Amperometric Titration of Potassium with Sodium Tetra phenylbo rate WAYNE R. AMOS and R. F. SYMPSQN Department of Chemistry, Ohio University, Athens, Ohio ,An amperometric method can b e used for detecting the end point in the direct titration of potassium with a standard solution of sodium tetraphenylborate. The anodic depolarization current of tetraphenylborate a t a dropping mercury electrode is measured a t intervals throughout the titration. The method gives good results for the titration of potassium in the concentration range of 2.2 1 0-3 to 2.0 X 10-2M. Chloride interferes at concentrations of 0.32M or higher.
x
S
Kittig, Keicher, Ruckert, and Raff (21) reported that the tctraphenylborate ion quantitatively precipitates potassium ion in aqueous solution, numerous procedures for the determination of potassium based on the precipitation reaction have been described. No attempt is made to refer to all of these determinations in this article. Most of the volumetric methods published have been indirect methods of two general types. One technique is to precipitate potassium by the addition of excess sodium tetraphenglborate. The precipitate is collected and dissolved in an organic solvent. The dissolved tetraphenylborate is then determined by one of several titration procedures (2-6, 16, 18). The second method is to add an excess of standard tetraphenylborate solution to a solution of potassium ion. The precipitate is filtered off, and the excess tetraphenylborate in the filtrate is determined by titration with a second standard solution (8, 17, 15, 20). Four methods for the direct titration of potassium with standard tetraphenylborate have been described. Raff and Brotz (15) determined potassium by a conductometric titration using a standard lithium tetraphenylborate solution. In a modification of this method sodium tetraphenylborate was used as the titrant ( 7 ) . Lane (12) has reported a high frequency titration of potassium with tetraphenylborate. Conductometric titrations cannot be applied to solutions containing high concentrations of electrolytes not involved in the titration reaction. Kirsten, Berggren, and Xilsson (5) have reported the potentiometric titration INCE
of potassium nitrate with standard tetraphenylborate using a silver metal indicator electrode. This technique would, of course, require dropwise addition of the titrant in the vicinity of the end point. The potential break a t the end point did not appear to be as sharp as desirable. The purpose of this investigation was to develop another method for detecting the end point in the direct titration of potassium with sodium tetraphenylborate. This would eliminate certain additional steps in the analysis, such as filtrations and back-titrations. It was known that mercury(I1) forms a very insoluble precipitate with tetraphenylborate (10, 21). This suggested that the tetraphenylborate ion should give an anodic polarographic wave a t a dropping mercury electrode due to the depolarization of the dropping mercury electrode by the tetraphenylborate. If a potential, corresponding to some point on the anodic wave of tetraphenylborate, were applied to a dropping mercury electrode immersed in a potassium solution being titrated with tetraphenylborate, no significant anodic current would flow before the end point. As tetraphenylborate is added beyond the end point, the anodic current would increase in proportion to the increase in tetraphenylborate concentration. The end point would be located by extrapolation of the two straight lines to their point of intersection. Exploratory experiments showed that tetraphenylborate gives an anodic polarographic wave, and that amperometric titration curves of the type shown in Figure 1 could be obtained by measuring the anodic current a t constant potential throughout the titration. EXPERIMENTAL
Reagents and Apparatus. The potential chosen for the titration was applied, and the currents, were measured with a Sargent Model I11 manual polarograph. 4 conventional dropping mercury capillary was used with an external calomel reference electrode. The calomel electrode was bridged to the titration solution through a sodium chloride-agar plug. The calomel electrode was prepared as an ordinary saturated calomel electrode. The solu-
tion was saturated with sodium chloride instead of potassium chloride to prevent contamination of the titration solution with potassium from the reference electrode. All chemicals employed were reagent grade, used without further purification. Titrations were performed in a 150-ml. beaker, and the titrant was added from a 10-ml. buret. After each addition of titrant, the solutions mere stirred with a magnetic stirrer. Reduction of oxygen did not occur a t the potential applied, so exclusion of air was not necessary. Titrations were performed in an acetate buffer, 0.8iM in both acetic acid and sodium acetate. The sodium tetraphenylborate titrant was weakly alkaline (pH-10); and in the absence of an acidic buffer the anodic current increased appreciably before the equivalence point, making the location of the end point difficult. This current was presumed to be an oxidation of mercury in the presence of hydroxide to form mercuric oxide. Titrating into the acetate buffer prevented this, and the sodium acetate also served as the supporting electrolyte. Standard solutions of potassium chloride, potassium nitrate, and potassium sulfate were prepared by direct weighing of the previously dried, reagent grade salts. Preparation and Standardization of Sodium Tetraphenylborate Solutions. Stock solutions of sodium tetraphenylborate were prepared as either approximately 0.15 or 0.05Llf. The procedure of IIuraca et al. ( I S ) was used in preparing the solutions. The solutions were standardized gravimetrically by a procedure essentially the same as that of Cluley ( I ) , but with several modifications. To three or four 10-ml. portions of 0.1M potassium chloride, 3 drops of 0 . l N acetic acid were added. To one sample, 5 drops of bromocresol purple were added, followed by enough 0.1N sodium hydroxide t o give a distinctly purple color. This solution n-as discarded, and the same volume of sodium hydroxide was added to each of the other solutions. A known volume of sodium tetraphenglborate was added dropwise from a buret with constant stirring. The amount of sodium tetraphenylborate added was sufficient to give a precipitate weighing a t least 0.1400 gram, but was less than the stoichiometric amount of potassium. The precipitate was digested for an hour, filtered through a fine-porosity, sinteredglass crucible, and washed with 0.001M potassium nitrate. The precipitate was dried for 1 hour a t 120' C. and weighed. VOL. 31, NO. 1, JANUARY 1959
133
I
Table 1.
Titration of Potassium Nitrate in Presence of Chloride
0
0.04 0.08 0.24 0.27 0.29 0.32
K Found, Mg. 7.83 7.82 7.83 7.86 7.75 7.75
No end point
I
I
I
I
I
I
I
I
I
\%
24
[Mg. K + taken = 7.82 ( 4 . 4 X 10-8M)]
C1- Concn., M
I
26
Error,
%
22
0
20
f0.51 -0.90 -0.90
18
+o. 13 ,
Figure 1. Amperometric titration of potassium chloride with sodium tetraphenylborate
+O. 13
2 s
16
9.78 mg. of K f titrated with 0.04849M NaB(CeHs)r
k
From the weight of the precipitate obtained, the concentration of the tetraphenylborate was calculated. Titration Procedure. Ten to 30 ml. of a potassium solution was diluted to 45 ml. with the acetate buffer in a 150-ml. beaker. The samples contained between 4 and 36 mg. of potassium. A potential of f0.080 volt us. the reference electrode was applied to the dropping mercury electrode. The solution was titrated with standard tetraphenylborate solution using a 10-ml. buret. Current measurements were made at frequent intervals throughout the titration. The measured currents were corrected for dilution, and the corrected currents were plotted against milliliters of titrant. The end point was located by the usual extrapolation procedure. It was usually necessary to disregard some points in the immediate vicinity of the end point. At least five current measurements were made before the end point and 12 to 17 after the end point. After the end point, it was necessary to obtain sufficient current measurements to give a straight line before the current became greater than 28 pa, RESULTS AND DISCUSSION
To demonstrate the applicability of the method, a number of solutions containing known amounts of potassiumadded as the nitrate, chloride, and sulfate salts-were titrated. Figure 1 is a plot of a typical curve. For each salt, nine titrations were performed a t different concentrations within the range to 2.0 X 10-*M potasof 2.2 X sium ion. I n all but three of these 27 titrations, the relative error was less than 0.81%. Two of the three titrations, that deviated more than this, were titrations performed a t the lowest concentration of potassium to which the method can be applied, 2.2 X 10-3M. The results in these two titrations were high by 1.20 and 3.35%. In the other titrations, positive and negative errors were equally frequent. The types of substances that could interfere with this titration could be grouped into three classes. The first group would be composed of any ions which reduce a t the dropping mercury electrode a t the potential applied during the titration. The number of ions re134
ANALYTICAL CHEMISTRY
5 14 LT 50 12 0 8 IO z
e 6 4
2 0 IO
20 30 40 50 6.0 M L OF SODIUM TETRAPHENYLBORATE
ducible a t a potential of f0.080 volt us. the reference electrode used is limited, and they could probably be quantitatively reduced or removed prior to titration. In the second group would be cations which also precipitate with tetraphenylborate. With the exception of cesium and rubidium, separation of other interfering cations of this type from potassium would not be difficult. The third type of interference would be from ions that form stable precipitates or complexes with mercury(1) or mercury(I1). In high Concentrations these would give large anodic currents which would obscure the anodic current due to tetraphenylborate. The most common of these would be the halides (except fluoride), thiocyanate, and cyanide. All of these form highly insoluble mercury(1) salts. To determine the amount of chloride that could be tolerated, increasing amounts of sodium chloride were added to a series of solutions containing a constant amount of potassium nitrate dissolved in the acetate buffer. These solutions were titrated amperometrically with sodium tetraphenylborate. The results are given in Table 1. No interference of chloride occurred as long as the chloride concentration was 0.24M or less. At chloride concentrations of 0.27 and 0.29M results were low by 0.9% relative error. At 0.32M chloride an accurate end point could not be determined. Similar experiments were performed to determine a t what concentrations
70
bromide and iodide would interfere. It was found that only trace amounts of these ions could be present. With a bromide concentration of 5.5 X 10-4M the relative error in potassium analysis was - 1.7%. Consistent results could not be obtained a t a bromide concentration of 1.1 X 10-3M. With an iodide concentration of 1.1 X 1 O - 6 M J the error was -1.1%, and a t higher concentrations the error became increasingly negative. The method described gives good results in the titration of potassium in the concentration range of 2.2 X to 2.0 X lO-*M. The standard deviation for 31 titrations was 5.6 parts per thousand. This does not include two of the titrations performed at the lowest Concentration of potassiuni. The accuracy is apparently not so good a t this concentration as a t s!ightly higher concentrations. The last titration in Table I and the titrations performed in the presence of bromide and iodide were also excluded in calculating the reported standard deviation. T h e n the concentration of potassium is below 2.2 X 10-3iM, equilibrium in the precipitation reaction is reached very slowly. The time required to obtain a single current measurement becomes prohibitive. The solubility of potassium tetraphenylborate in such dilute potassium solutions is also significant. This causes a deviation from linearity of the points for some distance before and after the end point. The extrapolation becomes more arbitrary.
KO titrations were attempted a t concentrations of potassium above 0.020M. It is possible that the titration could be performed a t higher concentrations by this procedure, but a practical limit would soon be reached. For higher potassium concentrations, a more concentrated tetraphenylborate titrant would be needed. This would make it difficult to obtain a sufficient number of current readings beyond the end point, before the current became unreasonably high. It mas found that when the concentration of ewess tetraphenylborate was large enough to give currents in excess of 28 pa., the currents were erratic and were not linear with respect to the volume of titrant added. This irregularity is probably due to precipitate formation on the surface of the drop. Precipitate formation on the surface of the drop would inhibit the oxidation of more mercury. The current then would not be proportional to the concentration of tetcaphenylborate. This phenomenon has been observed by others (11,14). For the titration of more concentrated potassium solutions an alternative method of locating the end point should prove more satisfactory. It should be possible to add the titrant dropwise in the vicinity of the equivalence point.
The end point would correspond to the first sharp increase in current. The chief disadvantage to the method as it was performed in this work was the time factor. Between 2 and 3 hours were required for the complete titration. The authors' chief interest in this investigation was in establishing the principle of the method and determining the accuracy which could be obtained. No extensive attempts were made to speed up the titration. A long time was required, because 17 to 25 current measurements were made in each titration, and after each addition of titrant, time was allowed for the current to reach a final steady value. Making fewer current measurements and reading the current a t some fixed time after the addition of titrant instead of waiting for the final, steady value would reduce considerably the time required. This would cause some loss in accuracy. LITERATURE CITED
(1) Cluley, H. J., Analyst 80, 354 (1955). (2) Findeis, A. F., DeVries, T., ANAL. CHEW28, 1899 (1956). (3) Flaschka, H., Chemist Analyst 44, 60 ( 1955). (4) Flaschka, H., Holasek, A,, Amin, A. bI., 2. anal. Chem. 138,241 (1953).
(5) Flaschka, H., Sadek, F., Chemist Analyst 45, 20 (1956). (6) Hahn, F. L., 2. anal. Chem. 145, 97 (1955). ( 7 ) Jander, G., Anke, A., Ibid., 154, 8 (1957). (8) Kemula, W., Kornacki, J., Rocznicki Chem. 28, 635 (1954). (9) Kirsten, W. J., Berggren, A., Nilsson, K., ANAL.CHEM.30,237 (1958). (10) Kohler, M., 2. anal. Chem. 138, 9 (1953). (11) Kolthoff, I. W., Miller, C. S., J. Am. Chem. Soc. 63, 1408 (1941). (12) Lane, E. S., Analyst 82,406 (1957). (13) Muraca, R. F., Collier, H. E., Bonsack, J. P., Jacobs, E. S., Chemist Analyst 43, 102 (1954). (14) Nyman, c. J., Johnson, R. A., ANAL. CHEM.29,483 (1957). (15) Raff, P., Brotz, W., 2. anal. Chem. 133, 241 (1951). (16) Rudorff, W.,Zannier, H., Zbid., 137, 1 119.53). (17) Ibid., 140, 1 (1953). (18) Ibid., p. 241. (19) Schall, E. D., ANAL. CHEBI.29, 1044 (1957). (20) Schmidt, H. J., 2.unal. Chem. 157, 321 (1957). (21) Wittig, G., Keicher, G., Ruckert, A., Raff, P., Ann. Chenz. Liebigs 563, 110 (1949). \ - - - - I
RECEIVEDfor review May 15, 1958. Accepted .4ugust 6, 1958. Abstracted from a thesis presented t o the Graduate College of Ohio University by W.R. Amos in partial fulfillment of the requirements for the degree of master of science.
Some Factors in Ultraviolet Densitometry of Amino Acid Chromatograms SISTER HELENE VEN HORST, HELEN TANG, and VERONICA JURKOVlCHl Department o f Chemistry, Marycrest College, Davenport, Iowa
b The limitations of this method of analysis have been studied and compared with results obtained by analysis in the visible region. Factors considered include: suitable solvents, suitable grades of filter paper, effect of time and temperature in the production of fluorescence, sensitivity and stability of fluorescence, amino acids which produce fluorescence, and reproducibility of results.
I
K A previous article (5) the authors
pointed out certain problems which were encountered in the use of microchromatography as a quantitative measure of amino acids. The variation of the color intensity due to atmospheric conditions and the difficulty in applying a uniform spray of chromogenic Present address, Department of Chemistry, Catholic University of America, Washington, D. C.
reagent are among the variables involved in the method of analysis. Because fluorometric techniques are often more sensitive than the corresponding colorimetric processes, it was believed that ultraviolet densitometry would present some advantages in amino acid analysis. Some of the earlier research on fluorescence on filter paper was performed by observation with the naked eye. Of the 20 amino acids which he analyzed, Woiwod (6) obtained marked fluorescence for tryptophan, histidine, and citrulline. He reports the others as giving only feeble fluorescence. Mavrodineanu, Sanford, and Hitchcock (2) studied the emission of indole-3acetic acid under ultraviolet light in a modified Photovolt densitometer. Shore and Pardee (4) used the increase in fluorescence resulting from spraying the amino acid chromatograms with a xylose solution and reading the intensity with a modified Beckman DU spectro-
photometer. Similar applications of fluorescence are described in the literature (1, 3 ) . Because amino acids depend for their fluorescence upon a reaction with some form of carbohydrate, and the intensity of fluorescence is a function of the time and degree of heating, the usefulness and limitations of this type of analysis were studied in some detail. The following points were considered : (I) the effect of irrigating solvents on the fluorescence of the filter paper; (2) the grades of filter paper most suitable from the point of view of uniform background and maximum fluorescence of the amino acid; (3) the time and temperature necessary for maximum fluorescence; (4) the sensitivity of the fluorescence of amino acids in the ultraviolet range as compared with the sensitivity of the ninhydrin compounds observed in the visible region; ( 5 ) the stability of the fluorescence of the developed VOL. 31, NO. l,.JANUARY 1959
135
