124
ANALYTICAL EDITION
In the experiments given in Table I, 25 ml. of 0.1 N potassium bromate containing 50 grams of potassium bromide per liter were used. This quantity of potassium bromate is suitable for samples containing from 10 to 100 mg. of furfural. The results show clearly that a t 0" C. and within about 3 to 10 minutes only one mole of bromine reacts with the furfural. The use of specially constructed Erlenmeyer flasks eliminates the loss of bromine during the reaction. This is shown by the fact that the blanks run on the reagents standing as long as one hour checked within 0.1 per cent. As a further precaution against the loss of bromine it is advisable to seal the ground-glass joint with sirupy phosphoric acid. The presence of direct sunlight should be avoided during the reaction, but no evidence of marked photochemical influence was observed under laboratory conditions in check experiments run in diffused light or in the absence of light. Comparative studies a t O", 5", lo", 21" C., and higher on both furfural and furoic acid indicate that the end point can be more quickly and accurately ascertained for the reaction of furfural with one molecule of bromine a t 0" C. than with two molecules a t higher temperatures. As furoic acid reacts very rapidly with more than one molecule of bromine under these conditions, it is suggested that furfural is converted
Vol. 6, No. 2
first into a bromo derivative such as 4,5-dibromo-2-furfural
($1. CONCLUSION
A method is given for the quantitative estimation of furfural by treating it 5 minutes a t 0" C. with an excess of 0.1 N potassium bromate plus potassium bromide in 3 per cent hydrochloric acid and determining the unused bromine with potassium iodide and 0.1 N thiosulfate. The furfural combines with one mole of bromine. LITERATURE CITED (1) Eok, P. N. van, Verslag. v. d. Verricht. v. h. Centr. lab. v. d. Volksgerondheid, (1918); Chem. A h . , 14, 509 (1920). (2) Gilman and Wright, J . Am. Chem. Soc., 52, 1170 (1930). (3) Kline, G. M., and Acree, S. F., B u r . Standards J . Research, 8, 25 (1932). (4) Magistad, 0. C., IND. ENG.CHEM.,Anal. Ed., 5, 253 (1933). (5) Pervier, N. C., and Gortner, R. A,, IND. ENG.CHEM.,15, 1167, 1255 (1923). (6) Powell, W. J., and Whittaker, H., J . Soc. Chem. I n d . , 43, 35T (1924). RBCEIVED December 5, 1933. Publioation approved by the Director of the Bureau of Standards, U. S. Department of Cornmeroe.
A Study of Synthetic Cryolite Analysis F. J. FRERE, Pennsylvania Salt Manufacturing Company, Philadelphia, Pa.
T
HE chief distinction between natural cryolite (Kryolith) and synthetic cryolite lies in the fact that all the constituents of the natural variety are combined in a definite chemical ratio and that this salt always occurs as an individual and never as a mixture. Therefore, in order to determine its purity it is necessary only to make a single determination of any one constituent. On the other hand, synthetic cryolite usually contains other compounds such as sodium fluoride, aluminum fluoride, and occasionally aluminum oxide, which may vary over a wide range, depending to a great extent upon the method of preparation. In view of the great differences which exist between these two products, an analytical procedure for the determination of each component in synthetic cryolite becomes highly desirable. The primary purpose of this investigation, however, has not been to set forth any definite analytical procedure, but rather to point out the differences between the two products, to identify some of the chief components, and to test the applicability of certain procedures which may ultimately lead to an acceptable method of analysis.
fluoride concentrations was calculated. The results of these experiments are given in Table I. TABLEI. SOLUBILITY O F NATURAL CRYOLITE IN SOLUTIONS OF SODIUM FLUORIDE NaF ADDED NaaAlFe FOUND NaF ADDED Gram/lOO ml. 0.0 0.006 0.015 0.025
Grarn/lOO ml. 0.0345 0.0263 0.0176 0.0059
Gram/lOO ml. 0.035 0.050 0.075 0.100
AQUEOUS
NasAlFs FOUND Gram/lOO ml. 0.0020 0.0011
0 . OOOb
0.0000
DETERMINATION OF SODIUM FLUORIDE The possibility of a procedure for the direct estimation of sodium fluoride seems rather remote, owing to the lack of a suitable solvent in which sodium fluoride is alone soluble. As a result, an indirect procedure based on the insolubility of cryolite in the presence of sodium fluoride has been developed, which succeeds only in the absence of soluble or reactable aluminum salts. Since aluminum fluoride is completely insoluble, its presence causes no interference. PROCEDURE. Weigh out 5 grams of the sample and transfer to a 500-ml. volumetric flask. Add about 250 ml. of water and a itate vigorously for about 0.5 hour t o insure complete solution SOLUB~LITY I N AQUEOUS SOLUTIONS OF SODIUM FLUORIDE o f the sodium fluoride. Add a sufficient quantity of a known As a basis for a possible quantitative estimation of sodium solution of sodium fluoride t o give this salt a concentration of fluoride, the solubility of cryolite in aqueous solutions of this about 0.10 gram per 100 ml. Make up to volume, mix thoroughly by shaking, and filter on a dry funnel. Transfer 100 ml. of the salt was determined. solution t o a 250-ml. beaker and make exactly neutral to phenolAdd a few drops of methyl red and titrate with a Samples of natural cryolite to which were added known varied phthalein. amounts of sodium fluoride were made up to a definite volume, standard solution of yttrium nitrate. The sodium fluoride added subtracted from that found represents the amount present in the stoppered tightly, and allowed to remain at room temperature (approximately 25" C.) for one week. Approximately a tenfold original sample. excess of cryolite was used to insure saturation. The flasks were In order to ascertain the applicability of such a procedure, shaken at frequent intervals in order that equilibrium might be established as com letely as possible. At the end of this period, synthetic mixtures were prepared from pure materials and the filtered portions o?each solution were treated with about 10 ml. sodium fluoride was determined. The fluorine was deterof perchloric acid and the fluorine was expelled by evaporating mined by means of yttrium nitrate as outlined by the author t o fumes in a platinum dish. The aluminum in the residues (3) in a previous article. The results of these experiments are was determined by means of 8-hydroxyquinoline and from the amount found the solubility of cryolite at the various sodium given in Table 11.
.
March 15, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
123
OF SODIUM FLUORIDE IN SYNTHETIC Natural cryolite obtained from Greenland was used for the TABLE11. DETERMINATION MIXTURES experiments. The material was selected very carefully by NaaAlFs ADDED ALF6 ADDED N a F ADDED N a F FOUND hand and analysis showed no trace of impurities. The results % % % % of these experiments are given in Table 111.
DETERMINATION OF MOISTURE The determination of moisture in synthetic cryolite may be accomplished with fair success, providing certain precautionary measures are taken. The water content usually varies DETERMINATION OF SODIUM over a wide range and is always bound rather firmly. HeatThe procedure outlined by Barber and Kolthoff ( I ) , eni- ing cryolite a t relatively high temperatures in the presence of ploying the use of uranyl zinc acetate as a reagent, has been water results in partial hydrolysis with the formation of alumina and the liberation of fluorine as hydrofluoric acid. found to be satisfactory for the determination of sodium. The volatility increases rapidly with an increase in impurities. SOLUTION OF SAMPLE.Weigh out a 1-gram sample and transFor further details, the observations of Fredotieff and Iljinsky fer to a platinum dish. Add 8 to 10 ml. of a 30 per cent solution (2) on the system sodium fluoride-aluminum fluoride may be of perchloric acid. A few milliliters of nitric acid aid in the decomposition. Place upon the hot plate and digest at a low heat consulted. I n general, it was found that synthetic cryolite could be at first in order to prevent any loss due t o spattering. Gradually increase the heat and, finally,'fume to dryness. Moisten the dried satisfactorily to constant weight a t a temperature of residue with a few milliliters more of perchloric acid and again 700" C. Practically no change in weight was observed after take to dryness. Repeat this operation at least three times. Add a small amount of water and digest on the hot plate until igniting for 1 hour, though occasionally a sample was found the residue has loosened from the dish. Transfer to a beaker, add which decomposed a t this temperature. However, there are about 25 ml. of hydrochloric acid, and heat until all salts are no means by which such a sample may be identified beforedissolved. Transfer t o a 500-ml. volumetric flask, cool, make hand; this can be done only by trial. At 800" C. practically u t o volume, and mix thoroughly by shaking. Transfer 10 ml. ofthe solution to a 100-ml. beaker and evaporate until it just all synthetic cryolites investigated showed considerable debegins to crystallize. Add a few drops of water t o dissolve any composition. crystals and precipitate the sodium according to the procedure A number of samples of synthetic cryolite, which are fairly of Barber and Kolthoff (1). representative of the product now on the market, have been analyzed according to the above outlined procedures. The 'TABLE111. DETERMINATION OF SODICM IN NATCRAL CRYOLITE total fluorine was determined as outlined by the author (S), SODIUM FOUND and the alumina, by means of 8-hydroxyquinoline after exHClO? HClOd + ,B203 His04 SODICM PRESENT decomposition decomposition decomposition pelling the fluorine. % % % % As will be seen, samples 5, 6, and 7 contain rather high per32.57 32.56 32,86 32.85 32 85 32.53 32.55 32.86 centages of free aluminum oxide. The fluorine found in sam32.86 32.84 32.49 32.52 ples 8,9, and 10 is in excess of that required for the compounds 32.84 32.47 32.51 32.86 32.45 32.50 32.86 32.84 which are most commonly present in synthetic cryolite. This would seem to indicate the presence of other fluorine comSulfuric acid, perchloric acid, and perchloric-boric acid pounds which are perhaps of a more volatile nature. The fact mixture (4) were used for decomposing the cryolite. The de- that sample 8 slowly decomposed a t 700 O C. substantiates composition was carried out in a platinum dish, with the ex- this belief to a certain degree. Since only a limited quantity ception of the perchloric-boric acid mixture which was done of these samples was available, a further and more complete in a Pyrex beaker. Although good results were obtained bl' investigation could not be made. TABLEIV. ANALYSISOF SYNTHETIC CRYOLITE SAMPLE
Na
F2
%
%
32.34 53.78 52.51 31.36 52.56 31.65 52.51 32.49 4 52.04 30.41 5 24.30 48.48 6 31.22 16.99 7 53.78 28.73 Sa 51.26 27.79 96 51.29 27.00 10 b Deoom osed slowly a t 700' C. b I g n i t e f a t 500' C. C N. D. Not determined.
AlzOa
% 24.19 23.79 23.52 22.64 25.06 30.60 42.00 24.10 24.53 24.90
NaaAlFs % 97.69 92.82 91.32 87.40 81.89 72.79 46.38 84.84 77.29 74.34
EXCESS Ah01
EXCESS Fz
NaF
AlzFs
%
%
%
%
0.81 2.09 2.24 2.37 5.42 12.67 9.95 5.80 9.53 11.31
0.0 0.0
0.0 0.0 0.0 0.0
0.46 1.56 3.00 6.88 6.40 0.70 3.20 1.56 4.37 4.69
0.0
0.0 1.91 5.35 24.72
0.0
0.0 0.0
0.0 0.0 0.0 3.07 0.85 1.14
H2O % 1.03 2.74 3.20 2.22 4.72
N.D.C 15.94 5.38 4.45 5.23
TOTA~
% 99.98 99.21 99.76 98.87 100.34 91.61 100.19 100.65 96.49 96.71
Q
all three methods, those obtained with the perchloric acid decomposition were considerably better and are to be preferred. The acidity of the solution from which the sodium was precipitated seemed to be of no great importance as long as there was sufficient acid present to hold the alumina in solution. As much as 1 ml. of concentrated hydrochloric acid and as little as 1 nil. of 0.5 N hydrochloric acid was used with equally good results. It was found necessary to use an improvised water bath when precipitating the sodium, so that the temperature of the solution a t the time of filtration was approximately the same as that of the reagent when filtered from the triple salt. The wash solutions should also be of the same temperature.
The basis for the calculations is as follows: The amount of sodium combined as free sodium fluoride, deducted from the total sodium, represents the cryolite content and the remaining fluorine and aluminum are calculated to aluminum fluoride, aluminum oxide, or excess fluorine as the figures permit. The results of these analyses are shown in Table IV.
LITERATURE CITED (1) Barber, H. H., and Kolthoff, I. M., J. Am. Chem. SOC.,50, 1625 (1928); 51, 3233 (1929). (2) Fredotieff and Iljinsky, 2. anorg. Chem., 80, 113 (1913). (3) Frere, F. J., IND.ENO.CHEM.,Anal. Ed., 5, 17 (1933). (4) Schrenk and Ode, Ibid., 1, 201 (1929). RECEIVED June 10, 1933.
