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Anal. Chem. 1981, 53, 762-786
then appears that the remaining 10 instruments, according to their calibration curves, may be divided into three classes. The first class contains the instruments for which a proper calibration at 291 nm yields a reading, correct at 491 nm, but too low at 220 nm (G, H, I, M, and N). Such a behavior, which resembles that of the instruments we studied, would be expected when a small static birefringence in the modulator interferes with the CD measurement. In the second class (J, K, L, 0) the calibration at 291 nm ensures that at 220 nm, but now the CD at 491 nm is underrated. This points to an interference, small in the ultraviolet and increasing toward the visible region, whose origin is much harder to guess. Possibly its nature relates to an improper modulator response toward driving voltage (which increases with wavelength). One expects that also in this class three points adequately describe the calibration curves, provided these are not too steep (as is perhaps the case with K). The last class contains a single, photoelastic modulator type, instrument (P). When scaled at 291 nm its CD readings at 220 and 491 nm both are in error, viz., too high. It is the latter aspect which leads us to suspect the very performance of the instrument for it is difficult to conceive of any interference which causes a principally sound CD apparatus, correctly calibrated at an intermediate wavelength, to yield too high readings at shorter as well as at longer wavelengths. We conclude that correct calibration of commercial CD apparatus can be performed by using the calibration curve obtained from the three standards PL, CSA or EA, and the cobalt complex. Use of the curve seems not warranted if its curvature is too large (deviations of, say, >20%). Obviously, for any calibration procedure to hold, the apparatus at hand must be basically well-performing,as may be judged from such well-known criteria as signal-to-noise ratio, flatness of base line, stability in time, etc. On the basis of our own experience we like to add a further one. An apparatus may perform well in the ultraviolet but fail to give correct readings in the visible region of the spectrum. In our case this situation was caused by a nonlinear response of the photomultiplier as a result of saturation effects, leading to CD signals that depend on the light intensity transmitted by the sample. Such a defect,
giving errors which are a capricious function of wavelength, is easily diagnosed by detecting the CD with and without neutral density filters. ACKNOWLEDGMENT We are grateful to C. Altona and H. M. Buck for the use of their CD spectrometers, to L. A. M. Bastiaansen and C. S. M. Olsthoorn for their help with the measurements on these instruments, and to H. J. C. Jacobs for critically reading the manuscript. LITERATURE CITED Tuzimura, Katura; Konno, Toshio; Meguro, Hiroshi; Hatano, Masahiro: Murakami, Tasuku; Kashlwabara, Kazuo; Saito, Kazuo; Kondo, Yoshikazu; Suzukl, Toshishige M. Anal. Blochem. 1977, 61, 167-174. Velluz, LBon; Legrand, Maurice; Grosjean, Marc ”Optical Circular Dlchrolsm”; Verlag Chemle/Academic Press: Weinheim/New York, 1965; Chapter 3. Davidsson, Ake; NordBn, Bengt Specfrochlm. Acta, Part A 1976. 32A, 717-722. NordBn, Bengt Acfa Chem. Scand. 1973, 27, 4021-4024. Moscowltz, Albert Adv. Chem. Phys. 1962, 4, 67-112. Emels, C. A.; Oosterhoff, L. J.; De Vrles, Gonda Proc. R. SOC. London, Ser. A 1967, 297, 54-65. Cassim, Joseph Y.; Yang, Jen Tsi Biochemistry 1960, 8, 1947-1951. DeTar, Delos F. Anal. Chem. 1969, 41, 1406-1408. Krueger, W. C.; Pschlgoda, L. M. Anal. Chem. 1971, 43, 675-677. Chen. G. Chi; Yang, Jen Tsi Anal. Lett. 1977, 10, 1195-1207. Gillen, Michael F.; Wllliams, Ross E. Can. J. Chem. 1975, 53, 2351-2353. Konno. Toshlo: Meauro. Hiroshl; Tuzimura. Katura Anal. Biochem. 1975, 67, 226-232: Schippers, Peter H.; Van den Beukel, Arie; Dekkers, Harry P. J. M., unpublished results. Richardson, Frederick S.; Riehl, James P. Chem. Rev. 1977, 77, 773-792. Kemp, James C. J . opt. SOC. ~ m1969, . 59, 950-954. Franklin, M. L.; Horllck, Gary; Malmstadt, H. V. Anal. Chem. 1969, 41, 2-10. Ingle, J. O., Jr.; Crouch, S. R. Anal. Chem. 1972, 44, 777-784. Broomhead, J. A.; Dwyer, F. P.; Hogarth, J. W. Inorg. Synth. 1960, 6 , 183- 188. McCaffery, A. J.; Mason, S. F. Mol. Phys. 1963, 6, 359-371. “The Merck Index”, 9th ed.; Merck: Rahway, NJ, 1976; p 909. Lowry, T. Martin “Optical Rotatory Power”; Longmans, Green and Co.; London, 1935; Chapter 22. Arvedson, Peter F.; Larsen, Edwin M. Inorg. Chem. 1966, 5, 779-783.
RECEIVED for review October 1,1980. Accepted January 21, 1981.
Continuous Measurement of Ammonia in Stack Gas by Wavelength-Modulated Derivative Spectrometry Takusuke Izumi” Research and Development Division, Anritsu Electric Co., Ltd., 7800 Onna, A tsugi-shi, Kanagawa-ken 243, Japan
Keigo Nakamura Air Pollution Control Division, National Research Institute for Pollution and Resources, Onogawa, Yatabe-cho, Tsukuba-gun, Ibaraki-ken 305, Japan
A wavelength-modulated derlvatlve spectrometer is used to make continuous and direct measurements of NH:, In stack gas with few Interferences. The measurement Is achieved wlthln an error of f 2 ppm. The error is due to Interference caused by SO,, but the spectrometer can be used for SO, concentrations of up to 1000 ppm In the sample gas. Some specific features of the spectrometer are as follows: a 1.6-nm spectral silt width, a 3-nm peak-to-peak amplitude for wavelength modulatlon, a 219-nm wavelength for detectlon of the degree of intensity modulation (magnitude of second derivative), and a 200 O C cell temperature. The limit of detection Is 0.5 ppm and the response t h e Is about 1 mln.
Derivative spectrometry based on wavelength modulation has been described in many previous papers (1-3) where it was used to resolve a particular spectrum among several overlapping spectra. Due to its increased sensitivity, derivative spectrometry has been applied to the analysis of trace gases (3).
Hunt and Williams (4) have shown that the second-derivative signal for ammonia (NH,) at 204.6 nm is free from interference from molecules in human breath and is suitable for the measurement of NH3. However, for mixtures containing sulfur dioxide (SOZ) or nitric oxide (NO), the wavelength-modulatedsecond-derivativespectrometerhas not been
0003-2700/81/0353-0782$01.25/00 1981 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6,MAY 1981
used for NH3 determination because of the large interference caused by these oxides. The derivative spectrometer discussed in this paper is designed to meet the current demand for a method to continuously monitor residual NH3 from a nitrogen oxide purging process. In the course of its design, the second-derivative ultraviolet absorption spectra for NH3, NO, and SO2 were examined carefully due to the weakness of the NH3 absorption band and its overlapping with stronger absorption bands of NO and SO2. The absorption cross section of SO2 at 219 nm is 1 X 10-ls cm2,being larger than that of NH3 (4 X lo-'' cm'). These values are obtained by using a spectrometer with a broad band slit function, and they are average values over the wavelength region between 217 and 221 nm. If the concentrations of NH3 and SO2in the sample gas are 10 and 500 ppm, respectively, the ratio of absorbance A ~ , o ~ / A Nis H ~estimated to be 125. This means that the usual derivative spectrometer cannot be used for measurement of NH3 in the sample gas. Some effort was made to investigate and to determine the optimum slit width, the optimum amplitude of the modulation, and the wavelength for the NH3 measurement without interference from SO2 and NO. Analytical expressions for the intensity of incident radiation and the degree of intensity modulation are developed as follows. Assume that radiation having a varying wavelength X ( t ) with time t in a sinusoidal way as X ( t ) = Xo + a sin w t about some specific wavelength Xo falls upon the molecule, where "a" and w are the amplitude and angular frequency of the wavelength modulation, respectively. Then the resulting intensity of the radiation P(t) is a function of time ( I ) , as m
P(t) = P
+ m=l C A,,
m
cos 2mwt
+ mC= lA2m-1sin (2m - 1)ut
(1) with a constant component of P and the superimposed sinusoidal intensity ripples of the angular frequency 2mw and (2m - 1)w. The amplitude A , of the ripple of angular frequency 2w is proportional to the second derivative of the intensity with respect to wavelength (I), d2P/dX2. When the central wavelength of modulation X,agrees with either an absorption maximum or minirnum wavelength of a band and the density of the molecules in small, the degree of modulation for the intensity ripple of angular frequency 20, A2/P, is shown as (5)
A,/P
(a(X0) -
(2) where a(X) is the absorption cross section of a molecule at the wavelength A, X1 is the wavelength at the peak point of wavelength modulation (namely, XI = A0 + a or XI = A0 - a), n is the density of the molecules, b is the optical path length, and it is assumed that the absorption profile of the band is symmetric with respect to the absorption maximum or minimum wavelength, inamely, a(Xo + a) s a(& - a). The degree of modulation A 2 / P is linearly proportional to the density of the molecules as long as the optical path length is constant, and it does not depend on the light intensity P, although the signal to noise ratio of A 2 / P depends on P. u(Xl))nb/2
THEORY For clarification of the NH3 spectrum in overlapping spectral bands of SOZ, particular consideration was made on three points, namely, slit width, amplitude of the wavelength modulation, and temperature of the sample gas in the cell. Slit Width. We know that the spectra of both NH3 and SO2 are of periodic structure with a period of oscillation AX,, (AX,, for NH3 z 4 nm, and AX,, for SO2 2 nm). A spectrum g(A) coming out from a spectrometer can be expressed as a convolution, g(h) = O(X)*f(X), where f ( X ) is a true incident spectrum and O(X) is a function of the slit used. If the slit
783
is of a triangular function with a half-width Ab, the oscillatory structure in f ( X ) appears and disappears in the spectrum g(X) as AX, varies. The structure disappears when AX, = AX,, or integer multiple of AX,, and the spectrum observed is a broad band spectrum. When AX, = (1/2)AXpp or odd multiple of (1/2)AX,,, the oscillatory structure appears markedly in the spectrum (6). Thus, it is expected that, when the slit width (AX,) is nearly equal to the full width of the half-maximum (FWHM) (corresponding to, (1/2)AXPp) of the absorption spectrum of NH,, which is nearly equal to double of FWHM (corresponding to AX,,) of the absorption spectrum of SO2, namely, FWHM "= 2 nm, only the second-derivativespectrum of NH3is detected and the SO2spectrum is not differentiated. Amplitude of Wavelength Modulation. Assume that f ( X ) is written by approximation with
where f b ( X ) is a broad band spectrum (and hence it can be assumed to be constant if the wavelength is considered in the limited small region), & is a center wavelength of a periodic spectrum line, and B is the amplitude of oscillatory structure. The intensity-modulatedlight signal (IMLS),P(t),is obtained as a result of the wavelength modulation X = XO + a sin ut (eq 4), where f b = fb (io a sin w t ) is an almost constant dc P(t) = 7b + B cos (x sin ut a) =& B{cos (x sin u t ) cos a - sin ( x sin w t ) sin a)
+
+ + = f b + B COS Cu{Jo(X)+ 2 C Jzn(X) COS 2nwt) n=l B sin 4 2 CJ2,+l(x)sin (2n + 1 ) w t ) n=l = P + B cos a(2C J&) cos 2 n d ) n=l m
m
m
B sin ai2
C~ n=l
~ ~ + sin ~ ((2n x )+ 1 ) w t j
(4)
component as long as the amplitude, a, is small, P = f b + B cos aJo(x),a = 2r(& - &)/A&,, and x = 2ra/AXpp. It is clear from eq 4 that the amplitude of IMLS of angular frequency 2w is shown by a Ressel function of the first kind J&), which has a maximum value at x s 3 and is zero at x 5. It is expected that, when the amplitude of wavelength modulation, a, is nearly equal to 1.6 nm (corresponding to x s 5 for SOz), the amplitude IMLS of angular frequency 2w for SO2is equal to zero, because AX,, for SOz is about 2 nm. Gas Temperature. In general, when the temperature of the sample gas in a cell is increased, the periodic structure of the spectrum suffers from loss of its sharpness due to thermal excitation of the gas molecules. Thus, it is expected that there is an optimum temperature at which the loss in sharpness for SOz is large enough to diminish the interference although that for NH3 is negligible due to the difference of the period AA,,.
EXPERIMENTAL SECTION The second-derivative spectra are obtained by the wavelength-modulated derivative spectrometer shown in Figure 1. The wavelength modulator (WM) is arranged on an exit image plane of the monochromator,and the modulator's function is to vibrate sinusoidally the exit slit (Sz)along the wavelength coordinate of the dispersed spectrum on the exit image plane. The slit S2is photoetched onto a phosphorus bronze sheet of 0.3 mm thickness. A U-shaped tuning fork and the driving circuit of modulator DCM are used to vibrate the slit plate. The phosphorus bronze sheet is brazed by hard solder on the free end of one leg of the tuning fork. The IMLS containing information of NH3 concentration is produced by the vibrating exit slit and is received by the photomultiplier PMT. Both ac and dc components are contained
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981 INLET
MONOCHROMATER
-2
I
L---J
205
Flgure 1. Block diagram of the spectrometer: D2,deuterium lamp of 30 W; L,, collimator lens; S,, entrance slR; G, diffraction grating of ruled grooves 1200 lines/mm; PRE-AMP, preamplifier.
in the output signal of the PMT. The ac component is intensity modulated with the various angular frequencies, i.e., 2mw and (2m - 1101,m = 1,2, ...,only when the molecules to be measured are in the sample gas. (The method may well be called selfmodulation spectroscopy.) After the dc component is eliminated by the capacitor (C), only the ac component of angular frequency 2w is detected and a dc signal (VaJ proportional to the amplitude of the ac component is generated by the lock-in-amplifierLIA. The time constant of the LIA used in 10 s. On the other hand, the dc output signal from the PMT is detected by a dc amplifier (DCA) and dc signal (VdJ is obtained as an output of a low-pass filter LPF with a time constant of 10 s. Division of VaC/Vdcis carried out by the division circuit DIVC, and the resultant signal is proportional to the degree of modulation A,/P. The output signal of the DIVC is recorded by an X-Y recorder, Four kinds of slits with slit width of 1.1, 1.4, 1.6, and 1.9 nm were examined for both entrance and exit slits, in order to determine the optimum slit width, that is, the slit width with minimum interference to the measurement of NH3 in stack gas. The amplitude of the wavelength modulation can be adjusted by a,variable resistor (not shown in the figure) of the driving circuit DCM, in order to determine the optimum amplitude of the wavelength modulation. The cell of 8.5- or 20-cm path length was heated by a electric heater, and the temperature can be variable by a electric thermocontroller in order to observe the broadning effect of spectrum due to the thermal excitation for molecules. The standard gases, NH3, SO, and NO, contained in each cylinder were used, and the dilution system employing the flowratio method was also used for calibration and measurement. Nitrogen gas in a cylinder was used for zero gas and for the dilution,
RESULTS AND DISCUSSION Results of a preliminary experiment showed that Ah, = 1.6 nm is the best condition to pick up the NH3spectrum out of the overlapping spectral lines with SO2. Although the oscillatory structure of SOz disappears almost completely when Ah, = 1.9 nm, or Ah, is nearly equal to AA,, of SO2,Ah, = 1.6 nm gives even better sensitivity for NH3 than Ah, = 1.9 nm. The optimum peak-to-peak amplitude of the modulation, 2a, obtained by experiment is 3 nm or a = 1.5 nm. The value is almost equal to the theoretical value, a = 1.6 nm, where the IMLS of angular frequency 2 0 for SO2becomes zero. Because Ah,, for NH3 is 4 nm, the amplitude of IMLS for NH3 obtained with a = 1.5 nm was only 10% less than that obtained with a = 1.9 nm (corresponding to x 1 3 in eq 3). As "a" is increased to 1.9 nm, the amplitude of IMLS for SO2 is increased as well as that for NH3and an undesirable interference becomes greater. When the sample gas in the cell is increased up to a temperature about 200 O C , the interference from SO2 is reduced
,
I 210
,
,
I
I
I 215
I
,
,
,
I
,
,
,
220
WAVELENGTH (nm)
Secondderivative spectra for various gases and gas concentrations (obtalned by using the cell of 8.5-cm path length). Flgure 2.
by about half compared to that at room temperature. This is due to broadening effect for SO2 spectrum by thermal excitation and the effect is saturated at temperatures above 200 "C. The second-derivative spectra for NH,, SO2, and NO, obtained under these optimum conditions, are shown in Figure 2. It is clear from Figure 2 that the degree of modulation at both wavelengths 209 and 219 nm (corresponding to near the maximum and minimum, respectively, of the absorption spectrum of NHJ, is almost maximal for NH3 and nearly zero or very small for both SO2 and NO. A broad band spectrum of SO2 still remains on the periodic NH3 spectrum in spite of the careful design of the slit width and the amplitude of modulation, although this superposing interference of SO2 is very small a t both wavelengths 209 and 219 nm. The interference at 209 nm is about double of or larger than that at 219 nm. Thus, the wavelength, 219 nm, is preferable for the measurement of NH3concentrationin the mixtures containing
sop
There is another favorable effect at high temperature, namely, the weak absorption at 217 nm and the weaker absorption at 221 nm of NH, spectrum (locating near the absorption edge a t long wavelength region for the NH3periodic spectrum) are increased due to red shift by thermal excitation. In fact the second derivative at 219 nm (being the center wavelength of 217 and 221 nm, namely, the absorption minimum) and at 200 "C becomes about 20% greater than that at room temperature. Enlarged second-derivative spectra for the broad band spectrum of SO2in the wavelength region neighboring 219 nm are shown in Figure 3 together with the second-derivative spectrum of NH3. Figure 4 is a translation from the results shown in Figure 3. The interference does not change linearly with the concentration of SO2. This is due to the fact that the degree of modulation changes linearly with the concentration for NH3 but not for S02, although the reason for this is not clear. However, the interference can be removed by use of electrical technique, namely, subtraction of the voltage (correspondingto the degree of modulation of IMLS generated by the broad band spectrum of S02) from the deteded voltage. One can determine the voltage to be subtracted by using a microcomputor with a memory in which information on the interference characteristic curves is stored and a monitor of the SO2 concentration. The same principle of the derivative spectrometer can be used for this monitoring the SO2 concentration a t the wavelength around 300 nm. In fact, both NH3 and SO2 concentration were measured simultaneously and continuously by one spectrometer with two vibrating slits.
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
\
785
concentrations up to 1000 ppm. The interferences from both NO and NO2are negligible for the sample gas containing less than lo00 ppm for NO and less than 50 ppm for NOz. The concentrations in stack gas are less than these values. For continuous measurements of NH3 in the oil-burnt stack gas, some modifications were made to the spectrometer shown in Figure 1, to provide (1)b = 20 cm; (2) Xo = 219.3 nm; (3) subtracting circuit, where a constant voltage (corresponding to about 7 ppm NH3) is subtracted at all times from the output signal voltage of the degree of modulation (This is to remove the interference caused by broad band spectrum of SO,.); (4) zero- and span-calibrationfacility; (5) an analog recorder with a scale indicated in parts-per-million concentrations. The results obtained by the measurement of NH3 in the oil-burnt stack gas are as follows. The regression equation is y = 0.995~ 0.9, coefficient of correlation y is 0.997, and the scattering deviation from the regression line s (standard deviation) is about 1ppm, where y is the measured value in parts per million. The concentration of NH3 in stack gas in parts per million, x , is calculated from the ratio of the amount of the injected NH3 into the stack to the total flow quantity of the stack gas. A very good correlation, the coefficient of correlation y being 0.996, is obtained between the analytical values by the spectrometer and those by the standard method (indophenol method). The scattering deviation for the oilburnt stack gas observed by the standard method is s = 3.7 ppm, and this is larger than that of the value obtained by an instrumental analysis using the spectrometer (about 1pprn). Although the concentration of SO2 in the stack gas fluctuates between 200 and 400 ppm, NH3 in the stack gas is analyzed exactly by the spectrometer using the constant voltage subtraction technique. Although the response time of the main apparatus itself is less than 1 min, the time obtained by using the 25-m gas intake hose (6 mm inside diameter, made of Teflon, and heated up to about 180 O C ) is about 3 min. This is fast enough to measure the concentration of residual NH3 in a nitrogen oxide purging process. The response time depends mainly on the adsorption of the NH3 in the inner surface of the gas intake hose. The limit of identification, which is defined as the level equal to the full width of the electronic noise recorded on an analog recorder, is 0.5 ppm. This is sensitive enough to measure the concentration of residual NH3 in a nitrogen oxide purging process, because it fluctuates between about 5 and 50 ppm in almost all cases. Although some minor interference from hydrocarbons was observed, it is practically insignificant. Because in general stack gas produced by perfect combustion contains only a very small amount of hydrocarbon, and this was also confirmed by the experiments for various kinds of stack gas, namely, LPG-burnt, oil-burnt, and coal-burnt stack gas.
+
Figure 3. The second-derivativespectra of NH3 (concentration 116 ppm) and of SO2 at various concentrations (obtained by using the cell of 20-cm path length), wavelength around 219 nm.
30
^n
I
219.6 nm
20
I
?
E, 2
IO
O N
ln
>m n
O
w
l3 n
3
-10
0
500
1000
CONCENTRATION OF SO2 1 ppm
Flgure 4. Interfererice caused by SO2 vs. concentration of SOp at various wavelengths.
However, noting that the characteristic curves show a concave with a very broad flat region of the width of about 300 ppm of SOz concentration (see Figure 4) and also that the fluctuation in SO2 concentration is in general 200 or 300 ppm so far as the same kind of fuel is combusted, a technique of the constant voltage subtraction from the measured voltage may well be used for the interference removal. By use of this technique, the measurement of NH3 is made within an error of f 2 ppm provided that the fluctuation in SO2 concentration is smaller than 300 ppm. This technique is valid for SO2
CONCLUSION The experimental investigations discussed in this paper demonstrate (1)that the wavelength-modulated derivative spectrometer can be used to detect the very weak absorption spectrum of NH3 at 209 and 219 nm with very small interference, even if the spectrum is overlapped by the strong absorption spectrum (band) of SO2, (2) that the principle of the operation is supported by the theoretical analysis, and (3) that the spectrometeroperatingat 219 nm with an interference removement technique (i.e., constant voltage subtraction) can measure NH3 concentration in a complex mixture of the SO2 concentration up to lo00 ppm within an error of f 2 ppm. The results obtained for an oil-burnt stack gas show the standard deviation of the scattered data is only 1ppm, and the spectrometer is useful for measurements of NH3 in stack gas with a high SO2 concentration.
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Anal. Chem. 1981, 53, 786-789
A further improvement in the analysis error can be possible by employing an automatic calculation method using a microcomputer to the interference-removal technique. Application of the spectrometer to much higher SOz concentration, namely, over 1000 ppm, is the subject for a future study. ACKNOWLEDGMENT The authors wish to thank R. T. Koike for his hehful discussion and critical reading of the paper before publicakon. The authors were favored to have the assistance of S. Sekino, M. Hamajima, N. Uno, and A. Shimizu who contributed their experimental skill. LITERATURE CITED (1) Bonflglioii, G.; Brovetto, P. Appl. Opt. 1964, 3 , 1417-1424. (2) Hager, R. N., Jr.; Anderson, R. C. J . Opt. Soc. Am. 1970, 60, 1444-1449.
Paper, No. 71-1045; American Institute of Aeronautics and Astronautics; New York, 1971.
(3) Hager, Robert N., Jr. AIAA
(4) Hunt, R. D.; Williams, D. T. Am. Lab. (Falrfleld, Conn.) 1977, 9 (6),
10-23.
(5) Izumi, T.; Nakamura, K. J . Phys. €1981, 14, 105-112. (6) Stewart, James E. Appl. Opt. 1965, 4 , 609-612.
RECEIVED for review March 3, 1980. Accepted December 29, 1980. Financial support by the Japanese Government is gratefully acknowledged. The paper is published by permission of the Director, Air Pollution Control Division, National Research Institute for Pollution and Resources, to whom the authors are grateful. Presented in part at the 25th Spring Meeting of the Japan Society of Applied Physics (27a-L-1, -2, and -3, Tokyo, Japan, March 1978) [in Japanese].
Quantitative Determination of Carboxylic Acids and Their Salts and Anhydrides in Asphalts by Selective Chemical Reactions and Differential Infrared Spectrometry J. Claine Petersen” and Henry Plancher Laramie Energy Technology Center, P.0. Box 3395, University Station, Laramie, Wyoming 82070
A method for determining the concentratlon of carboxylic acids and anhydrldes In asphalts and asphalt fractions has been developed. I n addltlon, carboxylale salts are determined independently from the free acids. The method is based on the selective Interaction of trfphenyltln hydroxide wlth free carboxylic acids, the hydrolysls of acids and anhydrides wlth sodlum hydroxide, and the silylatlon of free aclds and thelr salts. Dffferentlal spectra are used to make the quantftatlve determlnations.
A quantitative method for determining several compound types in asphalts that absorb in the carbonyl region of the infrared spectrum was described in a previous paper (I). In the method, anhydrides and carboxylic acids in asphalts were reacted simultaneously with added sodium hydroxide (NaOH), and the total combined absorbance of these two functional types was determined from the band area of the differential spectrum of NaOH treated vs. untreated samples. For determination of the carboxylic acid absorbance, silyl esters were formed from the acids in untreated samples, and the absorbance was estimated from the area of the acid band obtained from the differential spectrum of silylated vs. untreated samples. The absorbance of the anhydrides was then determined by difference. This procedure often presented problems because of band overlap between the free acid and silyl ester bands, making necessary the approximation of the area of the acid band. Determination of carboxylic acids separately using potassium bicarbonate (I) sometimes also presented problems because of the reaction of bicarbonate with some of the more easily hydrolyzed anhydrides. The present paper describes a modification of the earlier method (1) that obviates the problems outlined above. Triphenyltin hydroxide (TPTH) is used; this reagent interacts quantitatively and selectively with the carboxylic acids in asphalts at ambient temperature without interaction with
other functional groups that absorb in the carbonyl region. Free carboxylic acids and anhydrides are then easily determined independently using differential spectra. Carboxylic acid salts can also be determined independently of the free carboxylic acids.
EXPERIMENTAL SECTION Materials. A large number of asphalts and asphalt fractions have been examined by the modified method. Two fractions were chosen to illustrate the method: (1) Wilmington asphaltenes precipitated by pentane (2) and (2) an asphalt fraction that had been strongly adsorbed (SA) on the aggregate taken from an aged (air oxidized)road core. The SA fraction, which represented less than 1% of the asphalt, was the asphalt fraction not removed by extraction of the aggregate with benzene but that was removed by extraction of the benzene-extracted aggregate with pyridine. Details of the isolation of the SA fraction are found elsewhere (3). Benzene was analyzed reagent grade from J. T. Baker Chemical Co. Tetrahydrofuran (THF) from Eastman Chemical Co. was passed through a column of dry, activated basic alumina to remove possible interfering amounts of water and oxidation products, stabilized with 0.025% butylated hydroxytoluene, and protected from moisture by storing over activated, Type 4A molecular sieve from J. T. Baker and Co. Hexamethyldisilizane (HMDS) and trimethylchlorosilane(TMCS) were “specially purified grade” from Pierce Chemical Co. Sodium hydroxide (NaOH) was analytical reagent grade. Triphenyltin hydroxide (TPTH) of about 90% purity was obtained from Pfaltz and Bauer, Inc. The TPTH reagent was prepared as follows. Residual water was removed from 0.32 g of TPTH by azeotropicdistillation with about 10 mL of benzene followed by complete evaporation of the benzene. The dried TPTH was then diluted with 100 mL of THF and stored in a brown bottle containing 4A molecular sieve. The bottle was stoppered with an inert cap, and, thus prepared, the reagent was stable for several weeks. With time the reagent slowly develops an interfering infrared band at about 1775 cm-’. Model compounds used in the interaction with TPTH were standard laboratory grade. Sample Preparation. Triplicate samples of asphaltenes (0.125 g) or SA fraction (0.0312 g) were weighed into 25-mL Erlenmeyer
This article not subject to U.S. Copyright. Published 1981 by the
American Chemical Society
