18
ANALYTICAL EDITION
STANDARD SOLUTION OF YTTRIUMNITRATE. Dissolve apin one liter of water. roximately 16 grams of Yt(NOa)~4Ha0 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 54.13d 54.29 Cryolite 54.10d 64.13d 54.29 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. (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
LITERATURE CITED Batchelder and Meloche, J. A m . Chem. Soc., 53, 2131 (1931). Ibd., 54, 1319 (1932). Berzelius, Pogg. Ann., 1, 169 (1824). Hawley, IND.ENG.CHEM.,18, 573 (1926). Hempel and Scheffler, 2. anorg. Chem., 20, 1201 (1899). Hillebrand, U. 8. Oeol. Survey Bull. 700. Hoffman and Lundell, Bur. Standards J.Research, 3, 681 (1929). Kurtenacker and Jurenka, 2. anal. Chem., 82, 210 (1930). Metzger, J. Am. Chcm. Soc., 31, 523 (1909). Meyer and Schulz, 2. angew. Chem., 38, 203 (1925). Penfield, 2. anal. Chem., 21, 120 (1882). Pisani, Compt. rend., 162, 791 (1916). Reynolds, Ross, and Jacob, J. Assoc. Ofzcial Agr. Chem., 11, 225 (1928). Shuey, Ibid., 14, 126 (1931). Starok, 2. anorg. Chem., 70, 173 (1911). 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
