~~
Table 11.
~~
Recovery of Selenium Added to Herbage and Soil Samples
Selenium, pg. Found Recovered ... 0.06, 0.07, 0.07 1.o o a 1.10, 1.04 1.03 '0.97 0.50" 0.56 0.49 0.20b 0.28, 0.29, 0.30 0.21, 0.22, 0.23 Soil ... 0.36, 0.34 0 . 505 0.83, 0.80 0.48, 0.45 After digestion. * Before digestion. Sample Mixed herbage
Added
Selenium Content, P.P.M. 0.024, 0.028, 0.028
1.49, 1.41
0
fluorescence method, but the dithiol separation effectively eliminates them, including large amounts of vanadium which Cheng's procedure did not cope with. By digesting the residue from the ethylene chloride-carbon tetrachloride extract with nitric and perchloric acids, the last trace of any organic material from the sample that might produce a n interfering fluorescence is removed. Beath, Eppson. and Gilbert (1) reported a negligible loss of selenium from grass samples grown on seleniferous soils d i e n dried a t 50" t o 60" C. for 48 hours. Yo information is available for plant samples containing small amounts of selenium, but as a precaution against loss, drying in a draft of air a t a low temperature isrecommended.
Although the method is more sensitive and specific than others in current use, the conditions for the formation and measurement of the selenadiazole are being investigated further in order to improve the sensitivity. The procedure outlined can be used in the analysis of a wide variety of materials, and is reasonably rapid and precise. ACKNOWLEDGMENT
Thanks are expressed to E. B. Davies, XerT Zealand Department of Agriculture, for interest and encouragement, and to J. E. Allan, New Zealand Department of Agriculture, for advice and use of facilities.
Table 111.
Analysis of Different Plant Species
Selenium Content, P.P.M. Sample, Duplicate Leaves determinations Average White clover 0.035, 0.030 0.033 Grapefruit 0.033, 0.029 0.031 Maize 0.038, 0.044 0.041 Rye grass 0.022, 0.027 0.025 Silver beet 0.045, 0.050 0.048 Leek 0.050, 0.051 0.051 Leek bulb 0.010, 0.011 0.011 Onion bulb 0.038, 0.036 0.037
LITERATURE CITED
f l ) Beath, 0. A., Eppson, H. F., Gilbert, C. S., J . Am. Pharm. Assoc. 26, 394
(1937). (2) Cheng, IC. L., ANAL. CHEM.28, 1738 (1956). (3) C!ark, R. E. D., Analyst 82, 182 (1907); 83, 396 (1958). (4) Cousins, F. B., Medical School, University of Otago, private communication, 1959. (5) Gorsuch, T. T., Analyst 84, 133 (1959). (6) Hartley, W. J., Drake, C., Grant, A. B., New Zealand J . Agr. 99, 259 (1989). (7) Luke, c. L., ANAL.CHEM. 31, 572 (1959). (8) McXulty, J. S., Zbid., 19, 809 (1947).
RECEJVED for review December 22, 1959. Accepted April 26, 1960.
Photometric Titration of Aromatic Amines CHARLES A, REYNOLDS, SISTER FRANCIS HUGH WALKER, and EVELYN COCHRAN Department of Chemistry, University of Kansas, Lawrence, Kan.
A ,
procedure has been developed
for the photometric titration of aromatic amines which are unsubstituted or which are substituted with alkyl, alkoxy, hydroxy, and halogen groups. The amines are titrated directly with acetic anhydride in pyridine. Alcohols, phenols, and aliphatic amines do not interfere.
0
most common volumetric methods for primary and secondary amines involves the acetylation of the amine with an excess quantity of a standard solution of acetic anhydride in pyridine. After the acetylation reaction is complete, the amount of amine reacted is determined by titrating the acid liberated on hydrolysis of the reaction mixture ( 4 ) , or by measuring the amount of water consumed in hydrolyzing the excess acetylating solution by a Karl Fischer titration (7). The NE OF THE
method is applicable to both aliphatic and aromatic amines, but alcohols and phenols interfere. I n the present study the acetylation reaction has been adapted to the direct photometric titration of certain aromatic amines. The absorbance of the amine in the ultraviolet region of the spectrum is measured as a function of the volume of standard acetylating reagent. No back-titration is necessary, and aliphatic amines, alcohols, and phenols do not interfere. REAGENTS AND APPARATUS
An appro$mately 0.001M solution of acetic anhydride in dry pyridine was prepared b y dissolving reagent grade acetic anhydride in reagent grade pyridine which had been passed through a column packed with 4A Molecular Sieves. The solution was then standardized by titrating against a weighed amount of reagent grade aniline which
had been distilled over zinc dust. The titration procedure was the photometric one described below. The normality of a standard solution of acetic anhydride was checked daily for over 3 weeks, and remained constant within &0.5% provided i t was stored in a dark bottle protected from the atmosphere by a Drierite tube. The p-toluidine used was distilled twice over zinc dust. All of the solid aromatic amines were recrystallized a t least two times from m-xylene. The solvent employed for the titrations was reagent grade pyridine which had been dried in the manner described above. D r y hydrogen chloride gas mas passed into the pyridine until the hydrogen chloride concentration was -0.23M. The photometric titration apparatus consisted of a Beckman dual light source, a Farrand grating ultraviolet monochromator, a cubical glass cell of 126-ml. capacity, and a n Eldorado Electronics photomultiplier detector. A calibrated 5-ml. microburet was employed with magnetic stirring. VOL. 32, NO. 8, JULY 1960
0
983
PROCEDURES
The ultraviolet absorption spectra of each aromatic amine and its acetylated product were determined with a Beckman DK-1 spectrophotometer. I n each case examined, a wave length at which only the free amine absorbed could be found, and this wave length was chosen for the titration. Weighed samples of amine were dissolved in 100 ml. of dry pyridine which was saturated with dry hydrogen chloride, and the titration was performed using the 0.00lJf acetic anhydride solution according to procedures published previously (6).
~
RESULTS AND DISCUSSION
The results of the titration of different aromatic amines are given in Table I. Tlventy-five different neighed samples of aniline were titrated over a period of 2 months n i t h a n average deviation of 0.6%. The precision obtained on the other aromatic anlines was never greater than 0.8%. A typical titration curve for aniline is shown in Figure 1. The weight of aniine taken for analysis can be as low as 1 nig. with no loss in accuracy or precision.
.---
I 3
2
Vo urne of
Figure aniline
1.
4
Ac.0
Photometric
5
(mu
titration
of LITERATURE CITED
(1) Baumgarten, H. E., J . Am. Chem. SOC. 75, 1239 (1953).
Table I. Results of Photometric Titration of Aromatic Amines
No. of Samples Run
Av. Size of Sample,
Amine
Wave Length, MI
Mg.
%
Aniline m-Toluidine p - Anisidine p-Chloroaniline m-Aminoohenol 2-Amino-bphenylphenol p-Aminoacetanilide 2-Naphthylamine N-Methylaniline
320 350 340 370 320
7 11 7 7
5 8 8 6 5
99.8 99.7 103.0 104.2 107.4
The reaction between acetic anhydride and a n aromatic amine was catalyzed by the presence of a strong acid, such as perchloric acid or hydrogen chloride. Unless a small quantity of such a n acid is added t o the pyridine solution of the aromatic amine, about 15 minutes are required before a constant absorbance reading can be made after each increment of titrant added. I n the presence of a n acid catalyst, a constant absorbance reading can be obtained immediately. This same behavior has been observed in the acetylation of alcohols ( 3 ) . Although perchloric acid was first used as a catalyst, high results were obtained with some amines using this acid. This error was traced to the mater added to the system with the perchloric acid. If dry hydrogen chloride is used instead, this error is completely eliminated. Aniline and aromatic amines substituted with groups such as alkyl, alkoxy, hydroxy, and halogen can be successfully titrated by this method.
984
ANALYTICAL CHEMISTRY
relatively large quantities of a n aliphatic amine, such as n-butylamine, causes no interference in the aromatic amine titration: as there is enough hydrogen chloride in the solvent to convert the aliphatic amine t’oits conjugate acid which does not acetylate. There is considerable evidence (1 2 , 5 ) that the mechanism of the acetylation reaction of aromatic amines is a n electrophilic substitution reaction. Acetyl pyridiniuni ion, the probable acetylating agent when pyridine is used as solvent, acts as a n electrophilic substituting agent which attacks the amine ortho or para to the amino group. If the attack is ortho, the intermediate adduct can rearrange to form the corresponding acetylabed amine! pyridine, and acetic acid. I n t,his work, evidence was obtained for the presence of this intermediate in the form of a transient absorption peak a t 315 mp when acetyl pyridinium acetat’e was first added to a pyridine solution of aniline.
7
Purity,
Attempts to titrate paminobenzoic acid, p-aminoacetophenone, and o-nitroaniline produced no results because of the very slow rate of reaction between these compounds and acetic anhydride. However, the presence of these amines substituted by electron-withdrawing groups apparently has no effect upon the titration results obtained with the amines listed in Table I. For example, the titration curve obtained when a n equimolar mixture of aniline and onitroaniline is titrated with acetic anhydride is exactly the same as if aniline alone were titrated. Because both alcohols and phenols interfere with standard acetylation procedures, equimolar mixtures of aniline and other amines with both n-butyl alcohol and phenol were titrated. The titration results were identical with those obtained when the amines alone were titrated, indicating that the rate of acetylation of the amines is considerably greater than the corresponding rate for alcohols or phenols. The presence of
(2) Doering, W. v. E., hlcEwen, W. E., Zbid., 73,2104 (1951). (3) Fritz, J. S., Schenk, G. H., Abstracts, p. l5B, 136th Meeting, ACS, Atlantic City, N. J., September 1959. (4) Hillenbrand, E. F., Jr., Pentz, C. A., “Organic Analysis,” Vol. 3, p. 162, Interscience, Yew York, 1956. ( 5 ) McEwen, W.E., Terss, R . H., Elliott, I. W., J . AWL.Chem. SOC. 74, 3605 (19521. \
I
TUlU7LkZ 1
,i t , ( l Y O 6 j .
( 7 ) Mitchell, J., Jr., Hawkins, W., Smith, D. >! JI .. Am, , Chem. SOC. 6 6 , 782 (1947). RECEIVED for review January 20, 1960. Accepted .4pril11, 1960. Work supported in part by the United States Air Force under Contract AF49(638)-472 monitored by the Air Force Office of Scientific Research of the Air Research and Development Command. One of the authors, Sister Francis Hugh, was supported by a National Science Foundation summer research institute.
Correction
Gas Chromatography I n this review article by Stephen
Dal Nogare [ANAL.CHEM.32, 19R (1960)], on page 21R, column 1, line 33, t h e efficiencies for t h e capillary column described b y Lipsky, Lovelock, and Landowne [ J . A m . Chem. SOC. 81, 1010 (1959)], were erroneously quoted as 21 t o 200 theoretical plates per foot. The correct efficiencies ranged from 100 t o 1000 theoretical plates per foot.
