Colorimetric Determination of Fluorine in Waters and Soil Extracts W. M. SHAW The University
o f Tennessee Agricultural Experiment Station, Knoxville, Tenn.
T
HE reaction of zirconium-alizarin lake with fluoride ion is the basis of most colorimetric procedures for determination of fluorine in waters ( 2 , 7 , 17, 18). Colorimetric methods are simpler and more expeditious than the distillation and titration procedure of Willard and Winter (19), which is prescribed by the Association of Official Agricultural Chemists for fluorine in water (3). rlt this station the Sanchis procedure (IS) has been employed for the determination of fluorine carried by rainwater leachings from added fluoric materials and phosphate furnace slag in lysimeter experiments (8-10). I n the course of these determinations, a number of simplifications were introduced and checked against the original. Subsequently, however, these changes were found to be identical with those introduced by Scott (15) and later incorporated in the official methods of the American Public Health Association ( I ) . Lysimeter leachings collected in winter and early spring may be turbid, and in many instances are colored from dissolved organic matter. Because such waters and soil extracts could not be clarified through ordinary filtration, their fluorine content could not be determined accurately by colorimetric procedures. Consequently, it was necessary to devise a chemical treatment that would remove both turbidity and color from waters without affecting the accuracy of the colorimetric determination of fluorine.
standard solution. The filtrate from this solution should yield complete recovery of its fluorine content. Determination. Transfer approximately 200 ml. of water sample, rain mater, percolate, or soil extract into a 250-ml. Erlenmeyer flask. Add approximately 0.1 gram of calcium sulfate powder and approximately 0.5 gram of a tested decolorizing carbon. The charges may be measured by means of calibrated cups or spoons. Stopper the flask; shake it vigorously six times, 30 seconds each time, a t 10-minute intervals, or preferably agitate 30 minutes on a Ross-Kershaw type of shaker. Filter into an Erlenmeyer flask, using 12.5-cm. Whatman No. 12 folded paper. By means of a graduated cylinder, transfer 100 ml. of the clear filtrate into a tall (Berzelius) 100-ml. beaker. Prepare standards of 0, 0.2, 0.4, etc., up t o 2.0 p.p.m. of fluoride by delivering from a buret the requisite quantities of the standard fluoride solution and diluting to 100 ml. with water. Maintain the standards and unknowns a t the same temperature during color development and color matching. To the unknowns and standards add with stirring exactly 5 ml. of the acid-zirconiumalizarin reagent. If, within 15 minutes after addition of reagent, the unknown develops a yellow or greenish yellow color, discard the contents and replace successively with aliquots of 50, 25, etc., each diluted with distilled water to 100 ml. Use that dilution whose color, after addition of reagent, will best match with that of the standards. Let the solution stand 1 hour for color development, and compare with the standards on a sheet of white paper under a white fluorescent light in a position devoid of glare. Place the unknown between the two standards that afford the nearest match and interpolate to nearest 0.1 p.p.m. I n case of dilution, applv the appropriate factor to determine the parts per million of fluoride in the sample. Correct the results based on the value of the undiluted sample by subtracting 0.10 p.p.m. The glassware used for color development of zirconium-alizarin reagent should be washed soon after use.
PROCEDURE
The procedure is essentially as given in the American Public Health Association methods (1), but with the following modifications. The zirconium oxychloride reagent was increased from 0.3 to 0.43 gram. This increase was found necessary so that the quantity of zirconium (per determination) used as the oxychloride is the same as that in the nitrate of the original method and so that the equivalent range of color development is given with either salt. The color development and the comparisons with standards are made in lOO-ml., tall (Berzelius) beakers instead of Nessler tubes; the readings are thus obtained with equal accuracy in less time and with less eyestrain. Standards in steps of 0.2 p.p.m. of fluoride are recommended, although 0.1 p.p.m. differences are easily distinguishable. Removal of color and turbidity is introduced as an essential step, preliminary in the colorimetric determination of fluorine in waters that are colored or turbid, such as pond waters and soil extracts.
One person can start and complete 30 determinations per day, including a high proportion of repeats. Where the fluoride content is uniformly low (not requiring dilution) as manv as 40 to 50 determinations per day can be completed. EFFECTIVENESS O F V4RIOUS DECOLORIZING C4RBONS AND CLARIFYING AGEhTS
Reagents. ACID-ZIRCONIUhi-ALIZARIN REAGEKT. Dissolve 0.43 gram of zirconium oxychloride octahydrate in 50 ml. of distilled water in a 1-liter beaker. While stirring, add 0.07 gram of alizarin monosulfonate dissolved in 50 ml. of distilled water. Dilute 37 ml. of C.P. sulfuric acid (specific gravity 1.84) with 400 ml. of distilled water, and dilute to 500 ml. Dilute 112 ml. o! C.P. hydrochloric acid (specific gravity 1.19) to 500 ml., and mix the two acid solutions. While stirring, add 800 ml. of the mixed acid solution slowly to the zirconium-alizarin solution and transfer to a liter flask. Dilute to volume with the acid solution, and mix the contents. Store in a refrigerator, and filtei a volume sufficient for each day’s use. STANDARD SoDIuv FLUORIDE SOLCTIOX.Dissolve 0.221 gram of sodium fluoride in 1 liter of water and dilute 100 ml. of this stock solution to 1 liter. After dilution, 1 ml. of solution contains 0.1 p.p.m. of fluoride. CLARIFYING A N D DECOLORIZING AGEXTS. Calcium sulfate powder, c.P., and a decolorizing carbon are previously tested for nonadsorption of fluoride in the following manner: Introduce 0.1 gram of calcium sulfate and 0.5 gram of decolorizing carbon into 200 ml. of a soil extract, shake 0.5 hour, and filter; the filtrate should be clear and colorless. Treat in like manner a solution made to contain 10 p,p.m. of fluorine through addition of the
A highly turbid lysimeter leaching and a 1to 10 water extract of peat were used in the testing of the decolorizing and clarifying agents. The lysimeter leachings carried an amber-colored colloidal suspension, which failed to settle out even after months of standing, and which contained coloring matter that became apparent only after flocculation and removal of the suspended matter. The well known flocculating effects of calcium salts upon soil colloids and the use of decolorizing charcoals in chemical analyses of plant tissues and soil extracts (6, 11, 12, 14, 20) suggestel the possibility that a calcium salt in combination with decolorizing carbon might effect the removal of color and turbidity in soil-water extracts and leachings, preparatory to fluorine analysis. Alumina cream (4),copper sulfate, and ferric sulfate, with calcium hydroxide, have been employed to effect decolorization and clarification of soil extracts ( 5 ) , but those reagents were considered inadmissible in this study because of their precipitative effects upon solute fluorides. The specific functions of the calcium salt and of decolorizing carbon are illustrated in Figure 1. Flasks 1 and 2 show that the decolorizing carbon had virtually no effect upon the turbiditv of the leaching, whereas the calcium sulfate treatment (flask 3 ) resulted in complete removal of turbidity without affecting coloration. Both color and turbidity, however, were removed completely by the combined treatment with calcium sulfate and decolorizing carbon, as in flask 4. The presence of calcium
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V O L U M E 26, N O , 1. J U L Y 1 9 5 4
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is a differenceof opinion, however, as to the extent of the error and as to counteractive measures. Sanchis (IS) sought t o decrease the positive error 1 to 10 Water Extract of Peat of sulfate through introduction of hydrochloric and One volume* oi CalOium Sulfate + Hiq;:&bid sulfuric acid as a combination reagent for staudBrand of Deoolorizin~Carbonb 1 case. Reference Deeoioriving Carbon 4a 06 B Carbon ards and unknowns. Walker and Finlay ( 18) have shown, however, that with the Sanchis prooedure a concentration of 500 to 600 p.p.m. of sulfate in the water induced a positive error of 0.2 p.p.m. in the fluorine determination, and that a concentration of 1200 p.p.m. of sulfate caused an error of 0.5 p . p n On the other haud, the American e Results for o m volume oi osloium sulfate and 2 volumes of carbon were 811 too yellow &r
