1449
V O L U M E 2 2 , NO. 1 1 , N O V E M B E R 1 9 5 0 per test instead of the 0.0470 used for tryptophan alone ( 3 ) . Also in the determination of tryptophan in unhydrolyzed proteins the optimum concentration of sodium nitrite to develop maximum color varied with different proteins from 0.03 to 0.06% (3). A large excess of sodium nitrite over that required to develop maximum color, however, should be avoided because the quality of the color is thereby affected and, as shown in Table I\.' (Z), slightly less than maximal color is obtained when color is developed with even a relatively small excess of sodium nitrite in a single addition. For example, in the present work minimum transmittancies of 14.1,14.9,and 15.0% were obtained when the same tryptophan solution was analyzed by Procedure F and transmittancies were determined 30 and 15 minutes after addition of two successive 0.1-ml. portions of 0.04, 0.06,a d 0.08% sodium nitrite solutions, respectively. However, excellent recoveries of tryptophnn were obtained when the standard
curve was prepared using the same concentration of sodium nitrite as was used on test solutions. Thus as shown in Table 11, equally good recoveries of tryptophan were obtained when two 0.1-ml. portions of 0.04y0 or single 0.1-ml. portions of 0.06 or O.OSyosodium nitrite solution were used to develop the color of corresponding test and standard solutions. LIT-ERATURE CITED
(1) Lesuk, A., U. S. Pharmacopoeia Amino Acids Advisory Committee, "Report on Collaborative Study 011 Chemical Tests and Standards for Amino Acids." Letter 135 (Nov. 26,1948). (2) Spies, J. R.,and Chambers, D. C., AN.AL.CHEM., 20,30 (1948). (3) Ibid., 21,1249 (1949). (4) Ibid., 22,1209 (1950). RECEIVED January 21, 1950. Paper VI in a series entitled "Chemical Determination of Tryptophan."
For paper V see ( 4 ) .
Rapid Test for Fluoride Ion W. R. CRANDALL, M . Ewing Fox Co.,Znc., New York, N. Y . to prepare industrial casein solutions of relatively I lowORDER viscosity, sufficiently resistant a t room temperature to the pi
continued action of water and alkali, sodium fluoride is often added for the purpose of .decomposing calcium compounds nat,urally present in commercial casein. A qualitative test based upon a known reaction of alkali fluorides with hydrated alumina was devised and successfully used to control the excess of fluoride required for this purpose. The test was afterward found to be generally applicable and responsive to one drop of 0.017, sodium fluoride solution, or its fluoride equivalent, without interference bS sodium csseinate or low concentrat,ions of the more rommon anions, e?jcept oxalate ion. Initially the reaction forms AIF,--- and OH-, and it must also b e assumed that in the presence of sodium or potassium ion, precipitation of a fluoaluminate may occur, inasmuch as precipitation of potassium fluoaluminate (KSAIF6) and of sodium and potassium basic fluoaluminat es was observed hy Craig (1,3 ) under similar conditions.
AI(OH),
+ 6F-
AIF6---
Equilibrium values of pH were observed a t room temperature for aluminum hydroxide and clay, respectively, and are represented graphically by Figure 1. Sodium fluoride in known concentration was added to distilled water containing a large indefinite excess of suspended solid, and allowed to stand for 3 days a t 19' to 24" C. with occasional agitation. Observations of pH reproducible to i0.05 unit were made by means of a direcb reading glass electrode instrument, calibrated with standard buffer solutions at pH 4 and 8.
+ 3OH-
Though for convmicnre aluminum hydroxide may be indicated as the reacting substance, prec'ipit,ated and dried basic sulfate and basic acetate were also observed to react promptly. Perhaps all basic salts of aluminum react in the same manner if not too strongly dehydrat,et! : for even kiln-dried industrid clays reacted with 0.027, sodium fluoride solution. Pyrophyllite, a nonhydrous silicate, was inert. Similarly, the reaction of l a s i c alumipum sulfate with potassium fluoride was applied by Craig to the alkalimetric titration of basic sulfate in aluminum sulfate solutions, and the method was a f t c h a r d developed by Scott ( I , 2 ) .
Figure 1. Relation of Equilibrium pH to Molal Concentration of Sodium Fluoride
PRELIMINARY OBSERVATIONS
An alkalimetric titration of fluoride on the same principle was demonstrated, with only partial success. To neutral solutions containing up to O.ZOj, sodium fluoride not less than 2570 excess of 0.08M aluminum sulfate was added, and titrated back to pH 7 at room temperature with 0.2 A' sodium hydroxide. The designated excess was necessary t,o ensure rapid reartion. I t can be assumed that the precision attaincd (about +2%) would have been improved by titrat,ing a t thr tloiling point, but the method was abandoned as unsuitable upon observing large positive errors caused by phosphate ion and preripitated calrium fluoride.
Aluminum hydroxide, substantially anion-free, was prepared from amalgamated commerrially pure aluminum and distilled water. Suspensions, respectively, 0.0476,0.152,and 0.476 M in sodium fluoride attained ronsta'nt pH values in 19 to 24 hours; the initial change could be observed in a few seconds. At the end of 48 hours these mixtures, which had approached equilibrium from the acid side, were diluted with an equal volume of water and observed again a t the end of another 24 hours, during which period equilibrium had been approached from the alkaline side. The clay was a commercial webclassified kaolin substantially free of mrater-soluble matter. Its suspensions attained constant
ANALYTICAL CHEMISTRY
1450 values of pH in 3 to 4 days; but in this instance the equilibrium was not tested by dilution. R E A G E W AND INDICATORS
For reasons of convenience both reagent and indicators were incorporated in slips of filter paper. Indicator papers made by drying alcoholic dye solutions on qualitative filter paper were found to be somewhat more quickly reactive than the equivalent commercial indicator papers containing size. The preferred indicators, methyl red, bromocresol purple, qromothymol blue, and thymol blue, were chosen for their senaitiveness-i.e., the visibility of the color contrast. Methyl orange and phenolphthalein proved relatively insensitive, and are not recommended. Aluminum acetate paper was prepared by dipping filter paper in a cold solution of 1% ammonium alum and 1% sodium ?etste, blotting, dipping in boiling 0.2% sodium acetate solution, and drying at room temperature. Paper so prepared remains sensitive for at least a year. PROCEDURE
On a glrtss surface a slip of dry indicator paper is laid, and across it a slip of dry aluminum acetate paper. To the intersection a drop of solution is applied, previously brought within the middle or lower pH range of the chosen indicator--e.g., 6 to 7.5 if bromothymol blue is to be used. In the absence of interfering substances, one drop of solution containing 0.01% or more of sodium fluoride (or its fluoride e uivalent) regularly causes a perceptible color contrast between &e covered and uncovered parts of the paper, provided the test is made within the p H range 6 to 8. Outside this range the sensitivity is reduced, as must be expected from the mathematical relation of p H to ion concentration; but unbuffered or slightly buffered solutions containing 0.1% of sodium fluoride gave satisfactory positive results throughout the pH range 4 to 9 with appropriately chosen indicators. INTERFERENCE BY ANIONS
In the presence of o.25yO sodium phosphates a t pH 7 , 0.1% sodium fluoride solution regularly gave a positive result, but not
when the phosphate was increwxi tu 0.57,. Interference by the buffer function of the salt must be ~ w r n e d . That in this instance the formation of aluminum phosphate did not contribute to the effect was shown by soaking aluminum acetate paper in neutral phosphate solution before making the test. Paper thus treated and imperfectly wmhed with water reacted normally thereafter with O . O l ~ ofluoride solution. False positive results were’ol)tninetl with 0.1% ammonium oxalate solution. Aluminum acetate paper wetted with oxitlate solution and washed with water WHR thereafter insensitive to fluoride ion, but not if washed with dilute calcium chloride followed by wtrter-observations which suggest the formation of an aluminum oxalate decomposable by calcium ion. As a test for interference by other anions, 1 % salt solutions were prepared, neutralized approximately, and tested with and without the addition of sodium fluoride, 0.1% of the weight of Rolution. The following substances interfered slightly or not at all: sodium borate, thiosulfate, nitrate, nitrite, acetate, chloride, chlorate, sulfate, sulfite, and citrate; potassium iodide, iodate, bromide, phthalate, and ferrocyanide; and ammonium tartrate. ACKNOW LEIXMENT
The assistance of Judith Pavsikoff is gratefully acknowledged. LITERATURE CITED
(1) Craig, T. J. I., J . SOC.Chem. Id.. 30,185 (1911). (2) Scott, W. W., J . I d . Eny. Chem., 7, 1059 (1915); “Standard Methods of Chemical Analysis.” 5th ed., p. 16, New York, D.
Van Nostrand Co., 1939. (3) Seidell, Atherton, “Solubilities of Inorganic Compounds,”3rd ed., New York, D. Van Nostrand Co., 1940; references to work of Carter. 1930, and Frere, 1933. RECEIVED August 23. 1949.
Test for Microdetection of Parathion in Orange and lemon Oils F. A. GUNTHER A N D R. C. BLINN linivereity of Calvorniu Citrus Experiment Station, Riverside, Culif. A Q U I C K method for the detection of the presence of the insecticide, 0,O-diethyl 0-p-nitrophenyl thiophosphate (parathion) in microquantities of vegetable material should find numerous applications. Such a method has been developed for use with orange and lemon oils. The procedure here presented is a micro adaptation of the basic analytical method proposed by Averell and Norris ( I ) ,which is based upon the reduction of the nitro to the amino group with subsequent diazotization and coupling with N-1-naphthylethylenediamineto afford a magenta color. PROCEDURE
Reagents. Ethyl alcohpl, 95%. Hydrochloric acid, concentra&d. Zinc dust. Sodium nitrite solution, 0.25%. Ammonium sulfamate solution, 2.5%. N-1-Naphthylethylenediaminedihydrochloride solution, 1.0%. Procedure. One drop of the orange or lemon oil is placed in a 7 X 50 mm. test tube; then 2 dro s each of ethyl alcohol and water, 1 drop of concentrated hy&ochloric acid, and a trace (about 0.1 gram) of zinc dust are added. After being heated in a water bath for 5 minutes, the mixture is filtered into a 10 X 70 mm. centrifuge tube through a wisp of cotton in a 15-mm. funnel, and the test tube i s then washed with 2 successive drops of water. To the filtered solution is added 1 drop of a 0.25% solution of sodium nitrite, followed in 5 minutes by 1 drop of a 2.5% solution of ammonium sulfamate, then after 2 minutes with 2 drops of a 1% solution of N-1-naphthylethylenediamine dihydrochloride. A magenta color developing within 10 minutes indicates that
parathion is present. It is suggested that a sample of similar oil known to be free of parathion be run simultaneously for comparison (cf. 2, 3). DISCUSSION
With this procedure it is possible to detect parathion in these citrus oils in a concentration as low as 25 p.p.m. in 1 drop of oil, which is approximately 1 microgram of the insecticide per drop. Because this represents a value 3f about 0.5 p.p.m. of parathion in the fresh peel of the fruit, the 25 p.p.m. limit of the oil is not so high as it may first appear. By using 2 drops of oil, parathion in a concentration of 12 p.p.m. in the oil can be detected. The use of much larger quantities of the oil, however, increases interference with color interpretation and thus obviates this micro adaptation. It must be kept in mind that, even when compared with a proper check sample, the development of the characteristic magenta color in this test does not prove the presence of parathion. Many other nitro compounds and amines respond similarly. LITERATURE CITED
Averell, P. R., and Norris, M. V., ANAL.CHEM.,20, 753 (1948). (2) Blinn, R. C., and Gunther, F. A., Ibid., in press. (3) Gunther, F. A., and Blinn, R. C., Advances in Chemistry Series, No. 1, 72 (1950). (1)
RECEIVED April 28, 1950.
