1542
ANALYTICAL CHEMISTRY
acids, as it does in dilute aqueous solutions, that reaction could account for the difference in the calibration line at high nitrogen dioxide contents. At one stage of the work the absorbances a t 1.423 microns of about 30 samples containing up to 5% water were determined on each of four modified Beckman DU spectrophotometers. The variations among the instruments never exceeded 0.27, water equivalent, and the average spread was only about 0.1% water. Over the range of normal laboratory temperatures, the spectrophotometric determination of water in nitric acid is not significantly influenced by temperature. Early in the work, before the techniques were precise enough to show the nitrogen dioxide effect, the influence of dissolved nitrates of iron, nickel, and chromium was briefly investigated. KO influence on the water determination was detected. It is assumed, therefore, that dissolved nitrates do no more than suppress the extent of the self-dissociation of the acid. The method has been in use in several laboratories for 2 years or more.
assisted in developing the instrumentation and in making preliminary measurements. Twenty-three of the analyses used in establishing the final calibration were performed by the Kava1 Air Rocket Test Station, Dover, N. J., through the courtesy of J. D. Clark and H. G. Streim. The probable existence of the effects of ionization equilibria was predicted to the authors by H. E. Higbie of M. W. Kellogg Co., New York, K. Y.
ACKNOWLEDGMENT
RECEIVED for review September 28, 1955. Accepted June 21, 1956, This paper represents a part of the work done under Contract No. AF 18(600)-53 with the Air Research and Development Command, Wright Air Development Center, Wright-Patterson Air Force Base, Ohio.
Ruby James made the bulk of the measurements on which this paper is based, and most of the analyses. Walter B. Wade
LITERATURE CITED
(1) Dalmon, R., Freymann, R., Compt. rend. 211,472 (1940). (2) Ellis, J. W., Phys. Rev. 38, 693 (1931). (3) Gillespie, R. J . , Hughes, E. D., Ingold, C. K., J. Chem. SOC. 1950, 2552. (4) Goulden. J. D. S., Millen, D. J.,Ibid., 2620. (5) Ingold, C. K., hlillen, D. J., Ibid.. 2612. (6) “International Critical Tables,” vol. 111, p. 133, XlcGraw-Hill,
New York. 1933. (7) Kinsey,W. L., Ellis, J. W., Phys. Rev. 36, 603 (1930). (8) Ibid., 51, 1074 (1937). (9) Lynn, S., Mason, D. M., Sage, B. H., Ind. Eng. Chem. 46, 1953 (1954).
Spectrophotometric Determination of Potassium with Sodium Tetraphenylborate RONALD T. PFLAUM and LESTER C. HOWICK Department o f Chemistry, State University o f lowa, lowa City, lowa
Potassium tetraphenylborate was investigated spectrophotometrically in an acetonitrile-water system. The tetraphenylborate ion shows absorption maxima at 266 and 274 m,u, with molar absorptivities of 3225 and 2100, respectively. Beer’s law is obeyed over a concentration range from 5 X 10-6 t o 7.5 X 10-1 M . The results obtained on the determination of potassium in selected samples indicate the feasibility of employing the described system for such determinations. Spectrophotometric evaluations of the solubilities of tetraphenylborate salts in aqueous solutions were obtained.
Table I.
Summary of Potassium Determinations
Table 11.
ITHIX the past 5 years, sodium tetraphenylborate has come into prominence as a precipitant for potassium. Various methods for the determination of potassium based on the insolubility of the potassium salt in aqueous solution and its solubility in certain organic solvents have been advanced. Gravimetric (2, 7 ) , titrimetric (6, 9 ) , turbidimetric (8),conductometric ( 7 ) , and voltammetric ( 1 ) methods have been proposed. A spectrophotometric method is a logical and useful extension to this existing list of measurements. Tetraphenylborate salts are soluble in certain organic solvents. Dissolution in acetonitrile leads to a solvent medium that is especially well suited for spectrophotometric measurement. This work is concerned primarily with an investigation of the potassium salt in such an acetonitrile medium. I t was undertaken in order to elucidate the feasibility of a spectrophotometric determination of potassium. APPARATUS AND REAGENTS
All spectrophotometric measyements were made a t room temperature (approximately 25 C.) with a Cary Model 11
Potassium, Mg. Present Found
Sample
Solubilities of Tetraphenylborate Salts in Pure Water at 25” C. Salt “4
cs
K Rb T1
Solubility, M X 106 Experimental Literature 10.7 2.79 17.8 2.33 5.29
3 . 2 8 (20’ C.) (4) 1 8 . 2 (IO) 4 . 4 1 (20’ C.) ( 4 ) 2 . 9 (5)
recording spectrophotometer, using 1-cm. matched silica cells. A Beckman Model G pH meter was used for all pH values. Sodium tetraphenylborate was obtained from the J. T. Baker Chemical Co. It was used as received for all work except for obtaining the absorption curve of the reagent. I n this case, reagent recrystallized from an acetone-hexane mixture was used. Crystalline salts of ammonia, cesium, potassium, rubidium, and thallium(1) were prepared by reaction of the respective chlorides with the reagent in aqueous solution. Recrystallization of the precipitated material was effected from an acetonitrile-water system. Acetonitrile was obtained from the Matheson, Coleman & Bell Division of the Matheson Co. Purification was effected by treating with cold saturated potassium hydroxide, drying over
V O L U M E 28, N O . 10, O C T O B E R 1 9 5 6
1543
determination of 2 to 30 p.p.m. (5 X 10-6 t o 7.5 X 10-4M) of potassium in the measured sample with an accuracy of & 2%. 2.c DISCUSSION
:I W
0
z a m a
8
1s
m U
O.!
Figure 1. Absorption spectra of tetraphenylborate salts in acetonitrile Curve 1 2 3 4 5
Concn.,
.%f
x
10-4
1.2 2.4 3.6 4.8 6.0
Salt CS " I
Rb Na K
anhydrous potassium carbonate for 24 hours, refluxing over phosphorus pentoxide for 2 to 3 hours, then distilling from phosphorus pentoxide in an all-glass system. The fraction boiling a t 81-81.5' C. was used as the pure solvent. All other chemicals used were of reagent grade quality. SUGGESTED METHOD
After appropriate preliminary treatment of the sample to yield an aqueous solution of potassium ion, adjust the pH of the solution to 4.0 to 5.0 with dilute sodium hydroxide and dilute sulfuric acid. Prepare a stock solution of sodium tetraphenylborate by dissolving 1.0 gram of the reagent and 0.5 gram of aluminum chloride hexahydrate or aluminum nitrate hexahydrate in 100 ml. of water. Filter the solution to remove any turbidity that mag develop. Add 5 ml. of the reagent solution to 5 ml. of the sample solution in a 15-ml. graduated centrifuge tube. Centrifuge for 3 minutes in a high speed centrifuge and remove the supernatant liquid by pipet. Kash the precipitate twice with 3 ml. of a cold saturated solution of the potassium salt, again removing liquid by pipet. A constant volume of liquid (0.5 ml.) is left with the precipitate. Dissolve the precipitate by adding 5 ml. of a mixture of 7573 acetonitrile and 25% water. Transfer to a 25-ml. volumetric flask, rinse the centrifuge tube with additional solvent, and dilute the sample to 25 ml. Prepare a blank solution by an identical procedure, using the solvent mixture to yield the 25-ml. volume. Measure the absorbance a t 266 mp. Determine potassium concentration from the absorbance value and a prepared calibration curve. RESULTS
The results obtained with this method are shown in Table I. The values given are the average of multiple measurements on the various samples. The analyses indicate that potassium can be determined with an accuracy within that usually assigned to a spectrophotometric method. The method is applicable to the
Spectrophotometric Studies. il spectrophotometric investigation of various systems was undertaken as an initial step in this study. Absorption curves were obtained for weighed amounts of the various pure tetraphenylborate salts dissolved in pure acetonitrile. Curves of ammonium, cesium, potassium, rubidium, and sodium tetraphenylborate are shown in Figure 1, from n-hich it is evident that all of these salts have the same absorption characteristics. I t is concluded, therefore, that the absorption maxima of the tetraphenylborate ion occur a t 266 and 274 mp. The molar absorptivities a t these maxima are 3225 and 2100, respectively. It was found, in addition, that identical results were obtained when technical grade solvent was used without purification. Solutions containing varying concentrations of the potassium salt (5.0 X 10-6 to 7.5 X l O - * X ) were prepared and measured in order to determine the conformance to Beer's law. It was found that Beer's law was obeyed over the concentration range studied. Moreover, the solutions show a high degree of stability with no apparent changes even after 5 days. The effect of the addition of water to acetonitrile solutions of the salts was also investigated. Varying amounts of water were added to constant amounts of potassium tetraphenylborate in acetonitrile, with subsequent dilution to constant volume with the organic solvent. Absorption curves for a series of solutions prepared in this manner are shown in Figure 2. The curves have been displaced vertically, inasmuch as very little change occurs in the absorptivity of the absorbing ion. However, a loss in the definition of the curves results from the addition of increasing amounts of water to the system. There is no apparent change in the shape of the absorption curve up to 40% of water by volume. Thus, analytical measurements could be carried out in a solvent mixture of these proportions. Solubility Studies. A study of the solubility characteristics of tetraphenylborate salts was carried out. The recrystallized ammonium, cesium potassium, rubidium, sodium, and thallium (I) salts were studied. With the exception of the sodium compound, all are insoluble in water. All dissolve in acetone, acetonitrile, dimethylformamide, and dioxane. The thallium salt is the least soluble in these solvents. All are insoluble in benzene, carbon tetrachloride, and chloroform. 4 study of the solubility of the salts in aqueous solution was carried out by saturating conductivity water with the respective salts a t 25" C. After equilibrium was attained, portions of the solutions were analyzed for tetraphenylborate ion content by spectrophotometric measurement a t 266 and 27-1 mp. The results of this study are presented in Table 11. d comparison with reported literature data indicates that solubility values from spectrophotometric measurement are comparable to those obtained by conductometric (IO) and radiometric measurements ( 4 ) . Precipitation Studies. A study was carried out of the precipitation reaction of potassium and the reagent in aqueous media, as well as the effect of pH. For these purposes a stock solution of potassium ion, 2 X 10-2J4, was prepared by dissolving potassium chloride in conductivity water. A stock solution 4 X 10-*M in sodium tetraphenylborate was prepared by dissolving the reagent in water. Turbidity in the solution was removed by filtration. Portions of 5 ml. of the potassium solution were added to 5 ml. of the reagent solution. Changes in pH were made with dilute sulfuric acid and sodium hydroxide. It was found that quantitative precipitation of potassium occurred in a p H range from 1 to 9. Precipitation in cold solution is recommended for the p H range from 1 to 3 in order to prevent the decomposition of the reagent (6). The reagent shows a high degree of stability in neutral or basic solution.
1544
ANALYTICAL CHEMISTRY reaction are summarized in Table 111. The data were obtained a t pH 3.0 to 4.0 for solutions 4 X 10-3JI in potassium ion and 8 X 10-3M in reagent. Diverse cations vere added as the chloride or perchlorate salts. Sodium salts of the anions were employed.
Table 111. Ion C2H802Al+++
NHI
Gal-
+
cs Co++ Cut+ Fe+*' Lit Rlg t *
WAVE LENGTH, rnfi Figure 2.
Effect of addition of water to potassium tetraphenylborate in acetonitrile Curve 1 2 3 4 5
% Water 0 20 40 60 80
The insoluble potassium salt can be quantitatively separated from the aqueous phase by filtration with a fine-porosity sinteredglass filter. Separation can also be effected by centrifugation in a high speed centrifuge. The presence of aluminum ion in the solution, as proposed by Kohler (6) and Findeis and De Vries ( I ) , is helpful in causing the separation of the precipitate. Aluminum ion is apparently precipitated together with the potassium salt and is not soluble in pure acetonitrile. The mixed precipitate dissolves completely in an acetonitrile-water medium. The effects of the presence of diverse ions on the precipitation
Amount Permissible, P.P.M. 2000 2000 0 2000
0 1000 1000 1000 2000 2000
Effect of Diverse Ions Ion
++ 3+ + XO:-
Rb
SO, - X-(Br-, C1-, I - )
Amount Permissible, P.P.hI. 0 1000 2000 0 0 2000 0 2000 2000
Only a few ions interfere in the system. Interferences result from the interaction of the particular ion with the reagent. Of the ions interfering, silver, mercury(II), and thallium(1) can readily be removed by simple preliminary treatment of the sample. Certain of the amines and ammonia are likewise easily removed through preliminary treatment. Cesium and rubidium ions, although usually present only as traces in a sample, are precipitated together with the potassium. Certain amines, cesium, potassium, or rubidium can be quantitatively determined with sodium tetraphenylborate in the absence of the other species. LITERATURE CITED (1) Findeis, A. F., De Vries, T., AXAL.CHEM.28, 209 (1956), (2) Flaschka, H., 2. anal. Chem. 136, 99 (1952). (3) Geilmann, W., Angew. Chem. 66,454 (1954). (4) Geilmann, W., Gebauhr, W., 2. anal. Chem. 139, 161 (1953). (5) Hahn, F. L., Ibid., 145, 97 (1955). (6) Kohler, M., Ibid., 138, 9 (1953). (7) Raff, P., Brotz, W., Ibid., 133, 241 (1951). (8) Rubia Pacheco, J. de la, Blasco Lopez-Rubio, F., Chemist AnaEyst 44,58 (1955). (9) Rudorff, W., Zannier, H., 2. anal. Chem. 140, 1 (1953). (10) Rudorff, W., Zannier, H., 2. Naturforsch. 8b, 611 (1953). RECEIVEDfor review M a y 5 , 1956. Accepted July 18, 1956. Division of Analytical Chemistry, 129th meeting. I C s . Dallas, Tex., April 19;1G.
Spectrochemical Analysis of Thermionic Cathode Nickel Alloys by a Graphite-to-Metal Arcing Technique EDWIN
K. JAYCOX A N D BETTY E. PRESCOTT N. J.
Bell Telephone Laboratories, Inc, Murray Hill,
A technique is described for the determination of aluminum, cobalt, chromium, copper, iron, magnesium, manganese, silicon, and titanium in thermionic cathode nickel alloys, in the general concentration range from 0.003 to 0.2% for each element. In this procedure 10 mg. of nickel metal is placed in the crater of a graphite cup and burned to extinction in the direct current arc. The precision is adequate for the determination of the metals listed in the concentration ranges normally encountered in cathode nickel alloys. The speed of the analysis is considerably increased over that of the dry oxide powder technique.
T
HE chemical composition and the analysis of the nickel
alloys used for the thermionic cathodes in electron tubes has long been of primary concern to the manufacturers of these devices. The ease of activation, the ultimate degree of thermionic activity, and the emission life of thermionic cathodes are markedly dependent upon trace constituents present in the nickel base to which the alkaline earth emitter is applied. Trace constituents, particularly magnesium, silicon, aluminum, and titanium, react with the alkaline earth compounds (barium, strontium, and calcium oxides) of the coating to produce free alkaline earth metal thought to be essential for high thermionic activity. Other elements such as iron and manganese may
