ANALYTICAL CHEMISTRY
544 RECOMMENDED GENERAL PROCEDURE
Sample. Weigh, or' measure by volume, a sample such that the resulting neutral solution contains 0.001 to 0.1 mg. of zirconium per ml. Separation. Transfer a 20-ml. aliquot of this prepared solution to a 50-ml. centrifuge tube and add 5 ml. of concentrated nitric acid. Add 10 ml. of the standard Dhosphate solution. mix thoroughly, and allow the solution to itand-for 20 hours, ifpractical. Centrifuge for 15 minutes. ~~~~~f~~ a 25-ml, aliquot of the supernatantsolution to a 5 0 - ~ l volumetric . flask and tralize with 10 N sodium hydroxide to a phenolphthalein end p i n t . Add 5 ml. of a 2.8 lvnitric acid solution to the volumetric flask and dilute to the mark.
Measurement of Desired Constituent. Add 5 ml. of the molybdate solution to 50 ml. of the above-mentioned solution. ~~~~~~
~
~~
s
~~
it:~~
~~ ~; , ~ ~~ ~ ~' ~ ~ ~ i ~"
The use of distilled water in the reference cell is recommended. LITERATURE CITED
(1) Bolts, D. F., and liIellon, M. G . , . k - a t . CHEM., 20, 749 (194S). (2) Hillebrand, W. F., and Lundell, G. E. F., "Applied Inorganic Analysis," p. 446, New Yolk. John Kiley &- Sons, 1929. (3) Willard, H. H., and Hahn, R.B., . I s . < L . C H E ~ I21, . , 293 (1919). R E C E I V EJuly D 11, 3961.
-4ccepted October 1,1951.
Colorimetric Determination of Benzene Hexachloride MILTON S. SCHECHTER AND IRWIN HORNSTEIN Bureau of Entomology and Plant Quarantine, United States Department of Agriculture, Beltsville, Md. The need for determining benzene hexachloride in trace amounts has led to the development of a sensitive colorimetric method. Benzene hexachloride is dechlorinated to benzene in a special apparatus by means of zinc in acetic acid. The benzene is absorbed in a nitrating mixture and converted to m-dinitrobenzene, which, after extraction, is made to react with methyl ethyl ketone in the presence of strong alkali. The violet-red color which is produced is measured photometrically. A s little as 5 micrograms of benzene hexachloride can be determined. This method should prove useful in analyses of spray residue and in pharmacological investigations.
T
HE development in recent years of many halogen-containing organic insecticides such as DDT, benzene hexachloride, toxaphene, chlordan, dieldrin, and aldrin has created the need for sensitive, specific methods for their detection and determination. Such methods are an absolute necessity if any progress is to be made in studies requiring analyses of spray residues, or in pharmacological and other fields of investigations. The toxicity of many of the new insecticides to humans and livestock makes it important to find methods of analysis to ensure their being employed effectively and safely without causing contamination of foodstuffs. Several methods have been devised for the quantitative determination of benzene hexachloride. Methods previously described have been based on mass isotope dilution ( 2 2 ) ,ultraviolet (6) and infrared (4,5 ) spectrophotometry, total and hydrolyzable chlorine (IO),partition chromatography (1, 18), polarographic analysis ( 7 ) ,and biological assay (9). Recently a colorimetric method has been described by Fairing and Phillips (8). The sensitive colorimetric method herein described is based on the dechlorination of benzene hexachloride to benzene by means of zinc in acetic acid. Although the dechlorination of benzene hexachloride and of benzene heptachloride has been observed by others (13, 14, 19), so far as the authors know, this reaction has not been utilized as the basis of an analytical method. In the present method the dechlorination and the subsequent nitration are carried out in a specially designed all-glass apparatus (Figure 1). The benzene is absorbed in a nitrating mixture and converted to m-dinitrobenzene. After extraction, it reacts with methyl ethyl ketone in the presence of strong alkali ( 2 , S, 12, 17, 20, 23, 2 4 ) and the violet-red color produced is measured photometrically. The color of the condensation product of the ketone with m-dinitrobenzene is believed to be due to a quinoid structure ( 2 ) . Although the conversion to m-dinitrobenzene under the conditions employed is not quantitative, the results by this method are consistent and as little as 5 micrograms of technical benzene hexachloride or of lindane can be determined. Lindane, as available commercially, contains 99% or more of the gamma isomer of benzene hexachloride.
APPARATUS
Beckman spectrophotometer, Model B, and 1-em. absorption cells with glass covers. Glass Gooch crucible holders, body about 25 mm. in diameter by 75 mm. long, stem about 30 mm. long. Electric heating units of the type of Fisher No. 11-502-15 or equivalent, screwed into a porcelain socket. A specially designed all-glass digestion and nitrating apparatus (Figure 1). REAGENTS
Malonic acid, C.P. reagent grade. Zinc metal, granular 80-mesh. Glacial acetic acid, Nitrating acid. A mixture of C.P. fuming nitric acid (spccifii. gravity 1.49 to 1.50) and C.P. concentrated sulfuric acid (sl)ei.ific gravity 1.84), 1 to 1 by volume. Sodium chloride solution. Distilled nater saturated with C.P. sodium chloride. Sodium hydroxide solution, 2%. Ether, U.S.P. grade, freshly distilled and free from peroxides and aldehydes. Potassium hydroxide, 40% w./v. Dissolve 470 grams of C.P. potassium hydroxide (8570) in distilled water to make 1 liter or' solution. . ~~~~. Phosphoric acid, 85%. hlineral oil, refined. Nujol or its equivalent. Methyl ethyl ketone, fractionated and the cut boiling a t T981' C. used. Absorbent cotton. dried in an oven a t 110" for 2 hours. ~~~~
PREPAR4TION O F ST4NDARD CURVES
For lindane samples prepare calibration curve (Figure 2, curve 3) employing aliquots of a standard glacial acetic acid solution of lindane or of the pure gamma isomer. For technical benzene hexachloride samples prepare a standard curve (Figure 2, curve 4) with the same or a similar type of benzene hesachloride as to that for which analyses are to be run. In general, the recoveries of the gamma isomer are about 10% higher than of technical grades of benzene hexachloride. To the sample in the flask, made up to a volume of 10 ml. vith acetic acid, add 1 gram of granulated zinc and 2 grams of malonic acid. Lubricate the ground joint and stopcock of the apparatus with 85y0 phosphoric acid and attach the flask, making sure that
545
V O L U M E 2 4 , NO. 3, M A R C H 1 9 5 2 no particles of zinc adhere to the ground surface of the joint. Fill the nitration tube of the apparatus with 5.0 ml. of the nitrating mixture. Fill the outer jacket of the apparatus approximately to the one fourth mark with trichloroethylene and add a small amount of granulated zinc for smooth ebullition. Place the flask on the heating unit and reflux vigorously for a minimum of 2.5 hours. Apply just enough heat so that the trichloroethylene vapors reach the "cold finger" condenser (not shown in diagram) resting in tube A , Figure 1. The refluxing trichloroethylene serves to condense the acetic acid and prevents benzene from condensing before being swept into the nitrating tube. At the end of the run remove the heat source and quickly separate the reaction flask from the rest of the apparatus, so that the liquid is not sucked back from the nitrating tube. mash the contents of the nitrating tube into a 250-ml. separatory funnel set under the outlet of the apparatus, using the following procedure: Place 10 ml. of ice-cold distilled water in the separatory funnel and drain the nitrating acid into it, Rinse the apparatus three times with a total of 50 ml. of cold distilled water; sim,ilarly rinse with 50 ml. of ether and then with another 50-ml. portion of distilled water. Shake the separatory funnel vigorously, let the layers separate, and drain the acid portion into a second 250-ml. funnel. Extract the acid fraction with 30 ml. of ether and discard the aqueous portion. Wash the ether in the first and second funnels successively with 30 ml. of 2% sodium hydroxide solution; repeat with 30 ml. of saturated salt solution. Filter the ether extract in the first funnel through a 0.75-inch plug of cotton packed in a glass Gooch crucibje holder into a 250-ml. glass-stoppered Erlenmeyer flask. Drain the ether extract from the second funnel into the first funnel, use it as a rinse, and then filter through the cotton plug. Use three 15-ml. portions of ether as successive washes for the funnels and the cotton. To the ether solution add a glass bead and a drop of mineral oil. Remove the ether on a steam bath, sqirling the flask to start the bead bouncing and stopping the distillation when a few milliliters of ether are left If desired, the ether may be recovered through an a 1 I - g 1 a s s recovery condenser having a standard joint, as no rubber or cork stoppers should be used. Rotate the flask horizontally and allow any residual ether to evaporate. The mineral oil prevents volatilization of the nd i n i t r o b e n z e n e . Pipet into the flask 10.0 ml. of methyl ethyl ketone and swirl to complete solution of any residue. Add 1.0 ml. of 40% potassiuni hyd r o x i d e solution, shake vigorously for a t least 1 minute, and let the color develop for 20 minutes, preferably in the absence Figure 1. Apparatus for Deof bright light. chlorination and Nitration The two layers separate cleanly. Decant part of A . Outer jacketing tube 28-mm. 0.d. by 35 om. t o ring seal the methyl ethyl ketone B . Glass tube 16-mm. 0.d. by 26 layer into an absorption c m . to ring seal cell and cover. Read the C. Glass tube 9-mm. 0.d. D . 19/38 standard-taper groundtransmittance i m m e d i glass joint ately a t 565 mp, as reE. Reaction flask with 19/38 standmoval from contact with ard-taper ground-glass joint. Bushing adapters may be thestrongly alkaline phase used for large flasks accelerates fading. To set F . Kitrating tube, 9-mm. o.d., the photometer a t 100% flared a t top G . Bulb, 20-mm. diameter, center transmittance, use 10.0 ml. of bulb 11 cm. from top of of methyl ethyl ketone that apparatus has been shaken vigorously H . Solid glass beads 3-mm. diameter packed to height of apwith 1.0 ml. of 40% poproximatelv 15 cm. tassium hydroxide solution I . Glass wool piug and allowed to separate. J . Stopcock, 2-mm. oblique bore
NOTES ON APPLIC4TIONS OF THE PROCEDURE
Benzene hexachloride can be determined in many cases diiec tly on the solid material. If prior treatment is required, care rriu-t be taken to avoid loss of benzene hexachloride by volatilization. I n some cases, extraction with a low-boiling, nonaromatic solwnt such as carbon tetrachloride may be necessary. Certain solrt nts, especially petroleum fractions, unless specially purified are lihely to cause serious interference because of the presence of traces of aromatics (%O,21). Benzene, in particular, should be avoided. 100
90 80 7c
6C 5! 4c
3(
a
IC I
I
I
20
40
I 60 80 100 MICROGRAMS
Figure 2.
Standard Curves
120
1. m-Dinitrobenzene 2. Theoretical curve for benzene hexachloride calculated from curve 1 3. ?-Benzene hexachloride carried through method 4. Technical benzene hexachloride carried through method
If a solvent is present, add a glass bead and remove the solvent on a steam bath, avoiding prolonged heating or the use of an a i r stream. Then carry out the benzene hexachloride determination as described under the Preparation of the Standard Curve, with appropriate amounts of zinc, acetic acid, and malonic acid. For immiscible liquids and finely divided solids insoluble in acetic acid, use a volume of acid at leaht tnice the bulk volume of the liquid or solid, or, in any event, enough to avoid too thick a suspension. In a homogeneous system use a t le'ut one volume of the acid for one volume of the soluble material. On the basis of present experience, it is suggested that 1 gram of zinc be used for each 10 ml. oi acetic acid and 2 grams of malonic acid for the first 10 nil. of acetic acid and 0.5 gram for each additional 10 ml. of acid. \Vhen using larger flaqks, let the initial heating be gradual and not so rapid that nitrating acid is forced out of the absorber. \Iith soil samples, ground peanuts, and some extracts and oils, local overheating and charring can take place when electric heaters are used. In such cases an oil bath a t about 150" C is preferable. The remainder of the piocedure is identical with the steps described under the Preparation of the Standard Curve. From the per cent transmittance or absorbancy for a given sample, determine the amount of benzene hexachloride present from the appropriate standard curve. If an aliquot has been used a t any stage, multiply the amount of benzene hexachloride found by the appropriate factor. If the amount of benzene hexachloride in the sample is known to be above the range covered by the standard curve, use appropriate aliquots. This may be done
ANALYTICAL CHEMISTRY
546 either by adjusting the sample size or by adding more than 10.0 ml. of methyl ethyl ketone to the m-dinitrobenzene residue and withdrawing an aliquot. In general, unless analyzing for minimal amounts of benzene hexachloride near the limit of the method, it is preferable to add more than 10.0 ml. of methyl ethyl ketone to the m-dinitrobenzene residue and to withdraw an aliquot for color development. If the color is too dark to read satisfactorily, repeat the color development, employing a smaller aliquot diluted to 10.0 ml. with methyl ethyl ketone. With colorimeters and photometers that have the light beam passing through the test tube or cell above the level of the aqueous potassium hydroxide layer (Aminco Type F photometer and probably some others), a simpler procedure may be used. .80
t---
done, as acetic acid is not readily attacked by the nitrating acid and the nitration can still proceed satisfactorily. Some samples of acetic acid give a measurable blank. This blank can be minimized by refluxing the acetic acid with some granulated zinc for several hours, distilling off about 20% of the acid, and filtering the remainder for use in the method. It may also be desirable to wash the zinc, prior to its use, with a solvent to remove any adhering film of oil, but the authors have found no appreciable reagent blank from either zinc or malonic acid. A number of variations were tried of the basic method of developing a color from m-dinitrobenzene with a ketone and alkali. Tests were made by shaking acetone or methyl ethyl ketone with various concentrations and relative volumes of sodium and potassium hydroxide solutions, but the procedure described was adopted as the simplest for the purpose, RESULTS
*
20
c
\ WAVELENGTH, MILLIMICRONS
Figure 3. Absorption Curve of rn-Dinitrobenzene in Methyl Ethyl Ketone Plus Alkali
Pipet the methyl ethyl ketone and alkali directly into the photometer test tubes, preferably glass-stoppered, and shake thoroughly to develop the color. If rubber stoppers are used, boil them two or three times with 20% aqueous alkali to remove sulfur; wash thoroughly and dry. When using rubber-stoppered photometer tubes, shake the test tubes so that the liquid does not touch the stopper. Do not decant the methyl ethyl ketone layer; simply let the layers separate and 20 minutes later read the transmittance using the appropriate filter. The color is much more stable when produced in this manner than when the methyl ethyl ketone is decanted from the alkali. In using this method it is essential to run a control analysis on untreated material of the type being analyzed.
If the untreated sample gives an apparent benzene hexachloride value, subtract the results, expressed in the desired units-for example, in parts per million-from the benzene hexachloride values obtained on the treated samples expressed in the same units. In any case, apply the values obtained from control analyses as corrections to the regular analyses, giving due care to dilutions or aliquots and to the units in which concentrations are expressed. This control analysis on untreated material will automatically include the reagent blank, provided that the same reagents and solvents are used in the same quantities as in the analysis of the benzene hexachloride-treated samples. When a control or untreated sample is unavailable for analysis, a blank run on the reagents and solvents employed will furnish a correction, although this is not EO satisfactory. Acetic acid has definite advantages. In addition to being a useful solvent, it promotes the dechlorination reaction, and the boiling point is convenient. If some acetic acid gets into the nitrating tube, either because of the azeotropic distillation with benzene ( 1 1 ) or because it is carried over in the vapor state, no harm is
The absorption curve for the color produced by 51 micrograms of pure m-dinitrobenzene in 10.0 ml. of methyl ethyl ketone shaken with 1.0 ml. of 40y0 potassium hydroxide solution is shown in Figure 3. There is a broad peak and all quantitative measurements were made at a wave length of 565 mp, which appears to be the wave length of maximum absorption. Figure 2 shows curve 1 produced by direct reaction of pure ndinitrobenzene with methyl ethyl ketone and alkali and the theoretical curve, 2, for benzene hexachloride calculated from the mdinitrobenzene curve by assuming quantitative dechlorination to benzene and quantitative nitration to m-dinitrobenzene. The calibration curve, 3, obtained by the actual dechlorination and nitration of the pure gamma isomer of benzene hexachloride does not coincide with the theoretical curve, 2. There may be several reasons for this incomplete recovery in addition to possible slight mechanical and extraction losses. The nitration of benzene produces primarily m-dinitrobenzene and also some of the ortho and para isomers, The last two compounds do not produce any color with methyl ethyl ketone and alkali ( 2 ) . Although unlikely, there may also be incomplete conversion of some of the gamma isomer to benzene. At room temperature there appears to be about an 85y0 over-all conversion of the gamma isomer to m-dinitrobenzene by this procedure. Sormal fluctuation in room temperature does not appear to introduce any appreciable error in the values obtained. A series of analyses of four samples, each containing 74.7 micrograms of the gamma isomer, gave 73.5, 73.5, 74.4, and 74.7 micrograms, or an average of 74.0 micrograms with an average deviation of zk0.5 microgram. By jacketing the nitration tube and maintaining a constant temperature, this source of error could probably be minimized. In practice this procedure does not appear to be necessary except where high precision is desired. With the spectrophotometer and cells employed, the absorption curves follow Beer's law over the range studied, although with one filter photometer tested this was not the case.
Table I.
Results with Individual Isomers of Benzene Hexachloride Added to Acetic Acid
Isomer Alpha Beta Gamma Delta
Epsilon Technical benzene hexachloride
Melting Point,
Re0ux Time, Hours 158.5-159 2.5 2.5 308-309 2.5 113.5-114 138.5-139 2.0 2.5 3.0 5.0 16.0 16.0 218-220 2.5
C.
Micrograms .4dded 65
Micrograma Found 65 Varies 88 88 90 28 22 73 28 90 73 23 91.5 55 183 107 44 45
%
Found" 100 S b o u t 80-90 100 31 30 31 31 60 58 98
2.5 24 22 91 3.5 62 56 90 0 All results a r e given relative t o t h a t of gamma isomer taken as 100% Curve 3 of Figure 2 was used as standard.
547
V O L U d E 24, N O . 3, M A R C H 1 9 5 2 Table I shows typical results obtained with the various individual isomers of benzene hexachloride and with a technical product. With the gamma isomer as a standard, results for the alpha and epsilon isomers appear to be about 100% and for the delta isomer about 30%, although longer refluxing increased this last value to about 60%. With the beta isomer, results were somewhat variable and ranged from about 80 to 90%. A change in the conditions of the dechlorination reaction may improve the results with the beta and delta isomers, and work is under way to this end. However, the delta isomer, which gives the lowest results, accounts for only about 5 to 10% of technical benzene hexachloride, whereas the alpha, beta, gamma, and epsilon isomers account for most of the remainder. A typical sample of technical benzene hexachloride, when used to prepare a standard curve (Figure 2, curve 4),gives about 90% recovery relative to that of the gamma isomer (Figure 2, curve 3). The low recovery on the technical product may he due not only t o the low values for the beta and delta isomers, but also to impurities-for example, any heptachlorocyclohexanes present would probably interfere. Table I1 shows the I ecoveries of the gamma isomer added to a number of substances. Among the materials tested, untreated peanuts and certain types of soils gave appreciable apparent benzene hexachloride values. U-ork is under way to eliminate these difficulties
Table 11. Recovery of Gamma Isomer of Benzene Hexachloride Added to Various Materials
Sample
Weight oi Sample, Grams
Gamma Isomer ildded, y
Alfalfa 25 Butterfat 10 Sol1 low organic content 10 Peanut oil 10 Peanuts, finely ground 25 a Corrected by control analyses on
Gamma Isomer Recovered, y
Recovery % h-ot cor- Corrected recteda
100 100
96 99
96 99
95 98
100 100
100
100
100
98
98
106
98
98
100 106 untreated samples
T o ascertain the possible degree of interference from other chlorinated insecticides, a number of them were analyzed for benzene hexachloride by this method. The results are given in Table 111. Chlordan was the only insecticide that showed more than a trace of apparent benzene hexachloride, and even this interference was insignificant. One possible explanation for the apparent amounts of benzene hexachloride in these insecticides may be that some benzene hexachloride actually was formed by chlorination of traces of benzene present in the raw materials.
Table 111. Amounts of Apparent Benzene Hexachloride Found in Other Chlorinated Organic Insecticides
Insecticide Toxaphene Aldrin Dieldrin DDT Methoxychlor Chlordan
Weight of Sample, Mg. 57.4 41.6 41.6 40.0 74.5 42.4
Apparent Benzene Hexachloride Found, Mg.
0.002 0,002 0.002 0.002 0.005 0.238
Where benzene hexachloride is found and it is known or suspected that free benzene or another interfering aromatic compound is also present, the extent of this interference can be determined on a separate sample by running through the procedure exactly as described but omitting only the addition of granulated zinc-Le., without the dechlorination. When this was done with the chlordan sample in Table 111, 0.219 mg. of apparent benzene hexachloride was found, an indication that in this case the interference was largely due to the presence of some benzene or other
aromatic in the sample. If an appreciable amount of such an interfering aromatic is present, some preliminary treatment of the sample is necessary to remove it before benzene hexachloride is determined. As a niixrure of trichlorobenzenes, uith the 1,2,4- isomer predominating, is formed from technical benzene hexachloride upon reaction with alkali, an experiment was performed to determine the degree of interference. An analysis of 51 mg. of 1,2,4-trichlorobenzene gave only 35 micrograms of apparent benzene hexachloride, which is regarded as insignificant. Although the method seems to be specific for benzene hexachloride with regard to any significant interference by commonly employed insecticides, any material that can be reduced or dehalogenated to benzene under the conditions employed would give the same color. In addition, certain aromatics ($O), if originally present or eventually produced, might distill into the nitration mixture to yield polgnitro derivatives which would also interfere. During the development of this method a number of interesting observations were made. Because of difficulties in attempts to use carbon dioxide from a Kipp geneiator or a tank controlled by a needle valve, or in generatingenough hydrogen from the zinc tosweep the benzene formed into the nitrating mixture, malonic acid dissolved in the acetic acid was tried as an autogenous source of carbon dioxide. Carbon dioxide and some hydrogen are evolved smoothly a t a satisfactory rate for more than 3 hours. This process might have applications in other work where a slow stream of carbon dioxide is required. I t is obvious that any chloride formed in the dechlorination reaction can be determined after filtration of the zinc and that any benzene formed can be determined by ultraviolet spectrophotometry. In one experiment benzene Ras detected after the gamma isomer was refluxed in ethyl alcohol with granulated zinc, and the ultraviolet absorption was run on the distillate. In another experiment the gamma isomer was refluxed with zinc in ethyl alcohol, and the chloride was determined by electrometric titration of the filtrate after dilution with nitric acid, giving results corresponding to the elimination of six chlorine atoms. Titrations of chloride formed from the gamma isomer by zinc in acetic acid gave the same results; honever, in this case a trap containing aqueous alkali had to he used, since some hydrogen chloride escaped from the refluxing acetic acid. The delta isomer gave only about 30% of the theoretical amount of chloride even after 4 hours’ refluxing. The observation was also made that amalgamated zinc in hot absolute ethyl alcohol reacted a t the fastest rate with the gamma isomer of benzene hexachloride. I\ hen the alpha, beta, and delta isomers were tested, only the alpha isomer showed any appreciable activity, reacting considerably slower than the gamma isomer. The authors had hoped to acply this observation to the development of a specific method for the gamma isomer, but they encountered difficulties with mixtures of the isomers. Moreover, they found that traces of water accelerated the rates of dechlorination. The dechlorination reaction may possibly be of value in studying the configuration of the benzene hexachloride isomers and possibly of other polychloro compounds such as chlordan, toxaphene, dieldrin, and aldrin. The polarography of the gamma isomer ( 7 , 16) and its toxicity to insects may also be related to the ease of dechlorination (which is an oxidation-reduction reaction), since of all the isomers of benzene hexachloride, only the gamma isomer gives a half-wave reduction potential and is highly insecticidal. Since the submission of this paper, an article by Nakazima, Inagaki, and Tati (16), has come to the authors’ attention, in which the reaction rates of benzene hexachloride with various metal powders have been studied and in which benzene was formed quantitatively by reaction with zinc. These authors also noted that water had an accelerating effect.
548
A N A L Y T I CA L CHfM I STRY ACKNOWLEDGMENT
The authors are grateful to A. K. Klein, Food and Drug Administration, for checking the method and offering valuable suggestions. LITERATURE CITED
A\epli, 0 . T., Munter, P. A, 610 (1948).
and Gall, J. F., ANAL.CHEM.,2 0 ,
Baernstein, H. D., IND.ENG.CHru., ANAL.ED.,15, 251 (1943). Bost, R. W., and Nicholson, F., Ibid., 7 , 190 (1935). Daasch, L. W., Ax.4~.CHEM.,19, 779 (1947). Daasch, L. W., and Smith. D. C., U. 9. Naval Research Inst., Rept. P-3033 (Feb. 6 , 1947). Davidow, B., and Woodard, G., J . Assoc. Ofic.Agr. Chemists, 3 2 , 7 5 1 (1949).
Drayt. G., ~ ~ K A LCHEM., . 20, 737 (1948). Fairing, J. D., and Phillips, TV. F., Division of Agricultural and Food Chemistry, 119th Meeting - 4 ~CHEX. . SOC., Boston, Afass., 1951.
Furman, D. P., and Hoskins, W.AI., J . Econ. Entomol., 41, 106 (194s).
Goldenson. J., and Sass, S., .%SAL. CHEY..1 9 , 3 2 0 (1947). Horsley, H. L., Ibid., 19, 508 (1947). Janovsky, J. V., Ber., 2 4 971 (1891). Lfathews, F. E., J . Cheni. iSoc., 61, 103 (1892).
JIeunier, J., Compt. rend., 114, 75 (1892). Xakarima, Jl.,Inagaki, K., and Tati, T., Botyu-Ragaku, 16, 107 (1951); Englishresume, p. 110. (16) Nakazima, hf., and Oiwa, T., Ibid., 15, 114 (1950); English resume. p. 116. (17) Pearce, S. J., Schrenk, H. H., and Tant, TV. P., r.S.Bur. Mines, (14) (15)
Rept. Inreat. 3302 (1936).
Ramsey, L. L., and Patterson, IT. I., J . Assoc. Ofic. d g r . Chemists, 29, 337 (1946). (19) Riemschneider, R., and Ottman. G., 2. .\-utialfuo,.sch., 5b, 307 (18)
(1950).
Schrenk, H. H., Pearce, S.J., and Tant, I-.P., L-. S. Bur. Mines, Repts. Inwst., 3287 (1938, revised 1937). (21) Schrenk, H. H., Yant, JT. P., and Pearce, S. J., Ibid., 3293 (20)
(1935).
(22)
Trenner', K. R., Walker, R . IT-., and Buhs, R. P., . 4 ~ . 4 CHEM., ~. 21. 285 11849). I
~~~
j
~I
(23) Yant, W.P., Pearce, S. J., and Schrenk, H. H.. C. S.Bur. Mines, Repts. Inccst., 3323 (1936). (24) l a n t , W. P., Schrenk, H. H., and JIautz, P. H., Ibid., 3282 (1935).
RECEIVED for review July 6 , 1931. Accepted October 30, 1951. Presented before the Division of Agricultural and Food Chemistry and Analytical Chemistry, Symposium on Methods of Analysis for Micro Quantities of Pesticides, a t the 119th Meeting of the A X E R I C ACIIEMICAL ~ SOCIETY, Boston,
Mass.
Spectrophotometric Determination of Inorganic Fluoride A. D. HORTON, P. F. THORIASON, AND F. J. MILLER Analytical Chemistry Dirision, Oak Ridge National Laboratory, Oak Ridge, T e n n .
'The discovery that fluoride ion diminishes the color produced when thoron reagent, 1-(0-arsonophenylazo)-2-naphthol-3,6-disulfonic acid, is added to thorium nitrate solution. created further interest in thoron as a possible colorimetric reagent for the determination of microgram quantities of fluoride. Fluoride ion has been estimated spectrophotometrically in the range of 0 to 50 micrograms with an overall accuracy of zt4Yo and a mean standard deviation
R"
GENTLY, Willard and Horton (7') made a thorough study of indicators for the titration of fluorides with thorium soluLions. They developed an excellent photofluorometric titration method wing quercetin as a fluorescent indicator and thorium nitrate as a titrant. This method is useful in the range l t o 40 mg. of fluoride A colorimetric method By Nonnier et al. (3), based on the diminution of the color obtained with ferric ion and 5-sulfosalicylic acid, has been used t o determine 0.2 to 1.0 mg. of fluoride. More recently, Thrun (6) developed a colorimetric procedure for determining fluoride in waters, that is dependent upon the change of color when fluoride ion is added to a n aluminum lake of eriochromecyanine. This procedure covers a range of 0 1 t o 6 p.p.ni. I n the course of developing the colorimetric procedure for thorium, Thomason et al. ( 5 ) ,found that the presence of fluoride ion caused a diminution of the red color of thorium nitrate solutions containing thoron reagent, l-(o-arsonophenylazo)-2-naphthol3,Gdisulfonic acid, which was synthesized by this laboratory according t o Kuznetsov (1). This fact resulted in an attempt to develop a spectrophotometric procedure for fluoride. The feasibility of the method was demonstrated by Thomason and Miller (4),who added varying quantities of a standard sodium fluoride solution t o known volumes of standard thorium nitrate aolution, acidified with hydrochloric acid, and added thoron reagent, Comparison in the Beckman spectrophotometer with a reference containing only thoron resgent and hydrochloric acid 1
of 0.83 microgram. Separation of fluoride from interfering substances has been improved by an automatically controlled steam-distillation apparatus which allows distillation to take place unattended. The total time required for the analysis is about 40 minutes, which includes 15 minutes for development of color. The procedure providea a fairly simple means for determining micro quantities of fluoride, and does not require a skilled analyst.
shoxed that the diminution of color was directly proportional to the fluoride ion present. The standard calibration curve (Figure 1) does not fol~owBeer's law, but its shape indicates that it is us-
I
0
I
10
I
20 F- ION
(a)
I
I
30
40
Figure 1. Standard Calibration Curve
!
