Direct Determination of Potassium in Silicate Rock IIOB.SRT H. WILLARD, University of Michigan, Ann Arbor, Mich., L. M. LIGGETT State College, Ames, Iowa
T
HE advent of the triple acetate methods for the determination of sodium has made possible the direct determination of sodium in silicate materials. The silicate may be decomposed by treatment rvith hydrofluoric acid, the fluoride removed by evaporation with sulfuric acid, and the sodium precipitated immediately as magnesium or zinc sodium uranyl acetate, in which form it is weighed. Unfortunately, sulfate cannot be present in the determination of potassium when i t is iveighed as perchlorate. The Berzelius method, in which the sulfate is removed by precipitation as barium sulfate and the excess barium and other heavy metals are removed by precipitation with ammonia and ammonium carbonate, is long, and the coprecipitation of potassium with the barium sulfate makes the results somewhat uncertain. The J. Lawrence Smith method, in which the decomposition is accomplished by a fusion with ammonium chloride and calcium carbonate, is extremely time-consuming but yields results beyond reproach. Of the alternative ways of eliminating fluoride f o l l o ~ ~ i nthe g decomposition of the silicate by hydrofluoric acid, evaporation with perchloric acid woulrl appear feasible; actually, however, all fluoride cannot be removed even b y repeated evaporations with perchloric acid, probably because of the great stability of the fluoaluminate ion, AlFs---. Removal of fluoride by precipitation as calcium fluoride was suggested by Koenig (1) in 1933, but this method in the authors' hands has not been satisfactory. Another method of eliminating fluoride is now propoPetl in which the fluoride is removed by volatilization as hydrofluosilicic acid, HzSiFs, as in the method of Killard and Winter ( 5 ) for the determination of fluoride. Unfortunate results with the Koenig method led the authors to perform a number of determinations ivith the vieiv of locating the cources of error in the procedure.
AND
HARVEY DIEHL, Iowa
with either sulfuric or perchloric acid to elevate the temperature of distillation, the obvious choice is perchloric acid, which yields directly the solution of perchlorates needed for the determination of potassium by the perchlorate method and also furnishes a method of dehydrating any residual silica prior to the determination of the potassium. As no ammonium salts are introduced during the analysis their removal is obviated. Since in this case it is immaterial whether some perchloric acid is distilled with the hydrofluoric acid, the distillation was carried out at temperatures of 140-150' C.-that is, somewhat above the 135' recommended by Willard and Winterand the time-required for the distillation was thus shortened. The distillation is best effected b y injecting steam into the distillation flask and regulating the flame below the flask t o give the desired temperature. I n final form the all-Pyres distillation vessel shown in Figure 1 was employed. If the bulb mere attached t o the stem by a ground joint, the removal of the contents would be somewhat facilitated. A volume of distillate of 400 to 500 ml. is sufficient to remove all fluoride. As is customary in the analysis of the alkali metals, blanks must be run concurrently. Since the apparatus used is platinum and Pyrex and the time required for the determination is not great, the blank is very small and consistent. Twenty blank determinations run during the course of the work gave a n average value of 0.0005 gram of potassium perchloFIGURE1 rate, the minimum and maximum values being 0.0003 and 0.0009 gram. Table I indicates that the method tends t o give slightly high results. This was thought to be due possibly to the fact that fluoride was taken into the distilling flask in the samples but not in the blank, in which no aluminum was present; this fluoride might then attack the distilling flask, introducing potassium into the sample but not into the blank. This would account for high results. I n order to make the blank duplicate the determination as closely as possible, a special grade of cryolite, SaaAlF6 known to contain a negligible amount of potassium, !vas added to the blank in a series of determinations. The values obtained in this way were 0.0005,0.0004, and 0.0008 gram of potassium perchlorate, identical in size and variation with the blanks run in the normal manner. It thus appears that the slightly higher results cannot be explained in this manner. Incomplete dehydration of silica picked up during the distillation might also account for the higher results. Following the distillation, the silica was dehydrated by evaporation with a n additional amount of perchloric acid and the usual boiling after the appearance of perchloric acid fumes. It was improbable that any silica contaminated the potassium salt, but as a check the potassium perchlorate finally obtained and
In a series of determinations of the sodium plus potassium in Bureau of Standards Sample Xo. 70, a high-potassium feldspar, the results obtained xere highly erratic and invariably low, sometimes by as much as 50 mg. in the 0.2440 gram of sodium plus potassium chlorides which should have been derived fr6m a 1gram sample. The mixed chlorides obtained were also found t o contain appreciable quantities of fluoride as measured by the bleaching effect on pertitanic acid. Treatment with calcium hydroxide does not remove fluoride completely in the analysis of a silicate, again probably because of the stability of the fluoaluminate ion. The calcium oxide used was prepared by ignition of the special grade of calcium carbonate usually used for J. Lawrence Smith fusions. I n some experiments the calcium oxide was slaked slowly by treatment with steam prior to use, with the object of improving the physical character of the calcium hydroxide. This had no beneficial effects on the results, nor did more extensive washing during the filtrations. Finally, weighed portions of pure potassium chloride and mixtures of pure potassium chloride and pure sodium chloride were treated with hydrofluoric acid, the solutions evaporated t o dryness, dissolved, added t o the calcium oxide, and carried through the procedure exactly as in the silicate analysis. The results were erratic and low by as much as 30 mg., samples of about 1 gram having been taken. Xo fluoride was found in the chlorides obtained. It thus appears that the alkalies are retained somewhere in the process, undoubtedly with the calcium hydroxide. In view of these results the authors considered the problem as still lacking an adequate solution. The separation of fluoride b y distillation as hydrofluosilicic acid has received considerable study since i t was originally proposed and t h e conditions and completeness of the separation have been confirmed. As the distillation may be effected
234
ANALYTICAL EDITION
March 15, 1942
TABLE I. DETERYINATIOS O F .\nnlysis SO.
Wcight of Sample Crams
Weight of
KCIOa Gram
POI'AsSIUlf 13 BUREAU OF
K.0 I'rrsent Found B. of S. L-
/'
Analysis No.
(7
1 0000 1 0011
1 0000
1 0000 1 0000 1 0010
0,3707 0.3708 0.3730 0,3741 0.3676 0 . 8680 0.3680 0,3676
12 . .i!J 12.39 12.60 12.70 12.48 12.49 12 49 12.4b
>
3 4
:1 000 :i 000
:I oon :I 0011
0.0896 0.0412 0 0400 0 0836
0 44
d.llIPLE3
Weight of liCl0I Gram
I . 0000 I . 0010
K?O Present
Found
0.09J.i 0 0948
B.of 9 . Si,
B. of S
%
(70
3.23 3.21
SO Lcad-Bnriiitri Glass
2 000 2.000
1). 4I)OS I ) . 4ot;n
2,000
0.4915
8.33 8.43 5.34
13. of J. So.97 Flint Clay
13. of J. So. 99 Soda Frlil.p:ir 1
STAXD.4RDS
Weight of Sample Grams
B. of S. ?;o. 91 Opal Glass ( C o n t ' d )
B. of S.S o . 70 Fe1da~i:ir 1 0000 1 0004
235
0 . L1
0.4li n 4%;
1 7
3
0 . :Is
13. o f J. Yo. 91 Opal G1a.r;
Procedure Weigh 1 to 2 grams of the sample, de ending on the potassium content, into a 20-ml. platinum crucifle, moisten with water, and add 3 ml. of 70 per Fent perchloric acid and 10 ml. of hydrofluoric acid, Stir well with a platinum wire or the rubber end.of a policeman, taking care not to let th? acid touch the glass; rinse, remove the stirring rod, place in a Hillebrand evaporator in a good hood, and allow to evaporate to dryness. Moisten with water, add 2 to 5 ml. more of hydrofluoric acid, depending.on the size of the sample taken, and 3 ml. of perchloric acid, stir well, and again evaporate to dryness. Transfer the salts to a 125-m1. Pyrex distilling flask, using a long-stemmed funnel and making sure that all insoluble material is transferred to the flask. Blanks should be started with the samples and carried through the entre procedure. The use of a two-hole rubber stopper carrying the thermometer and steam inlet tube is permissible only if care is taken to avoid concentration of the acid, which would then attack the rubber, perhaps violently. With this precaution no trouble has been experienced from this source. The all-glass distilling apparatus previously described is convenient, and eliminates the possibility of danger of the hot perchloric acid coming into contact with the rubber stopper. Add a few pieces of broken quartz or Pyrex to the flask and connect the flask to a steam generator and to a water-cooled condenser. Four of these may be set up and run simultaneously by one person. With the steam generator disconnected and the inlet tube closed with a piece of rubber tubing and a pinch clamp, concentrate the liquid in the flask to a volume of about 12 ml. by boiling gently. Cool somewhat, add 8 ml. of 70 per cent perchloric acid, heat to 140-150", and pass steam through the mixture a t a moderate rate until 300 to 500 ml. of distjllate have been collected. The larger volume is necessary only in case a large sample is employed. Shake the flask occasionally to wash down material spattered on the walls. Transfer the contents of the flask to a 250-ml. beaker and wash out well nith hot water, leaving behind the pieces of glass or quartz. Evaporate on a hot plate to strong fumes of perchloric acid, adding 2 ml. or enough perchloric acid so that the mixture may be boiled without spattering. Continue the evaporation a t a somewhat higher temperature to dehydrate completely the silica and to expel the excess of perchloric acid. Evaporate until the residue is just moist but not to dryness; otherwise insoluble basic salts of aluminum will be formed.
0 0176 0.0174 0.0178
0.58
1)
.i I
0.57 0.59
B . of 9 . So. 95 Plastic C h y 1 2 3 4
weighed was dissolved in water and the crucible dried and weighed. I n a few cases in earlier work some silica was found but none if the dehydration was properly performed. Results b y students on the silicate samples used as unknowns at the University of Michigan and at Iowa State College have also shown t h a t t h e method tends t o give slightly higher values for potassium than the J. L a w e n c e Smith fusion followed by the perchloric acid method. The method is more rapid than the Smith method and requires less applied time. Sulfates must, of course, be absent.
1.0013 1 . O00b 1.0010
2.0000 2,0010 1. ,5000 1.5013
0.1896 0.1891 0.lPll 0 . 143s
3.22 3.21 3.19 3.25
:i 17
A little silica will sepmatc 11 itli the potassium perchlorate. Cool the residue by immersing the beaker in cold water. Add 20 to 30 ml. of anhydrous ethyl acetate (2, 4), and stir until the calcium and aluminum perchlorates dissolve and the potassium perchlorate remains. Cool in cold water and filter through a small, dry, fine filter paper, transferring most of the precipitate with ethyl acetate from a wvnsh bottle. Wash three or four times with 2-ml. portions of ethyl acetate and discard the filtrate. Dissolve the potassium perchlorate remaining in the beaker with hot water, and pour it through the filter, collecting the filtrate in a 150-ml. beaker. Wash with hot water until all the potassium perchlorate has dissolved and at least ten times more, using small portions. Any silica will remain on the filter. Evaporate to gentle dryness. Cool, add 15 ml. of anhydrous ethyl acetate, warm slightly, and stir to extract the small amount of soluble perchlorates remaining. Cool, transfer to a weighed filtering crucible, and wash with five or six portions of ethyl acetate of about 1 ml. each. Pyrex sintered-glass crucibles of medium porosity are very satisfactory. Dry the beaker and scrape loose any adhering salt crystals with a bright metal spatula and brush into the crucible. Dry in an oven at 110' for 20 to 30 minutes and then heat the covered crucible to 310" ( 3 ) for 20 minutes, using a muffle or oven. Cool and weigh. Reheat at 310", cool, and weigh to ensure constant weight. The crystals burst during the heating and leave a fine powder. Subtract the weight of the potassium perchlorate in the blanks, usually 1 mg. or less.
Summary The metals in insoluble silicates are completely converted into perchlorates b y evaporation of the silicate with hydrofluoric and perchloric acids, followed by a steam-distillation a t 140" to 150" C. to remove fluoride as hydrofluosilicic acid. After dehydrating the small amount of silica, the solution is evaporated nearly to dryness and the potassium separated twice as perchlorate by extracting the soluble perchlorates with ethyl acetate.
Acknowledgment The authors wish to acknowledge the contribution of James B. Montgomery of Purdue University, who carried out some of the preliminary work of this investigation, and t o thank H. V. Churchill of the Aluminum Company of America for providing the special grade of cryolite used in the work.
Literature Cited (1) Koenig. E. W., ISD. ENG.CHEM.,ANAL.ED.,7, 314 (1935). ( 2 ) Smith, G. F., J . Am. Chem. SOC.,47, 762 (1925). (3) Smith, G . F., and Ross, J. F., Ibid., 47, 774 (1925). (4) Willard, H. H., and Smith, G. F., Ibid., 45, 293 (1923).
(5) Willard, H. H., and Winter, 0. B., IND.ENG.CHEV.,ANAL.ED., 5, 7 (1933).
