Distribution of Atoms in an Atomic Absorption Flame C. 5. R A N N and A. N. HAMBLY Chemistry Department, School of General Studies, Australian National University, Canberra, A. C. T., Australio
A study has been made of the distribution of metallic atoms in their ground state when a salt solution i s sprayed into the flame of a burner with a IO-cm. slat such a s is used in analysis b y atomic absorption spectrometry. The distributions of OH radical and temperature within this flame have alsa been measured. The results suggest that atoms a r e released b y pyralysis of the salt in the hottest region of the flame, then removed b y secondary chemical reactions. The distributions indicate that in most cases the sensitivities of atomic absarption measurements can b e increased b y the use of "small-area" absarbance and of spectral saurces elongated in the vertical direction.
T
sensitivity of the atomic ahsorption technique is very dependent on the flame used to vaporize the solution. The chemical reactions within the flame are imperfectly understood; hence the optimum conditions needed to produce ground-state atoms within the flame are not obvious. Buell (2)and Gibson, Grossman, and Cooke (6) have described the distribution of emission spectra from solutions sprayed into the oxyhydrogen flame of a Beckman burner. Gibson el al. ( 6 ) have shown that the emission intensity is closely related to the ground-state concentration of the atom Concerned; hence these investigations are also useful to workers interested in absorption. Most emission measurements are made with small hnrners of circular cross section, whereas in the absorptionmethod slot burners giving a long path through the flame are favored. Gatehouse and Willis (4) have described a typical burner which is similar to the 10-em. burner used with the Perkin-Elmer and the Techtron atomic absorption units. The sensitivity of the absorption method is not dependent on a high flame temperature, so that acetylene, propane, and coal gas, together wit,h air, are the fuels most often used. This investigation is concerned with the distribution of ground-state atoms in the flame from a IO-cm. slot burner. The work was undertaken in an endeavor to improve the sensitivity of atomic ahsorption measurements and to help elucidate the mechanisms involved in the formation of atoms in such a flame. HE
entrance slit
Ea+ collimated beam
pin hole
flame
source
Figure 1. Optical arrangement for measurement of distribution of atoms in flame EXPERIMENTAL
Distribution of Ground-State Atoms. The burner and holder were manufactured by S. R. Skinner Pty., Ltd., Melbourne. The slot used for acetylene-air mixtures was 10 em. X 0.25-mm. An EEL (Evans Electro Selenium, Ltd., London) spray chamber, without the baffle plates, was situated below the burner. The unit holding the spray chamber had both vertical and horizontal movements which positioned the flame relative to the outic axis. Air at 30 p.s.i. was applied to the spray unit, the air flow being 9.25 liters per minute, The acetylene flow was varied from 1.0 to 1.7 liters per minute. The portion of the sprayed solution actually entering the flame did so at the rate of 0.5 cc. per minute. An E.M.I. 62565 phototube multiplier (E.M.I. Electronics, Inc., Middlesex, England) was used to measure the signal, which was then fed to a Hewlett-Packard, Model 302A, wave analyzer (Hewlett-
Packard Co., Palo Alto, Calif.). Output was registered on a Varian G-10 recorder. The monochromator was a Zeiss Type SPM. 1 with a silica prism. Two Ultrasil silica lenses (Heraeus, Hanau, Germany), matching the aperture of the monochromator, were used in the arrangement shown in Figure 1. The spectral sources (except that for selenium) were hollow-cathode lamps from Ransley Glass Instruments, Melbourne. For a selenium source a small electrodeless discharge tube was made in the laboratory and excited by a 15-Mc.per-second, 50O-watt, radio transmitter. The distribution of the ground-state atoms in the flame was measured by atomic absorption. Figure 1 shows an idealized arrangement of the experimental conditions. The light passing through the flame was not parallel, because of the finite width of the spectral source. This light was refracted by the flame, the extent of the refraction being dependent on the part of the flame through which th? light traveled-
Figure 2. Patterns resulting from refraction of collimated light in parsing along 1 0-cm. flame Left.
Rich Rome
Right.
Leon Rams
VOL. 37, NO. 7, JUNE 1965
a79
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0 .
0
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a Copper
c
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Rich
Lean
Lean b - Molybdenum
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0
E
0.
i U ._
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c-Magnesium
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d- Chromium
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Lean e- Calcium
880
ANALYTICAL CHEMISTRY
L
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f - Silver
Lean
i
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ECII
.4
A.
Rich
A
.A
g-Strontium
Lean
Rich
1
h-'B a ri u m
A.
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Rich
Lean
I-Sodlum Figures 3 and 4.
Rich
Lean
1- Selenium
Lean
Distributions of atoms in 10-cm. air-acetylene flame
Contours drawn at intervals of 0.1 absorbance unit, with maximum absorbance in center
e.g., light through the center of the flame remained undeflected. A refraction pattern (Figure 2 ) was formed in the plane of the pinhole aperture (1 mm. in diameter). This refraction pattern was scanned past the pinhole by movement of the flame, accomplished by horizontal and vertical transversing screws attached to the burner support. An absorbance reading was taken a t each 1 mm. of traverse. =It each point, distilled water and then the test solution were sprayed. The concentration of the test solution had been previously adjusted so that the positions of highest absorbance would have an absorbance of about 1.0. These distributions are shown in Figures 3 and 4.
RESULTS
AND DISCUSSION
A source of error in the absorbance estimation could be produced by light emitted from the excited atoms in the flame. This error, usually assumed to be negligible when modulated light and an a.c. amplifier are used, can often be appreciable. The distributions in Figure 3 were measured by mechanically chopping the incident light a t 300 C.P.S. and using an amplifier of +3-c.p.s. band width. This system virtually eliminated the emission signal from the flame, since the a x . component of the emission light from the flame, within this band width, is extremely small.
The use of a 1-mm. pinhole resulted in severe attenuation of the light from the spectral lamp. Only 1,400 of the light normally available was used. This led to a poor signal-noise ( S I N ) ratio for the measurement. However, the narrow band amplifier and further smoothing of the signal with capacitors enabled results, reproducible to about 0.004 absorbance unit, to be obtained. Distribution of OH Radical. This distribution was obtained by using an absorption technique similar in most respects t o that described b y Alkemade (1). A copper hollow cathode lamp, which was found to emit a strong OH band a t 3060 A. (presumably due to VOL. 37, NO. 7, JUNE 1965
881
Table I. Ratio of Peak Absorbances for Rich and Lean Flames Na Ag Cu Mo Mg Ba Cr Ca
Arieh Alean
0.80
1.00
0.93
1.13
water or organic solvent contamination) was used as the source. The monochromator was set at the band head, but wide slits (0.15 mm.) were used to reduce the spectral resolution, and thus minimize any complications due to a temperature difference between the source and the flame. A check on an unignited mixture of acetylene and air, together with a water spray, showed no appreciable absorption or scattering at this wavelength. There were two complications not found in atomic absorption. The OH content of the flame could not be controlled; spraying water into the flame, for instance, made little difference. The absorption was so great that the measured absorbance approached 2.0, thus limiting the accuracy of the measurement. Secondly, Io, the value of the light transmitted through the flame when there is no absorption due to OH radical, cannot be measured because OH radicals are always present. Because of refraction, the value of Iovaries from point to point in the cross section of the flame, but as this variation changes only slowly with wavelength, the copper line a t 3274 A. (which was not absorbed by OH radical) was used to obtain the relative magnitude of IO at the many points used in plotting the distribution. The value of I o was calculated as
where I’ values are measurements of the source with the flame off, and 13274 is the intensity of the copper line measured through the pinhole with the flame burning. Temperature Gradient. The temperature in the flame was measured by the sodium line reversal method. The image of a tungsten lamp (filament 1 x 1 mm.) was focused into the middle of the flame being investigated, This image and the center of the flame were then focused onto the entrance slit of the monochromator. A mechanical chopper was placed between the second lens and the slit, as an a.c. amplifier was used to record the signal from the multiplier phototube. When a concentrated solution of sodium chloride was sprayed into the flame and the monochromator was scanned across the sodium doublet a t 5890/5896 A , , the sodium line appeared 882
ANALYTICAL CHEMISTRY
1.08
1.41
1.00
1.37
Sr 1.20
either in emission or absorption. The current in the lamp was varied until no sodium line was apparent. At this temperature the emission and absorption lines were balanced and it was assumed that the lamp filament was a t the same temperature as that part of the flame being examined. The temperature of the lamp filament was estimated with an optical pyrometer (Leeds and Northrup), with the tungsten filament viewed through a 2.0 neutral density filter. The true temperature was then calculated from the apparent temperature given by the neutral density filter. No corrections were made for the light lost in the first lens, or for the red comparison field in the optical pyrometer, as these errors [Gaydon and Wolfhard (ti) ] almost cancel each other. In this method serious errors can occur if the aperture of acceptance a t the monochromator slit is not identical for the light from the flame and from the lamp. The extensive refraction of light within the 10-cm. length of the flame could cause serious errors and, therefore, the usual point by point determinations in the flame were not attempted. A vertical distribution was obtained by rotating the flame so that the optic axis was normal to the plane of the flame, with a light path of a few millimeters through the flame. Readings were taken at varying heights in the flame above the slot. Only light from an area of approximately 1-mm. diameter within the flame was effective in the temperature determination. Formation of Atoms in Flame. The results presented here are not inconsistent with the hypothesis t h a t the absorbing atom is formed in the flame by pyrolysis. The lifetime of the atom thus formed is then dependent on the environment through which it travels. The type of fuel and the air-fuel ratio are the main factors controlling this environment. The height of the area of maximum absorption in the flame will then depend on the rates of formation and depletion of the atoms. The distributions, shown in Figure 3, for the air-acetylene flame suggest that the same number of atoms is released into the flame regardless of the flame conditions. The peak absorbances in the distributions show little difference between the rich and lean flames. The ratios of these absorbances are shown in Table I. The distribution of molybdenum, shown in Figure 3, b, has a very
small area of high absorption in the lean flame. Table I shows a value of 1.13 for molybdenum; hence both the rich and the lean flames have approximately the same concentration of molybdenum atoms a t the position of the peak absorbance in the flame. David (3) and Gatehouse and Willis (4) report little or no sensitivity for molybdenum in a lean flame. A technique of “small-area” absorbance measurement is required to detect absorption in the lean flame. I n the rich flame the atoms last longer; hence there are more atoms in the whole flame. If a “large-area” absorbance technique is used, a rich flame will give the greatest sensitivity. An indication of the total number of atoms a t any particular height can be obtained by taking the line integral of the distribution a t that height [Gibson et al. (S)]. This approach suggests ai1 interpretation of the patterns in terms of the following two opposing phenomena which cause a variation with height of the number of atoms in the flame. There is an increase of the number of atoms, with height, until the volatilization is complete; there is a simultaneous decrease in the number of atoms due to loss of atoms by chemical combination together with a dilution due to the expanding volume of the products of combustion. The distributions of magnesium, silver, sodium, and copper show a high concentration of atoms in the upper parts of the flame; hence the chemical recombination reactions are relatively slow. Calcium, strontium, and barium show a marked depletion of atoms in the upper regions of the flame due to a more rapid recombination. Molybdenum in a lean flame has the most rapid recombination rate. Variation of the rate of release of atoms is best shown by comparing the magnesium distribution with that of the other elements. Line integrals show a progressive increase of magnesium atoms to a height of 1.5 cm. above the slot; then the number remains constant to the limit of measurement permitted by the apparatus. This type of distribution could be explained by a slow release of atoms, together with a very slow recombination rate. Variation of Temperature with Height in Flame. The variation of temperature with height is shown in Figure 4. This curve was obtained from a lean acetylene-air flame into which was sprayed a sodium chloride solution a t a rate identical with t h a t used during the experiments on the distribution of atoms in the flame. The maximum temperature is a t a height of 0.5 cm., which is the region of maximum atomic concentration for most distributions in the lean flame. If the variation of the number of atoms
’t \ 1600
1700
1800
T “c.
1900
2000
2100
Figure 5. Variation of temperature with height in flame
with height is dependent on a balance between the rate of volatilization of the solution and the formation of hydroxides or similar compounds, a maximum would be expected near thc hottest position in the flame. The zone ot maximum temperature in the case of the rich flame was higher and more diffuse. Typical distributions of temperature with height are shown for rich and lean flames by Gibson, Grossman, and Cooke (6). Distribution of OH Radical. The distribution of the O H radical, shown in Figure 5 , indicates that this radical is present a t high concentration a t all positions in the flame. Buell ( 2 ) , using an oxyhydrogen flame, noted that emission bands of the metallic hydroxides appeared in the higher regions of the flame. Emission measurements were not performed with the 10-em. burner, but the results oi Buell @), together with the OH distribution, suggest that reaction with the OH radical could reduce the number of atoms in the upper part of the flame. Oxide formation could also be important in this region, especially with lean flames. The rate of volatilization and release of atoms could depend on the anions present in the solution. Copper chloride and copper sulfate solutions gave identical distributions, as did selenium dissolved in NaCN, ?;anSOa, and a mixture of H S 0 3and HzS04. It is felt that the anion, in general, does not affect the distribution of the atoms in the flame. Systems forming refractory compounds were not tested. The results of Gibson et al. (6) indicate that the distribution as well as the sensitivity is altered in such cases. For a few elements which were investigated with a propane.air flame there was general agreement with the distributions found for the acetylene-air flame. The propane-air distributions are not easily correlated with the acetylene-air distributions because propane required a burner with a much nider slot. This burner gave a low velocity, fluctuating flame which resulted in a much wider distribution and less precise absorbance measurements.
Small-Area Absorbance. Most of the distributions show a small region of high absorbance. ,4n analyst using the common optical arrangement of a wide beam of light passing through the flame would average the absorbance over the cross section of the light beam and thus obtain an absorbance value somewhat less than the maximum absorbance in the flame. This type of measurement is referred to as large-area absorbance whereas absorbance measured with the pinhole is referred to as small-area absorbance. The distributions also explain why increasing the path length by multiplepass techniques often leads to less than the expected increase in sensitivity. As most multiple-pass arrangements use relatively wide beams of light, most of which pass through areas of low absorbance, the optics are inefficient and the improvement in sensitivity is not proportional to the number of passes. An efficient multiple-pass technique would use a light beam of small cross section. Such systems are difficult to design without an excessive loss of light from the spectral source. When the distributions shown in Figure 3 were being studied, measurements were also made of the large-area absorbance, by adjusting the burner to the position where the small-area absorbance was a maximum, then recording the absorbance of the flame with the pinhole removed. This gave an extreme value of large-area absorbance, as the collimated light beam passing through the flame was 1 inch in diameter-a wider beam than that used by most workers, If the atoms in a flame are distributed so that there are no steep gradients of concentration, the large-area absorbance is almost as high as the small-area absorbance. If, however, the atoms are concentrated in a small area of the flame, these absorbances differ markedly. iln indication of the improvement that can be expected from using a small-area technique is shown in Table 11. The figures shown in this table are the ratio of small-area absorbance-large-area absorbance; thus a large number indicates that small-area techniques should be used for the element concerned, if better sensitivity is required. There are very few flames in which the large-area absorbance is similar to the small-area absorbance. It would seem, therefore, that there is little to be lost, and much to be gained, by using small-area techniques, Unfortunately, this cannot be advocated unreservedly, because of the serious loss of light entailed in isolating the narrow beam, The use of high intensity spectral lamps, which do not show self-reversal of the resonance lines, would allow this technique to be used more fully.
E
V
Figure 6. Distribution of OH radical in flame Contours representing equal concentration of OH radical are drawn every one-third absorbance unit. Maximum absorbance of 2.0 near reaction zone
The Shape of Spectral Source. The hollow cathode tubes normally used as spectral sources have cathodes with a circular cross section. The distribution of atoms in the flame, and the entrance slit t o the monochromator, are both elongated in the vertical direction. It is suggested that an elongated spectral source should be used for the following reasons. For cathodes of a given cross-sectional area more light will enter the monochromator if the image focused on the slit is elongated in the vertical direction. Also, a typical distribution of atoms in the flame shows a steep gradient of density on the horizontal direction but a low gradient in the vertical direction. If large-area absorbance is to be used, greater sensitivity will be obtained by concentrating the light in a vertical direction. If the cross section of the light beam is very wide, less of the light beam will pass through the areas of high density in the flame, and sensitivity will be lost.
Table II. Values of Small-Area Absorbance-Large-Area Absorbance
Element Copper Molybdenum Xagnesium Chromium Calcium Silver Strontium Barium Sodium
Rich flame
Lean flame
1.7
1.7 14.6 4.0 7.4
6.4 1.9
6.5 3.1
1.6
3.4 2.9 1.3
VOL. 37, NO. 7,JUNE 1965
4.2
2.3 6.0 6.7 1.8
0
883
This effect was tested by constructing a copper hollow cathode lamp containing an elongated cathode. This cathode, which was machined from a block of copper without using lubricating fluid, had the same cross-sectional area as the circular hollow cathodes, but the height had been made five times greater than the width. Measurements of the absorption Of ‘OWer atoms in flame with the cathode oriented vertically
or horizontally confirmed the above suggestions. LITERATURE CITED
(1) Alkemade, C. T. J., “Proceedings of Xth Colloquium SpectroscopicumInternationale,” p. 143, Spartan Books, Washington, 1963. (2) Buell, B. E., ANAL. CHEJI. 35, 372 (1963). (3) David, D. J., Nature 187, 1109 (1960). (4) Gatehouse, B. M., Willis, J. B., Spectrochim. Acta 17, 710 (1961).
( 5 ) Gaydon, A. G., Wolfhard, H. G.,
“Flames, Their Structure, Radiation and Temperature,” Chapman & Hall, London, 1960. (6) Gibson, J. H., Grossman, W. E. L., (1963). Cooke, W. D., AXAL. CHEX.35, 266 RECEIVED for review November 3, 1964. Accepted January 25, 1965. Support received by one of us (C.S.R.) from an Australian National University PostGraduate Scholarship, during the tenure of which this investigation was made, is gratefully acknowledged.
Heats and Entropies of Formation of Metal Chelates of Polyamine and Polyaminocarboxylate Ligands DONALD L. WRIGHTtl JAMES H. HOLLOWAYt2 and CHARLES N. REILLEY Department of Chemistry, University of North Carolina, Chapel Hill, N. C. The heats of reaction of diethylenetriaminepentaacetic acid, trans-cyclohexanediaminetetraacetic acid, Nhydroxyethylethylenediaminetria ce t i c acid, ethyletherdiaminetetraacetic acid, and ethyleneglycol-(bis-p-amino ethyl ether) N,N’-tetraacetic acid with Mg+2, Ca+2, Srf2, Ba+2, Mn+2, Fef2, CO’~, Ni’z, CU+~,Zn12, Cd+Z, Hg+2, and Pb+2 and triethylenetetramine and tetraethylenepentamine with Nif2, Cu”, Zn+: and Cd+2 have been determined calorimetrically at 25’ C. in a salt medium of 0.1M KN03. The heat of reaction of (ethylenedinitril0)tetraacetic acid with Fe+2 was also measured. The entropies of reaction have been calculated from these heats of reaction and stability constants previously reported in the literature. The results were interpreted on the basis of several effects including changes in the hydration of the various species, chelate ring strain, number and nature of the bonding donor groups, and characteristic properties of the metal ions.
T
HE PROPERTIES AND APPLICATIONS
of metal chelates of the analytically important polyaminocarboxylic acids and polyamines have been extensively investigated in recent years. Yet the only thermodynamic data generally available are the free energies of reaction (AF)-usually expressed as stability constants. The heats of reaction ( A H ) and the entropies of reaction ( A S ) have received little attention primarily because the applicabilities of most chelometric methods, such as visual, potentiometric, and photometric titrations, are sufficiently described by the stability constants. The values of AH and A S are of interest, however, 884
ANALYTICAL CHEMISTRY
because they suggest certain features concerning the nature of the reacting species and products which are of importance in understanding the relationship between structure and the net reaction tendency and in selecting reagents and conditions for thermometric titrimetry. In this work AH was determined calorimetrically for the reaction of diethylenetriaminepentaacetic acid trans-cyclohexanediamine(DTPA), tetraacetic acid (CyDTA), AT-hydroxyethylethylenediaminetriacetic acid (HEDTA), ethgletherdiaminetetraacetic acid (EEDTA), and ethyleneglycol - (bis - 0 - aminoethyl ether)N,N’-tetraacetic acid (EGTA) with representative alkaline earth, transition, and heavy metals: Mg+2, Ca+2, Sr+2, Ba+2, Mn+2, Fe+2, C O + ~Ni+2, , CU+~, Zn+2, Cd+2, Hg+2, and Pbf2. The heats of reaction of triethylenetetramine (trien) and tetraethylenepentamine (tetren) with Ni+2, C U + ~Zn+2, , and Cd+2 were also measured. Subsequent to our work with the polyamines (16) the heats of reaction for Ni+2, C U + ~ , Zn+2 were reported (13) and were in good agreement with the values obtained in this laboratory. The heat of reaction for (ethy1enedinitrilo)tetraacetic acid (EDTA) was determined for Fef2, which had not been previously reported; heats of reaction for this chelating agent were redetermined for Ba+2, Ni+2,C U + ~Zn+2, , Cd+2,and Hg+2 in order t o check our experimental results with those of other workers ( 1 , 7 , 9). The A S values for the various chelate reactions were then calculated from the experimental AH values and the corresponding stability constants previously reported in the literature (3,5,6,8, 14, 17, 27-30, 33).
EXPERIMENTAL
CHELATIKG AGENTS. The acid forms of DTPA, CyDTA, HEDTA, EGTA, and EEDTA were used as obtained from Geigy Chemical Corp. Purity was checked by titration with metals and with KOH. Subsequent nuclear magnetic resonance studies did not ind cate the presence of detectable impurities. H4EDTA was prepared from reagent grade NazHzEDTh 2H20 using a modified version of the procedure of Charles (9). The trien solution was prepared from reagent grade trien 2H2S04 obtained from J. T. Baker Chemical Co. The sulfate salt of tetren was prepared from the crude amine (Union Carbide Chemicals Co.) by a previously described method (25). The polyaminocarboxylic acids were dissolved in reagent grade POtassium hydroxide solution. Excess potassium hydroxide, approximately 5% more than was necessary for complete neutralization of the chelating agents, was added to prevent hydrolysis of the chelon. The preparation of the polyamine solutions was identical to that for the polyaminocarboxylic acids except that reagent grade 50% sodium hydroxide was used. All of the solutions were 0.671-l4 in chelon except for those of tetren, which were 0.514M, and some of the EDTA and EGTA solutions, which were 0.964M. Both the polyaminocarbovylate and polyamine solutions were 0.1Min KNOa. METALIONSOLUTIONS.All metal ion solutions, except Fe+2, were prepared from reagent grade metal nitrates. The Fe+2 was freshly prepared each time from reagent grade Fe(NH&Reagents.
1 Present address, Rleasurements and Control Laboratory, Research Triangle Institute, Durham, N. C. 2 Present address, J’allecitos iltomic Laboratory, General Electric Co., Pleasanton, Calif.
