Colorimetric Determination of Fluorine with Ferron JOSEPH J. FAHEY Geological Survey, U. S. Department of the Interior, Washington, D. C.
HE difficulties encountered the quantitative determination of fluorine in rocks and minerals can be pointed out in no better way than by reference
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The colorimetric determination of fluorine, using the ferron-iron reagent herein described, is applicable to a wide range of materials, including rocks and minerals having up to 10 per cent of fluorine and natural waters with a minimum fluorine content of 1 part per million.
Mix the sample (usually 0.5 gram) with 5.0 grams of sodium carbonate and fuse the mixture in a Covered Platinum crucible over a Bunsen flame, taking care to keep the cover of the crucible After fusion attained further heating is needless and may cause a loss of fluorine. Cool and leach overnight with 300 ml. of water in a platinum dish. I n the morning bring just to incipient boiling, covering the dish with a &ratch glass, and filter hot through a 7-cm. Whatman No. 41 paper. Wash once with hot water and transfer the ' residue back to the dish with a jet of water, Add water until the volume is approximately 100 ml., boil for about 1 minute, filter through the same paper and wash well with hot water, Reserve residue A for the silica determination. To the filtrate in a large platinum dish add a solution containing 1.0 gram of zinc oxide in 30 ml. of hydrochloric acid (1 to 3). Cover with a watch glass and heat to boiling. Allow the Precipitate to settle and filter through a 12.5-cm. Whatman No. 41 paper. Wash the platinum dish and the residue on the paper once with hot water. Transfer the residue back to the dish, boil with about 75 mi. of water, filter using the same paper, and finally wash several times with hot water. Reserve residue B also for the silica determination. Carefully measure the volume of the solution and divide it into two equal parts. To one part add methyl orange indicator and hydrochloric acid (1 to 19) to the end,point from a buret. To the other part add the same volume of hydrochloric acid (1 to 19) using no indicator, followed by a solution of 0.5 gram of zinc oxide and 1 gram of ammonium carbonate in 2 ml. of concentrated ammonium hydroxide and 10 ml. of water. Cover the platinum dish with a large glass cover and boil gently until the odor of ammonia is no longer noticed. This requires reducing to a volume of about 75 ml. Add 50 ml. of hot water, stir +ell, allow the precipitate to settle, filter through a 9-cm. Whatman NO. 41
to the prolific literature on this subject. From the work of Berzelius (1) in 1816 to that of Stevens (9) in 1936-a period of 120 years-there have been published more than twenty methods and modifications thereof relating to the determination of fluorine and embracing gravimetric, volumetric, colorimetric, and nephelometric methods of analysis. As a review of the literature was made by Stevens (9), further efforts in that direction are unnecessary. However, Steiger's (7) method as modified by Merwin (6) is the most widely used in the analysis of rocks and the Hoffman-Lundell(6) lead chlorofluoride procedure is probably employed more than any Other where high Percentages of fluorine, characteristic of some minerals, are to be determined. The method herein described consists in matching in a comparison solution the yellowish hue of green produced by the reaction of fluorine in the unknown Solution on the ferron-iron reagent.
The Ferron-Iron Reagent Ferron (7-iodo-8-hydroxyquinoline-5sulfonicacid) was described by yoe(11) in 1932 as a co~orimetricreagentfor ferric iron. In Using this compound a6 a reagent for fluorine, a saturated water solution is combined with a water solution of ferric chloride and hydrochloric acid. The proportions of
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ferron, ferric chloride, and hydrochloric acid finally decided on rvere the result of some nineteen attempts to Produce a tive and stable fluorine reagent. When the iron content was too lo~v,the green color was too weak, and conversely, when iron than necessary was in the solution, the color was darker than desired. Hydrochloric acid was found to be the best of the three acids tried in the reagent; sulfuric acid causes a partial discharge of color and nitric acid a complete decolorization. As the concentration of hydrochloric acid was increased beyond the desired point, the color became paler, whereas a muddy green resulted from a deficiency of the acid. The iron as ferric chloride and the hydrochloric acid are contained in a water solution that is 2 N to hydrochloric acid and 0.1 N to ferric chloride. The composition of the ferron-iron reagent used is as follows : ~
Saturated water solution of ferron Ferric chloride and hydrochloric acid solution (described above) Distilled 'water
i
Residues C, A, and B contain the silica of the sample. Make up the filtrate to 250 ml. This volume will be adjusted during the course of the analysis, so that a 25-ml. aliquot portion will contain from 0.1 to 1.5 mg. of fluorine. When the fluorine content of the sample is less than 0.20 per cent, a 1-gram sample should be used and the final volume after removing residue C reduced to 50 ml. By fusion with sodium carbonate and subsequent leaching with water only about per cent of the fluorine in phosphate rock is rendered soluble.
Procedure for Determination of Fluorine
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9o
10 100
indefiIt is believed that this reagent Will remain nitely. No change was noticed after the reagent had stood for 6 months in the light of the laboratory.
Extraction of Fluorine as Sodium Fluoride The preliminary extraction Of fluorine from a rock follows with few changes that outlined b y Hoffman and Lundell (6). 362
A 25-m1. aliquot portion of the solution containing the fluorine of the sample as the sodium salt is pipetted into a 50-ml. beaker; this will be referred t o as the unknown solution. Into another beaker of the same size are measured 25 ml. of a solution having the same pH and the same quantity of sodium chloride per milliliter as the solution containing the fluorine; this will be called the comparison solution. To each beaker 2.00 ml. of the ferron-iron reagent are added. Unless the fluorine content of the sample is very low, a difference in color of the solution will be noticed in the two beakers without resorting to the colorimeter for comparison. The colorimeter used was a Klett top reader of the plunger type, having glass cups 65 mm. deep, with black opaque sides and transparent bottoms. A 0.02 N solution of sodium fluoride is now slowly added from a buret graduated to 0.05 ml. to the greener or comparison solution until the color almost matches that of the unknown. An equal quantity of distilled water is added to the unknown solution in order to maintain the same volume in each of the two beakers.
JULT 15, 1939
ANALYTICAL EDITION
TABLEI. RESULTS WITH SYNTHETIC SAMPLES Sample No.
Weight of Sample Fluorine Present Fluorine Found Error Mg. Gram Mg. % Mg. % 1” 0.1000 10.00 10.00 9.88 9.88 -0.12 2 0.5000 25.00 24.30 4.86 5.00 -0.70 3 0.5000 12.80 2.50 2.55 12.50 +O. 30 4 0.5000 5.00 5.10 1.03 1.00 +0.10 0.5000 5 0.52 2.50 2.60 0.50 +O.lO 0.5000 6 0.27 1.25 1.35 0.25 +0.10 1.0000 7 0.100 1.10 0.110 $0.10 1.00 1.0000 8 0,046 -0.04 0.50 0.050 0.46 9 0.014 0.10 0.14 1.0000 0,010 $0.04 lob 0.1000 26.41 26.41 25.84 -0.57 25.84 11 17.60 3.50 17.35 0.5000 3.47 -0.15 12 6.10 1.22 6.10 0.5000 1.22 None 1.0000 13 0.90 0.090 1.00 0.100 +0.10 14 1.0000 0.30 0.030 0.40 0.040 +o. 10 150 0.5000 24.30 4.86 4.80 -0.30 24.00 Nos. 1 t o 9, inclusive, sodium fluoride in distilled water, prepared by J. J. Fahey. b Nos. 10 t o 14, inclusive, sodium fluoride in distilled water, prepared by R. E. Stevens of the Geological Survey. No. 15, microcline 90 per cent and fluorite 10 per cent, prepared by J. J. Fahey.
TABLE11. COMPARISON OF RESULTS Sample NO.
Fluorine Found by Other Analysts
Fluorine Found by J. J. Fahey
% 5.72 2b 7.76 a Bureau of Standards standard sample 91, opal glass. b Lepidolite from San Diego mine, Mesa Grande. Calif. Stevens ( 8 ) , using Stevens’ nephelometric method. la
% 5.78 7.52
Analysis by R. E.
The two cups of the colorimeter are filled with the comparison and the unknown solutions, respectively, and the plungers inserted to the point where the depth of each liquid observed is 50 mm. If the end point has not been reached, the color of the unknown contains more yellow than that of the comparison solution. Repeated additions of the 0.02 N sodium fluoride solution are made to the comparison solution until it attains the same color as the unknown solution in the same volume. From the volume of 0.02 N sodium fluoride solution required to make the match is computed the amount of fluorine in the aliquot portion and in the sample.
A difference in color between the comparison and unknown solutions is evident when the difference in fluorine content of the two solutions is as little as 0.05 mg. If the unknown solution is a natural water containing no more than 1000 parts per million of total solids, the sensitivity is about twice as great as this, making it possible to measure 0.025 mg. of fluorine in the 25-ml. aliquot portion. This greater sensitivity obtained in estimating fluorine in natural waters, over that to be had with rocks, is due to the relatively small quantity of salts in solution in the 25 ml. of the water. TABLE111. COMPARATIVE RESULTSOBTAINED ON WATERS (Grqund-water samples from Avoyelles Parish and Rapides Parish, La., obtained from the Water Resources Laboratory, U. S. Geological Survey) Water Resources Results by Results by Laboratory No. M. D. Foster J. J. Fahey 20,325 20,094 20,327 20,342 20,331 20,093 20,349 20,360 20,363
P. p . m.
P.p . m.
7.9 4.5 1.8 1.2 0.8 5.4 0.9 1.4 2.4
7.5 4.6 1.9 1.1 0.8 6.1 1.2 1.5 2.3
Evaluation of the Method Table I gives results obtained on synthetic samples. Samples 1 to 14, inclusive, were treated as aliquot portions of weighed samples, making it possible to compute the concentration of sodium chloride in the aliquot portion and to express the results in percentage.
363
Table I1 gives comparisons of results by the method herein described with those obtained by other methods. Table I11 gives results obtained on waters by this method compared with those obtained by the Foster ferrithiocyanate method (4). The determination of fluoride in the waters was made without preliminary concentration. Both the comparison solution (distilled water) and the unknown water samples were brought to pH 4.2 before adding the ferron-iron reagent. This acidity is very satisfactory for the determination of fluorine in natural waters. The percentage error inherent in the ferron-iron colorimetric determination permits its use in the analysis of samples having no more than 10 per cent of fluorine. With rocks or minerals that have higher fluorine contents than this, the absolute error becomes sufficiently great to render the method unsuitable for accurate work. The method as herein described is not sensitive enough to measure accurately less than 1 part per million of fluorine in natural waters. However, it is possible that by using 0.005 N sodium fluoride and a ferroniron solution that contains less hydrochloric acid than the one used in these determinations this limit can be lowered.
Fluorine Content of Rocks and Minerals An estimate of the quantity of fluorine likely to be found in rocks and minerals can be obtained from the compilations of Clarke (9)and Wells (IO), which record the rock and mineral analyses made in the U. S. Geological Survey from 1880 to 1936. I n 1229 mineral analyses reported, there were 18 analyses having more than 10 per cent of fluorine; 27 between 5 and 10 per cent; and 17 from l to 5 per cent. In the 3093 rocks analyzed, the highest percentage of fluorine found was 1 per cent. Only 18 samples in 4322 examined had more than 10 per cent of fluorine.
Interfering Elements
It is obvious that colored salts will interfere with obtaining accurate results by this method. However, all such salts, except chromates but including vanadates, as pointed out by Fairchild (S),are removed by controlling the pH in the treatment with zinc oxide outlined in this paper. Chromates, if present in quantities large enough to impart a noticeable color, must be removed. Phosphates are completely taken out by the zinc oxide procedure. I n the analysis of rocks and minerals, it is necessary that the sodium chloride content and the p H of the comparison and unknown solutions be approximately the same, because a large difference would introduce an error.
Summary A quantitative method for the determination of fluorine, applicable to rocks, minerals, and natural waters, is based on the use of ferron. The green color of the ferron-iron reagent assumes a yellowish hue when fluorine is present. Matching this color with a solution of known fluorine content makes possible an accurate measurement of the percentage of fluorine in the sample.
Literature Cited (1) Berzelius, Schweigg. J., 16, 426 (1816). (2) Clarke, U. S. Geol. Survey, Bull. 591 (1915). (3) Fairchild, J. Wash. Acad. Sci., 20, 141 (1930). (4) Foster, IND. E N Q . CHEM.,Anal. Ed., 5, 324 (1933). (5) Hoffman a n d Lundell, Bur. Standards J. Research, 3, 581 (1929). (6) Merwin, Am. J. Sci., 141 28, 119 (1909). (7) Steiger, J. Am. Chem. SOC.,30,219 (1908). (8) Stevens, Am. Mineral., 23, 615, Analysis 9 (1938). ENQ. CHEM.,Anal. Ed., 8, 248 (1936). (9) Stevens, IND. (10) Wells, U. S. Geol. Survey, Bull. 878 (1937). (11) Yoe, J. Am. Chem. Soc., 54, 4139 (1932). PRESENTBID before the Division of Physical and Inorganic Chemistry a t the 97th Meeting of the American Chemical Society, Baltimore, Md., under the title, “A Dicolorimetric Method for the Determination of Fluorine.” Published by permission of the Director, C . S. Geologioal Survey.
