (assumed to be independent of potential in interval aE) = concentration of metal in mercury C” = bulk concentration of metal ion in solution D = diffusion coefficient of metal ion in solution D,M = diffusion coefficient of metal ion in mercury F = faraday I , = limiting current density for metal deposition I , = diffusion current density for stripping n = number of electrons in electrode reaction = time since beginning of stripping t z = distance from electrode
c
Le., AE varies linearly with t112 when t 0. (This conclusion also follows immediately from Equation 6.) This conclusion holds approximately when T >> t. Since I , is proportional to the bulk concentration Co of metal ion in solution, AI3 for given t and 7 is proportional to CO. Sensitivity should be a t least as high as in conventional anodic stripping, but the coulostatic method has the advantage that the difficulty associated with double layer charging is eliminated. Experimental results and, possibly, extension of theory to spherical electrodes will be published. -f
=
L E = shift of potential 7 = duration of metal deposition LITERATURE CITED
(1) Carslaw, H. S.: Jaeger, J. C., “Conduction of Heat in Solids,” p. 230, Oxford University Press, London, 1947. (2) Delahay, P., ANAL.CHEM.34, 1267 (1962). ( 3 j Delahay, P., ilnal. Chim. Acta 27, 90 (1962). (4) Delahay, P., Ide, Y., Ax.41,. CHEnl. 34, 1580 (1962). (5) Mamantov, G., Papoff, P., Delahay, P., J . Am. Chem. Sac. 79, 4034 (1957).
PAUL DELAHAY Coates Chemical Laboratory Louisiana State University Baton Rouge 3, La. RECEIVEDfor review July 23, 1962. Accepted August 22,1962. Research supported by grant G-10006 from the National Science Foundation.
Preparation of a Standard Chromium(lll) Solution by Controlled -PotentiaI Coulometry SIR: A novel application of controlled-potential coulometry %‘as developed for the preparation and simultaneous standardization of chemically pure cationic species a t a specific valence. The technique, applied to the preparation of chromium(II1) solutions, is fast and simple and eliminates the use of a chemical species that must subsequently be removed a t the risk of reosidizing or contaminating the standard. Chromium(V1) solutions of known concentration have been prepared by constant current coulometry with a chromium anode (S), and chromium has been determined by reduction in acid solution with controlledpotential coulometry ($) ; however, the use of controlled-potential coulometry has not been reported for the simultaneous preparation and standardization of solutions. The method of chromium(II1) preparation involves the controlled-potential reduction of the dichromate ion a t a gold electrode with a coulometer similar to that described by Booman (1). Approximately 70 mg. of National Bureau of Standards potassium dichromate was dissolved in 10 ml. of 1M HzS04. This solution (1 ml.) was
added to 10 ml. of 131 previously reduced at 0.0 volt v?. a Hg/HgSO4 ( I f i f H2S0,) reference electrode. The dichromate solution was then reduced a t 0.0 volt until the electrolytic current reached background. The amount of chromium(II1) produced was calculated from the integrator voltage reading, The integrator was previously calibrated against precision electrical standards. The contents of the cell were then quantitatively transferred to a 50-ni1. volumetric flask and diluted t o volume. A spectrophotometric comparison of this standard agreed within 1% with a chromium(I11) standard prepared by the sulfite reduction of dichromate (4). The sulfite reduction required 2 hours; the coulometric reduction required only 15 minutes. Highly precise standards can be prepared by controlled-potential coulometry since the coulometer measures the quantity of electricity consumed in the primary process of oxidation or reduction in accordance with Faraday’s law. The coulometa is calibrated against electrical standards (precision but, in a practical sense, random errors of sample transfer and volumetric measurement limit the pre-
cision of the preparation to an estimated 0.1%. The only disadvantage is that the quantity of a standard that can be prepared is limited by the electrical capacity of the instrument,. The maximum integrator capacity of the coulometer used in this work is 18 coulombs. Work is now under way to extend this technique to the preparation of ruthenium standards. LITERATURE CITED
( I ) Booman, G. L., ANAL. CHEM. 29, 213-18 (1957). (2) Ludering, H., Arch. Eisenhuettenw. 29, 173-8 (1958). (3) Monnier, D., Zwahlen, P., Helv. Chim. B C ~ 39. U 1863-76 (1956). (4) Saltzman, B’. E., Ax&. CnEif. 24, 1016-20 (1952).
D. E. HARRINGTON R. C. PROPST R. D. BRITT,JR. E. I. du Pont de Kemours & Co. Savannah River Laboratory Aiken, S. C.
The information contained in this article was developed during the course of work under contract AT(07-2)-1 with the U. S. Atomic Energy Commission. VOL. 34,
NO. 12, NOVEMBER 1962
1663
