January 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
DelBpine (a), who found that the base was precipitated by HgC12, HgBrz, HgI2, KBiI4, and other salts. Following are the approximate molar concentrations a t which precipitation of various metallic derivatives of allyl iodourotropine take place. SALT SbCla Bi(NOda FeCla PbClr
MOLARCONCN. 5 x 10-4 5 x 10-3 7 x 102 . 4 x 10-7
SALT Hg(N0a)r SnCL CUClZ AsC1a
MOLAR CONCN. 4 x 10-4 6 . 7 X 10-8
x 1x
5
1010-
Pentavalent arsenic and antimony do not yield precipitates when treated with the reagent. Likewise molar solutions (or in some cases saturated) of LiC1, NaCl, KCl, BaC12, CaC12, SrClz, Mg(N03)2, ZnSO4, A1 (NO&, La(N03)3, Ce(KO&, Ce(N03)*, ZrOC12, Th(NO&, SnC14, MnC12, KReO?,
17
CrC13, NiCI2, and FeSOc do not form insoluble complex allyl iodourotropine derivatives. The use of allyl iodourotropine as a precipitant for cadmium is, therefore, not a reliable method because of solubility and adsorption errors. The interference of other metals has been indicated. LITERATURE CITED (1) D a t t a , R. L., J.Am. Chem. Soc., 36, 1006 (1914). (2) DelBpine, Bull. soc. chim., [3] 17, 293 (1897). (3) Evrard, Ann. chirn. anal. chim. a p p l . , 11, 322 (1929). (4) Reilly, J., “Physico-chemical Methods,” p. 607, Van Nostrand, 1926. RECEIVED March 17, 1932. L. C. Hurd‘s present addrees is Marienstr. 33, Hannover, Germany. This paper is from the senior theais of R. W.Evans, University of Wisconsin, 1932.
Determination of Fluorine in Cryolite F. J. FRERE,Pennsylvania Salt Manufacturing Company, Philadelphia, Pa.
T
HE determination of fluorine in minerals containing appreciable amounts of this element has always been accomplished by difficult and tedious procedures which, a t their best, have left much to be desired. Many compounds have been proposed for the gravimetric determination of fluorine, the most common of which are: calcium fluoride (S), lanthanum fluoride (IO), thorium fluoride ( l a ) ,and lead chlorofluoride (16). While good results have been reported in some cases, preliminary trials proved none of these methods applicable to complex compounds of high fluorine content such as cryolite, except that in which the fluorine was precipitated as lead chlorofluoride. The volumetric methods of Hempel and Scheffler (6), Wagner and Ross (16)’ and Penfield (11), which depend upon the evolution of the fluorine as silicon tetrafluoride, are impractical, as it is not possible to recover all the fluorine by such distillations. According to Reynolds, Ross, and Jacob ( l a ) , 92 per cent is about all that can be obtained by the best available procedures. Shuey (14) has made an investigation of the recovery of fluorine from sodium fluoride and states that the average of the results obtained would suggest the possible use of a factor in placing the recovery of fluorine on a 100 per cent basis. His results, for the most part, are rather inconsistent and it is felt that the use of a factor would be unreliable. Kurtenacker and Jurenka (8) have proposed the use of cerous nitrate as a reagent for the direct titration of fluorine using methyl red as an indicator. More recently Batchelder and Meloche ( I ) have outlined a procedure involving the use of ampho magenta as an adsorption indicator for the direct titration of fluorine by means of cerous nitrate. Data have been given showing a comparison of results obtained by this method and those obtained by the method of Kurtenacker and Jurenka. According to Batchelder and Meloche, methyl red gave slightly better results in the case of smaller quantities of fluoride, and in a later publication (9) they have given further details concerning this reagent. It has been the writer’s experience that methyl red is the more satisfactory of the two indicators. Kurtenacker and Jurenka reported difficulty in obtaining results in agreement with the theoretical values based upon the cerium content of the solution added. Differences amounting to about 4 per cent have been observed by them. This has been the writer’s experience in nearly all cases. Batchelder and Meloche reported no such discrepancy.
DETERMINATION OF FLUORINE USINGYTTRIUM NITRATE The writer has found that yttrium nitrate may be used as a satisfactory reagent for the direct titration of fluorine using methyl red as an indicator. Quantities of sodium fluoride ranging from a few tenths of a milligram to 0.3 gram were titrated with equally good success. I n pure solutions the results were in close agreement with the theoretical values based upon the yttrium content of the solution added. However, titrations could not be made a t 80” C. as in the case of cerous nitrate. It was found that potassium chloride and nitrate produced errors varying from -3.3 per cent at 1 gram per 100 ml. to -6.2 per cent at 4 grams per 100 ml. with practically no increase up to 8 grams per 100 ml. The corresponding sodium salts produced an error varying from -1.2 per cent to -3.8 per cent under the same conditions. Mixtures of potassium and sodium salts gave errors varying from -2.3 per cent at 0.5 gram per 100 ml. to - 6.2 per cent a t 4 grams per 100 ml. Sulfates caused an over-titration in concentrations greater than 0.1 gram per 100 ml. MATERIALSUSED. The sodium fluoride used as a primary standard was furnished through the courtesy of V. W. Meloche, of the University of Wisconsin. Several sodium determinations, as well as tests for impurities, showed the material to be of excellent quality. Natural cryolite obtained from Greenland was used for these experiments. The material was very carefully selected by hand and gave the following analysis: THEORETICAL Na AliOa
FOUND
%
%
32.86 24.26
32.85 24.25
White reagent-grade cerou8 nitrate was used. The solution was standardized according to the method of Metzger (9) and showed a purity of 99.0 per cent as compared to the cerium determined gravimetrically. Reagent-grade yttrium nitrate was used, which tests showed to be of a good quality. This reagent may be readily obtained in pure form.
PROPOSED METHOD After a thorough investigation of the methods already described, it was decided that the use of yttrium nitrate for the direct titration of fluorine in cryolite offered the best possibility.
18
ANALYTICAL EDITION
STANDARD SOLUTION OF YTTRIUMNITRATE. Dissolve aproximately 16 grams of Yt(NOa)~4Ha0in one liter of water. beat the solution to boiling and continue to boil for 2 to 3 minutes. After allowing t o stand overnight the solution is filtered and standardized against a neutral solution of pure sodium fluoride containing approximately the same amount of sodium and potassium nitrates as will be present in the unknown. The titration is made at room temperature using methyl red as indicator. The volume of the solution and the amount of fluorine in the blank should be approximately the same as that .~ in the unknown. FUSION OF SAMPLE.Mix intimately in a platinum crucible, by means of a platinum wire, 0.5 grim of the sample with a mixture of 6 grams of sodium and potassium carbonates and 0.5 t o 0.6 gram of finely ground quartz. Cover the crucible with a platinum lid and fuse at a moderate heat for 1 to 1.5 hours. Transfer the crucible containing the cooled melt t o a 400-ml. beaker, cover with water, and digest on the hot plate until disintegrated. Filter and wash thoroughly with hot water. NEUTRALIZATION OF THE SOLUTION.Add a few drops of phenolphthalein and add dilute nitric acid (1 to 9) dropwise and with rapid stirring until the pink color is datroyed. Heat the solution to boiling and continue the successive small additions of nitric acid until the sodium carbonate has been reduced to about 0.5 per cent. The volume of the solution should be reduced to about 200 t o 250 ml. Now add, dropwise and with rapid stirrin a neutral solution of zinc nitrate (10 per cent) until the pint color of the indicator no longer reap ears. Continue the boiling for 2 to 3 minutes. Filter and w a 8 thoroughly with hot water. Add dilute sodium hydroxide (0.25 N carbonate free) from a buret until the solution is just a faint pink. Evaporate the solution t o a volume of about 150 ml. and transfer to a 200-ml. volumetric flask. Cool to room temperature, make up to mark, mix well, and filter into a dry flask. Transfer 100 ml. of the solution TITRATION OF THE SOLUTION. to a 250-ml. beaker and make exactly neutral to phenolphthalein. Add 3 to 4 drops of methyl red and titrate a t room temperature with a standard solution of yttrium nitrate. The error produced by alkali salts may be easily corrected by adding to the blank determination the approximate amount of each corresponding salt which will be present in the unknown or by adding to both sufficient potassium nitrate to give this salt a concentration of 4 to 5 grams per 100 ml. I n concentrated fluoride solutions the sensitivity of methyl red to yttrium nitrate was far superior to that of cerous nitrate, l to 2 drops of a 0.04 N solution being sufficient to cause a very sharp color change, If, on the other hand, the fluoride concentration was less than 0.025 gram per 100 ml. the sensitivity of the indicator to cerous nitrate was greatly improved, while in the case of yttrium nitrate it was diminished to such an extent as to make its use impractical. FUSION OF SAMPLE. Cryolite may be satisfactorily decomposed by fusing a t a moderate heat with a mixture of sodium and potassium carbonates. Silica must be added to aid in the decomposition. The melt disintegrates easily and offers no difficulty in dissolving. For materials containing appreciable amounts of fluorine, Hillebrand (6) recommends that the insoluble residue be ignited and the fusion and extraction repeated, I n this work only occasionally was as much as 1mg. of fluorine found in the residues. Hawley (4) recommends that the insoluble residue be boiled with a strong solution of sodium carbonate to extract the last trace of fluorine. This is only a precautionary measure and is of no value unless the original fusion was properly carried out. NEUTRALIZATION OF SOLUTION. It was found that neutralization could be effected completely and .in a relatively short time by adding dilute nitric acid a t boiling temperature, using phenolphthalein as an indicator until the sodium carbonate in the solution was reduced to about 0.5 per cent, and finally completing the neutralization by the addition of a small excess of a neutral solution of zinc nitrate. After the solution was filtered, the excess zinc was removed by the addition of dilute sodium hydroxide until the phenolphthalein just turned pink.
Vol. 5 , No. 1
After transferring the solution to a volumetric flask, filtered portions were titrated with standard solutions of yttrium nitrate and cerous nitrate. Another portion of the solution was used for precipitating the fluorine as lead chlorofluoride, which was dissolved and the chlorine titrated according to the method of Hoffman and Lundell (7). Fluorine in a Bureau of Standards sample of opal glass, No. 91, was also determined in a further effort to test the adaptability of the method to glasses and enamels. Yttrium nitrate gave unsatisfactory results and could not be used. The results are given in Table I. TABLE I. COMPARISON OF Yt(NO&, Ce(NO&, AND PbClF METHODS FOR DETERMINATION OF FLUORINE (Results calculated on basis of 0.26-gram sample for Yt N0a)a and Ce(NOa)a, and 0.1875-gram sample for PbClh) FLUORINE FLUORINE FOUND= MATERIAL PRESENT Yt(N0a)sb Ce(N0a)ac PbClF
%
%
%
Cryolite 54.29 54.29 54.15 54.29 64.24 Cryolite 54.08 64.29 64.24 Cryolite 54.05 64.29 Cryolite 64.22 53.93 54.29 Cryolite 54.19 63.91 54.29 54.13d Cryolite 54.10d 54.29 64.13d Cryolite 64.06d 5.758 Opal glaas 5.76 5.75 Opal glass ... 5.74 6.75 Opal glass ... 5.71 6.75 ... Opal glass 5.71 5.75 ... Opal glass 5.70 0 No determinations made where blanks are shown. b Titration made at room temperature. c Titration made at 80° C. d Residue boiled with 50 ml. of 2 er cent sodium carbonate. e Certificate value of Bureau of Etandards opal glass No. 91.
...
% 53.43 63.33 53.27 53.27 53.23
... ... ... ... ... ...
...
SUMMARY A volumetric method has been outlined for the determination of fluorine in cryolite, using yttrium nitrate as a reagent. The method did not succeed when the amount of fluorine was less than 0.025 gram per 100 ml. The effects of alkali salts are shown. ‘Neutral solutions must be used in all cases. The lead chlorofluoride method gave low results. Cerous nitrate is recommended as a reagent for the determination of fluorine in glasses. A rapid method has been outlined for the neutralization of the solution.
LITERATURE CITED (1) Batchelder and Meloche, J. A m . Chem. Soc., 53, 2131 (1931). (2) Ibd., 54, 1319 (1932). (3) Berzelius, Pogg. Ann., 1, 169 (1824). (4) Hawley, IND.ENG.CHEM.,18, 573 (1926). (5) Hempel and Scheffler, 2. anorg. Chem., 20, 1201 (1899). (6) Hillebrand, U. 8. Oeol. Survey Bull. 700. (7) Hoffman and Lundell, Bur. Standards J.Research, 3, 681 (1929). (8) Kurtenacker and Jurenka, 2. anal. Chem., 82, 210 (1930). (9) Metzger, J. Am. Chcm. Soc., 31, 523 (1909). (10) Meyer and Schulz, 2. angew. Chem., 38, 203 (1925). (11) Penfield, 2. anal. Chem., 21, 120 (1882). (12) Pisani, Compt. rend., 162, 791 (1916). (13) Reynolds, Ross, and Jacob, J. Assoc. Ofzcial Agr. Chem., 11, 225 (1928). (14) Shuey, Ibid., 14, 126 (1931). (15) Starok, 2. anorg. Chem., 70, 173 (1911). (16) Wagner and Ross, J. IND.ENG.CHEM.,9, 1116 (1917). RECEIVED August 2, 1932.
As a result of a study of methods of solution of CORRECTION. cubic equations, which fit the curves for certain potentiometric titrations in the region of the end point, I deduced the formula for finding the inflection published in IND.EKG.CIEEM.,Anal. Ed., 4, 144 (1932). Not until after publication of that paper was it brought to my attention that in Z . anal. Chem., 69, 417 (1926), Hahn employs what is essentially the same method. I regret that I did not know of his solution earlier; recognition of it would have saved me considerable work. FLORENCE FENWICK
