air/organic liquid fuel flames as air/acetylene > air/benzene air/hexane > air/isooctane > air/acetone.
>
DISCUSSION The results given above indicate that aspirated organic liquids can be used to provide flames that are of use in atomic absorption spectrometry. The sensitivities determined for the metals under consideration have been found to be, for
most cases, equal to o r greater than those obtained using the more conventional air/acetylene flame. Though the flames produced using aspirated organic liquids appear to be colloidal, it has been shown that this characteristic can be minimized to the degree that the flame noise level falls within an acceptable range.
RECEIVED for review June 23, 1970. Accepted November 5 , 1970.
Some Physical Characteristics of Flames Produced from Organic Liquid Fuels B. W. Bailey and J. AI. Rankin Dicision of Laboratories and Research, New York State Department of Health, New Scotland Acenue, Albany, N . Y . 12201 Investigation of some physical properties of colloidal flames produced by the combustion of air-aspirated organic liquids shows them to be cooler and slower burning than those commonly used in atomic absorption flame spectrometry. Emission spectra indicate that colloidal flames are lower in background radiation than the air/acetylene flame. Results of these investigations are discussed with regard to the function of the flame in atomic absorption measurements.
RECENT STUDIES ( I , 2 ) have shown that flames resulting from the combustion of aspirated organic liquids can be produced in such a manner as t o allow their application to atomic absorption flame spectrometry. It has further been shown (3) that such flames are colloidal in type and that the sensitivities and detection limits they provide are in many instances equal to o r greater than those provided by the more commonly used air/acetylene flame. Colloidal flames are classified as premix in terms of the manner in which the fuel and oxidant are presented t o the reaction zone ( 4 ) . The differences, therefore, between colloidal flames and the premix flames which result from the combustion of gaseous fuels are not due to variations in flame structure but rather to the physical processes which occur before the fuel-oxidant mixture reaches the reaction zone. In premix flames supplied by gaseous fuels, the fuel-oxidant mixture passes through a preheat zone which raises its temperature to the ignition point. The preheat zone extends, in this case, from the reaction zone to the burner face. I n colloidal flames, the fuel arrives at the burner face as a fine mist and the “preheat zone,” extending from the envelope of the fuel mist to the reaction zone, must provide for evaporation and mixing as well as preheating. Figures 1 and 2 show the general structural features of both types of premix flames. In view of previous results from the application of colloidal flames to atomic absorption flame spectrometry, it was felt that further characterization of flame physical properties (1) B. W. Bailey and J. M. Rankin, Spectrosc. Lett., 2 ( 6 ) , 159-164
would provide information useful for additional applications of these flames to other areas of flame spectrometry. I n this communication the results from the determination of flame temperature and flame velocity for a variety of colloidal flames are compared with values obtained from the literature o r from our laboratory determinations of a variety of premix flames which employ gaseous fuels. In addition, flame emission spectra are presented for a variety of colloidal flames and compared with spectra from a n air/acetylene flame. EXPERIMENTAL
Flame Temperature Measurements. Flame temperatures were determined by means of sodium line reversal. The principle of this technique has been described elsewhere ( 4 , 5 ) and will not be restated here. The apparatus used to obtain the measurements is shown in Figure 3. The tungsten filament lamp was a 6-volt G.E. Model 2331. Lamp current was supplied by means of a step-down transformer with a 25.2volt, 2-ampere secondary. Voltage to the primary winding was controlled by a rheostat connected to 110-volt ac line current. L I and L? are 1-inch diameter fused quartz lenses with a focal length of 10 cm which were used to focus the image of the tungsten filament, first on the center of the flame and then on the entrance slit of the monochromator. The diaphragm D reduced the aperture of lens L? so that only a limited area of the flame would be observed at any time. The light from the tungsten source and the radiation emitted by the flame were synchronized by means of a Princeton Applied Research Model 222 variable speed chopper. The monochromator used was a Jarrel-Ash 0.5 Meter, Ebert Model 82020 equippe( with curved slits and a n 1180 line/mm grating blazed at 1900 A. The radiation leaving the exit slit was monitored by a 1P28 photomultiplier. The output from the photomultiplier was fed to a Princeton Applied Research Type A preamplifier and then to a Princeton Applied Research Model HR-8 lock-in amplifier which was tuned to the lamp and flame frequency with a reference frequency generated by the chopper. The output from the amplifier was displayed o n a Sargent-Welch Model SRLG recorder.
(1969).
(2) B. W. Bailey and J. M. Rankin, !bid., (8), 233-237. (3) B. W. Bailey and J. M. Rankin, ANAL.CHEM., 43, 216 (1971). (4) R . M. Fristrom and A. A. Westenberg, “Flame Structure,” McGraw-Hill Book Co., New York, N. Y., 1965.
( 5 ) J. A. Dean and T. C . Rains, “Flame Emission and Atomic
Absorption Spectrometry,” Vol. I, Marcel Dekker, New York, N. Y., 1969.
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2,000
A-
-
Y 0
Flame gases
i Figure 1. Structural features of a premix flame produced from the combustion of gaseous fuels
5
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M i l i m e t e r s A b o v e Burner Figure 4. Temperature profile of air/hexane flame
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Y
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a 0
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Figure 5. Temperature profile of air/ acetone flame
Flame
LI
Figure 3. Block diagram of apparatus used in the determination of flame temperatures by sodium line reversal Lamp calibration was accomplished by means of a PYROmicropyrometer manufactured by the Pyrometer Instrument Company. Calibration measurements were made through lens L1to obviate correcting for the transmission of the lens. The burner used in the flame temperature measurements was a Techtron Model F.E.-1 flame emission burner. The burner was attached to a Perkin-Elmer Model 303 mixing chamber equipped with the dual nebulizer attachment previously described ( I , 3). Flame temperature measurements were made by means of the following procedure. The fuel under cosideration was aspirated into the mixing chamber by means of one of the nebulizers of the dual nebulizer assembly. The fueljair mixture passed through the chamber and was ignited at the burner face. The fuel aspiration rate was adjusted to yield a stable, nonluminous flame. A 500-ppm sodium solution was then 220
aspirated into the flame by means of the second nebulizer. The tungsten lamp was lit an; allowed to stabilize. The wavFlength region of 5885-5900 A was scanned a t a rate of 10 A/ minute. The lamp temperature was then changed and the scan repeated. This process was continued until the reversal temperature was bracketed and then determined by interpolation. The flame temperature T, was obtained from the lamp brightness temperature TIby the following equation (15)
where X is the wavelength, Cs the second radiation constant, E the emissivity of tungsten, and 7 the transmission of the lens L1. In these measurements, T was set equal to unity since the lamp calibration was made through lens LI. Temperature measurements were made at various heights in the flame for airlhexane, airlacetone, and airjbenzene flames. These results were used to construct the flame temperature profiles shown in Figures 4-6. Temperature measurements were made for the air/acetylene flame at a point 5 mm above the reaction zone which yielded a temperature of (6) Bernard Lewis and Guenther von Elbe, “Combustion, Flames and Explosions of Gases,” 2nd ed., Academic Press, New York, N. Y . . 1961.
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2.000 r
y 7 mI y
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.
Monochromolor
I
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1
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1
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Figure 6. Temperature profile of air/benzene flame
Figure 7. Block diagram of apparatus used to obtain flame emission spectra
Table I. Temperatures of Colloidal Flames and Common Premixed Flames TemperaFuel Oxidant Flame type ture, O K Reference 1850 This work Acetone Air Colloidal This work Air Colloidal 1825 Hexane 1960 This work Benzene Air Colloidal 2270 This work Acetylene Air Premix Acetylene Air Premix 2300 (5) Hydrogen Air Premix 2275 (5) Methane Air Premix 2150 (5) Natural Gas Air Premix 1970 (5) Table 11. Limiting Velocities of Air/Organic Liquid Flames AspiraMaxtion rate. imum cm error,a Fuel min-l R,, cmBsec-l Vi, cm sec-I 1.2 1 . 0 8 x 102 22.2 5.5 Acetone Acetone 2.0 1 . 5 0 x lo2 30.7 6.4 3.1 1.75 x lo2 35.8 8.4 Acetone Hexane 0.7 1.08 x I02 22.1 1.8 Hexane 0.9 1 . 4 2 X lo2 28.8 1.7 1.7 1 . 7 5 x 102 35.7 2.3 Hexane Benzene 1.2 1 . 3 3 x 102 27.2 3.9 Benzene 1.5 1 . 6 6 x lo2 34.0 3.9 2.2 2.08 X IO2 42.5 4.5 Benzene Acetylene --> 6 . 5 x loZb >133 -__ a Assuming total vaporization and no losses. Total volume of gas (C2H2-k air) supplied to the burner.
2270 OK. As can be seen from the flame temperature profiles, the maximum temperatures of the air/organic liquid flames were considerably lower than those of the air/acetylene flame. Table I gives the flame temperature values obtained in this work as well as literature values for a variety of common premixed flames. As can be seen from the table, the temperatures of the colloidal flames are lower than those of the most commonly employed premix flames. In a previous communication (3), the temperatures of air/organic liquid flames were ordered according to their ability to function as atom reservoirs in the determination of calcium by atomic absorption. The absorbance values and the corresponding ground state atom population were related to the efficiency with which the flame served as a dissociative medium for the metal oxides formed during the atomization process. This relationship
was then used to determine the relative temperatures of the flames employed and to establish the following temperature order: air/acetylene >air/benzene >air/hexane >air/acetone. In the present work, line reversal measurements indicate that the air/acetone flame is some 25 degrees hotter than the air/ hexane flame. This seeming contradiction can be resolved if one recalls that a flame is not an equilibrium system and, therefore, temperature has meaning only with reference to the process used in its determination. In line reversal measurements, one determines an electronic excitation temperature whereas the method used in Reference 3 is based upon ground state enhancements resulting from a dissociation process. There is no particularly compelling reason why the two should give the same results. I n addition, acetone is the only oxygencontaining fuel examined. One would, therefore, expect some enhancement in oxide formation due to oxygen enrichment of the flame gases. Flame Velocity. Since the interest in this investigation was centered on relative rather than absolute velocities, it was felt that sufficient information could be obtained by considering only the velocity limit or lift-off velocity. This parameter is defined by the flow rate above which convective velocities are generated which exceed the burning velocity and cause the flame to blow off and extinguish. Velocity limits were determined for the airlhexane, air/acetone, and air/benzene flames by using a 1-inch diameter Meker type burner, modified for use with a Perkin-Elmer mixing chamber. Measurements were taken at constant fuel aspiration rates by increasing the air flow through the burner until a flow rate was reached at which the flame was completely lifted but stationary. The velocity limit Y , was calculated from the maximum flow rate R, by dividing the flow rate by the cross sectional area of the burner. It was found that a velocity limit for the air/acetylene flame could not be determined since the corresponding value of R, is beyond the upper flow limit of the apparatus employed. The value reported indicates the maximum flow rate obtainable. Difficulties arose with attempts to evaluate the contribution of the fuel vapor to the total gas flow in the determination of R, for the air/organic liquid flames. If one assumes complete vaporization with no fuel losses, the maximum error resulting from ignoring the contribution of fuel vapor was less than 10% at the highest fuel supply. It was thought that in view of the fuel losses due to the inefficiency of pneumatic
ANALYTICAL CHEMISTRY, VOL. 43, NO. '2, FEBRUARY 1971
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Figure 8. Emission spectrum of the air/hexane flame. Scan rate 100 Ajmin; 25-p slits
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nebulization the actual error would be considerably below the theoretical maximum and would fall well within the range of experimental error. Therefore, we decided to ignore the vapor contribution and to calculate the velocity limits based o n the air flow rates. Results of their determination, with the experimental conditions and maximum error calculations given in Table I1 show the velocity limits for the air/organic liquid flames to be quite low, approximately one fifth the value for the air/acetylene flame. It should be remembered that the velocities given above are limiting velocities and the flames employed in analytical work will be somewhat slower. 222
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The burning velocity reported for the air/natural gas flame (one of the slowest commonly employed) is given as 5 5 cm sec- 1 (5)which is probably higher than that of colloidal flames. Flame Spectra. Emission spectra were obtained for the air/acetylene, air/hexane, air/acetone, and airlbenzene flames over the wavelength range 2000-5500 A with the apparatus shown in Figure 7. The components of this apparatus are the same as those used in the flame temperature determinations described previously. The resulting spectra are very similar. They all exhibit the band emissions from C?and O H that are characteristic of hydrocarbon combustion.
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The most striking and important dissimilarities are the reduction of the background emission intensities and the decreased emission from the OH band at 3064 A which were found for the organic liquid flames. This decrease in background and band emission is even more evident when the spectra are run at sensitivities which allow the full intensity of the emission from the air/acetylene flame t o be seen (Figures 8 and 9). DISCUSSION
The results of these investigations indicate that colloidal flames produced by the combustion of air-aspirated organic liquids are cool, slow burning, and exhibit relatively low background radiation. It is apparent that these features provide a n ideal system for many of the elements commonly determined by atomic absorption measurements if one considers the processes occurring in such determinations.
I n general, one selects a flame with a high enough temperature t o provide efficient atomization. I n contrast, one might as well speak of choosing a flame with the lowest temperature consistent with efficient atomization. By so doing, one also obtains the flame which most effectively enhances the ground state population (the parameter being determined). Flame velocity is important since it determines the residence time of the analyte atom in the analytical region of the flame. By using lower velocity flames, one increases the probability that a n atom in the flame will absorb incident radiation. Colloidal flames are therefore seen t o possess physical properties that, for many elements, offer a distinct advantage over the premixed flames most commonly used in atomic absorption flame spectrometry.
RECEIVED for review June 23, 1970. Accepted November 5 , 1970.
Automated Thermoanalytical Techniques: An Automated Thermobalance’ W . S. Bradley and W. W . Wendlandt
Thermochemistry Laboratory, Department of Chemistry, Unicersity oj’Houston, Houston, Texas 77004
An automated thermobalance is described which is capable of recording the TG curves of eight samples in a sequential manner. The instrument consists of a recording top-loading balance, a furnace and temperature programmer, and an automatic sample changer. Each sample in the sample holder disk is positioned into the furnace automatically, heated to a preselected temperature, then removed. After the furnace is cooled back to room temperature, the cycle is repeated with a new sample. Operation of the thermobalance is completely automatic and it requires no operator attention, once the cycle is begun.
HONDA ( I ) WAS PERHAPS the first investigator to use the term “thermobalance” t o describe a n apparatus which was used to determine the continuous weight-change of a sample as the sample was heated to elevated temperatures in a furnace. Although the instrument was rather crude, it enabled him to obtain weight-change curves of a number of inorganic compounds and also t o establish a Japanese school of thermogravimetry, the results of which have been summarized by Saito (2). In 1923, a similarly crude thermobalance was described by Guichard (3) which was to be the first of a large number of instruments used by French workers in this field. The historical development of the modern thermobalance has been adequately described by Gordon and Campbell (4), See reference (14) for Part I. (1) K. Honda, Sei. Rep. Tohoku Unic., 4, 97 (1915). (2) H. Saito. “Thermobalance Analysis,” Gijitsu Shoin, Tokyo,
1962. (3) M. Guichard, Bull. SOC.Chim. Fr., 33, 258 (1923). (4) S. Gordon and C. Campbell, ANAL.CHEM., 32, 271R (1960).
Duval (3, Wendlandt (6), Keattch (7), Saito (8), and others (9, 10). The modern instruments have been described in well-known textbooks in the field (5,6, 9) and other sources (7, 11). By far the most sophisticated multifunction instrument is the Mettler thermobalance, as described by Weidemann (12). Besides recording the weight-change curves of a sample at two different sensitivities, it also records the derivative of the weight-change and the D T A curve. Another multifunction instrument, the Derivatograph, has previously been described by Paulik et al. (13). The modern thermobalance is an automatic instrument in that the weight-change of a sample can be recorded over a wide temperature range. None of the instruments are capable of introducing a new sample automatically into the furnace chamber o r studying multiple samples in a sequential manner. We wish to report here a n automated instrument which is ( 5 ) C. Duval, “Inorganic Thermogravimetric Analysis,” Second
Ed., Elsevier, Amsterdam, 1963. (6) W. W. Wendlandt, “Thermal Methods of Analysis,” Tnterscience, New York, N. Y . , 1964. (7) C. Keattch, “An Introduction to Thermogravimetry,” Heyden,
London, 1969. (8) H. Saito, “Thermal Analysis,” R. F. Schwenker and P. D. Gam, Ed., Academic Press, New York, N. Y . , 1969, pp 11-24.
(9) P. D. Garn, “Thermoanalytical Methods of Investigation,” Academic Press, New York, N. Y., 1965, Chap. 10. (10) H. C. Anderson, “Techniques and Methods of Polymer Evaluation,” P. E. Slade and L. T. Jenkins, Ed., Dekker, New York, N. Y . , 1966, Chap. 3. (11) W. W. Wendlandt, Lab. Matiagemenf, October, p 26 (1965). (12) H. G. Wiedemann, Achema Congress paper, Frankfurt, Germany (June 26, 1964). (13) F. Paulik, J. Paulik, and L. Erdey, 2. Atzal. Chern., 160, 241 ( 1958).
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