Use of Fluoboric Acid for the Direct Determination of Potassium HAROLD M. MANASEVIT Armour Research Foundation of Illinois Institute
oi T e a no:ogy, Chicago 76, 111. ice bath until the solution has reached a constant temperature of about 3' C., from 45 minutes to an hour. The supernatant liquid is decanted, with suction applied, through a fritted-glass Gooch-type crucible of medium porosity, which has been inserted into a soft-rubber holder and placed in a funnel and surrounded by chips of ice. After the decantation process, the beaker is returned to the ice-water bath until the filtrate has passed through the crucible. The outside of the beaker is wiped free of water and the bulk of the precipitate is added carefully to the crucible and allowed to drain free of liquid. The precipitate remaining in the beaker is washed quantitatively into the crucible with a fine stream of ice-cold (3" C.) 1 to 1 methanol-ethyl alcohol solution from a wash bottle. The crucible and contents are washed with three or four 10,ml. portions of the alcohol solution and dried in an oven a t 105 to 110' C. for 30 minutes. The crucible is cooled to room temperature and weighed. The potassium content is calculated by multiplying the weight of potassium fluoborate by the stoichiometric factor, 0.3105. I n a mixture in which more than 0.50 gram of sodium chloride is expected in the mixed chlorides, the precipitate should be rewashed with 30 ml. of 1 to 1 methanol-ethyl alcohol solution, redried, and reweighed until a change in weight no greater than 0.5 mg. is reached between successive weighings. The drying time may be reduced considerably by rrashing the excess alcohol from the DreciDitate with ice-cold ether and drawing air through the precTpitaie until the ether odor is gone. The Gooch crucibles can be cleaned by suction filtering about 100 ml. of hot water through them after the bulk of the precipitate has been removed. After numerous determinations only very slight weight changes and etching of the glass have been noticed. Alcohol to Sample Solution Ratio. In order to establish the correct conditions for precipitating potassium fluoborate, the ratio of alcohol to reagent to sample solution was studied. Varying amounts of 95y0 ethyl alcohol and fluoboric acid were added to 5-ml. portions of a potassium chloride solution ( 5 ml. equivalent to 0.0200 gram potassium chloride) a t room temperature. It was found that a greater than 1 to 1 ratio of alcohol to sample solution gave quite reproducible but low results. The amount of fluoboric acid added (2 to 10 ml.) likewise had no effect on the reproducibility of the determination a t this alcohol concentration. Temperature of Precipitation. It was suspected that the low results were due t o the effect of temperature on the solubility of the potassium fluoborate precipitate. Therefore, the precipitations were made in an ice-water bath. Table I shows that the precipitation is more nearly quantitative when carried out a t about 3" C.
4 practical analytical method is proposed for the direct determination of potassium by precipitation as potassium fluoborate from an ice-cold solution. Solutions containing 20 to 250 mg. of potassium chloride in the presence of up to 500 mg. of sodium chloride have been analyzed, with a relative error in potassium chloride of less than 1 'j&and with good reproducibility. Moderate to comparatively high ratios of the chlorides of copper(II), zinc, cadmium, cobalt, nickel, manganese(II), iron, aluminum, chromium, calcium, lithium, or magnesium to potassium do not interfere. Combinations of these chlorides are permissible; however, aluminum and calcium must not be present in the same solution with potassium. The ammonium, barium, and sulfate ions interfere.
0
F THE various methods that are available today for the de-
termination of potassium ( 7 ) )the tx-o most often used are the chloroplatinate and perchlorate methods ( 5 ) . The chlorcplatinate method generally is considered to be the most accurate for determining potassium in the presence of sodium, but consid~raliletime is required to regenerate the expensive chloroplatinic acid. The perchlorate method, which requires a much less expensive reagent, has been adopted by many laboratories and found to give good results, but it requires more attention to details and involves more steps in the procedure. Considerable interest has been stimulated recently in more rapid and more specific methods for potassiun~-especially the use of sodium tetmphenylboron (e-ql, but the reagent is expensive. In this paper an inexpensive and simple routine method is propowd for the rapid direct determination of potassium in the presence of the mixed chlorides of the alkali metals (except cesium and rubidium), with a prepared alcoholic solution of fluoboric acid as the precipitating agent. T h r preparation of potassium fluoborate dates back to the early 1820's vihm it was made by mixing fluoboric acid with a solution of a potassium salt, such as the carbonate, nitrate, or chloride ( I ) . In 1915, Mathers et al. (6) developed a qualitative trpt for the presence of sodium in potassium, lithium, and magnesium mixtures. During the course of their work the potassium was removed as potassium fluoborate. However, appnrentlJ-no one has utilized the insolubility of potassium fluoborate as H means for the quantitative determination of potassium in the presence of the other common alkalies, as well as many other elements. This is a report of the development of a workable method based on the insoluhilitl- of potassium fluoborate.
Table I.
[Precipitating solution. 5 ml KC1 solution (0 0200 gram KCl), 10 ml. HBFa, and 25 ml. CzHsOH] KC1 Recovered, G. At room temp A t 3' C. (ice bath) o.ni9i n. ozoo 0.0192 0.0197 0,0201 0.0192 0.0199 0.0192 0.0200 0.0194
EXPERIMERTAL
'
Effect of Temperature on Precipitation of Potassium Fluoborate
Reagents. Throughout t,he invest.igation reagent-grade salts were used. Solutions of t,he following specifications were used: 48 to 5oY0 purified Baker and Adamson fluoboric acid; absolute methanol, analytical reagent, 99.5% assay; and 95% ethyl alcohol. Preparation of Precipitating Solution. A 0.50-gram sample of sodium chloride is dissolved in 40 ml. of distilled water. To it are added 250 ml. of fluoboric acid, 500 ml. of 95% ethyl alcohol, and 500 ml. of met,hanol. The mixture is placed in a chipped ice bath until the temperature of the solution is about 3" C. and then filtered by suction through an asbestos-padded porcelain Gooch crucible until the filtrate is clear. The prepared solution is stored in polyethvlene bottles. Procedure. To"a 100-ml. borosilicate glass beaker containing 10 ml. of a cold solution of the mixed chlorides of the alkali metals (except cesium and rubidium) are added 30 ml. of the prepared alcoholic solution of fluoboric acid. The beaker is kept in an
Effect of Sodium Chloride. An important requirement of any method for potassium is that the procedure be applicable in the presence of large amounts of the most common alkali metal, sodium. Therefore, increments of sodium chloride (0.10 to 0.50 gram) were added to several standard potassium solutions and the solutions were analyzed. I n all cases, consistently high results (15 mg. per 10 ml. of fluoboric acid reagent used) were obtained. A blank on the reagent plus sodium chloride showed that the high values were due to impurities in the fluoboric acid in combination with 81
82
ANALYTICAL CHEMISTRY
the sodium chloride. However, this difficulty was eliminated by pretreating the fluoric acid reagent as follows: +4stock solution containing 0.5 gram of sodium chloride dissolved in 5 ml. of water, 30 ml. of fluoboric acid, and 125 ml. of ethyl alcohol was cooled in ice (- 3" C.) and filtered through an asbestos-padded, porcelain Gooch crucible. The minimum amount of sodium chloride required to remove the impurity will vary with each batch of fluoborio acid reagent; however, 0.5 gram of sodium chloride was sufficient to purify a stock solution containing up to 250 ml. fluoboric acid. The impurities removed by the pretreatment of fluoboric acid with sodium chloride are probably sodium fluosilicate and sodium sulfate. Effect of Other Cations. d series of acidified solutions containing moderate to excessive proportions of other saltP to potassium chloride were analyzed for potassium with good results. For each 10 ml. of sample solution 30 ml. of the stock solution were used. Table I1 shows that 2 grams each of cadmium, lithium, zinc, magnesium, manganese(II), iron, cobalt, calcium, aluminum, and copper(I1) salts show little interference in the determination of 0.04 gram of potassiumchloride. Good potassium iecoveries were obtained from potassium chloride solutions containing combinations of 0.2 gram each, of the chlorides of cadmium, lithium, cobalt, sodium, magnesium, zinc, manganese(II), chromium, nickel, aluminum, and iron.
Table IV.
(Precipitating solution.
"
When fluoboric acid was added to an alcoholic calcium chloride solution, a white cloudiness would sometimes result, but when the alcohol was added to an aqueous calcium chloride solution containing fluoboric acid, no cloudiness was encountered. iicidification of the calcium chloride-alcohol mixture with 1 ml. of hydrochloric acid before the addition of the fluoboric acid prevented the cloudiness without affecting the solubility of the potassium fluoborate. If calcium salts are known to be present, the solution should be acidified before the precipitating solution is added. It was found that the addition of an alcoholic fluoboric acid mixture to a solution containing both aluminum and calcium chlorides produced a precipitate if the alcohol solution ratio was slightly greater than 1 to I . Sumerous other solvents were tried (1-butyl alcohol, acetone, methanol, ethyl acetate, chloroform, ether, etc.), but precipitation still occurred. The ammonium ion interferes with the determination because of coprecipitation and solubility effects and its removal by ignition of the mixed chlorides is required. Effect of Other Anions. The insolubility of the sodium and
...
... ,
,..
0.5 1 0 2 0
G. Recovered 0.0194 0.0392 0.0974 0,2485 0.0395 0 0390 0 0391
-~ __ - ~ ~ ~ --_ Table V. Effect of Ethyl Alcohol plus Methanel on Analysis of Potassium with Fluoboric Acid (Precipitating solution 0 5 gram NaCl, 40 ml CzHsOH, and 500 ml Ratio of Chloride Soln.: Added t o Pptg. S o h . G Pptg. Soln. 10:30 .. ...
15:45
-
... -..~
KCl, G . Recovered 0.0199 0.0402,0.0397 0.0999,0.0991 0.2496,O. 2497 0.0399 0.0403 0.0202,0.0197 0,2499 0.2904 0.0200 0.0409,0.0406 0.2492 0.2495 0.0198 0.2501 0,2502
1.0
2.0 2.0
0.0200 0.0200
0.0200 0.0206
2.0 0.25 2.0 2.0
0,2500
0.2499
0.0200
0.0198,O. 0202
2.0
0.2500
0.2499
..
Naci ' ' h-aCl SaCl NaCl NaCl NaCl NaCl NaCl CaClz CaClz CaClz CaClz NaCl CaClz CaClz NaCl CaCL NaCl NiClz. 6 Hz0 CrCla, 6 Hz0 SiClz. 6 Hz0 CrCla. 6 Hz0 NaCl
ml HzO, 250 ml HBFI, 500 CHsOH) Added 0.0200 0.0400 0.1000 0.2500 0.0400 0.0400 0.0200 0,2500 0.2900 0.0200 0.0400 0.2500 0.2500 0.0200 0.2500 0.2500
..
, . .
...
1000 ml. CZHKOH;compound dissolved in 10 ml. stock solution containing 0.0400 gram KCI) Recovered Recovered Compound, 2 G . KC1, G . Compound, 2 G. KCI, G. CdClz 2 5 H z 0 0.0401 CoClz. 6 HzO 0.0406 ZnClz 0.0400 AICls. 6 H z 0 0.0400 hIgC1z 6 Hz0 0.0402 Ca(NOd9.4 H z 0 0.0403 hInClz.4 Hz0 0,0404 Cu(SOs)z.3 HzO 0,0401 FeCls. 6 HzO 0 0400 LiCl 0.0401
I
KC1, ~-
Added 0.0200 0.0400 0.1000 0.2500 0.0400 0 0400 0 0400
(;
.
Effect of Foreign Cations on Potassium Analysis with Fluoboric Acid (Precipitating solution. 0.5 gram NaCI, 40 ml. HzO, 250 ml. HBF4,. and
(Precipitating solution. 0.5 gram NaCl, 40 ml. HzO, 250 ml. HBFI, 500 ml. CzHaOH, and 500 ml. CHaOH) Ratio of KCI, G. Soh: Acid, NaC1, Pptg. S o h . 1 M1. G. Sdded Recovered 11:30 Hap01 0.0200 0.0206 HsPOi 0125 0.0200 0.0208 0.2500 0.2490 HaPo4 0.25 "01 0.25 0.0200 0.0199 HNOa 0.25 0,2500 0.2492 HSOs 0.0200 0.0196 12:30 HaPo11
-
Sac1
20 :40 20 60
Table 11.
Effect of Foreign Anions on the Determination of Potassium with Fluoboric Acid
1 grain NaCI, 40 ml. HzO. 250 ml HBF,, and 1000 ml. CHsOH)
Ratio of Soln.: I'ptg. S o h 10:30
11:30
Table 111.
Effect of lMethano1 on Determination of Potassium with Fluoboric Acid
0 : io
0.20 0.25 0.23 0.25 0 50 0.50 0.50
0.25 1.0 1.0 0.25
0.23
2.0 0.25
- ~~. ~
potassium sulfates in alcohol necessitates the absence of the sulfate ion but the addition of 1 ml. each of phosphoric and nitric acids to the chlorides had no effect on the determination of potassium under the conditions presented here (Table 111). -4lthough this point was not investigated, it is probable that the fluoride ion should not be present with the alkalies, for it has been reported (3) that when potassium fluoride reacts with fluoboric acid, a potassium fluoborate other than KBF4 is formed. Ethyl Alcohol versus Methanol. Ethyl alcohpl was used initially as a solvent in this investigation and it proved to be satisfactory for the determination of as much as 0.25 gram of potassium chloride in the presence of 0.25 gram of sodium chloride. Since both sodium and potassium chloride are more soluble in absolute methanol, the effect of methanol was studied. It was found that although larger amounts of sodium chloride could be tolerated in the presence of potassium chloride, the results were slightly low (Table IV). A 1 to 1 mixture of the alcohols was used next in the precipitating solution as well as the wash liquid, and acceptable results were obtained in the presence of a t least 0.50 gram of sodium chloride. Table V shows that 20 to 250 mg. of potassium chloride can be determined in the presence of up to 500 mg. of sodium chloride with a relative error of less than 1%and that moderate amounts of calcium, nickel, and chromium chlorides do not interfere in the determination of potassium. Thus, a 1 to 1 mixture of ethyl alcohol-methanol seems superior to ethyl alcohol alone and it appears that a higher concentration of methanol to ethyl alcohol also will give good results in the presence of even greater amounts of sodium chloride. The 1 to 1 mixture proved much more efficient than the ethyl
_
V O L U M E 2 7 , N O . 1, J A N U A R Y 1 9 5 5 alcohol alone for washing the potassium fluoborate precipitate free of occluded salts; only when high amounts of sodium chloride or other salts were present was more than one washing found to be necessary. The precipitate was considered free of impurities when washing with 30 ml. of the 1 to 1 ice-cold solution caused no greater than a 0.5-mg. change in weight. When ethyl alcohol or the 1 to 1 mixture of ethyl alcoholmethanol is used for the determination of potassium fluoborate, the precipitate has a white, almost gelatinous appearance, but when methanol alone is used, the potassium fluoborate appears to be very fine, granular, and almost transparent. The precipitate from methanol requires more careful handling during transfer to the crucible, and technique difficulties may be the reason for the slightly lower results.
83 The necessity of having to work with ice-cold solutions is a slight disadvantage but when more is known about the solubility of potassium fluoborate in other organir solvents, it is conceivable that a method could be developed for precipitating potassium fluoborate quantitatively a t room temperature. ACKNOWLEDGMENT
The author gratefully acknowledges the encouragement and advice of William A. Dupraw during the course of this investigation. LITERATURE CITED
Booth, H. S., and Martin, D. It., “Boron Trifiuoride and Its Derivatives,” pp. 99-106, New York, John Wiley & Sons, 1949.
Flaschka, H., and Amin, A. XI., Chemist Analyst, 42, No. 4, DISCUSSION
78 (1953). I h i L ‘ 4 3 . N o. .. _ 1.4 _..__,._,_ , _(1954). ~
Except for the difficulty encountered when calcium and aluminum are present in the same solution, the determination of potassium as potassium fluoborate in many materials is simple by this rapid method in the presence of the common alkalies and other salts. The method has its advantages over the perchlorate and chloroplatinate methods in speed and costR, respectively, and Rhould he ideal for routine work.
Gloss, G. H., Ibid., 42, No.3,50 (1953). Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” New York, N. Y., John Wiley & Sons, Ino., 1929. Mathers. F. C.. Stewart, C. O., Housernann, H. V., and Lee. I, E..
J. Am. Chem. Soc.. 37. 1515-17 (1915). Robinson, J. W., C h a . Age (London), 66,’447-50,467-9, 507-10, 573-7 (1952). RbrmvED
for review April 19. 1954.
Arcepted September 17, 1954
Semiautomatic Gas Separation Equipment CHARLES W. HANCHER’ and KARL KAMMERMEYER Chemical Engineering, State University of lowa, lowa City, lowa
With manual operation of apparatus for gas separation, the chance of introducing errors is great. A simultaneous-sampling, double automatic gas buret apparatus was developed whereby one operator could handle the equipment with a greater degree of accuracy because the timing and pressure control are automatic. Experimental data with the described instrument agreed well with data obtained from the manually operated apparatus and less time was required per determination.
F
ROhf 1820, lvhen Graham ( 4 ) carried out his initial work 011 gas separation, which resulted in the statement of Graham’s la\v, until 1945, gaseous phase separation was only a laboratory phenomenon. The first and foremost important application of gas separation, using a porous membrane, was in the separation of uranium isotopes. When a membrane or barrier, Khcthcr it be plastic, porous glass or ceramic, or metal is considered for use as a separating membrane, two characterist’ics should be determined: the rate of gas flow for a given pressure drop across t,he membrane. and the amount of enrichment in one or more of tlrr componerit,sof the gas as it permeates through the membr:tne. The theory and the literat,ure of the gaseou~phase separation have been well covered in previous publicat,ions (1, 2. 6, 7-9). The membranes under consideration will permit one or more of the three types of Row which are normally encountered \Then gases flow through membranes: molccular streaming or Knudsen flow, viscous or Poiseuille flow, and a mixture of niolecular streaming and viscous flow. Recent publications ( 3 . 5) have emphasized t,he fact that another flow phenomenon must be considered when vapor flow through rnicroporous membranes is included-that is, the occurrence of adsorbed (or condensed or surface) flow, resulting from presumable formation of a sorbed liquid phase in the microporous structure of the membrane. I
Prpsent a d d r a n . Oah Ridge National Laboratory, Oak Ridge, Tenn.
TYPES OF M E M H R i N E S
Membranes available for wparat’i(iii are essentially oi t n-o types: plastic films and porous bodies. While a plastic rnernbrane undoubtedly possesses a porous structure, it has been found helpful to differentiate between plastic films on one hand antl microporous membranes on the othcr hand. In general, microporous membranes are considrrctl t,o h r t v ~pores wit.h diamcters of about, tmhemean free pat’h size of gases, while the much smaller porous structure in the plastic membranes is caused by the spacing between the molecules of tlic plastic. JIicroporous membranes are actually capillary systems with interconnected pores. Such membranes can be prepared by two inethotls--producing microporee bj- rcnioving an interdispersed phast. or component or by reducing lwge holes which already exist in the membrane. A good example of the removal of a dispersed phasc is the preparation of porous glass (6). The technique of reducing the size of esisting holes in a membrane would be represented by ceramic pract.ice antl powder metallurgy. I t was shonn (1) that the test, gas mixture hydrogen-carboii dioside may behave very different~lyn hen plastic membranes arc used than when microporous membranes are used. The carbon cliositl(~i~ often selectively rnriched n.hrri the gas permeates through mstiy of the plastic inembrarics. This selective enrirlirneiit ia c.onsit1ert.d to he causcd by solubility pheiiomena (1). Sepai,atiniis i i r rnicroporous menibrsnes essentially obey Graham’s diffusion h t v , which iii it,s simplest form states that the separation is R funct’ion of the square root of the inverse ratio of the molecular weights. Under cerbain conditions the phenomenon of condcnsed flow will be encountered even with such a gaslikc substnncc a.s c : t i h ) t i dioxide. THEORE1‘IC:&L (;OVSII)ERATIONS
F r o m the rate equations for gas diffusion and a material balance, Reller and Steiner (8, 9) developed the equations for the binary system, providing a method for predicting separation results for two somewhat different caws of flow conditions. Th(,
