Determination of Fluoride in Water
A
Modified Zirconium-Alizarin Method WILLIAM L. LAMAR
Geological Survey, United States Department of the Interior, Raleigh,
A convenient, rapid colorimetric procedure using the zirconiumalizarin indicator acidified with sdfuric acid for the determination of fluoride in water i s described. Since this acid indicator i s stable indefinitely, it i s more useful than other zirconium-alizarin reagents previously reported. The use of sulfuric acid alone in acidifying the zirconium-alizarin reagent makes possible the maximum suppression of the interference of sulfate. Control of the p H of the samples eliminates errors due to the alkalinity of the samples. The fluoride content of waters containing less than 500 parts per million of sulfate and less than 1000 p.p.m. of chloride may be determined within a limit of 0.1 p.p.m. when a 100-ml. samplt is used.
N. C.
sample and standard, mix well, and compare in 1 hour or better after the samples and standards have stood overnight. When the alkalinity of the samples exceeds 100 arts per million as calcium carbonate (bicarbonate 122 p.p.my, neutralize the alkalinity with the 0.164 N nitric acid, make up samples and standards to 105 ml., and add exactly 10 ml. of the acid indicator. Comparisons are conveniently made in a %hole colorimeter in which each sample is compared with the two closest standards. For moderately colored waters compensate for the color in the sample as follows: Place the sample to be compared above a Nessler tube containing distilled water, place the standard above a duplicate sample which has been acidified with 5 ml. of 2.1 N sulfuric acid, and make up to volume with distilled water. The volume in each tube should be the same. DISCUSSION OF METHOD
A sensitive color range is obtained for amounts of fluoride ranging from 0.0 to 0.16 mg. which is a range from 0.0 to 1.6 p.p.m. when a 100-ml. sample is used. For waters containing more than about 1.6 p.p.m. of fluoride smaller samples diluted to 100 ml. should be used. It is generally Satisfactory to use sulfuric acid that is approximately 2.1 N in the preparation of the acid indicator, but to obtain the most satisfactory fluoride range the normality of the acid should be close to 2.1. A sensitive color range is dependent upon the strength of the reagents used in the preparation of the acid indicator. Although samples and standards can be satisfactorily compared after one hour, comparisons after the samples and standards have stood overnight frequently give a little greater accuracy. This is true for the fluoride methods employing the acid reaction of the zirconium-alizarin indicator because equilibrium conditions are not reached in one hour. The time consumed in adding the acid indicator and in mixing allows slight differences in the stage of the reaction. When comparisons are made after one hour the samples and standards should be of the same temperature and the acid indicator should be added as quickly as possible. The color change is complete for samples and standards that are allowed to stand overnight. In the latter case as much as 2 hours’ difference in the time of adding the acid indicator does not affect the determination. This fact can be used to advantage when inspection shows that some of the samples are out of the range of the standards. Smaller samples can be taken, so that when comparisons are made the next day all samples will be within the range of the standards. By using the sulfuric acid zirconium-alizarin indicator the interference of sulfate is decreased. On the basis of 100-ml. samples the error that may be introduced by sulfate or chloride is as follows: 500 p.p.m. of sulfate are equivalent to about
V
ARIOUS methods and modifications using the zirconiumalizarin reagent for the determination of fluoride in water have appeared in the literature (1, 2, 3, 6, 6 ) . The procedure published in 1941 for thedetermination of fluoride in water, using the zirconium-alizarin indicator and sulfuric acid (d), has been revised to make the method more convenient and adaptable for use in water laboratories. The sensitive fluoride range of the indicator has also been increased. The acid indicator used is prepared with sulfuric acid and the zirconium-alizarin reagent. This acid indicator is stable indefinitely, is always ready for use, and is therefore more satisfactory than other previously reported zirconium-alizarin solutions which are not stable. The use of sulfuric acid alone in acidifying the zirconium-alizarin reagent makes possible the maximum suppression of the interference of sulfate. The alkalinity of a sample may interfere with the accurate determination of fluoride by ita effect on the p H of the solution. Neutralization of the alkalinity of the samples with nitric acid is employed to prevent errors caused by the alkalinity. The method reported here embodies a stable indicator solution, the maximum suppression of the interference of sulfate, a sensitive color range, and a more accurate and convenient procedure for the determination of fluoride in water. REAGENTS
Zirconyl nitrate, 1.84 grams of zirconyl nitrate dihydrate in 250 ml. (filter). Alizarin red S, 0.37 gram of alizarin monosodium sulfonate in 250 mi. Sulfuric acid, 2.10 N (to 2.12 N ) . Acid indicator. Add 25 ml. of the zirconyl nitrate solution to 50 to 100 ml. of distilled water and add slowly with constant stirring 25 ml. of lhe alizarin solution and make up to 500 ml. Mix well and add 500 ml. of 2.1 N sulfuric acid. The acid indicator is ready for use in about one hour. Nitric acid, 0.164 N ( 1 ml. will neutralize 10 mg. of bicarbonate); or use nitric acid that is ten tunes the strength of the acid used in titrating the alkalinity of the samples. Sodium fluoride. Stock solution, 0.2210 gram of sodium fluoride in 1liter. Standard solution, dilute 100 ml. of the stock solution to 1 liter (1 ml. equals 0.01 mg. of fluoride).
Table I. Interference of Sulfate, Chloride, and Unneutralized Bicarbonate in Determination of Fluoride in Water (Using the zirconium-alizarin reagent with 2.1 N sulfuric acid) Sulfate
PROCEDURE
Transfer 100 ml. of the clear samples to matched Nessler tubes. Make up to 100 ml. in matched Nessler tubes the standards that are needed, Standards may be made up in 0.02-mg. intervals from 0 to 0.16 mg. of fluoride or in 0.01-mg. intervals from 0 to 0.10 mg. of fluoride and in 0.02-mg. intervals from 0.10 to 0.16 mg. of fluoride. Add exactly 10 ml. of the acid indicator to each
20 30
40 50
eo 100
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Fluoride Error (Plus) 0.003 0.005 0.008
0.010 0.012 0.020
Fluoride Error Chloride (Minus) Milligrams per 100 ml. 40 0.002 60 0.005 80 0.008
100 200
...
0.010 0.017
...
Fluoride Error Bicarbonate (Minus)
10 20 30 40 50
0.003 0.008
0.012 0.017 0.021
...
March, 1945
ANALYTICAL EDITION
+0.01 mg. of fluoride; and lo00 p.p.m. of chloride are equivalent to about -0.01 mg. of fluoride. Since the errors introduced by sulfate and chloride are plus and minus, respectiv&ly, the effect of the one tends to counteract the effect of the other. However, the effects of interfering ions are not completely additive. On the basis of 100-ml. samples an alkalinity of 200 p.p.m. (bicarbonate 244 p.p.m.) is equivalent to about -0.01 mg. of fluoride. For accurate results it is necessary to neutralize the alkalinity with nitric acid when it exceeds about 100 p.p.m. (bicarbonate 122 p.p.m.). The effect of nitrate in the nitric acid and of nitrate present in natural waters is negligible. Table I shows the interference of sulfate, chloride, and unneutralized bicarbonate. Bicarbonate may be converted to alkalinity as calcium carbonate by multiplying the bicarbonate by the factor 0.82. The interference of sulfate causes the fluoride measurement to be high and the interference of chloride and unneutralized alkalinity cause the fluoride measurement to be
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low by the amounts shown in Table I. Neutralization of the alkalinity as prescribed eliminates the error caused by the alkalinity of the samples. Fluoride corrections may be applied instead of neutralizing the alkalinity of the samples. However, neutralization of the alkalinity is recommended. LITERATURE CITED
(1) Am.Public Health Assoc. and Am. Water Works Assoc., “Standard Methods for the Examination of Water and Sewage”, 8th ed.. pp. 36-8.1936. (2) Am. Water Works Assoc., Committee Report, J. Am. Water Work8 ASSOC., 33, 1966-2017 (1941). (3) Elvove, Elias, Pub. HeaZth Repts., 48,1219-22 (1933). (4) Lamar, W. L.,and Seegmiller, C. G . , IND.ENG.CHEM.,ANAL. ED., 13,901-2 (1941). ( 5 ) Sanchis, J. M., Ibid., 6,134-5 (1934). (6) Ssott, R.D., J. Am. Water Works Assoc., 33,2018-20 (1941).
PUBLISHED by permission of the Director, Geological Survey, United Stated Department of the Interior.
Quantitative Estimation of DDT and of Dust Deposits
DDT Spray
or
FRANCIS A. GUNTHER University of California Citrus Experiment Station, Riverside, Calif.
A method for the quantitative estimation of the amount of DDT in a wmple containins DDT is bared upon the quantitative dehydrohalogenation of DDT under certain conditions. A specific application to the determination of spray or dust deposits is discussed in detail. Data are presented to show both the accuracy and the reproducibility of the method, and possible sources of error are discussed.
M
molecule of hydrochloric acid per molecule of parent compound upon boiling with alcoholic potassium hydroxide. Recently Brand and Busse-Sundermann (1) synthesized and studied D D T and many new analogs. That phase of their work concerned with dehydrohalogenation indicated that 50 ml. of 0.5 N alcoholic potassium hydroxide completely dehydrohalogenated 5 grams of D D T within 2 to 3 minutes under reflux. These studies form the basis for the analytical method herein described. When the sample containing the D D T (I) is treated with excess alcoholic potassium hydroxide, the following transformation is effected:
ANY workers have shown D D T [2,2-bis-(p-chlorophenyl)l,l,l-trichloroethane] to be such a promising insecticidal material that the lack of a method of analysis suitable for its quantitative estimation has become a serious hindrance to its further insecticidal evaluac1 tion. (The alphabetical symbol derives from II the loosely descriptive name, dichlorodi)C = C-C1 KC1 Hz0 -C-C1 KOH 3phenyltrichloroethane.) A total halogen determination on a materid containing D D T c l ~ / c1 appears impractical because of the probable contamination with extraneous chlorides, (1) such as those found in hard waters, for example. I n addition, all organic halogen-conSince one, and only one, chloride ion is liberated from each taining contaminants, such as those found in technical DDT, molecule of DDT, the ,remainder of the method involves deterwould also respond to such a drastic method of analysis. mining the quantity of free chloride ion in the products of hyIt has been brought to the attention of the author, and subsedrolysis; this is accomplished by precipitating the chloride ion quently verified by him, that technical D D T may contain some with excess silver ion, and then determining the excess of the of the o,p‘ isomer. Theoretically, both isomers should yield one latter by means of a Volhard titration with ferric nitrate as inmole of hydrogen chloride under the experimental conditions dicator. described herein. Actually, however, slightly more than one mole may be obtained from the purified o,p’ isomer. This behavior is not without justification, and i t will be discussed in dePROCEDURE tail in a later report. These considerations led to the development of a more specific e i ~ ~ ~ i P b ~ h ~ ~ ~ ~ ~ ~ ~ method based upon dehydrohalogenation. In 1874, Zeidler (4) washing them individualIy with a stream of benzene from a wash bottle. The quantity of stripping solvent used is immaterial, reported that long boiling of the compound now popularly known as ’ D D T (I) with alcoholic potassium hydroxide resulted in for the entire Sample Will be used in One analysis. Filter the benzene strip solution through a plug of cotton or of solid foreign matter, and catch the filtrate dehydrohalogenation, yielding 2,2-bis-(p-chlorophenyl)-l,l-di- glass wool to cuoroethylene (11). Fischer (2) extended this study to several in a standard-taper 500-ml. Erlenmeyer flask. If preliminary analogs of DDT and reported that most of them readily lost & experiments have shown benzene-soluble inorganic chlorides to be
I
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