Estimation of Potassium in Mixtures with Lithium and Sodium by Nonaqueous Titration Iswori La1 Shresta and Mihir N a t h Das Physical Chemistry Laboratories, Jadacpur Unicersity, Calcutta-32, India
PIFER,WOLLISH,AND S C H ~ ~ A(1) LL found that when a mixture of potassium or ammonium acetate with sodium or lithium acetate is potentiometrically titrated in a n acetic acid-chloroform medium with perchloric acid in dioxane, two inflections are obtained, the first one corresponding to potassium. They also found that by this method potassium can be differentiated from many other metallic acetates. The present method for the determination of potassium in the presence of sodium and lithium is essentially based on this principle and consists in converting the alkali metal salts to their acetates and then titrating in acetic acid-chloroform with perchloric acid in dioxane.
KF + N a F tCa(0ACh
EXPERIMENTAL
Method. For mixtures of alkali halides (chloride, bromide, and iodide) the sample was treated with a slight excess of mercury(I1) acetate, whereby the anion was tied up with mercury as a n undissociated halide (2-4). The acetate formed was then potentiometrically titrated in acetic acidchloroform (1 :20) medium with perchloric acid in dioxane. Two inflections were obtained, corresponding t o potassium and sodium or lithium, respectively (Figure 1). The mixture of alkali fluorides was treated with calcium acetate, whereby the alkali acetates were formed; the fluoride was precipitated as calcium fluoride, with excess calcium acetate remaining unchanged. For mixtures of alkali sulfates, barium acetate was used to convert the alkali salts to acetates, the sulfate being precipitated as barium sulfate. In either case, titration in acetic acid-chloroform medium with perchloric acid in dioxane gives two inflections, the first one corresponding t o potassium (Figure 1). Procedure. A measured volume of the aqueous mixture was transferred to a 250-ml beaker and treated with a slight excess of an aqueous solution of the corresponding metallic acetate (Hg, Ca, or Ba). After evaporating the solution t o dryness on a water bath, 5 ml of glacial acetic acid was added. The solution was stirred well to dissolve the metallic acetates and then 100 ml of chloroform was added. The solution was titrated with perchloric acid, using a magnetic stirrer. For the chloride and bromide, the mercury halides which formed dissolved in acetic acid, but mercury iodide dissolved only on adding chloroform. In the case of sulfate and fluoride, the barium sulfate and calcium fluoride remained in suspension which, however, did not interfere with the titration. The titration was performed with a Cambridge p H meter with glass electrode and lithium chloride saturated calomel (sleeve type) electrode, using the pH scale, because steady readings were not obtained on the emf scale. The recorded pH values have no significance, but this is immaterial for titration purposes. Because even slight diffusion of KCI ( 1 ) C. W. Pifer. E. G. Wollish, and M. Schmall, ANAL.CHEM., 26,215 (1954). (2) C. W. Pifer and E. G. Wollish, Ibid., 24, 300 (1952). (3) Ibid., p. 519. (4) C. W. Pifer and E. G. Wollish, J . Am. Pharm. Assoc., Sei. Ed., 42, 509 (1953).
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ANALYTICAL CHEMISTRY
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Figure 1. Titration curves for mixtures of potassium with sodium and lithium from the calomel electrode interferes with the estimation of potassium, KCI was replaced by LiCI. Reagents. All chemicals and solvents used in the work were of analytical reagent grade (Merck). Mercury(I1) acetate was prepared by dissolving mercury(I1) oxide in glacial acetic acid and carefully evaporating the solution o n a water bath. Separate solutions (0.03-0.05N) of the individual alkali metal salts (chloride, bromide, iodide, fluoride, and sulfate) were prepared in redistilled water, Mixtures of the respective potassium and sodium (or lithium) salts were prepared by mixing aliquot portions of the individual solutions. Aqueous solutions ( E 0 . 0 2 M ) of the acetates of Hg, Ca, and Ba were used to convert the alkali salts to the acetates. Perchloric acid ( s 0 . 0 2 5 N ) in p-dioxane was used as the titrant, being potentiometrically standardized against potassium acid phthalate in glacial acetic acid medium. RESULTS
Typical titration curves are shown in Figure 1. F o r the chloride, bromide, and iodide, potassium is obtained from the first inflection and sodium or lithium from the second. For the fluoride and sulfate, however, the second inflection gives the sodium or lithium acetate plus the excess calcium or barium acetate. The second inflection is useless for analytical purposes, so that potassium alone in the mixture can be estimated and it is unnecessary to continue the titration beyond the first inflection. Analytical results indicate that potassium in a mixture with sodium or lithium can be estimated with a reasonable degree
of accuracy. For the chloride and iodide, the error was negative in all six annlyscs performed, the mean error being of the order of -22, whereas for the bromide the error was positive ( E -1- 0.774). For the estimation of the other constituent (sodium or lithium), however, the method is not very suitable; high results (even up to 7 x ) are sometimes obtained, because of the leakage of lithium chloride from the calomel electrode. As the amounts involved in the analyses are relatively small (0.1-0.2 mmole), even slight leakage tends to vitiate the results, so that only potassium can be accurately estimated even for chloride, bromide, and iodide. For the sulfate, potassium in the presence of sodium and lithium can be estimated with a mean error of - 2 x . F o r the fluoride, only two sets of analyses were performed, the error being - 1.S and $0.8 respectively. The method is not suitable for estimating larger amounts, because it would require inconveniently large volumes of solvent to
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keep the acetates in solution. The titration medium should contain acetic acid and chloroform in the ratio 1:20. A smaller proportion of chloroform impairs the sharpness of the first inflection. The method affords a rapid and reasonably accurate means for analyzing binary mixtures of potassium with sodium or lithium. After determining the total alkali metals by the usual methods--e.g., as sulfate or chloride-the potassium content may conveniently be determined by differential titration as described, the other constituent being obtained by difference.
RECEIVED for review January 30, 1967. Accepted May 10, 1967. Work supported by a fellowship awarded under the Colombo plan by the Ministry of Education, Government of India t o I. L. Shresta. Necessary facilities for him were provided by the Government of Nepal.
Concentration Gradients in Film and Spherical Mercury Electrodes under Variable-Flux Plating Conditions Sidney L. Phillips and Lawrence F. Karr International Business Mucliines Corp., Systetns Decelopment Division, Box 390, Poughkeepsie, N . Y . I2602
THETHEORY of plating amalgam-forming metals a t film and spherical mercury electrodes has been treated by many workers (1-5). Amalgamation is invariably carried out under constant flux conditions, either from a stirred solution as in the pre-electrolysis step in anodic stripping analysis (2), o r from a n unstirred solution as in chronopotentiometric plating (4). In both cases, the equations describing the concentration gradients existing within both film and spherical electrodes permit predictions to be made concerning the effects of such parameters as plating time, electrode radius, and diffusion coefficient. Thus, the effect of finite electrode size can be characterized reasonably well when metal deposition takes place under conditions of constant flux a t the electrode surface. O n the other hand, the case in which the flux of the amalgamforming metal ion is a function of time has only been considered in regard to the effect of curvature of a spherical electrode o n the magnitude of the plating current (6). Here, extension of the theoretical treatment to include a description of the concentration gradients within the electrode is desirable because important experiments such as cyclic stationary electrode polarography often involve formation of amalgams under variable-flux plating conditions. As in the stirred solution case, knowledge of the concentration gradients would then permit estimates to be made of the effect of finite electrode volume o n cyclic polarography, square-wave voltammetry, and alternating-currentpolarography. In the present work, the theory of amalgam formation is extended t o include time-dependent plating conditions. I n particular, emphasis is placed on the flux of the metal ion being (1) (2) (3) (4) (5) (6)
W. H. Reinmutli, ANAL.CHEW,33, 185 (1961). I. Shah and J. Leninson, Zhid.,p. 187. W. 7.DeVries, J . Elrctroariul. Cliern., 9, 448 (1965). S. P. Perone and A. Brumfield, Zbid.,13, 124 (1967). D. K. Roe and J. E. A. Toni, ANAL.CHEM., 37, 1503 (1965). W. G. Stevens and I. Shain, Zbid.,38, 865 (1966).
proportional t o t-1Iz: the case corresponding to chronoamperometric deposition. The calculated concentration gradients also describe the amalgam distribution during the cathodic portion of a cyclic experiment, so that an estimate of the effect of finite electrode volume on the anodic portion of this experiment can be made. The film electrode is considered first. CONCENTRATION-TIME RELATION
Planar Film Electrode. The initial-boundary value problem for the film electrode may be formulated in terms of the usual Fick's law equation for diffusion between two parallel planes :
Here, CR is the concentration of amalgam-forming substance within the film; t i s the time; DE is the diffusion coeficient; and x is the linear distance within the electrode. F o r the planar mercury film electrode, the bounds o n x are the solidmercury interface defined by x = 0, and the mercury-solution interface a t x = 8. The electrode then has a finite film thickness, P. Initially, we assume the mercury film is free of amalgamforming substances so that t = 0,0
