Spectrophotometric Determination of Pyridine in Aromatic Hydrocarbons with p-Phenylenediamine H. G. Higson, R. F. Raimondo, and E. W. Tunstall Analytical Section, MacMillan Bloedel Research Ltd., Vancouver, Canada
METHODS FOR DETERMINING pyridine using the Konig reaction ( I ) have been previously reported (2-4). According to Konig, equimolar amounts of cyanogen halide and pyridine undergo reaction to form a quaternary salt, A dyestuff is subsequently produced by allowing this intermediate product to react with an aromatic amine. Benzidine and aniline have previously been used as the aromatic amine but neither is completely acceptable, since benzidine is carcinogenic (5) and the sensitivity of methods employing aniline is low. A comprehensive study of several aromatic amines was made by Bark and Higson (6) in a paper describing the use of the Konig reaction for the determination of cyanide. Their study considered the color of dye produced, the wavelength of maximum absorbance, the color development time, the approximate strength of absorbance, and a brief summary of the applicability of the amine pertaining to analysis. In a later paper, the same authors (7) describe a method for the determination of cyanide using p-phenylenediamine. pPhenylenediamine was chosen as the dyestuff precursor for the present method since it is safe to handle (5)and produces a strong color intensity. Most analytical adaptations of the Konig reaction require close pH control. Buffer control of pH is superior to addition of NaOH which is normally used. Methods for pyridine determination using benzidine as dyestuff precursor, reported by Kroner, Ettinger, and Moore (3) and Piibyl (4), involve reaction in aqueous alcoholic media. No influence on the color reaction in the pH range 3-8.75 was reported. The deficiency of these methods is that in partially alcoholic media the reaction is inhibited, and the color does not become stable until four hours after addition of benzidine. EXPERIMENTAL Apparatus. Absorbance measurements were made using a Beckman DU spectrophotometer with silica 1-cm cells, and distilled water in the reference cell. Reagents. A phthalate buffer solution was prepared from 10 g of potassium hydrogen phthalate and 425 ml of 1.ON NaOH in a total volume of 1 1. of distilled water, Aqueous cyanogen bromide, lo%, was stable when stored in a tightly stoppered bottle. Aqueous p-phenylenediamine, 0.2 %, must be prepared immediately prior to initiation of the color reaction. A standard pyridine solution was prepared by diluting 10 g (accurately weighed) of redistilled pyridine to 1 1. with distilled water. (1) W. Konig, J . Prakt. Chem., 69, 105 (1904). ( 2 ) A. Goris and A. Larsonneau, Bull. Sei. Pltarmacol., 28, 497 (1 921). (3) R. C. Kroner, M. B. Ettinger, and W. A. Moore, ANAL.CHEM., 24, 1877 (1952). (4) M. Piibvl. Collect. Czech. Ckem. Commuiz.. 27. 1330 (1962). _
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02
400
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425
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475
WAVELENGTH,
500
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Figure 1. Absorption spectra of dyestuff
Procedure. A weighed amount of hydrocarbons containing 0.5-1.0 mg of pyridine is extracted twice with 10-ml portions of 0.5N HC1. To the combined acid extract, 25 ml of buffer solution is added, and the solution is diluted to 100 ml with distilled water. A suitable aliquot of the above solution (up to 3 ml depending on the amount of pyridine present), is added to a 25-ml volumetric flask. For aliquots less than 3 ml, compensation is made with dilute buffer solution (20 ml of 0.5N HC1 and 25 ml of buffer, in a total volume of 100 ml), and the solutions are made up to volume with distilled water. Standards are prepared by adding a 5-ml aliquot of the standard pyridine solution to 20 ml of 0.5N HC1 and 25 ml of buffer, and the solutions are then made up to 100 ml with distilled water. Aliquots of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 ml of standard pyridine solution are pipetted into 25-ml volumetric flasks. Sufficient dilute buffer is added to compensate for the amount of buffer in the highest standardi.e., addition of 2.5, 2.0, 1.5, 1.0, 0.5, and 0 ml, respectively, to each flask. Distilled water is added to make up the volume to 25 ml. The flasks at this stage contain 10, 20, 30, 40, 50, and 60 Mg pyridine/ml, respectively. The final standard and sample solutions, plus a reagent blank, are transferred to stoppered 125-ml flasks. Using a safety pipet, 0.5 ml of 10% aqueous cyanogen bromide is added to each flask, and the flasks are allowed io stand for 15 min. Three milliliters of 0.2 aqueous p-phenylenediamine is added to each flask and after exactly 30 min, the absorbance of the samples, standards, and blank is recorded at 480 nm. After substracting the blank readings, the absorbance is plotted us. pg/ml pyridine to obtain a calibration curve, and the concentrations of the samples are obtained from this curve.
I
i S j T. S .
Scott, “Carcinogenic and Chronic Toxic Hazards of Aromatic Amines,” Elsevier Publishing Company, Amsterdam, 1962. ( 6 ) L. S. Bark and H. G. Higson, Tala/?@ 11, 471 (1964). (7) Ibid., p 621. 1474
ANALYTICAL CHEMISTRY
RESULTS AND DISCUSSION Extraction of Pyridine from Aromatic Hydrocarbons. Since pyridine forms salts with acids, it is easily extracted from aromatic solvents with dilute HCl. This fact has been made use of in several methods reported for pyridine. As
4e5 I
20
30
40
50
60
PYRIDINE I y g / m l )
Figure 3. Calibration curve of pyridine standards
, 470
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20 TIME
A . Before compensation for phthalate buffer
~
30
43
B. After compensation for phthalate buffer
IMIN)
Figure 2. Time effect on wavelength stability
well as from benzene, complete extraction of pyridine from 0-,rn-, p-xylene and toluene was also confirmed in our work. Spectral Characteristics. The visible absorption spectrum of the dyestuff in aqueous media is shown in Figure 1. A well defined absorption maximum occurs at 480 nm. A shift in the wavelength of the maximum occurs with respect to time elapsed after addition of p-phenylenediamine. Figure 2 shows this shift for 10 pg/ml and 60 pg/ml pyridine standards, and indicates that stabilization of the wavelength (maximum) occurs after approximately 20 min. The absorption maximum of the 10 pg/ml and 60 pg/ml pyridine standards stabilizes at 478 nm and 480 nm, respectively. However, because of the breadth of the absorption band in the lower concentration samples, any error due to this 2-nm wavelength difference in absorption maximum would be negligible. Consequently, all readings were taken at 480 nm. In aqueous media, the reagent blank had very little absorption at 480 nm. Conformity to Beer's law was observed for a pyridine concentration range of 0 to 80 pg/ml. The optimum concentration range was from 10 pg/ml to 60 pg/ml pyridine and gave an absorbance spread of 0.1-0.5 unit. Some difficulty was encountered with concentrations of pyridine greater than 60 pg/ml, since precipitation of the dyestuff sometimes occurred. Stability of Dyestuff. Shortly after the addition of pphenylenediamine to the intermediate quaternary salt, an intense orange color developed which rapidly faded. The dyestuff rapidly declined in absorbance over the first few minutes following addition of p-phenylenediamine. A stabilization of the color to constant absorbance occurred 20 min after addition of p-phenylenediamine. The absorbance of the dyestuff remained constant from 20 to 60 min after addition of p-phenylenediamine, over the pyridine concentration range required for the analysis. Effect of Concentration of Cyanogen Bromide and p-Phenylenediamine. Use of excess cyanogen bromide made the dyestuff absorbance independent of cyanogen bromide concentration. No significant changes in absorbance of the reagent blank o r pyridine samples were observed with higher concentrations of p-phenylenediamine. pH Control and Interfering Ion Effect. As in other reported analytical modifications of the Konig reaction, pH control
was found to be critical. After extraction with HC1, results of analyses without pH control were found to be nonreproducible. Initially, attempts were made to control acidity by adjustment to pH 5 with 1% aqueous sodium carbonate solution. While the results were less scattered, satisfactory reproducibility was still not achieved. Also, sodium carbonate produced a marked depressant effect on the absorbance of the samples. Similar depressant effects and nonreproducible results were observed using 1 aqueous sodium hydroxide for control of pH. The scattered results obtained using sodium carbonate and sodium hydroxide indicated that fine control was necessary in the addition of basic ions. A phthalate pH 5 buffer was chosen to achieve this control. Although not as severe as for sodium carbonate and sodium hydroxide, the phthalate buffer also induced a depressant effect. From an aqueous pyridine solution in HC1 which had been adjusted to pH 5 with phthalate buffer, aliquots were withdrawn and each was made up to a final volume to correspond to the final concentration range for the analysis. Figure 3A shows the nonlinearity which results from higher concentrations of the phthalate buffer in the higher standards depressing the absorbance more than that buffer present in the lower standards. A second series of aliquots were compensated for the discrepancy in the amount of buffer before making up to final volume. Figure 3B shows the linear relationship obtained after compensation for the phthalate buffer. It is therefore essential that the same amount of buffer be present in the final solutions of both standards and samples.
Table I. Determination of Pyridine in Benzene - Pyridine (g/100 ml Benzene) Present Found Error 0.51 1.04 1.52 2.01 0.52 1.10 1.88 2.03
0.52 0.95 1.50 2.01 0.53 1.15 1.75 1.99
Re1 error,
i-0.01 -0.09 -0.02 0 -0.01 +0.05 -0.13 -0.04
2.0 8.6 1.3 0 1.9 4.5 6.9 2.0
Av re1 error 3.4
VOL. 41, NO. 11, SEPTEMBER 1969
1475
Interfering ions including zinc, iron(II), nickel(II), copper(II), and thiocyanate were reported by Bark and Higson (7) to interfere in the method for determination of cyanide using pyridine and p-phenylenediamine. The same species may be expected to interfere in this analysis for pyridine. Precision and Accuracy. The reproducibility of the determination was checked by recording the absorbance of several pure pyridine standards over several days. Over a 0.1 to 0.5 absorbance range, the average deviation was no more than =tO.Ol absorbance. However, greater discrepancies were noted when results were compared with analyses
carried out with a freshly prepared buffer solution. It is advisable, then, to check the standard calibration curve after a fresh buffer solution has been prepared. Table I shows the results of analysis for pyridine in benzene. ACKNOWLEDGMENT
The authors thank W. G. Howells for his assistance in the preparation of the manuscript. RECEIVED for review April 1, 1969. Accepted June 16, 1969.
NQmd ispersive Atomic FIuorescence Analysis T. J. Vickersl and R. M. Vaught Department of Chemistry, Florida State University, Tallahassee, Fla. 32306 SINCEthe introduction of atomic fluorescence as a method of chemical analysis ( I , 2), it has been customary to isolate the atomic fluorescence lines by means of a prism or grating monochromator. It has been known, however, since the time of the earliest measurements of atomic fluorescence [most of which have been described in the monograph by Mitchell and Zemansky (31 that the atomic vapor itself could serve to isolate spectral lines. At temperatures of 3000 “K or less, almost all of the atoms of most elements will be present in the ground state. Hence, only absorption transitions originating in the ground state can contribute significantly to excitation of the fluorescence, and, no matter what the complexity of the spectrum of the excitation source, only the so-called resonance lines will appear in the fluorescence spectrum. The isolation by the atomic vapor of the few lines of the resonance spectrum from the many source lines of the element of interest has been used by Sullivan and Walsh (4-6) in the design of resonance monochromators for atomic absorption measurements and other applications. Only for the Group 2 elements-Le., Be, Mg, Ca, Sr, Ba, Zn, Cd, and Hg-will the resonance spectrum consist almost entirely of a single line and, hence, provide almost complete spectral isolation. For other elements, the resonance spectrum will consist of several lines. However, if the source of excitation is a source of lines of only the element of interest, then all of the lines in the fluorescence spectrum will be due to the element of interest and can still provide useful information for analysis. The types of resonance spectra have recently been discussed by Sullivan and Walsh (7). Monochromators have been used in atomic fluorescence measurements chiefly to isolate the fluorescence radiation from the thermal background radiation of the flame. For a number of elements of analytical interest, atomization can be 1
Author to whom requests for reprints should be sent.
(1) J. D. Winefordner and T. J. Vickers, ANAL.CHEM., 36, 161
(1964).
(2) J. D. Winefordner and R. A. Staab, ibid.,p 165. (3) A. C. G. Mitchell and M. W. Zemansky, “Resonance Radiation and Excited Atoms,” University Press, Cambridge, 1961. (4) 3 . V. Sullivan and A. Walsh, Specrrochim. Acta, 21, 727 (1965). (5) Ibid.,22, 1843 (1966). (6) Ibid.,23B, 131 (1967). (7) J. V. Sullivan and A. Walsh, Appl. Opt., 7, 1271 (1968). 1476
ANALYTICAL CHEMISTRY
1 Black Felt
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Figure 1. General schematic of apparatus
achieved under conditions in which the thermal flame background emission is low. Under these conditions, the use of a monochromator may be a disadvantage. In this paper we describe an instrument for the atomic fluorescence determination of elements meeting the above criterion. This instrument makes use of the inherent spectral resolution of atomic fluorescence and does not make use of either filters or a conventional monochromator. We describe the use of‘ this instrument in observing the atomic fluorescence of Cd, Zn, and Hg. EXPERIMENTAL Apparatus. A general schematic of the instrument is shown in Figure 1. As may be seen, the source-to-burner and burner-to-detector distances are quite short. Since no dispersing devices are used, a large cone of radiation can be accepted by the detector. It is only necessary to ensure that the cone of radiation accepted by the detector does not include primary radiation from the source of excitation. The baffles shown in Figure 1 serve this purpose. None of the dimensions of the apparatus appear critical, but care must be taken to minimize the possibility of stray light from the source reaching the detector. To minimize reflections, the interior of the instrument is painted flat black and black felt covers the end opposite the source. Except for the source compartment, which is enclosed to minimize stray light, the instrument is open at the top and bottom.
