Spectral Determination of Fluorine in Water A. W. PETREY, Aluminum Research Laboratories, New Kensington, Pa.
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HE occurrence of fluorine in potable waters and the probabilities of its physiological effect have aroused considerable interest in the past few years. Churchill ( I ) , Dean (8, S), Sebrell, Dean, Elvore, and Breaux (8), Kempf and McKay (4), Smith, Lana, and Smith (9), and others have made comprehensive investigations on the subject. The general interest has stimulated activity among analytical chemists toward the development of methods for the accurate estimation of fluorine in the quantities which occur in natural waters. The amounts reported have varied from zero to as much as 14 p. p. m. of fluorine. Among the recent developments are the works of Willard and Winter (II), Thompson and Taylor (IO), Sanchis (?'), and Kolthoff and Stansby ( 5 ) .
FIGURE1. ELECTRODE HOLDER PARTIALLY FILLEDWITH ELECTRODES
Under certain conditions, difficulties which are not easily obviated, may be encountered in the chemical determination of fluorine in water. For this reason, the possibility of using spectral methods for the determination has been investigated in Aluminum Research Laboratories. DESCRIPTION OF METHOD The detection or estimation of fluorine in any material by spectral methods is based on the appearance of the band spectrum of calcium fluoride when a substance containing both fluorine and calcium is excited in the electric arc or spark, The band with head at A5291 is the most sensitive and is the one the author has used for the examination of water. Papish, Hoag, and Snee (6) have made use of the band spectrum of calcium fluoride for the detection of fluorine in gems and other minerals. Fluorine may be detected either by the photographing or visual observation of its spectrum. When the dissolved mineral matter consists essentially of calcium and magnesium compounds, the estimate can usually be made by arcing the dry residue. However, when large amounts of sodium compounds are present, calcium must be supplied externally, The most dependable means found in these laboratories for supplying the calcium is the impregnation of the lower electrode which supports the sample, with calcium chloride. The calcium salt in the electrode eliminates the variable intensities which result from the more or less selective volatilization of the various constituents of saline water residues.
spectra with the spectrograph while observing visually the performance of the fluorine bands with the spectrometer. By this means the author was enabled to reach the decision that visual estimates are possible where the base of the water residue is essentially calcium and magnesium compounds, but that photographic methods are necessary with saline residues. All spectrograms are examined by projection upon an aluminum screen by means of a Bausch and Lomb lantern slide projector. The projector has been equipped with a special plate holder to fit the 3 inch (7.62 em.) X 10 inch (25.4 cm.) spectrograms, and the lens mount has been extended for short-range projection.
EXPERIMENTAL By a study of the spectrum of calcium carbonate containing known added amounts of fluorine, it has been found that when photographed the calcium fluoride band will show a satisfactory density gradient when the fluorine lies between the approximate limits of 0.05 and 1.5 per cent of the calcium carbonate when using a sample weighing 12.5 mg. By varying the amount of material used for the arc, the limits can probably be broadened. For water analysis this range is satisfactory. The series of standards used for this investigation were prepared to contain 0.025, 0.05, 0.10, 0.25, 0.50, 1.00, 1.50, and 2.00 per cent of fluorine. When examined visually during the burning of the sample, the calcium fluoride band was found to disappear after a few seconds, the time varying with the amount of fluorine present. By plotting the time in seconds against the percentage of fluorine in the base material, a satisfactory curve can be drawn. For analysis of water of the same approximate composition as the standards used for plotting the curve, the estimate requires only the observation of the time required for the disappearance of the calcium fluoride band. 3
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CaF2 band
% 0.025
0.05
0.10 0.26 0.50
1.00 1.60
APPARATUS Aluminum Research Laboratories has available a large Littrow quartz spectrograph and a Hilger constant-deviation spectrometer, either of which may be used for the detection and estimation of fluorine. The spectrometer may be used visually or photographically. In a large part of the experimental work on fluorine both the quartz spectrograph and the glass spectrometer were used simultaneously, recording the
FIGURE2. SPECTRA OF STANDARDS CONTAINING VARIOUS AMOUNTS OF FLUORINE
Some of the western waters examined contained as much as 2000 p. p. m. of dissolved solids, mainly sodium chloride. With these waters, the best arc performance resulted when specially prepared electrodes were used. The electrodes, 0.25 inch (6.35 mm.) diameter graphite, were prepared by
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ANALYTICAL EDITION
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cutting to 1.25-inch (31.8-mm.) lengths. A shallow cavity, about 0.078 inch (1.98 mm.), was drilled in one end of half the electrodes. The other half, to serve as upper electrodes, were not drilled, nor were they impregnated. The drilled electrodes were placed in a 50 per cent solution of calcium chloride and under a bell jar which was then evacuated in order to fill the voids of the graphite with the solution. When no air bubbles were observed a t the surface of t h e elec140 trodes, they were removed from the 1.20 jar and the calcium chloride, dried in IO0 an oven a t 175 O C., and stored in a 80 desiccator. STANDARDS For the prepara40 tion of standards, c . P. calcium car20 bonate may be used as the base. 1x1 1 1 1 1 1 I 1 1 1 1 Freedom from fluorine is first asVolatilizing Time-Seconds. sured by a spectral FIGURE 3. CURVEFOR ESTIMATING analysis of the calFLUORINE BY VISUALOBSERVATION cium c a r b o n a t e . Fluorine in suitable amounts is added from an aqueous solution of a fluorine salt. Sodium or calcium fluoride may be used, although sodium fluoride is preferable because of its greater solubility. Both sodium and calcium fluoride have been used in these laboratories, since there is some doubt about the combination of the fluorine as it exists in water. However, the agreement in results of fluorine determined using either salt is such that the choice is considered as optional. A few grams of calcium carbonate are pulverized to a fine powder in an agate mortar. The desired amount is weighed from the ground material and placed in a platinum dish. The fluorine solution is next added and the volume diluted to approximately 100 cc. This solution is evaporated to dryness in any convenient manner. The residue is removed from the dish and thoroughly ground to insure intimate mixing of the calcium carbonate and fluorine salt. The entire series is usually prepared a t one time, adjusting all volumes to 100 cc. before evaporation. A word regarding the preparation of special standards may Le desirable a t this point. In any spectral analysis where an estimate of quantity is desired, it is essential that the composition of the standards and the sample under investigation be very similar. If the mineral residue of a water is radically different from the typical calcium carbonate residue, a set of standards should be prepared, the base of which approximates t h a t of the water residue itself. This information is usually available from the chemical analysis of the water. If such .analysis cannot be obtained, the spectrographic analysis of the residue will establish the metals and simple chemical tests can b e made to identify the acid radicals. 60
ANALY~IS OF SAMPLES Since the spectral method for fluorine in water consists of estimating the fluorine in the residue, it becomes apparent that the sensitivity of the test depends largely on the ratio of fluorine to residue. The experimental work has shown that the accurate range for estimating the fluorine extends from 0.05 to 1.5 per cent of the residue. For a great many waters
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all that is needed may be evaporation of the water to dryness and immediate examination of the residue. However, there may often be cases where the fluorine is present in too great a quantity to allow direct use of the residue, and dilution will be necessary. For example, assuming that a water contains 200 p. p. m. of “ignited residue,” fluorine between 0.1 and 3.0 p. p. m. could be estimated by evaporating the necessary volume to provide the required amount of residue. If such a water contained more than 3.0 p. p. m. of fluorine, the addition of calcium carbonate as a diluent would be necessary to reduce the fluorine to a determinable amount. Electrodes impregnated with calcium chloride are satisfactory for any water and in order to avoid any shortage of calcium are recommended for all analyses. PROCEDURE: Evaporate enough of the water in a platinum dish to provide 50 mg. of ignited residue. Standard methods of water analysis should be followed in all details except in the quantity of water used for the sample. The ignited residue is merely the mineral matter which is left after the organic matter has been decomposed. A temperature of 500” C. is used for the ignition. Remove the residue from the dish with the aid of a wood spatula, using a little distilled water if necessary. If water is used, the residue must of course be dried. Transfer the residue to an agate mortar and pulverize thoroughly. Pulverizing is essential for a thoroughly homogeneous sample. Place 12.5 mg. of the pulverized residue in the cavity of the calcium impregnated graphite electrode, packing in tightly. A convenient method of filling the electrodes is t o deposit the residue on a flat surface and place the electrode over the sample, cavity downward, in a manner similar to that used by druggists for filling gelatin capsules. This usually results in a very satisfactory electrode. Place the filled electrode in the arc stand, using a solid unimpregnated up r electrode. The lower electrode containing the sample is anode of the circuit. If the photographic method is being used, burn the residue until it has been completely volatilized. This will require about one minute with a current of 15 amperes. The time will of course vary, depending upon the composition of the residue. If a visual determination is being made, the current is interrupted as soon as the calcium fluoride band disappears. All determinationp should be made in duplicate. Develop the plate if the spectrograph was used, and project the spectrum, matching the intensities of the calcium fluoride bands with those of the standards. Since the difference between adjacent standards is not great, estimation t o the nearest standard will usually be sufficiently accurate. Interpolation is possible if greater accuracy is desired. Because some residues have been found t o be slightly hygroscopic, the filled electrodes are placed in an aluminum holder drilled with holes t o fit the electrodes. The holder is kept at a temperature of about 200’ C. by a gas flame. This precaution often prevents considerable annoyance and delay from the loss of a sample, which is likely t o happen if the residues pick up a little moisture. Figure 1 shows the holder filled with electrodes.
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Reproduction of the spectra of a series of fluorine standards is shown in Figure 2. The gradation is clearly evident, although considerable detail is lost in the various stages of reproduction, This set was synthesized for determining the fluorine in a certain water containing 200 p. p. m. of residue, principally calcium carbonate. The amounts of fluorine are 0.025,0.05,0.10,0.25,0.50,1.0, and 1.5 per cent of the residue, or, respectively, 0.05,0.10,0.20,0.50, 1.0,2.0, and 3.00 p. p. m. of fluorine in the water. This same set of standards would serve for the estimation of fluorine in other waters whose residues are similar, by calculation of the amount of fluorine in the residue to a water basis. Figure 3 is a curve typical of the results derived from the visual examination of fluorine-bearing waters. When the water residue is such that the visual instrument can be used, this affords the most rapid means of making the determination. SUMMARY Spectral methods for estimating fluorine in the mineral residue of water have been found to yield satisfactory results. Photographic methods are suitable for all waters. Visual
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INDUSTRIAL AND ENGINEERING CHEMISTRY
methods are satisfactory for waters whose residues consist essentially of calcium and magnesium. The conditions for most accurate estimation exist when the fluorine is between 0.05 and 1.5 per cent of the residue. LITERATURE CITED (1) Churchill, H. V., IND.ENQ.CHEW, 23, 996-8 (1931). (2) Dean, J. Am. Dental Assoc., 20, 319 (1933). (3) Dean, U. S. Pub. Health Serv., Pub. Health Rept. 48, 703 (1933). (4) Kempf, G. A., and McKay, F. S., Ibid., 45, 2923 (1930).
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(5) Kolthoff, I. M., and Stansby, M. E., IND.ENG.CHEM.,Anal. Ed., 6, 118-23 (1934). (6) Papish, J., Hoag, L. E., and Snee, W. E., Ibid., 2, 263-4 (1930). (7) Sanehis, J, M,, Ibid., 6, 134-5 (1934). (8) Sebrell, Dean, Elvore, and Breaux, U. S. Pub. Health Serv., Pub. Health Rept. 48, 437 (1933). (9) Smith, M. C., Lana, E. M., and Smith, H. V., Univ. Aria. Coll. Agr., Tech. Bull. 32 (1931). (10) Thompson, T. G., and Taylor, H. J., IND.ENQ.CHEM,Anal. Ed., 5, 87-9 (1933). (11) Willard, H. H., and Winter, 0. B., Ibid., 5, 7-10 (1933). RFJCEIYED January 30, 1934.
Analytical Uses of %Propanol G. W. FERNER AND M. G. MELLON, Purdue University, Lafayette, Ind. I n considering 2-propanol as a substitute for ethanol in analytical procedures there are relatively few unfavorable factors. There is very little difference in the physical properties of the two alcohols. As a solvent for analytical reagents, particularly organic reagents, 2-propanol is a satisfactory substitute for ethanol. For inorganic reagen's in rather high concentrations the absolute alcohol must be used, since two liquid phases are formed with the constantboiling mixture. As a reagent for the qualitative detection o j certain elemen's or radicals 2-propanol is not effective,
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HE extent and variety of the uses of ethanol in analytical chemistry, indicated in a preliminary paper ( 7 ) , include the preparation of materials for analytical work, such as analytical devices, reagents, and samples; and the determination of constituents, involving reactions such as the separation of materials, the reduction of the solubility of precipitates, and the removal of adhering liquids. In view of the cost of ethanol (including tax) and of the inconvenience of the precautions necessary to prevent its diversion, it would be of advantage to find a suitable substitute. The selection of such a substitute should be made on the basis of low cost and of similarity in properties. Of the lower members of the series of aliphatic alcohols having physical and chemical properties similar to those of ethanol, 2-propanol is most nearly like ethanol. It is completely mi+ cible with water and recent commercial production has brought the price considerably lower than that of taxed ethanol. The data in Table I, obtained from International Critical Tables, indicate the great similarity in the properties of these two compounds.
TABLE I.
PHYSICAL PROPERTY Boiling points, at 760 mm. Heat of combustion
Heat of vaporization at boiling point Va or gressure at 200 C. Refractive index. Na D line Surface tension at 20' C. Viscosity
PROPERTIES OF ETHANOT, AND 2-PROPANOL ETHANOL 2-PROPANOL 100% CzHsOH 78.4' 100% CaHiOH 82.26" C. B. CzHsOH,' 78.15' C. B. CSHIOH,'80.37' 328 kg.-cal./mole 474.8 kg.-cal./mole 7.13 kg.-cal./grem 7.91 kg.-cal./gram 855 joules/ ram 43.9 mm. f i g
667 i 2% joules/pram 32.4 mm. Hg
1.36242 at 18.35' C. 22.27 0.1 1.716 X 10-2
1.37757 at 20' C. 21.7 ;t 0 . 3 2.101 x 10-2
*
As a laboratory reagent 2-propanol has had some application. Griffin (10) successfully substituted it for ethyl alcohol in histological work, the preparation of reagents, and general laboratory use. Gilson (8) states, "During several years of
especially where the chemical properties of the alcohol are sign6cant. Thus the flame test f o r boron and the ester test for acetatesfail to work with 2-propanol. Certain inorganic compounds can be separated with 2-propanol. The advantage of the lower solubiliy of salts in 2-propanol is canceled by the decreased solubility of the soluble as well as fhe insoluble salt. However, 2-propanol is a satisfactory substitute for ethanol in this type of separation, and can be used in determinations to decrease the solubility of precipitates, and as a washing medium for precipitates. biochemical research the writer has found many instances where isopropanol could be substituted for ethanol in laboratory work. It is cheap and there are no restrictions governing its use, nor is it likely to be an object of theft." Schuette and Smith (18),using 2-propanol in the determination of acid numbers, obtained more satisfactory results than with ethanol. Schuette and Harris (17) made the same substitution in the determination of saponification numbers, using 2propanol in the preparation of solutions of potassium hydroxide. In references to the use of %propanol as a substitute for ethanol, there are few data of value in predicting its applicability as an analytical reagent. Neither International Critical Tables nor Seidell's "Solubilities of Inorganic Compounds" contains any appreciable amount of information regarding the solubility of inorganic salts in isopropyl alcohol, data which would be of importance in predicting the behavior of the reagent in inorganic analysis. Four articles have recently been published regarding the solubility of compounds in %propanol. Kim and Dunlap (IS) studied the solubilities of the alkali chlorides and sulfates in anhydrous alcohols, including isopropyl alcohol. These salts are slightly more soluble in ethanol than in 2-propanol. Ginnings and Chen (9) investigated the ternary systems, water, 2-propanol, and salts, obtaining qualitative results with seventy-five common inorganic salts and quantitative results with ten salts. Hopkins and Quill ( l a ) , in a study of the use of nonaqueous solvents in the rare earth group, stated that isopropyl alcohol is a very poor solvent for the rare earth chlorides. In determining the solubility of silver bromate in mixtures of alcohols and water Neuman (15) found that the values in mixtures of isopropyl alcohol and water fall between those in ethanol-water mixtures and those in n-propanol-water mixtures.
