Analysis of Primary Aromatic Amines and Nitrite by Diazotization and Pyrolysis Gas Chromatography Anthony Savitskyl and Sidney Siggia Department of Chemistry. University of Massachusetts, Arnherst, Mass. 07002
The use and study of amines is widespread in chemistry. It is frequently necessary to determine the primary aromatic amine content of complex amine mixtures. Existing methods for determining primary aromatic amines in mixtures include acid-base titration in a differentiating solvent ( I ) , separation and analysis of volatile amines directly or following derivatization by gas chromatography ( 2 4 , formation of a Schiff base (6),and diazotization methods where amines are converted to diazonium salts and coupled with phenols or amines to produce azo dyes for colorimetric analysis (7, 8). These methods all suffer from interferences or limitations. Acid-base titrations fail when bases in addition to amines are present. Gas chromatography is limited to volatile amines. Colorimetric procedures based upon diazotization are often unsuccessful because of highly colored N-nitroso and C-nitroso products from reaction of secondary and tertiary aromatic amines with nitrous acid. A pyrolysis gas chromatographic method for analysis of diazonium salts has recently been developed in this laboratory (9). Since only primary aromatic amines produce stable diazonium salts upon diazotization and only diazonium salts should liberate nitrogen upon mild pyrolysis, adaptation of the pyrolysis gas chromatographic method has been investigated as a specific method for measuring primary aromatic amine content of mixtures. The method developed during this investigation demonstrates the feasibility for determining primary aromatic amines. The method has been applied to mixtures of nonvolatile amines. Modification of the technique has enabled analysis of nitrite ion in nitrite-nitrate mixtures.
EXPERIMENTAL A p p a r a t u s . T h e analyses were performed in the same unit previously described for pyrolysis of diazonium salts (9). T h e s a m ples were pyrolyzed in platinum boats (11 m m long) supplied by Fisher Scientific Co. Reagents. All amines were obtained from Eastman Organic Chemicals or Matheson Coleman a n d Bell in the purest grade available a n d were further purified by recrystallization or vacuum sublimation if necessary. Primary aromatic amines were analyzed by titration with perchloric acid in glacial acetic acid, sodium nitrite, or aqueous sodium hydroxide (for some aromatic amino acids) t o determine purity. Baker Analyzed Reagent grade sodium nitrite, reagent grade concentrated hydrochloric acid (Fisher Scientific Co. j , or trifluoPresent address, Procter and Gamble. Ivorydale Technical Center. Cincinnati, Ohio 4521'7. (1) S Siggia, J. G . Hanna. and I . R Kervenski, Ana/. Chem.. 22, 1295 (1950). ( 2 ) C . E. Boufford. J. Gas Chromafogr.. 6 , 438 (1968). ( 3 ) D.E. Campbell. J. Pharm. Pharmacol.. 21, 129 (1969). (4) H . V. Street, J. Chromafogr.. 37,162 (1968) ( 5 ) W J. lrvine and M . J. Saxby. J . Chromatogr.. 43, 129 (1969). (6) W Hawkins. D. M . Smith, and J. Mitchell, Jr., J . Amer. Chem. S o c . . 66, 1662 (1944) (7) F. 2 . Bandelin and C. R . Kernp, lnd. Eng. Chem.. Ana/. Ed., 18, 470 (1946) (8) S. Siggia. "Quantitative Organic Analysis via Functional Groups," 3rd ed., Wiiey. New York. N . Y . . 1963, p 51 1, (9) A . C.Savitsky and S. Siggia, A n a / . Chem., 46, 149 (1974).
roacetic acid (95 per cent, Eastinan Organic Chemicals) were used for diazotizations. Practical grade sulfamic acid (Eastman Organic Chemicals), purified by recrystallization from water, was used t o destroy excess nitrous acid. Triply recrystallized sodium nitrite ( I O ) was used for preparing synthetic nitrite-nitrate mixtures. Procedure. Diazotization of A m i n e s . Two diazotization procedures were used depending upon the solubility of t h e resulting diazonium salt. Method A. When amines produced water soluble reaction products, 1.0-1.5 millimoles of primary aromatic amine was weighed or pipetted into a 10-ml volumetric flask. Solid samples were dissolved in a minimum volume of distilled water or ethanol. Aromatic amino acids were dissolved in dilute sodium hydroxide solution. One milliliter of 2.OM sodium nitrite was added and the flask cooled to 0-5 "C in a n external ice bath. Two milliliters of cold 4.OM hydrochloric acid was added rapidly. T h e reaction was allowed t o proceed 10-15 minutes with stirring. Cold saturated sulfamic acid solution was slowly added to destroy any excess nitrous acid until gas evolution ceased, and the solution was diluted t o volume with cold distilled water. Ten-microliter aliquots were pipetted into platinum boats using a calibrated Hamilton syringe. T h e boats were dried in a black Bell jar by applying vacuum gradually a t first to prevent splashing. Method B. When reaction produced insoluble diazonium salts or other insoluble products, diazotization was run directly in the platinum boats. One to 1.5 millimoles of primary aromatic amine was weighed or pipetted into a 10-ml volumetric flask and diluted to volume. T e n microliters of amine solution was pipetted into platinum boats containing about 50 mg of sea sand (to prevent creeping of t h e solution u p the walls of the boat). T e n microliters of 2.OM sodium nitrite was added to each boat. (A large excess of nitrous acid was necessary because of loss by volatilization a n d lack of stirring.) T h e boats were cooled on a copper plate in contact with ice-acetone. Twenty microliters of cold 4.OM hydrochloric acid was added and 10-15 minutes was allowed for reaction. T e n microliters of cold saturated sulfamic acid solution was slowly added. T h e boats were dried as before. Reaction of Nitrite Ion. A sample containing 1.0-1.5 millimoles of nitrite was added to a 50-ml volumetric flask. If a solid sample was used, it was dissolved in distilled water. Five milliliters of 0.29M sodium anthranilate (prepared by neutralizing 4.0 grams of anthranilic acid with sodium hydroxide and diluting to 100 ml) was added and the solution cooled to 0-.5 "C. Five milliliters of cold trifluoroacetic acid was added, t h e solution stirred, and 10-15 minutes allowed for reaction. The solution was diluted t o volume with cold distilled water. Forty-microliter aliquots were added t o each platinum boat and t h e boats dried a s before. girolysis. Up t o six sample boats could be stored in the pyrolysis unit. After the samples had been dried, loaded into t h e blackened pyrolysis chamber, a n d the detector stabilized, they were pyrolyzed a t 170 "C. T h e volatile pyrolysis products were trapped on t h e large volume of sea sand in the pyrolysis chamber and on the column. Only nitrogen passed through t h e chromatographic system to the detector. Chromatographic conditions were as follows: injection port, 55 "C; detector, 200 "C with 225-mA filament current; columns 6-ft X %inch copper packed with 60/80 mesh Molecular Sieve 5A a t 65 "C. T h e nitrogen peak eluted a t 2.5 minutes with a helium flow rate of 60 ml/min. T h e peak was well formed with little tailing. Calibration. Calibration was accomplished by pyrolyzing known quantities of 4-N,N-diethylaminobenzenediazonium tetrachlorozincate of known purity under conditions identical to the analysis. Sample boats for calibration were prepared by adding (10) 0. R . Gottlieb and M . T. Magalhaes, Anal. Chem., 3 0 , 995 (1958)
Table I. Analysis of Primary Aromatic Amines by Diazotization and PGC Analysis O/O std deva
Amine
p-Anisidine
Sodiurnsulfanilate Metanilic acid Anthranilic acid p-Aminobenzoic acid Benzidine
100.7 100.4 95.9 99.2 98.6 94.5
*
*
0.9 ( 6 ) b f 1 . 1 (5)c f 2.2 ( 6 ) b f 0.5 ( 6 ) b f 3.7 (6)c f 1.7 (12)b
Table I l l . Analysis of Nitrite in Sodium Nitrite-Sodium Nitrate Mixtures by Diazotization and PGC
Check method
Recovery, YO
100.Od 99.6f 98.0e 98.6e 99.7e 99.3d
100.7 100.8 97.9 100.6 98.9 95.1
*
Number in parentheses indicates replicate analyses. Diazotization in solution, aliquots taken for analysis. Diazotization in platinum boats. Nonaqueous titration (perchloric acid in glacial acetic acid). e Aqueous sodium hvdroxide titration. f Sodium nitrite titration. a
Table I I. Analysis of Primary Aromatic Amine Content of Amine Mixtures by Diazotization and PGC Mix- Amine ture type
1
1"Ar 2' A r
1' AI
1"Ar
2
2' A r 3" Ar
1'Ar
3
2" Ar 3" Ar
1'Ar
4
2'
Ar
3" Ar
1" AI 2" AI
5
1"Ar 2" Ar
3" Ar
1"Ar
6
2" Ar 3" Ar
a
Compound
itY
Analysis, molarity primary aromatic amine i std dev
Anthranilic acid p-Diphenyl amine sulfonic acid Methyl amine Anthranilic acid N-Phenyl anthranilic acid Triphenyl amine p-Aminobenzoic acid N-Phenyl anthranilic acid Triphenyl amine p-Aminobenzoic acid N-Phenyl anthranilic acid Triphenyl amine Methyl amine Diethyl amine Metaniiic acid N-Methyl anthranilic acid p - ( Dimethylamino) benzoic acid Sodium sulfanilate N-Methyl anthranilic acid p- (Dimethyiamino) benzoic acid
0.130
0 . 1 3 1 f 0.002 ( 6 ) a
Molar-
0.067 0.076 0.127
0.128 f 0.003 ( 6 )
0.073 0.065 0.162
0.162 f 0.003 (6)
0.082 0.065 0.081
0.084 f 0.003 ( 6 )
0.042 0.033
0.101 0.468 0.142
0.143 f 0.004 ( 6 )
0.135
0.139 0.145
0.145
k 0.002
(6)
0.143
0.111
Number of replicate analyses.
aliquots of a cold standard m e t h a n o l solution of t h e diazonium salt to each boat a n d v a c u u m d r y i n g a t r o o m temperature in a B e l l jar. Occasionally t h e calibration curve was checked b y a p p l y i n g a gas sample loop t o t h e injection p o r t a n d injecting k n o w n pressures of nitrogen gas using a 150-j~lsample loop. T h i s check m e t h o d guarded against possible decomposition of t h e standard but was unnecessary for d a y to d a y operations. T h e d i g i t a l intergrator was used for peak area measurement.
RESULTS AND DISCUSSION In order to achieve a successful method of analysis for primary aromatic amines based upon diazotization and subsequent pyrolysis gas chromatography. two factors must be considered. First, the stability of diazonium salts 154
Mixture, YONaN02
Pure NaNOz 0.94
10.5 40.7 56.9 78.4 (I
Analysis, % NaN02 std dev
*
100.4 f 0.4 ( 6 ) a 0.95 0.03 ( 6 ) 10.4 k 0.1 ( 6 ) 40.5 f 0.5 ( 6 ) 58.0 f 0.5 ( 6 )
*
80.5
f 0.4
(6)
Number of replicate analyses.
in solution in the presence of excess nitrous acid has been found t o be reduced (11).This presents no problem if the diazotization goes to completion fairly rapidly, so that the excess nitrous acid can be destroyed. Stability of the diazonium salt solution is further enhanced by running the diazotization a t low temperatures. This retards the onset of hydrolysis of the diazonium salt. Second, there is little quantitative evidence on the stability of unstabilized diazonium salts in the solid state a t room temperature, although their sensitivity t o heat and shock is well documented (12, 13). It was found during this investigation that the unstabilized diazonium salts could be vacuum dried at room temperature and stored in the helium atmosphere of the pyrolysis unit for considerable lengths of time without decomposition. On occasion, samples were stored overnight with no detectable decomposition. Most diazonium salts are light sensitive to widely varying degrees. The Bell jar used for drying the samples was painted black, and the pyrolysis unit was draped with a black cloth to avoid any photolytic decomposition. These precautions should be taken routinely, although no problems were encountered with the diazonium salts studied. T h e effect of allowing organic residues formed during pyrolysis to be trapped on the sea sand packing the front of the pyrolysis chamber, as well as on the Molecular Sieve column, was investigated. The slope of a calibration curve obtained after one month of continuous use had declined 1.39% from a calibration curve obtained immediately after conditioning the column and replacing the sea sand packing. This indicates little adverse short-term effect. Eventually, after about two months of use, the nitrogen peak began to tail significantly and the column was replaced. Conceivably, it could have been reconditioned. The feasibility of the pyrolysis gas chromatographic technique was demonstrated by analysis of primary aromatic amines of known purity. The per cent recoveries (Table I) by pyrolysis indicate that diazotization proceeds quantitatively and that the manipulation of the diazonium salts preserves them for analysis. Low, though reproducible: results were obtained for benzidine. This may be due to an inability to quantitatively diazotize the second amine group. An increased reaction time of one hour plus attempts to stabilize the diazonium salt with zinc chloride failed to increase the recovery. Generally the precision and accuracy of the analysis was on the order of 1-2%. The analysis of mixtures for primary aromatic amine content was carried out by diazotizing in platinum boats. This was necessary because of insolubility of the products of some secondary and tertiary aromatic amines. Table I1 demonstrates the ability of pyrolysis gas chromatography to selectively determine the diazonium salts produced (11) C. Schwalbe, Ber. Deut. Chem. Ges.. 38, 2196 (1905). (12) H. Wichelhaus, Ber. Deuf. Chem. Ges.. 34, 11 (1901). (13) E. Bamberger. Ber Deut. Chem. Ges.. 28, 538 (1895).
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 1 , J A N U A R Y 1974
upon reaction with nitrous acid. Attempts to analyze such mixtures by titration with sodium nitrite would fail because of consumption of nitrous acid by compounds other than the primary aromatic amine. The highly colored Nnitroso and C-nitroso compounds formed with secondary and tertiary amines similarly might interfere with colorimetric methods. Simple gas chromatography would be unsuitable for nonvolatile amines. Yet, only the diazonium salt liberates nitrogen upon mild pyrolysis, thus providing a specific measure of primary aromatic amine content. Analysis of nitrite in nitrate required special modification of the diazotization conditions. Anthranilic acid was chosen for diazotization because its diazonium salt has good water solubility. Hydrochloric acid was unsuitable
for reaction in the presence of high concentrations of nitrate because of the potential formation of aqua regia. Since a strong acid was required to achieve rapid, quantitative diazotization, trifluoroacetic acid was substituted. Trifluoroacetic acid is completely dissociated in aqueous solution and is readily removed upon vacuum drying. It was necessary to run reactions in dilute solution rather than platinum boats to prevent small, but significant losses of nitrous acid by volatilization. The results of analysis of synthetic nitrite-nitrate mixtures are given in Table III. Received for review March 19, 1973. Accepted May 9, 1973. This work was supported by the National Science Foundation. Grant Number GP-28054.
Determination of Cacodylic Acid (Hydroxydimethylarsine Oxide) by Gas Chromatography C. J. Soderquist, D. G. Crosby, and J. B. Bowers Department of Environmental Toxicology, University of California. Davis. Calif. 95676
Despite the introduction of synthetic organic pesticides during the last 30 years, arsenic compounds still find extensive use in agriculture. In 1972, over 1.2 million pounds of arsenicals were applied by permit in California, about 70% as inorganics including sodium arsenite and lead arsenate and the remainder as the herbicides hydroxydimethylamine oxide (cacodylic acid) and salts of methanearsonic acid (MSMA and DSMA) ( I ) .Although most of the inorganic arsenicals were used for agricultural and structural pest control, about 3090 of the organic arsenicals were used by irrigation, flood control, and water resource organizations. Significant residues of the arsenicals have been found in soil and water (2, 3 ) . Most methods for the determination of arsenic compounds are based on their conversion to arsenic trioxide (Asz03) which subsequently is reduced to arsine (AsH3) and quantitated by colorimetry (2, 4 ) , atomic absorption spectrophotometry ( 5 , 6), or emission spectroscopy (7). Still others [atomic absorption (8, 9), single-sweep polarographic ( I O ) , neutron activation ( 3 ) , and X-ray fluorescence ( I I ) ] measure the original arsenical directly. While the majority provide adequate recoveries and sensitivity, the determination represents only total arsenic; contributions from arsenite, arsenate, cacodylate, or methanearsonate cannot be differentiated. Paper chromatography followed by colorimetric determination of the separated arsenicals is the only selective analysis reported (12). "Pesticide Use Report," California Department of Agriculture (1971). D. L. Johnson, Environ. Sci. Techno/.. 5,411 (1971). R . E. Wilkinson and W . S. Hardcastle, WeedSci.. 17, 536 (1969). "Official Methods of Analysis," 1 0 t h ed.. Association of Official Agricultural Chemists, Washington, D.C.. 1965, Section 24.006. R. C. C h u . G . P. Barron. and P. A. W. Baumgarner, Ana/. Chem.. 44, 1476 (1972). W. Holak.Ana/. Chem.. 41, 1712 (1969). F. E. Lichte and R. K . Skogerboe. Anal. Chem.. 44, 1480 (1972) A. Ando. M . Suzuki, K. Fuwa, and B. L. Vallee, Anal. Chem.. 41, 1974 (1969). 0. Menis and T. C. Rains, Ana/. Chem., 41, 952 (1969) G. C. Whitnack and R . G . Brophy. Anal. Chim. Acta, 48, 123 (1 969). F. J. Marcie, Enwron. Sci. Techno/.. 1, 164 (1967). R . M . Sachs, J . L. Michael. F. B. Anastasia. and W . A. Wells, WeedSci.. 19,412 (1971).
This nonselectivity may be prohibitive. For example, determination of applied cacodylic acid in a waterway may be hindered by a background of other arsenicals arising from natural water content and runoff from treated fields areas. Furthermore, some cacodylic acid formulations contain nearly equal amounts of MSMA. We report here a procedure whereby cacodylic acid and its salts can be determined rapidly with a detectability limit below 0.05 ppm in water and 0.5 ppm in soil. The method, which excludes other arsenicals, is based on conversion of cacodylic acid (I) to iododimethylarsine (11) with hydriodic acid (Equation l ) , followed by determination with electron-capture gas chromatography.
1
HI
CH3-As-OH
4 CH,-As-I I
I
EXPERIMENTAL Apparatus. Gas-liquid chromatography was performed with a Varian Model 1700 gas chromatograph equipped with a tritium electron-capture detector (ECD) and a 15-ft X %-in. (0.d.) stainless steel column containing 10% DC-200 on 60/80 mesh Gas Chrom Q. Oven temperature was 105 "C; injection port, 125 "C; detector, 200 "C; carrier gas (nitrogen) flow was 20-30 ml/minute. Iododimethylarsine had a retention time of 5 minutes. Mass spectra were obtained with a Finnigan Model 3000 gas chromatograph-mass spectrometer operated a t 70 eV and equipped with a 4-ft X %-in. (i.d.) glass column containing 270 OV-1 on 60/80 mesh Chromosorb G. Reagents. Hydriodic acid (Fisher Scientific Co.) was a 57% w/v certified grade. Cacodylic acid ( K & K Laboratories) was recrystallized twice from aqueous ethanol, m p 194-6 "C [reported (23)200 "C]. All solvents were redistilled twice before use. Standard iododimethylarsine was prepared ( 24) by combining 5.0 g cacodylic acid, 16.0 g potassium iodide, and 20 ml of water in a small separatory funnel; sulfur dioxide (Matheson Gas Products) was bubbled in for about 10 minutes until the solution was saturated, and 5-ml portions of 6 N hydrochloric acid were added Herbicide Handbook 2nd ed Weed Science Society of America Geneva N Y 1970 p 251 (14) G J Burrowsand E E Turner J Chem Soc 117. 1376 (1920)
(13)
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