ANALYTICAL CHEMISTRY
998 treated a s shown in Table 11, and titrated by the general procedure. Effect of Hydrogen Chloride. This was observed by preparing a standard solution of hydrogen chloride in acetic acid (or ethylene chloride) and diluting with acetic acid to various concentrations. In one series of runs, when acetic acid saturated with air was used, and the hydrogen chloride concentration was varied, in six steps, from 0 to 1.3 meq. (is the standard volume of solvent used for the titrations), the blanks rose progressively from 0.02 to 0.06 meq. of released iodine; with the nitrogen-swept solutions, a similar series of determinations did not reveal such a trend. ACKNOULEDGMEKT
The authors are indebted to the National Science Foundation for partial support, and to Ronald Swidler for developing the method of synthesis of 2,4-dinitrobenzenesulfenylbromide cited above.
LITERATURE CITED Bohme, H., a n d Schneider, E., Ber., 76, 483 (1943). Foss, O., Acta Chem. Scand., 1 , 310 (1946). Hubacher, &I. H., "Organic Syntheses," Coll. Vol. 11, p. 455, Wilev. Piew York. 1943. Kharasch, N., Buess, C. AI., a n d Strashun, S. I., J. Am. Chem. SOC.,74, 3422 (1952). Kharasch, S . ,Gleason, G . I., and Buess, C. %I.,Ibid., 72, 1796 (1950). Kharasch, N., a n d Havlik, A. J., I b i d . , 75, 3734 (1953). Kharasch, N., LlcQuarrie. D . P., and Buess, C. hl., I b i d . , 75, 2658 (1953). Kharasch, S . , a n d Swidler. K., unpublished work. Kharasch, S . , Wehrmeister, H . L.. and Tigerman, H., J . Am. Chem. Soc., 69, 1612 (1947). Orr, W . L . , a n d Kharasch, S . ,Ibzd., 75, 6030 (1953). Wald, Vi. &I.,and Kharasch, iY., Division of Organic Chemistry, 126th Meeting ACS, New Y o r k , S. Y.. 1954. for review July 17, 1954. Accepted December 8, 1954. Part xx in the series "Derivatives of Sulfenic Acids." For preceding papers see J. O r y . Chem., 19, 1704 (1954). and J . Am. Chem. SOC.,Vols. 69 t o 77. REChIVED
Volumetric Determination of Fluorine Involving Distillation from a Sulfuric Acid Solution OLIVER
D. SMITH
and THOMAS
D. PARKS
Stanford Research Institute, Stanford, Calif.
The use of sulfuric acid and distillation at 150" C. in the Willard and Winter method decreases the retarding influence of aluminum and silicon upon the volatilization of fluorides in analysis of vegetation, soils, and particulate materials. A modified steam tube minimizes contamination of the distillate.
T
HE Willard and Winter (6) method is widely accepted as
the most satisfactory means of estimating the concentration of fluorine in a wide variety of materials and over a large range of concentrations. In the first publication the authors pointed out several limitations-for example, gelatinous silica, boron, aluminum, phosphates, and sulfates prevented the distillation of fluosilicic acid from a perchloric acid solution. Winter and Butler (7) obtained unsatisfactory results when they attempted to determine the fluorine content of a plant ash. Since the first announcement of the method a great deal of work has been carried out both by individual investigators and by committees sponsored by such organizations as the Association of Official Agricultural Chemists ( 1 , 5 ) . I t was found that when the temperature of distillation from a perchloric acid solution was increased above 135" to 139" C., decomposition products and perchloric acid were carried over, which interfered with the titration. For this reason a doubledistillation method was introduced for use on materials that were difficult to analyze by the standard method. The solution was distilled from sulfuric acid a t 165" C. to free as much of the fluorine as possible from the interfering substances, the distillate was then evaporated to a suitable volume, and a second distillation was made from perchloric acid a t 135" C. to free the fluorine from the interfering sulfuric acid carried over in the first distillation. Recently Remmert and Parks ( 3 ) and Rowley and coworkers ( 4 ) published methods for the determination of fluorine in plant materials, which overcame some of the difficulties encountered by Willard and Winter. They found that the ash from certain types of plants did not give satisfactory results, even when the AOAC double-distillation procedure was utilized. Introduction
of a sodium hydroxide fusion of the plant ash prior to distillation by perchloric acid at 135" C. gave greatly improved recovery of fluorine. Subsequent work a t Stanford Research Institute has shown that certain samples do not yield satisfactory results by distillation with perchloric acid at 135" C., even after a fusion Kith sodium hydroxide. The work described in this paper %--as carried out in an effort to improve the recovery of fluorine from samples containing silica and alumina, without a long and tedious procedure such as that involved in the double-distillation method. Two problems were immediately recognized in attacking the distillation with sulfuric acid. First, lime is normally added to samples in the field to prevent the loss of hydrogen fluoride. With a sulfuric acid distillation there tends to be precipitation of calcium sulfate nTith subsequent "bumping." The second problem is the carrying over of enough sulfuric acid to interfere with the titration m-ith thorium nitrate. Both problems have been overcome and a satisfactory working method has been developed for the determination of fluorine in the types of sample which yielded low results when perchloric acid was used for dietillation. RE4GENTS
-411 reagents must be tested for fluorine content, in order to assure low blank values. Analytical reagent chemicals were found suitable for all reagents except magnesium oxide. Magnesium Oxide, low fluorine content. Dissolve 750 grams
of magnesium sulfate in 2000 ml. of distilled water in a 4000-ml. beaker. Heat to SO" to 100' C. and add 50 grams of sodium carbonate dissolved in 200 ml. of hot distilled water. Remove from
the hot plate, stir occasionally, and let settle overnight. Filter through an 18-cm. S o . 1 Whatman filter paper, using a Buchner funnel and a suction pump. Discard the precipitate. Dissolve the precipitate adhering to the 4000-ml. beaker with a few milliliters of dilute hydrochloric acid and discard. Return the filtrate to the 4000-ml. beaker. Heat the filtrate again to 80" to 100" C. Dissolve 400 grams of sodium carbonate in 1000 ml. of distilled water, using heat to effect solution. Add the sodium carbonate solution to the magnesium sulfate solution with stirring. Remove from the hot plate, let stand for 30 minutes, stirring occasionally, and again filter off the precipitate through an 18-cm. Buchner funnel using a
V O L U M E 27, NO. 6, J U N E 1 9 5 5 suction pump. Discard the filtrate. Wash the precipitate eight or ten times with distilled water. Keep the precipitate wet enough when draining after each wash so that it does not crack. Transfer the precipitate to B large Inoonel dish, and dry on B hot plate or in an oven a t 105" C. Transfer to a muffle furnace a t not more than 300" C. and slowly increase the temperature to 600" C. Heat for 2 hours. Remove and cool in a desiccator. Pulverize and transfer to 8 tightly stoppered bottle.
999 T a b l e I. Recovery of Fluorine by Distillation in Perchloric Acid a t 135' to 139' C. (Volume of distillates, 500 nil. 250 7 of fluorine added to each sample) A h 0 1 Added, SiOn .4ddcd, F Recovered. Mg. >I& % Nolle
None
25
None Nono None
so
75 100
4PPARAlUS
Metal crucibles and dishes, of 1 5 , ZOO-, and 1000-ml. cappacity, Nickel or Inoonel. Stmm Generator. Erlenmeyer flask of 6000-ml. capacity Electric heater, 2000 watts, 115 volts, 8.5-inch diameter. Three-heat switch control. Distillation Apparatus. Modified Claissen distillation flasks, 250-ml. capacity (bottoms flattened t o fit electric plate and top opening of the side arm sealed). Electric heater, 550 watts, 115 volts with rheostat. West condenser, length of jacket, 300 mm. Erlenmeyer titration flasks, wide-mouthed, 250-ml. capacity. Koch miomburet with two-way stopcock, capacity 5 ml., subdivisions, 0.01 ml. Steam tube, constructed of glass tubing 6 mm. in outside diameter, completely sealed 1.5 inohes from lower end with opening of 2 to 3 mm. blown through the side of the tube iust above the seal (Figure 1). Distillation rack, constructed to support 16 t,o 24 distilling units (Figure 2). The steam is eonducted by rubber tubing from a steam manifold to 8 glastss capillary tube inch long-and with S. cspillary 1.25 mm. in inside diameter. The emillarv is connected to the glass dteam"tube entering the distilling flask by means of rubher tubing. A pinchcla.mp fitted over this rubber tubing is used to turn the steam off and on during the distilling operation. The distilling flask is fitted with %. two-hole stopper, through which me inserted a thermometer and the Rteam tube.
None
100
173
150
Sone
91. 89 94.91 84.8.75.5 79.8.79.4 74.6,78.5 63.4,63.7 63.4,611.9. 73.0
EXPERIMENTAL METHODS
The effect of alumina and silica on the recovery of fluorine from a O.D-6mm. I D - l n m . perchloric acid distillation is illusOPENING 2 T O 3rnrn.DIAUETLR trated in Table I. The standard e. TUBE COMPLETEL" SEALED known amount of sodium fluoride Figure 1. Modified was distilled, without fusion or steam tube eonashing, using the distillation equipstructed of glass ment previously described (S),and tubing a volume of distillate of 500 ml. was collected in each case. When increasing amounts of alumina were added, the recovery of fluorine decreased progressively. When silica wy&~added, together with the alumina, the effect was additive and produced poor results. Preliminary tests using sulfuric acid in the distillation pot a t a temperature of 150' C. gave high and erratic results. The distillates contained considerable amounts of sulfate ion. These erratic results were obtained even after magnesium oxide was substituted for citloium oxide in the ashing procedure on forage samples. AB this reagent was not commercially available, a procedure for its preparation is given under "Reagents." A change was made in the distillation apparatus, which was a radical departure from the previous method of introducing steam into the solution. I n the usual apparatus of this type the steam is allowed to purge through the solution to agitate i t and to prevent bumping. It was felt that such a procedure permitted droplets of sulfuric acid to be carried over with the steam, and a change was therefore made in the steam tube which led into
Figure 2. Distillation rack for fluorine determination
Ij
the solution. The tube was closed about 1.5 inohes from the lower end and an opening of 2 to 3 mm. in diameter blown through the side of the tube just above the seal. This is illustrated in Figure 1. This arrangement permits the steam to enter the Baak and sweep out the gases above the solution without bubbling through the solution and causing the spray of the solution to be carried over into the distillate. The chamber formed Eelow the seal allows steam to form in it and bubble through the lower extremity of the tube to agitate the solution mildly. This prevents the tendency of the solution to bump. Using the modified apparatus.for the sulfuric acid distillation, a series of samples was analyzed by distillation with sulfuric acid at 150" + 5" C. and with perchloric acid a t 137" =t2" C. In each case 500 ml. of distillate was oollected. The sample chosen for the test was a Sudan grass hay which had previously been found to be very difficult to analyze. To it were added various amounts of alumina, calculated to exceed the amounts found in normal vegetation. As can be Seen in Table 11, t h e sulfuric acid distillation gave satisfactory results, whereas the perchloric anid distillation gave very poor results when the alumima was present. The Sudan grass hay, No. 47, which was used for the experiments described in Tahle 11, was known to contain a large
1000
ANALYTICAL CHEMISTRY
amount of silica; when analyzed it was found to contain more than 50% silica in the ash. A series of experiments was carried out, therefore, to determine if silica and alumina together caused more interference than when either was present alone, as would be indicated from the experiments described in Table 11. Solutions containing known amounts of the fluoride ion with varying amounts of silica and alumina were distilled both from sulfuric acid at 150" C and from perchloric acid at 136" C. (Table 111). Here again, the combined effect of silica and alumina indicates that fluorine is tied up and is not distilled at 135" C. from perchloric acid. However, very satisfactory results were obtained with the sulfuric acid distillation a t 150" C.
appearance of a pink color. Just discharge the pink color by adding 0.05N hydrochloric acid drop by drop. Add 1 ml. of monochloroacetic acid buffer solution and titrate with 0.01N thorium nitrate solution to the appearance of a faint pink color. Titrate all subsequent standards by comparing to the color of this blank. Subtract the volume of thorium nitrate used on the blank from the volume used on each standard. Construct a graph to relate the quantity of fluorine taken to the net volume of thorium nitrate solution used. Take a suitable aliquot of the distillate from each sample and dilute to 200 ml. with distilled water, if the volume of the aliquot is less than 200 ml. Add indicator, sodium hydroxide, hydrochloric acid, and buffer solution in the same manner as described for the standardization of the thorium nitrate solution. Titrate the aliquot of the sample to the same color as that of the titration blank. Subtract from each aliquot of the sample titrated the volume of thorium nitrate used to titrate a similar aliquot of the reagent blank. From the above graph determine the micrograms of fluorine contained in the aliquot of the sample.
Table 11. Comparison of Results Using Sulfuric Acid at 145" to 155' C. and Perchloric Acid at 135' to 139' C. in Distillation (Volume of distillates, 500 ml. 5 grams of unrtshed hay used in all cases. Procedure includes ashing and fusion) A1 Added Fluorine, P.P.M. as 41a01, HzSOd HClOi Mg. None 44 44 100 10 24 100 38 26 200 43 7 200 46 0
DISCUSSION
PROCEDURE FOR FLUORINE IN VEGETATION
Include a reagent blank with each group of samples. In making a determination of fluorine in vegetation, determine the moisture on a separate sample. Transfer 10 to 15 grams of sample to a suitable Inconel dish. Add magnesium oxide in the ratio of 1 gram of magnesium oxide to 30 grams of sample. Cover with water, mix, and test for alkalinity. Maintain an alkaline condition by adding more magnesium oxide if necessary and evaporate to dryness. Ash the dried sample in a furnace in an excess of air, starting at a temperature of 300' C. and gradually increasing to 600" C. Continue heating until the ash is light gray in color. A small amount of carbon in the ash will not affect the analysis.
Table 111. Recovery of Known Amounts of Fluorine
Fluorine Present, Y
(Volume of distillates, 500 ml.) ~ l Fluorine ~ Recovered, ~ Silica Present Distilled Distilled (as .41203), in HzSOd in HC104 Present, at 135' C. Mg. af 150' C. hZg.
Transfer the ash (or an aliquot of it) to a 200-ml. Inconel dish, add 10 grams of sodium hydroxide, and heat 5 to 10 minutes at a temperature of 600' C., swirling the dish occasionall\. to effect complete mixing of the ash with the melt. Cool and add 50 ml. of water. Boil 20 to 30 minutes to disintegrate the fusion and transfer to a 250-ml. distilling flask. Add a few glass beads, sufficient silver sulfate t o precipitate the chlorides present, and 60 ml. of 24N sulfuric acid. Insert a two-hole rubber stopper containing the steam tube and a thermometer (range 125" to 200' C.) into the neck of a distilling flask. Connect the flask to the steam source and to the condenser. Heat the flask and collect the distillate in a 500-ml. volumetric flask. When the temperature in the distilling flask reaches 145" C. admit the steam and maintain the temperature within the range of 145' to 155 C. in the distilling flask. Collect 500 ml. of distillate over a period of 2 to 3 hours. Standardize the 0.OliV thorium nitrate solution by pipetting aliquots containing 10 to 300 y of fluorine into titrating flasks. Dilute to 200 ml. with distilled water. Titrate a blank containing 200 ml. of distilled water first. Add 1 ml. of 0.05% alizarin red indicator and add 0.05s sodium hydroxide solution to the O
i
In comparing distillation from perchloric acid with that from sulfuric acid, the following differences are found. Samples which have a high calcium content are more difficult to distill in sulfuric acid solution because of the tendency to bump in the presence of precipitated calcium sulfate. The addition of 1 or 2 grams of ammonium sulfate decreases this tendency and about 1 gram of calcium oxide can be tolerated under these conditions. The presence of potassium salts causes precipitation of potassium perchlorate and consequent bumping in the perchloric acid distillat ion. The use of the modified steam tube has a definite advantage in decreasing the tendency to bump in the presence of excessive amounts of gelatinous silica, encountered when analyzing siliceous soils. When this tube is used, the steam can be turned on or off at any time during the distillation. This is advantageous when the temperature in the distilling flask exceeds 155" C. and it is desirable to stop the distillation until the temperature is reduced. In practice it has.been found that if the steam and the electric heater are turned off: only a few drops of distillate \Till continue to condense. Hence temperatures above 155" C. have produced no noticeable increase in the sulfuric acid content of the distillate when the above changes %ere immediately made. Thus ~while the~temperature range of 145" to 155' C. should be ~ generally maintained, temporary deviations of 5" C. above and below this range do not affect the analysis. For this reason 20 or 24 distillations can easily be controlled by one operator. Dahle and Wichman (2) found that the presence of the thorium ion greatly retarded the volatilization of fluorine at 135" C. in a perchloric acid distillation. The possibility exists that other ions may retard the volatilization of fluorine. In general, this retarding effect will be less at 150" than a t 135' C. Aluminum, which is commonly present in varying amounts in most materials such as soils and dusts, has a very marked influence upon the voltalization of fluorine at 135" C. and practically none at 150" C., as indicated in the above results. LITERATURE CITED
Offic.Agr. Chemists, "Official Methods of Analysis," 7th ed., pp. 40-1, 54-9, 389-96, 1950. (2) Dahle, D.,and Wichman, H , J . Assoc. Ofic.Agr. Chemists, 19, (I) Assoc.
320-7 (1936). (3) Remmert, L. F., and Parks, T. D., AXAL. CHEM.,25, 450-3 (1953). (4) Rowley, R. F., Grier, J. G., and Parsons, R. L., Ibid.,25, 1061-5 (1953). (5) Slivey, G.A., J . Assoc. Ofic.AQT.Chemists, 16, 153-5 (1933). (6) Willard, H. H., and Winter, 0. B., ISD. ENG.CHEM., .ANAL. ED., 5.7-10 (1933'1. . , (7) Winter, 0. B., and Butler, Lillian, J . B s s o c . Ofic.Agr. Chemists. 16, 105-7 (1933).
RECEIVED for review
May 6,
1954. Accepted December 1. 1954
