Temperatures of Some Typical Flames Used in Flame Photometry J. D. WINEFORDNEF!, C. T. MANSFIELD, and T . J. VICKERS Department of Chemistry, Universify o f Florida, Gainesville, Ha.
b Flame temperatures of Hz/Oz and C Z H ~ / O Zflames, iri which aqueous solution i s introduced via total consumption atomizer-burners, are measured as a function of solution flow rate into the flame for a variety o f flame compositions and heights in the flame. A simple method of flame temperature measurement, based on the measurement of the ratio of intensities of two iron lines (Fe 3737.1 3 A. and Fe 3734.87 A.), i s used to measure the flame temperatures. From the measured temperatures, empirical equations are determined which can b e used t o calculate the flame temperature of a given flame compo!;ition at a given height above the inner cone. The use o f the derived equations and the effect o f flame composition, solution flow rate, and flame heiqht on flame temperature o f Hz/02 and C ~ H Z / O Z flames are discussed.
T
HE txw.r{ C O T E of a flame can be considered to bc approximately in thcrinal equilibrium ( I ) , and the most important parameter describing a thermal source is its temperature, T . The intenqity of a cpectral line of a flame source coritaining j,paseouc atoms is euponcntially dependent upon the temperature of the flame (Boltzmann factor) The ahsorption of radiation t x the same flame is nearly independent of the flame temperature 19) Howmer. because the :ample is usually introduced into the flame as a salt colution, the flame temperature also influences the proceses of dispersion of introduwd salt wlui ion into droplets, evaporation of solvmt to produce a qalt mist, diqsociation of the salt into ilom-. a-qociation of the atoms in [ onwin nith flame gas producti and ioniz,ztion of the I toms For thesc rt'asons. it I-. iinportar i t that the aiqxoumatc tcrnprraturc of any giJ en flame he knonn nhen either absorption or eniii.ion men-urements are to be made. Moit flame emiqsi in and absorption n i m w r e m ( ~ i i t ~are performed n ith fT2 0 , or C'& 'Or flanw's. Therefore. flainr tcinprraturrs ot HA/'02 aiitl C'JT? 0: flanir~- arc nieawwl a\ a tunction of flaiiic. :.ai conipositiou, solution flow I ate, elhrieriry of sample introtlurtion, anti height abox e the
inner cone of the flame. From the experimental measurements, empirical equations for the flame temperature of a number of flame gas compositions are determined as a function of the above factors. The measurement method employed in this paper is based on the Ornstein tno-line method (3), and is similar to the method described by Broida and Shuler (6). This method involves the measurement of the intensity of t n o iron lines, F e 3737.13 A. and Fe 3734.87 A., from which the temperature can be found by a simple calculation. Other methods for the measurement of flame temperatures have been fully described by Gaydon and Wolfhard (8) and Broida (h), and so will not be discussed here. For flam: temperatures above 2800" K., the twoline method is the simplest and most rapid method, and gives good precision. Flame photometry involves a study of the electronic deactivation of excited atoms or the electronic activation of atoms from lower to higher electronic statey and so a method which measures an electronic temperature is desired. Temperatures measured using the twoline method are electronic temperatures. EXPERIMENTAL
and Solutions. The same evperimental equipment as previously described (IO) was used for all temperature measurements. However, in this case no condensing lenses were used. The flame was placed 2 em. from the spectrometer entrance slit. Between the entrance slit and the flame (1 em. from each), a flat black baffle with a n opening 0.5 em. by 0.5 em. was centered and permanently positioned on the same rider base as the burner. A -tock solution of ferric chloride containing 500 p.p.m. of iron mas prepared for use in all temperature mea-urements. Equipment
Procedure. T o make a temperature measurement, the flow rate of iron solution ($=io) into the flame, the flon rates of oxygen (+oJ and of hydrogen ($AZ) or acetylene ($CJH2) \+ere adjusted to any desired value, arid the \allies nere recorded. The t\*o iron line- nere scanned manually, and the sensitivity of the photometer \ \ a b adp t e d YO that the Fe 3737.13-A. line was about 80 to 90% of full scale.
Then the two iron lines were scanned automatically at the rate of 2 A. per minute, and the output signal was recorded. This process was repeated at least twice for each flame temperature measurement, and the heights of the two peaks were measured. The ratio of the intensities was determined, and the flame temperature, T , was calculated using the two-line method ( 3 ) . The intensity of a spectral line emitted by atoms in a flame in thermal equilibrium is given by Broida and Shuler (6). The temperature, T, can be found by taking the ratio of the intensities and solving for T. With the experimental setup used, it was found that for a given set of conditions the relative standard deviation of the intensity rat'io was =t5% even if the absolute intensity of the two iron lines varied. The relative standard deviation of all temperature measurements was about i10%. Minor fluctuations in solut.ion flow rate may cause appreciable changes in intensity but only small changes in temperature, T. The proper choice of a line pair for application of the Ornstein two-line method to the measurement of flame temperatures is governed by a number of criteria. The two lines must be negligibly self-absorbed or t'he selfabsorption must be known and approximately the same for both lines under the same conditions. Crosswhite (7) has indicated the two lines in concern show negligible self-absorption, Broida and Lalos ( 5 ) have discussed the selection of spectral lines with respect to the value of the transition probabilities and statistical weights. -4 number of instrumental factors also limit the choice of a line pair. The lines must be close enough together that the two lines can be scanned in a relatively short time and at such a slow speed that the recorder responds accurately to both lines. Also, the two lines must be close enough together that the photocathode has essentially the same response to both lines or else the spectral response must be correct'ed for. Finally, the two lines must be of sufficient intensity as to be accurately measurable a t the sensitivity setting used. Using the experimental setup described, the two iron lines chosen are nearly ideal. Numerous other iron lines for which spectral data, are readily available ( 7 ) can be used. For example, when usirig t'he two-line method with the Beckman DU monochromator or with similar VOL. 35, NO. 11, OCTOBER 1963
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161 1
Table 1.
Evaluation of Constants in Empirical Equation for Determination of Temperatures of Hz/Oz Flames
Flame conditions
Constants
402, CC.
@O~/@H~
0.2 0.4
0.5 0.6 Table 11.
per min. 2500 2500 2000-3500 2500
T T K . 2700 2750 2760 2630
0.05
0.05
C
40 40
50 40
Evaluation of Constants in Empirical Equation for Determination of Temperatures of CzHz/02 Flames $02,
4oa/4c2~, 1.o
per min. 2500 2OOC-3000 4000
3000-4500 4000
Tttp, ' K. 3050 3050 3050 3050 3050
a
The symbols a, b, and c are constants and have been empirically evaluated from the measured temperature us. solution flow rate curves for a variety of flame conditions. The values of a, b, c, and Ttip (the temperature of Hz/Oz flames a t the tip of the inner cone) for several fuel-rich, for a stoichiometric, and for one oxygen-rich flame are given in Table I. The term x is the solution flow rate expressed as number of moles ANALYTICAL CHEMISTRY
5 5 7
20
5
__ C
70 70 70 70 70
f
0.04
0.05 0.08 0.08 0.08
of solution per mole of oxygen gas introduced into the flame, and 6 is the efficiency of sample introduction as defined in a previous paper (IO). The empirical equation listed below was found to give C2H2/02flame temperatures with mean errors from the measured values of less than 50" K.
RESULTS AND DISCUSSION
The two lines used in the temperature measurements were thoroughly checked for self-absorption a t various concentrations of iron. lu'o measurable selfabsorption was found for concentrations of iron of 2000 p.p.m. or below. Using the two-line method, flame temperatures were measured as a function of aqueous solution flow rate for a large number of H2/02 and CSH2/02 flames a t various ratios of flow rate of oxygen to fuel, a t various absolute values of d~~ or $czaz and @02, and a t various heights, h, in centimeters above the inner cone of the flame. The average result of duplicate measurements of flame tcmperatures was plotted us. solution flow rate for certain values of $ O 2 / @ H 2 or ~ o ~ / @ ~ for ~ H certain absolute values of Q~,,and for various values of h. It is, however, unnecessary to give the measured data in numerous graphs because the following empirical equation was found to give R2/02 flame temperatures with mean errors from the measured values of less than 50" IC.
Constants b 10 10 10 10
5
monochromators, the separation of the lines would have to be greater for accurate measurement of the line intensities.
1612
0.0% 0.02.5
0.Oi 0.07 0.09
Flame conditions
1.5 2 .o 2.5 3.0
b 0.00
a
~
The symbols a, b, c, and f are constants which have been empirically determined for the flame conditions listed in Table 11. The term T,,,is the flame temperature of a C2H2/02 flame of given composition a t the tip of the inner cone. The terms x and 6 have the same meaning as given above, and log, is the natural logarithm of the term in brackets. To determine a flame temperature of a given Hn/02 or C2H2/02 flame the ,constants are evaluated from Table I or 11, respectively, the value of h is approximated for the flame in concern, the value of 6 is found from the data given in a previous paper (IO), and x is calculated from the following equation which can be readily derived from a dimensional analysis. The solution flow
rate,
and the oxygen flow rate,
Q~,,are in cc. per minute. The flames
studied and listed in Tables I and I1 were considered to be the most important flames used when employing total consumption atomizer-burners in analytical emission or absorption flame photometry. I n the event an analyst wishes to know the temperature of a flame whose composition is not listed in Tables I or 11, an approximate flame
temperature can still be found as long as the ratio of 02 to fuel is in the ranges listed in Table I (0.2 to 0.6) or in Table I1 (1.0 to 3.0). I n this case, the value of 6 is estimated from data previously given (IO),and Ttipsa, b, c, and f (for CJ&/02 flames) are estimated from data in Tables I or 11. Because the values of 6, a, b, c, f,and TSipdo not differ greatly even when the ratio of O2 to fuel is varied, estimates of flame temperatures with mean errors less than 100" K. should be readily possible. The authors have had excellent success with such approximations on flames other than those listed in Tables I and
11. The denominator of the above empirical equation for a Hz/02 flame is quite similar to the denominator of the theoretically derived equation given by Baker and Vallee (2) for a stoichiometric H2/Oz flame. The denominator of the empirical equation for a C~H2/02 flame is similar to the denominator of the theoretically derived equation given by Baker and Vallee (2) for a stoichiometric (Ci\')2/Ozflame. The numerator of the empirically derived expressions differs from that predicted by Broida and Lalos (6). They found that the flame temperature of a stoichiometric H 2 / 0 2 flame and of a stoichiometric C2H2/O2flame decreased linearly with height above the inner cone. They used a slit 1 mm. high by 0.01 mm. wide and so measured an adiabatic temperat u r e 4 . e . . the flame temperature over the height and width of thc slit was approximately a constant value. In this paper, a baffle 5 mm. high by 5 mm. wide was used to approximate actual flame photometric conditions. When using such a baffle, the flame temperature within the baffle will certainly vary both vertically and laterally as pointed out by Broida and Lalos ( 5 ) , and so an average flame temperature rather than a true adiabatic temperature is measured. The use of an outer flame sheath would minimize the lateral variation and to some extent the vertical. However, most total consumption burners are used for analytical purposes without an outer flame shield, and so the measurements in this paper were made without the use of one. It was empirically determined that the average flame tcmperature measured by the two-line method dropped with the square of the height above the inner cone. The use of an outer flame shield and/or the use of a much smaller baffle probably would result in the linear decrease of temperature with height above the inner cone. I n most H 2 / 0 2 flames used the inner cone tip was about 0.5 cm. above the burner tip, and in most C~H2/02flames used the inner cone tip was about 1 cm. above the burner tip. The values of the tip tcmperaturr,
T,,,, were found by extrapolation t o zero height. The values found compare quite favorably with similar values given by Broida and Lalos (6). The empirical equations gave accurate values of temperature when using small, medium, or 1:trge bore capillary atomizers. The differences in flame temperature resulting from different size atomizers can be attributed primarily to the difference in efficiency of introduction of sample into the flame-i.e., differences in the dispersion of droplets and rates of evaporation of water. The use of the 6 factor seems to fully account for these processes. -4 number of interesting conclusions result from observation of the empirical equations for the fla ne temperatures of Hz/Oz and C2H2/02 flames. I n all cases the flame temperature decreased with solution flow rehe. However, this decrease is not nearly so rapid as theoretically predicted by Baker and Vallee ( 2 ) . This is a result of several factors. I n the relationship derived by Baker and Vallee it was assumed that sample introduction was 100% efficient which is not experimentally valid. I n addition, in this pap:r an average flame temperature is measured rather than a single flame temperature of a small portion of gases. These factors would tend to moderate the change in temperature with flow rate of solution. The variation of flame temperature over the region of measurement results in a system which deviates slig’itly from being a true adiabatic system. However, the
number of excited atoms to the number of ground state atoms per cc. of flame gases in each portion of the flame as well as in the region of observation can still be represented by a Boltzmann factor whose value is determined by a local temperature in the former and by an average temperature in the latter. From the empirical equations, it can be shown that in most cases the flame flames decrease by temperature of H2/02 about 100” K. when the solution flow rate is increased from 0.5 to 3.0 cc. per. minute. A similar change in solution flow rate for an CzHJ02 flame gives a decrease of about 200” K. I t should be pointed out that the temperature above the inner cone of fuel rich flames is essentially the same as for stoichiometric flames (see Tables I and 11). This was also observed by Broida and Lalos (6). This phenomenon is probably a result of entrainment of air from the atmosphere. Although few measurements as a function of solution flow rate have been made using total consumption atomizerburners, the flame temperatures measured by the twc-line method in this paper compare favorably with flame temperatures measured by other methods (line reversal and rotational methods). Insufficient data are usually given in other papers to allow direct comparison of flame temperatures with ones calculated from the above equations. Using the empirical equations given in this paper approximate electronic
temperatures of almost any Ht/Oz or C2H2/02flame can be determined. This should allow the analyst the opportunity of selecting any desired flame temperature and flame condition. The selection of flame temperature is particularly important when ionization and compound formation are important processes in the flame. LITERATURE CITED
(1) Alkemade, C. T. J., International
Conference on Spectroscopy, College Park, Md., June 1962. ( 2 ) Baker, M. R., Vallee, B. L., ANAL. CHEM.31, 2037 (1959). (3) Bockris, J. O’M., White, J. L., MacKenzie, J. D., “Physiochemical ,-urements a t High Temperatures, Butterworths, London, 1959. (4) Broida, H. P., “Temperature,. Its Measurement and Control in Science and Industry,” H. C. Wolfe, ed., Vol. 11, pp. 265-85, Reinhold, New York, 1955. (5) Broida, H. P., Lalos, G. T., J. Chem. Phys. 20, 1466 (1952). (6) Broida, H. P., Shuler, K. E., Ibid., 27, 933 (1957). ( 7 ) Crosswhite, H. M., “The Spectrum of Iron I,” Johns Hopkins Spectroscopic Rept. No. 13, August 1958. (8) Gaydon, A. G. Wolfhard, H: G., “Flames, Their gtructure, Radiation and Temperature,” pp. 234-301, Chapman and Hall, Ltd., London, 1960. (9) Walsh, A,, Spectrochim. Acta 7, 108 (1955). (10). Winefordner, J. D., Mansfield, C. T., Vickers, T. J., ANAL. CHEM.35, 1610 (1963). RECEIVEDfor review January 9, 1963. Financial Accepted August 1, 1963. support from the h’ational Science Foundation (NSF-G19754) is gratefully acknowledged.
Spectrophotometric Determination of Uranium with 4- (2- ~yri(=Iy Ia z 0)resorcinoI T. M. FLORENCE and YVONNE FARRAR Australian Atomic Eiiergy Commission Research Establishment, lucas Heights, N.S. W., Australia
b A highly sensitiive and selective method is described for the spectrophotometric determination of uranium. The chromogenic reagent is 4-(2pyridy1azo)resorcinol (PAR), and the determination is carried out at pH 8.0 in the presence of a mixed complexing solution containing (1,2-cycIohexylenedinitri1o)tetraacetic acid, SUIfosalicylate, and fluoride. Most metals, including thorium, are masked by this complexing solution. Uranium forms a 1 : 1 complex with PAR, which has a molar absorptivity of 38,700 at 530 mp. A preliminary separation of uranium from gross amounts of impurities may be accomplished by a rapid chelating-re!;in ion exchange procedure.
A
reagents have been proposed for the spectrophotometric determination of uranium, none of the methods a t present available is completely satisfactory. Many of the more sensitive chromogenic agents such as dibenzoylmethane, oxine, and arsenazo are nonselective, while thiocyanate and hydrogen peroxide, which are quite selective, are relatively insensitive (9). Thorium, in particular, is a serious interference in most of these methods. The pyridine-azo dyes l-(2-pyridy1azo)-%naphthol (PAN) (2, 7) and 4-(2-pyridylazo)resorcinol (PAR) (1, 8) are among the most sensitive uranium reagents yet investigated. The molar absorptivity of the PAN-uranyl comLTHOUOH NUMEROUS
plex is 23,000 (560 mp) and that of the PAR complex, 39,000 (530 mp). A definite advantage of PAR is that both the free dye and the uranyl complex are water-soluble. Cheng has shown recently (3) that the selectivity of both these reagents toward uranium can be greatly improved by the use of (1,2cyclohexylenedinitri1o)tetraacetic acid (CyDTA) as masking agent. This paper presents a detailed investigation of the use of PAR for the spectrophotometric determination of uranium. Full sensitivity of the reagent has been retained, but the method has been made highly selective by including a mixed complexing solution containing CyDTA, fluoride, and sulfosalicylate. An important advantage is that a 50VOL 35, NO. 11, OCTOBER 1963
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