did not react with manganese(I1) and excellent titrations of manganese were obtained. Fluoride in large amounts does not interfere (7). Similarly, large amounts of chloride or perchlorate do not interfere (9). Bromide in amounts at least up to 500 mg. was without influence but iodide consumed permanganate with the formation of free iodine. Others have demonstrated that iodate even in trace amounts catalyzes the air oxidation of manganese prior t o titration and causes low results (6), unless titration is performed under nitrogen. Group VIII. Large amounts of iron, cobalt, and nickel have been studied and do not interfere (9, 4). Ruthenium trichloride (500, 250, and 100 mg.) imparted positive errors of 0.23, 0.09, and 0.08 ml. respectively, even after a blank correction. KO precipitate formed and the pyrophos-
phate solution was green at p H 7 before the start of the titration. The kfluence of pH on the ruthenium error was not studied. Rhodium (I I I) chloride, 3-hydrate (500 mg.) formed a precipitate at pH 6.5 but dissolved to form a deep red solution when more pyrophosphate waa added to bring the p H to 7. Manganese was successfully titrated. Palladium(I1) chloride, %hydrate formed an amber pyrophosphate solution at p H 6.5. Again, manganese was successfully titrated. Both rhodium and palladium were dissolved in 50 ml. of 1 : 9 hydrochloric acid. Osmium(VI11) oxide catalyzes the air oxidation of mangancse prior to titration (6),and somewhat low results are obtained. Iridium(1V) chloride (1 gram) was dissolved in 100 ml. of 1 : 9 hydrochloric acid upon boiling. I t was not possible to obtain an end point break in
the potentiometric titration of manganese in the presence of 40- or 10-ml. portions of the iridium solution. Potassium chloroplatinate(1V) waa dissolved in 50 ml. of 1:9 hydrochloric acid and presented no difficulties with the titration of manganese at pH 7.0 LITERATURE CITED
(1) .Bjerrrlm,
J., Schwarzenbach, G., Sillen, L., “Stability Constants, Part 11: Inorganic Li ands,” p. 61, The Chemical Society, kondon, 1958. (2) Duyckaerta, G., Anal. Chzm. Acla 5, 233 (1951). (3) Lingane, J. J., Karplus, R., IND.ENG. CHEM..ANAL.ED. 18. 191 (1946). (4) Scribner, W. G., ANAL.‘CHEM. 32, 9%(1960). p. 970. ( 5 j i‘hd., (6) f3talzer, R. F., Vosburg, W. C., Zbid., 23, 1880 (1951). (7) Tertoolen, J. F. W., Buijze, C., van Kolmeschate, G. J., Chim. anal. 42, 9 (1960). RECEIVED for review December 7, 1960. Accepted February 10, 1961.
FIa me Photo metric M etho ds of Determining the Potassium in Potassium Tetraphenylborate M. G. REED and A. D. SCOTT Agronomy Department, Iowa Sfate University, Ames, lowa
b Flame photometric methods for determining potassium precipitated os potassium tetraphenylborate in aqueous systems are described. In one method acetone-water solutions of the potassium tetraphenylborate are used directly. In the others, the potassium i s converted to a water-soluble form b y heating the tetraphenylborate salt for 20 minutes in a furnace a t 350” C. or in boiling aqueous solufions of mercuric chloride. The application of these methods to the determination of potassium extracted from soils or micaceous minerals b y aqueous sodium tetraphenylborate solutions is also discussed. With these methods, this potassium can b e separated from the mineral residues and determined quantitatively without interference from excess sodium tetraphenylborate, decomposition products of the tetraphenylborate, or other materials in soils and micaceous minerols that are soluble in acetone or water.
F
OF POTASSIUM from soils and micaceous minerals with aqueous salt solutions can be greatly enhanced by adding sodium tetraphcnylborate (NaTPB) to the solution to precipitate the extracted potassium XTRACTION
_J
(6). Extensive application of this extraction method has been limited, however, by the lack of a rapid method for determining the precipitated potassium. The published methods of determining potassium first precipitated as K T P B are not directly applicable to the determination of the precipitated potassium in aqueous micaceous mineralNaTPB systems because mineral residues, excess NaTPB, and decomposition products of the T P B ion that develop during long extraction periods are also present. Consequently, in preliminary studies of this extraction method, the K T P B was dissolved and separated from the mineral residues by adding acetone to the aqueous system and filtering (6). The potassium in the filtrate was then determined by flame photometry after the T P B salts had been destroyed with aqua regia. However, this procedure was cumbersome and the acetone treatment did not ensure complete removal of the precipitated potassium. Thus, other methods were investigated. Three rapid flame photometric methods of determining the potassium in K T P B and their applicability to aqueous micaceous mineral-NaTPB systems are described in this paper.
EXPERIMENTAL
Acetone-Water Solutions. The K T P B precipitate in aqueous systems can be dissolved by adding an appropriate amount of acetone. Thus, a simple approach to t h e determination of the potassium in K T P B by flame photometry would appear t o be thc direct use of this acetone solutioti in the flame photometer. Recently, considerable attention has been given to the use of organic solvent, extractions in flame photometry (1). I n fact, potassium as potassium chloride in acetone-water solutions has been determined by flame photometric met.hods (3). Thus, to apply this method to the determination of potassium as K T P E in acetonic solutions i t was only necessary to determine the effects of the acetone-mater ratio and the TPB ion. When acetone-water solutions are used In a flame photometer, the acetone affects both the background radiation and the flame emission of the element being determined ( 4 ) . Thus, the acetone-water ratio in the unknown potassium solutions and the potassium standards should be comparable. The seiection of a suitable acetone-water ratio, however, depends upon the amount of KTFU to be dissolved, the VOL. 33, NO. 6, MAY 1961
773
possibility of changes in the ratio during the experiment, and the flame photometer. The solubility of KTPB in acetonewater solutions incresses with the acetone-water ratio (6). The evaporation of acetone and the resulting changes in the ratio during the experiment, however, also increase with the acetone concentration. Thus, t o provide a basis for the selection of a n acetonewater ratio that can be used in the determination of potassium in acetonic solutions of KTPB, the effects of changes in the ratio as well as the effects of the ratio itself on the operation of the flame photometer were determined. A Perkin-Elmer Model 52-C flame hotometer with propane gas was caligrated with an aqueous solution of potass um chloride that contained 100 p.p.m. of potassium and 500 pap.m. of lithium. Internal standard potentiometer readings were then determined with acetone-water solutions of potassium chloride that contained 20 p.p.m. of potassiri,m, 500 p.p.m. of lithium, and different amounts of acetone. The results of this experiment are shown in Figure 1.
When the acetone concentration approached 30%, the flame lacked oxygen. Thus, higher concentrations of acetone were not used in this investigation. Higher acetone concentrations have been used successfully in the Beckman flame photometer with a hydrogenoxygen burner and a total consumptiontype of atomization system (3). The Perkin-Elmer instrument has a discharge-type of atomization system. High concentrations of acetone are not
Table 1. Effect of Tetraphenylborate Concentrations on Flame Photometer Readings at Different Potassium Concentrations
Potassium Concentration, P.P.M. 0
5
10
15
20
0
25.2
50 3
75.4
100.0
NaTPB :K Internal Standard Ratio Potentiometer Reading, % ' 0 0.5 1.0 2.0
. . . 24.9 50.4 74 8 101.1 . . , 25.2 49 9 7 4 . 9 100.5 . . . 2 1 . 8 5 0 . 2 75.1 100.1
detcrrnination of potassium in acetonic solutions (Table I). Aqueous Solutions-after
22
'
'0 5 ACETONE
10 I5 20 CONCENTRATION
25
r/. BY
30 VOL)
Figure 1 . Effect of acetone concentrations in acetone-water solutions of potassium chloride on internal standard potentiometer readings
required, however. I n fact, the data in Figure 1 show that errors from changes in the acetone-water ratio can be serious if the acetone concentration exceeds 15%. Furthermore, even a 16% acetone solution will dissolve enough KTPB to give a 44-p.p.m. potassium solution (6). Thus, if higher acetone concentrations are needed to ensure the initial solution of all the KTPB in a system, the resulting solution can be readily diluted in regard to both potassium and acetone. I n this manner the possibility of acetone evaporation and errors due to the resulting changes in acetone-water ratio can be reduced. To evaluate the effect of the T P B ion. the flame Dhotometer was calibrated with an acetone-water solution of potassium chloride that contained 20 p.p.m. of potassium, 500 p.p.m. of lithium and 20% acetone. Internal standard potentiometer readings were then determined with acetone-water solutions that also contained 20% acetone but different amounts of potassium chloride and NaTPB. The results obtained with solutions that contained 5, 10, 15, or 20 p.p.m. of potassium and either 0.5, 1.0, or 2.0 times as much T P B on an equivalent basis are given in Table I.
A linear calibration curve was obtained with the acetonic solutions of potassium chloride. Furthermore, with aretone present, there was greater stability in the meter readings because the acetone enhanced the flame emission and reduced the amplification required. TPLl conrcntrations up to twice that of the potassium present had no effect on the accuracy of the flame photometric
Table II. Recovery of Potassium in Aqueous Solutions after Heating in Platinum and Borosilicate Glass for Different Periods of Time at Several Temperatures TemPlatinum, Min. Borosilicate Glass, Min. __ perature, 10 20 40 60 10 20 40 60
" c. 200 300 350 400 500
774 *
Potassium Recovered, %
1.2 Y2.6 1#:2 100.2
1.1 97.2 99.6 99.8 100.4
ANALYTICAL CHEMISTRY
0.8 98.8
2.4 97.4
1bb:O 99.8
99.8 99.8
...
1.8 33.4 97.4 98.8 86.8
1.4 43.4 100.0 98.4 82.4
1.8 61.2 100.6 97.0 80.6
1.7 99.0 100.0 96.0 78.4
Heating.
According to Flaschka et al. (a), KTPB is converted quantitatively to potassium metaborate by heating the dry salt at full Bunsen flame for 1 minute in platinum. Since potassium metaborate is soluble in water, the possibility of using this approach to obtain aqueous solutions that can be used in a flame photometer to determine the potassium in KTPB was investigated. Instead of an open flame, however, an electric furnace was used and the effects of temperature and duration of heating were determined. Also, since platinumware may be limited in experiments involving a large number of samples, the possibility of using borosilicate glass containers was investigated. Five milligrams of potassium as potassium chloride was precipitated in platinum and borosilicate glass containers by adding a threefold excess of NaTPB. Acetone was then added to dissolve the precipitate and to simulate the conditions encountered in the separation of the KTPB from mineral residues or other potassium bearing materials. The acetone-water solution was dried on a steam plate and the residue was placed in a preheated muffle furnace for different periods of time. The heated samples were then cooled and 20 ml. of 0.5N hydrochloric arid was added to dissolve the potassiur? The solutions were diluted and filtered and the potassium in the filtrate was determined by flame photometry. The results of this experiment are given in Table 11. When the heating temperature was C. or less, the potassium in both the borosilicate glass and the platinum containers was not completely recovered. Also, a t temperatures above 350" C. the recovery of potassium from the borosilicate glass containers decreased with increasing temperatures and heating time. Consequently, t o use this method with borosilicate glass containers the temperature must be maintained near 350" C. and a heating time of at least 20 minutes must be used, If platinum containers are used, a wider range of temperatures and heating periods are satisfactory as long as the temperature is 350" C. or grcatpr. I n fact, all of the potassium can be recovered from platinum Containers that have been heated for 1 hour at temperatures as high as 500" C. Since the recovery of potassium from both borosilicate glass and platinum containers was essentially complete when the T P B salts were heated for 20 minutes a t 350' C., this procedure was used in a study of the precision of this method of determining the potassium in KTPB. I n this ease 10 samples of K T P B were heated in platinum and borosilicate glass con300"
tainers and the potassium recovered in the aqueous solution was determined by flame photometry as before. There was a mean recovery of 99.5% of the potassium in K T P B and a standard deviation of 0.9% when platinum was used. With borosilicate glass the mean recovery was 99.4% with a standard deviation of 1.3y0. Aqueous Solutions-after Boiling.
T P B is not stable in boiling aqueous systems and the precipitated potassium in NaTPB-micaceous mineral mixtures can, therefore, be determined by simply boiling the aqueous system, filtering, and determining the potassium in the filtrate by flame photomet r y (7). The only difficulty encountered with this method was the boiling time required for complete solution of the potassium. In some cases, long boiling periods were needed and in others i t was not possible to estimate visually when the K T P B precipitate disappeared. Thus, to improve this method of determining the ,potassium in K T P B the effect of several factors on the recovery of potassium in relatively short boiling periods was determined. Wittig el al. (8) have shown that the T P B ion can be destroyed completely by the following reaction with mercuric chloride:
+
-+
LiB(CaH& 4- 4HgCl1 3H20 LiCl 3HC1 4Hg(CsHs)C1
+
+
HIBOl
Therefore, the effect of mercuric chloride additions on the clarification of boiling aqueous K T P B systems was determined. One of the main advantages of the boiling method of determining potassium in K T P B is that large amounts of sodium or ammonium salts can be added t o aqueous NaTPB-micaceous mineral aystems before the potassium is brought into solution. In this manner the adsorption of the potassium by the degraded mineral can be blocked. Excess NaTPB in the aqueous system would, of course, .precipitate some of the added ammomum. Thus, in this study of the recovery of potassium by boiling, the effect of sodium and ammonium additions and excess NaTPB waa determined. Five-milligram samples of potassium in 5 ml. of potassium chloride solution were precipitated with a 2.3-fold excess of NaTPB in 15 ml. of I N sodium acetate adjusted to p H 5. Some of these systems were filtered to remove the excess NaTPB; some were diluted with water; and some were treated with sodium, ammonium, or mercuric chloride additions. They were then boiled for different periods of time, filtered, and the potassium in the filtrate was determined by flame photometry. The percentage of the added potassium recovered is shown in Table 111.
Table 111. Recovery of Potassium from Potassium Tetraphenylborate in Boiling Aqueous Solutions as Affected by NaTPB and Cation Concentrations, Dilution, Mercuric Chloride, and Boiling Time
Excess NaTPB Present
Absent Present Absent Present
Absent a
Final Vol-
Final BoilK Ne+ ing Re ume, Concn., Time, covered, M1. N Min. % 20 20 20 20 100 100 100 100 100 -.. 100 100 100 100 100 20 100 100
0.75 0.75 0.75 0.75 0.15 0.15 0.15 1 1 1 2 2 2
2" 0.75b 0.15' 0.15b
1 5 20
40
1 1 20 20 20
240
1 20 240 20 20 20
20
0.4 0.4 7.0 71 .O 1.7 25.9 99.9 16.6 40.0 99.8 1.2 13.9 100.0 100.0 99.4 99.9 99.6
0.54 gram of HgClr added. "4; 0.54 gram of HgClr
* 0.5N
added.
There was an increase in potassium recovery when the boiling period was increased and the other factors were held constant. In fact, by heating the diluted samples long enough, complete recovery of the potassium was obtained even though excess NaTPB or large amounts of other salts were present. Dilution of the T P B concentration and the removal of excess TPB, however, greatly increased the recovery of potassium in a given boiling period. The high sodium or ammonium concentrations, on the other hand, reduced the recovery of potassium in short periods of boiling. Only the mercuric chloride treatment was effective in allowing complete recovery of potassium in 20 minutes when large amounts of salt were present. Also with mercuric chloride present i t was not necessary to remove the excesa NaTPB or to dilute the system before boiling. The results of this experiment show that the potassium in K T P B can be readily determined by simply diluting the system, boiling for 20 minutes in the presence of mercuric chloride, filtering and determining the potassium in the aqueous filtrate by flame photometer. If sodium or ammonium ions are needed to block the adsorption of the potassium by other materials in the system, the dilution can be.accomplished with 2 N sodium chloride or 0.5N ammonium chloride solutions without interfering with the recovery of the potassium.
To determine the precision of this method of determining the potassium in KTPB, ten 5-mg. samples of pot-sium in 5 ml. of potassiuni chloride solution were precipitated with a 2.3fold excess of NaTPB in 15 ml. of 1N sodium acetate adjusted to p H 5. These systems were diluted to 100 ml. with 0.5N ammonium acetate and boiled for 20 minutes after adding 0.54 gram of mercuric chloride. They were then filtered and the potassium in the aqueous filtrate was determined by flame photometry. The mean recovery of potassium was 99.9% with a standard deviation of 0.3%. CONCLUSIONS
These experiments prove that potassium that has been precipitated as K T P B in aqueous systems can be quickly and accurately determined once this potassium has been separated from other potassium bearing materials in the system. If the separation of KTPR is made with acetone, i t may be necessary to evaporate the acetonic solution to dryness and heat the residue for 20 minutes in either a furnace a t 350' C. or a boiling aqueous solution of mercuric chloride to obtain an aqueous solution of the potassium. However, the more rapid method of using the acetonic solution in the flame photometer directly would probably be used if the acetone-water ratio can be adjusted to a known level within the operating conditions of the flame photometer. The boiling aqueous solution method should be particularly useful for the determination of potassium that has been extracted from soils and micaceous minerals by NaTPB solutions. With this method the precipitated potassium can be separated from the rest of the system without using acetone and large amounts of sodium or ammonium can be added to block the adsorption of the potassium by the degraded minerals. LITERATURE CITED
(1) Dean, J. A. Am. SOC.Testing Materiak Philadelphia, Pa., Spec. Tech. Publ. hio. 238, 43 (1958). (2) Flaachka, H., Holasek, A., Amin, A. M. Z.and. C h a . 138, 161 (1953). (3) Ifingsley, G . R., Schaffert, R. R., J . Biol. Chem. 206,807 (1954). (4) Rathje, A. O., ANAL.CHEM.27, 1583 (1955). (5) Scott, A. D., Humiker, H. H., Hanway, J. J., Soil Sci. Soc. Am. Proc. 24, 191 (1960). (6) Scott, A. D., Hunziker, H. H., Reed, M. G., Chemist Anal st 48, 11 (1959). (7) Scott, A. D., R&,' M. G., sot! sci. Soc. Am. Proc. 24, 326 (1960). (8) Wittig, G. Keicher, G., Ruchert, A., Raff, P., Ann. 563, 110 (1949).
RECEIVEDfor review October 17, 1960. Accepted March 8, 1961. Journal Paper No. 5-3986 of Project 1234 of the Iowa Agricultural and Home Economics Experiment Station, Ames, Iowa. VOL. 33, NO. 6, MAY 1961
775
