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Tin atomization in hydrogen supported flames
Bpectrochimica Acta, Vol.29B.pp.263to268. Pergamon Press 1974.hintedInNorthern Ireland
Tin atomization in hydrogen supported flames* I. RUBESKA Geological Survey of Czechoslovakia,Kostelni 26, 17000 Prague 7, Czechoslovakia (Received7 March 1974) Abstract-Relative atomization efficienciesof tin in the acetylene-air, hydrogen-air and the argon-hydrogen diffusion flames using the same nebulizer-burnersystem were estimated from peak absorption measurements. The results indicate that the argon-hydrogen diffusion flame despite its low temperature (1130 K) is the most efficient tin atomizer. This fact may be explained by assuming that free tin atoms are formed through reactions of tin oxide with free hydrogen atoms. Some additional supporting, though indirect evidence for this hypothesis, is also given. INTRODUCTION IT IS WELL known that tin exhibits higher sensitivity in hydrogen supported flames than in the hotter acetyleneair flame [l]. This fact has been commented on by WALSH [2] and referred to as ‘the tin mystery’ by SLAVIN[3]. DE GAUN and SAMAEY [4] when estimating atomization efficiencies of a number of elements from total absorption measurements in different premixed flames found an almost eight times higher tin atomization in the hydrogen-air, than in the acetylene-air flame [4]. What is even more intriguing is that in hydrogen supported flames tin atomization seems to increase with decreasing temperature. Thus with a direct injection burner higher sensitivity was found in the hydrogen-air flame than in the hydrogen-oxygen flame [6] and in premixed flames in the argon-hydrogen diffusion flame than in the hotter hydrogen-air flame [6,7], L’vov in a recent paper [8] suggested that the higher sensitivity of tin in the hydrogen-air flame reported by CAPACHO-DEMADO and MANNING [l] was due to the depletion of the ground level of tin *P, at higher temperatures due to the population of the two energy levels *P, and *P, (see Fig. 1). In order to check this assertion we have estimated the relative values of atomization efficiencies of tin in the acetylene-air, hydrogen-air and argon-hydrogen diffusion flames from peak absorption values of three tin lines corresponding to transitions from the three ground term energy levels of tin.
Relative atomization
e,@iency from peak absorption
When only relative values of atomization efficiencies of one element in Merent flames using the same nebulizer and burner system are estimated, a number of * Presented at the 17th Colloquium SpectroscopicumInternationsle. Florenoe, 1973. [l] [2] [3] [4] [5] [6] [7] [8]
L. CAPACHO-DELQADO and D. C. wu, Spedvchim. Acta aS, 1606 (1960). A. W-H, Appl. Opt& 7, 1259 (1968). W. SLAV-IN, Atomic Absorption Specko.9copy. p. 176 Wiley, New York (1967). L. DE GALANand G. F. SAMAEY,Spectrochim. Acta 25B, 246 (1970). J. H. GIBSON, W. E. L. GROSSMAN and W. D. COOKE, Anal. Chem. 35,266 (1963). H. L. KAJXNand J. E. S&ALLIS, Atomic AbsorptionNewaktter 7, 6 (1968). I. RUEE&A and M. MIK~~OVSKY,Atomic Abso-rptiolaNewdetter 11,67 (1972). B. V. L’vov, J. Quad. Spectry. Radiat. Transfer 12, 661 (1972).
2
263
264
lV 6
4-
3-
l-
l-
oFig. 1. Term diagram of tin. The lines 2863,2706 and 2840 were mwmred.
factors may be assumed equal. These are the nebulization efficiency, the fraction desolvated and for volatile compounds even the fraction volatilized. Different element concentrations in the flame gsses may result only from different flame gas volumes at the:psrticular flame temperature, and the following relation holds: %0&t
= IV
(1)
where qOt is the total concentration of the element (cm-a) and V the flame volume (cm8 8-l). If the emission line width from the hollow cathode is negligible compared to the absorption line width, ‘peak absorption’ is measured and the density of free atoms at level i is related to this peak absorbance by equation 4(&J
= (~+n4f,n,I~(&)
(2)
where A,(&,) is the peak absorbance, fi the oscillator strength, 12,the density of free atoms at level i, 2 the absorption path length, P(A,,) the peak value of the profile function, and e, m and c have their usual meaning. When more than one level is populated, the particular peak absorbancea must be taken into account. If the hyperfine structure (h.f.s.) of the psrticular lines may be neglected and the aparameter of the lines is assumed equal, the profile functions will be equal as well and the free atom density may be derived by summing up the peak absorbancea of lines originrtting from all the populated levels
266
Tin atomization in hydrogen supported flames
If we wish to compare free atom densities in different flames, the temperature dependence of the peak value of the protie function must be included. Beoause the h.f.s. has been neglected, the line pro& may be described by the Voigt function which depends on the mutual contribution of collisional and Doppler broadening. With growing temperature the Doppler line width will increase with Tlla. The temperature dependence of collision broadening is more difficult to assess, its component due to adiabatic collisions varying with T-l/6 [9]. For quenching collisions, data are scarce but, in general, a slight decrease with temperature may be expected rather than an increase. Let us therefore assume the unfavourable conditions that only Doppler broadening contributes to the temperature effect. Then the peak value of the profile function will be inversely proportional to the square root of the temperature.
(4) Here dashed symbols refer to the second flame. Substituting from equations (l), (3) and (4) into the expression for the ratio of atomization efficiencies (E) in two different flames we derive the following expression :
(6) EXPERIMENTAL All measurements were performed on a Perkin-Elmer model 306 atomic absorption spectrometer. Solutions of tin (100 ,ug/ml) in a 1:20 hydrochloric acid solution were sprayed and absorbances of three tin lines Sn 2863, Sn 2706 and Sn 2840 A were recorded. The flame conditions are given in Table 1. Table 1. Flame conditions Gas flows
(ew Flame C,H,air H&r AI-H,
Fuel 4.3 24.4 13.6
Oxidant di<
Volume after combustion mm
Observation height (=I
168 186 126
6 6 10
16 14 20
2340 1090 1130
The flame gas volumes were estimated including molar changes by the combustion reactions C&H, + 20, + 8Np +J 2CO + H, + SN, and and temperature expansion, but neglecting entrainment and Fusion from the surrounding atmosphere. No combustion reaction was therefore assumed for the argon-hydrogen diffusion flame. [Q] W. BEEMENBURQ,J. Quant.Spectry.
Radkzt.
Transfer
4, 177 (1964).
266
I. RUBE~KA
The temperatures reported in Table 1 were calculated from the peak absorbances of the two tin lines Sn 2863 and 2840 A according to the method of L’vov [8]. The value found depends both on the free atom distribution and the temperature distribution. L’vov calls it ‘effective’ temperature whereas REIF calls it ‘apparent’ temperature [lo]. However, the low values found for the three flames reflect mainly the fuel-rich reducing conditions used. The heights of observation were deliberately chosen equal for the acetyleneair and hydrogen-air flames. For the argon-hydrogen diffusion flame the observation height chosen was double that of the other flames to allow for the diffusion and combustion reactions to start. Absorbances of the three tin lines in the different flames are given in Table 2. Table 2. Absorbance8 of tin lines in the three flames
Line
Lower
(A) 2863 2706 2840
BP,, SP, SP,
state (cm-l)
frel
0 1692 3428
100 58 98
A C,H&r 0.115 0.081 0.0686
A Hz-air
A Ar-H,
0.169 0.075 0.0447
0.553 0.12 0.0346
Relative atomization efficiency %,/“CnH*
= 1.05
EArE,h~ = 1*2g EA&"C&lp = l-35
The relative atomization efficiencies were calculated according to equation (5). The results show clearly that the argon-hydrogen diffusion flame has the highest atomization ehlciency despite the fact that in this flame tin may not be completely volatilized. The vapour pressure of SnO reaches 1 atm at about 1650K [ll], whereas the flame temperature at the observation height is only 1130K. Atomization efficiencies in the hydrogen-air and acetylene-air flames are almost equal. However, keeping in mind that the temperature effect on the peak values of the profile functions was overestimated, the former should still be better as a tin atomizer. These results confirm that the higher tin sensitivity in hydrogen supported flames is not solely due to the higher population of the ground level in the cooler flames. DISCUSSIONOF THE RESULTS BULEW~Zand PADLEYhave recently shown that tin catalyses radical recombination in hydrogen supported flames [12,13]. If we assume that the reactions involved produce at some stage free tin atoms, this would explain the high atomization et&iency in the cool argon-hydrogen diffusion flame. It is known that the concentration of free radicals produced in hydrogen supported flames does not decrease with temperature below some 2000 K, due to the rapidly branching radical producing reactions between hydrogen and oxygen [14,15]. The catalytic effect of tin as found by Bulewicz and Padley is temperature independent whereas the catalytic effects of all other metals (Cr, U alkali earth metals) or [lo]
I. REIF,~. A. FASSEL and R. N. KNISELEY,&J~C~POC~~WL Acta 28B, 105 (1973). [ll] J.C. PLATTEEUW and G. MEYER, T~ane.FaradaySoc.52, 1066 (1956). [12] E. M. BIJLEWICZand P. J. PADLEY, Trans.Faraday Sot. 67, 2337 (1971). [13] E. M. BIJLE~ICZand P. J. PADLEY, Proc. 13th Int. Symp. Combustion, p. 73. Printed in the U.S. by the Combustion Institute, Pittsburg in 1971. Salt Lake City (1970). [14] E. M. BTJLEWICZ, C. G. JAMES and T. M. SUQDEN, Proc. Roy.Soc. A255, 89 (1956). [15] C. P. FENIMORE: Chemistry in Premixed Flames, p. 40. Pergamon Press, Oxford (1964).
Tin atomization in hydrogen supported flames
267
compounds (SO,, NO) decrease with decreasing temperature. Thus in the cool argon-hydrogen diffusion flame, all competing radical recombining reactions are suppressed, leaving tin as the most prominent catalyst. On the other hand, combustion of hydrogen with nitrous oxide does not involve rapidly branching reactions and the concentration of radicals produced in the reaction zone is lower [15]. Correspondingly, at the high temperature of the nitrous oxide-hydrogen flame no concentration of free radicals above equilibrium may be expected. The assumption that tin atomization is related to free hydrogen atoms is supported by interferences on tin observed in different flames under different conditions. In the acetylene+air flame, tin atomization may be assumed to be controlled mainly by the oxygen concentration in the flame gases and the temperature. Under these conditions interferences are relatively small. Numerous puzzling interferences have been observed in hydrogen-air flames where free hydrogen atoms play an important role according to the proposed assumption. Thus the strong depressive effects of sulphuric and nitric acids [16,17] may be due to the removal of excess hydrogen atoms by the catalytic effects of SO2 and/or NO on radical recombination. Further supporting evidence is seen in observations on tin absorption in long path absorption tubes [18]. Using a hydrogen-air flame with a direct injection burner the depressive effects of several elements in equal molar concentrations (0.005 M) were found to be in reasonable correlation with their reported catalytic efficiency [13] as seen in Fig. 2. The interferences of nonvolatile elements, e.g. platinum, exhibit strong memory effects, i.e. the depression takes place even after stopping aspiration of the interferent. The only explanation so far found is that platinum deposited on the tube walls catalyses radical recombination, the hydrogen atom concentrations in the gases fall and so do the fractions of tin atomized. Several published papers deal with interferences on tin in the argon-hydrogen and nitrogenhydrogen diffusion flames [7,19,20-J. Depressive as well as enhancing effects were observed. However, using a twin nebulizer we have found that both depression and enhancement were solute volatilization interference. The enhancement was caused most probably by augmented heat transfer through radical recombination on the particle surfaces resulting in accelerated volatilization [il. In inert gas diluted hydrogen-oxygen flames particle temperatures several hundred degrees above the temperature of the ambient flame gases have indeed been observed [Zl]. CONCLUSIONS The assumption that tin atomization is linked with excess hydrogen radicals thus fits well all experimental evidence. The reaction mechanism is, however, far from [ 16) B. J. HEFBERNAN,R. 0. ARCHBOLDand T. J. VICKJZRS, Proc. Amt. Imst.Mila. Met. No 223, 65 (1967). [17] B. MOLDAN,I. RWESKA, M. MIK&OVSKYand M. HUKA, An&. Chim. Actu 63,91 (1970). [18] I. RUBEBEA, paper presented at the 4th International Conference on Atomic Spectroscopy, Toronto (1973). [ 191 R. F. BROWNER,R. M. DAGNALL and T. S. WEST, Anal. Chim. Acta 46, 207 (1969). A, M. M~MORI and S. MTJSHA, Anal. Chim. Acta 82, 267 (1972). [20] T. NAKAHAR [Zl] E. M. BULEWICZ and P. J. PADLEY, Proc. Roy.Soc. Lendon A323, 377 (1971).
I. RUBEI&A
3OC’
I-’
OS’ M 0Fe &hi 4 Oco NiMO I 10 20
I 30
I 40
1 50
% depression Fig. 2. Correlation between catalytic efficiency on radical recombination and tin depressionin long path absorption tubes. Tin: 2 pg/ml. All interfereuts in 0.006 M solutions, added aa chlorides, only MO added as ammonium molybdate.
clear. BTJLEWICZ and PADLEY [13] were able to show, that with a high degree of probability, the Crst step of the catalytical mechanism of tin involves formation of tin hydroxide in an excited state SnO + H + SnOH* For the next step, they sssume a scheme which, according to conclusions based on kinetic considerations, is “the only scheme so far found to fit all essential features of the catalytic effect.” SnOH* (in state A) crosses to SnOH* (state B) and this then reacts with another hydrogen radical SnOH*(B) + H + SnO + H, Although this may be the prevailing reaction there might be a simultaneous reaction forming free tin atoms as sn intermediate product SnOH* +H+Sn
+H,O+SnO
+H,
The actual reaction path should depend on the probability of the formation of two different activated complexes [SnOH..
.H]*
or
ii: 1
SnOH z [
Unfortunately, &u&l knowledge at present available does not allow even a rough estimate of the particular heats of formation. Aokwx&dgeme~~ are due to Dr. Z. Herman from the Institute of Physical chemistry and Electrochemistry of the Czechoslovak Academy of Sciencea for several enlightening discussions.
Tin atomization in hydrogen supported flames* I. RUBESKA Geological Survey of Czechoslovakia,Kostelni 26, 17000 Prague 7, Czechoslovakia (Received7 March 1974) Abstract-Relative atomization efficienciesof tin in the acetylene-air, hydrogen-air and the argon-hydrogen diffusion flames using the same nebulizer-burnersystem were estimated from peak absorption measurements. The results indicate that the argon-hydrogen diffusion flame despite its low temperature (1130 K) is the most efficient tin atomizer. This fact may be explained by assuming that free tin atoms are formed through reactions of tin oxide with free hydrogen atoms. Some additional supporting, though indirect evidence for this hypothesis, is also given. INTRODUCTION IT IS WELL known that tin exhibits higher sensitivity in hydrogen supported flames than in the hotter acetyleneair flame [l]. This fact has been commented on by WALSH [2] and referred to as ‘the tin mystery’ by SLAVIN[3]. DE GAUN and SAMAEY [4] when estimating atomization efficiencies of a number of elements from total absorption measurements in different premixed flames found an almost eight times higher tin atomization in the hydrogen-air, than in the acetylene-air flame [4]. What is even more intriguing is that in hydrogen supported flames tin atomization seems to increase with decreasing temperature. Thus with a direct injection burner higher sensitivity was found in the hydrogen-air flame than in the hydrogen-oxygen flame [6] and in premixed flames in the argon-hydrogen diffusion flame than in the hotter hydrogen-air flame [6,7], L’vov in a recent paper [8] suggested that the higher sensitivity of tin in the hydrogen-air flame reported by CAPACHO-DEMADO and MANNING [l] was due to the depletion of the ground level of tin *P, at higher temperatures due to the population of the two energy levels *P, and *P, (see Fig. 1). In order to check this assertion we have estimated the relative values of atomization efficiencies of tin in the acetylene-air, hydrogen-air and argon-hydrogen diffusion flames from peak absorption values of three tin lines corresponding to transitions from the three ground term energy levels of tin.
Relative atomization
e,@iency from peak absorption
When only relative values of atomization efficiencies of one element in Merent flames using the same nebulizer and burner system are estimated, a number of * Presented at the 17th Colloquium SpectroscopicumInternationsle. Florenoe, 1973. [l] [2] [3] [4] [5] [6] [7] [8]
L. CAPACHO-DELQADO and D. C. wu, Spedvchim. Acta aS, 1606 (1960). A. W-H, Appl. Opt& 7, 1259 (1968). W. SLAV-IN, Atomic Absorption Specko.9copy. p. 176 Wiley, New York (1967). L. DE GALANand G. F. SAMAEY,Spectrochim. Acta 25B, 246 (1970). J. H. GIBSON, W. E. L. GROSSMAN and W. D. COOKE, Anal. Chem. 35,266 (1963). H. L. KAJXNand J. E. S&ALLIS, Atomic AbsorptionNewaktter 7, 6 (1968). I. RUEE&A and M. MIK~~OVSKY,Atomic Abso-rptiolaNewdetter 11,67 (1972). B. V. L’vov, J. Quad. Spectry. Radiat. Transfer 12, 661 (1972).
2
263
264
lV 6
4-
3-
l-
l-
oFig. 1. Term diagram of tin. The lines 2863,2706 and 2840 were mwmred.
factors may be assumed equal. These are the nebulization efficiency, the fraction desolvated and for volatile compounds even the fraction volatilized. Different element concentrations in the flame gsses may result only from different flame gas volumes at the:psrticular flame temperature, and the following relation holds: %0&t
= IV
(1)
where qOt is the total concentration of the element (cm-a) and V the flame volume (cm8 8-l). If the emission line width from the hollow cathode is negligible compared to the absorption line width, ‘peak absorption’ is measured and the density of free atoms at level i is related to this peak absorbance by equation 4(&J
= (~+n4f,n,I~(&)
(2)
where A,(&,) is the peak absorbance, fi the oscillator strength, 12,the density of free atoms at level i, 2 the absorption path length, P(A,,) the peak value of the profile function, and e, m and c have their usual meaning. When more than one level is populated, the particular peak absorbancea must be taken into account. If the hyperfine structure (h.f.s.) of the psrticular lines may be neglected and the aparameter of the lines is assumed equal, the profile functions will be equal as well and the free atom density may be derived by summing up the peak absorbancea of lines originrtting from all the populated levels
266
Tin atomization in hydrogen supported flames
If we wish to compare free atom densities in different flames, the temperature dependence of the peak value of the protie function must be included. Beoause the h.f.s. has been neglected, the line pro& may be described by the Voigt function which depends on the mutual contribution of collisional and Doppler broadening. With growing temperature the Doppler line width will increase with Tlla. The temperature dependence of collision broadening is more difficult to assess, its component due to adiabatic collisions varying with T-l/6 [9]. For quenching collisions, data are scarce but, in general, a slight decrease with temperature may be expected rather than an increase. Let us therefore assume the unfavourable conditions that only Doppler broadening contributes to the temperature effect. Then the peak value of the profile function will be inversely proportional to the square root of the temperature.
(4) Here dashed symbols refer to the second flame. Substituting from equations (l), (3) and (4) into the expression for the ratio of atomization efficiencies (E) in two different flames we derive the following expression :
(6) EXPERIMENTAL All measurements were performed on a Perkin-Elmer model 306 atomic absorption spectrometer. Solutions of tin (100 ,ug/ml) in a 1:20 hydrochloric acid solution were sprayed and absorbances of three tin lines Sn 2863, Sn 2706 and Sn 2840 A were recorded. The flame conditions are given in Table 1. Table 1. Flame conditions Gas flows
(ew Flame C,H,air H&r AI-H,
Fuel 4.3 24.4 13.6
Oxidant di<
Volume after combustion mm
Observation height (=I
168 186 126
6 6 10
16 14 20
2340 1090 1130
The flame gas volumes were estimated including molar changes by the combustion reactions C&H, + 20, + 8Np +J 2CO + H, + SN, and and temperature expansion, but neglecting entrainment and Fusion from the surrounding atmosphere. No combustion reaction was therefore assumed for the argon-hydrogen diffusion flame. [Q] W. BEEMENBURQ,J. Quant.Spectry.
Radkzt.
Transfer
4, 177 (1964).
266
I. RUBE~KA
The temperatures reported in Table 1 were calculated from the peak absorbances of the two tin lines Sn 2863 and 2840 A according to the method of L’vov [8]. The value found depends both on the free atom distribution and the temperature distribution. L’vov calls it ‘effective’ temperature whereas REIF calls it ‘apparent’ temperature [lo]. However, the low values found for the three flames reflect mainly the fuel-rich reducing conditions used. The heights of observation were deliberately chosen equal for the acetyleneair and hydrogen-air flames. For the argon-hydrogen diffusion flame the observation height chosen was double that of the other flames to allow for the diffusion and combustion reactions to start. Absorbances of the three tin lines in the different flames are given in Table 2. Table 2. Absorbance8 of tin lines in the three flames
Line
Lower
(A) 2863 2706 2840
BP,, SP, SP,
state (cm-l)
frel
0 1692 3428
100 58 98
A C,H&r 0.115 0.081 0.0686
A Hz-air
A Ar-H,
0.169 0.075 0.0447
0.553 0.12 0.0346
Relative atomization efficiency %,/“CnH*
= 1.05
EArE,h~ = 1*2g EA&"C&lp = l-35
The relative atomization efficiencies were calculated according to equation (5). The results show clearly that the argon-hydrogen diffusion flame has the highest atomization ehlciency despite the fact that in this flame tin may not be completely volatilized. The vapour pressure of SnO reaches 1 atm at about 1650K [ll], whereas the flame temperature at the observation height is only 1130K. Atomization efficiencies in the hydrogen-air and acetylene-air flames are almost equal. However, keeping in mind that the temperature effect on the peak values of the profile functions was overestimated, the former should still be better as a tin atomizer. These results confirm that the higher tin sensitivity in hydrogen supported flames is not solely due to the higher population of the ground level in the cooler flames. DISCUSSIONOF THE RESULTS BULEW~Zand PADLEYhave recently shown that tin catalyses radical recombination in hydrogen supported flames [12,13]. If we assume that the reactions involved produce at some stage free tin atoms, this would explain the high atomization et&iency in the cool argon-hydrogen diffusion flame. It is known that the concentration of free radicals produced in hydrogen supported flames does not decrease with temperature below some 2000 K, due to the rapidly branching radical producing reactions between hydrogen and oxygen [14,15]. The catalytic effect of tin as found by Bulewicz and Padley is temperature independent whereas the catalytic effects of all other metals (Cr, U alkali earth metals) or [lo]
I. REIF,~. A. FASSEL and R. N. KNISELEY,&J~C~POC~~WL Acta 28B, 105 (1973). [ll] J.C. PLATTEEUW and G. MEYER, T~ane.FaradaySoc.52, 1066 (1956). [12] E. M. BIJLEWICZand P. J. PADLEY, Trans.Faraday Sot. 67, 2337 (1971). [13] E. M. BIJLE~ICZand P. J. PADLEY, Proc. 13th Int. Symp. Combustion, p. 73. Printed in the U.S. by the Combustion Institute, Pittsburg in 1971. Salt Lake City (1970). [14] E. M. BTJLEWICZ, C. G. JAMES and T. M. SUQDEN, Proc. Roy.Soc. A255, 89 (1956). [15] C. P. FENIMORE: Chemistry in Premixed Flames, p. 40. Pergamon Press, Oxford (1964).
Tin atomization in hydrogen supported flames
267
compounds (SO,, NO) decrease with decreasing temperature. Thus in the cool argon-hydrogen diffusion flame, all competing radical recombining reactions are suppressed, leaving tin as the most prominent catalyst. On the other hand, combustion of hydrogen with nitrous oxide does not involve rapidly branching reactions and the concentration of radicals produced in the reaction zone is lower [15]. Correspondingly, at the high temperature of the nitrous oxide-hydrogen flame no concentration of free radicals above equilibrium may be expected. The assumption that tin atomization is related to free hydrogen atoms is supported by interferences on tin observed in different flames under different conditions. In the acetylene+air flame, tin atomization may be assumed to be controlled mainly by the oxygen concentration in the flame gases and the temperature. Under these conditions interferences are relatively small. Numerous puzzling interferences have been observed in hydrogen-air flames where free hydrogen atoms play an important role according to the proposed assumption. Thus the strong depressive effects of sulphuric and nitric acids [16,17] may be due to the removal of excess hydrogen atoms by the catalytic effects of SO2 and/or NO on radical recombination. Further supporting evidence is seen in observations on tin absorption in long path absorption tubes [18]. Using a hydrogen-air flame with a direct injection burner the depressive effects of several elements in equal molar concentrations (0.005 M) were found to be in reasonable correlation with their reported catalytic efficiency [13] as seen in Fig. 2. The interferences of nonvolatile elements, e.g. platinum, exhibit strong memory effects, i.e. the depression takes place even after stopping aspiration of the interferent. The only explanation so far found is that platinum deposited on the tube walls catalyses radical recombination, the hydrogen atom concentrations in the gases fall and so do the fractions of tin atomized. Several published papers deal with interferences on tin in the argon-hydrogen and nitrogenhydrogen diffusion flames [7,19,20-J. Depressive as well as enhancing effects were observed. However, using a twin nebulizer we have found that both depression and enhancement were solute volatilization interference. The enhancement was caused most probably by augmented heat transfer through radical recombination on the particle surfaces resulting in accelerated volatilization [il. In inert gas diluted hydrogen-oxygen flames particle temperatures several hundred degrees above the temperature of the ambient flame gases have indeed been observed [Zl]. CONCLUSIONS The assumption that tin atomization is linked with excess hydrogen radicals thus fits well all experimental evidence. The reaction mechanism is, however, far from [ 16) B. J. HEFBERNAN,R. 0. ARCHBOLDand T. J. VICKJZRS, Proc. Amt. Imst.Mila. Met. No 223, 65 (1967). [17] B. MOLDAN,I. RWESKA, M. MIK&OVSKYand M. HUKA, An&. Chim. Actu 63,91 (1970). [18] I. RUBEBEA, paper presented at the 4th International Conference on Atomic Spectroscopy, Toronto (1973). [ 191 R. F. BROWNER,R. M. DAGNALL and T. S. WEST, Anal. Chim. Acta 46, 207 (1969). A, M. M~MORI and S. MTJSHA, Anal. Chim. Acta 82, 267 (1972). [20] T. NAKAHAR [Zl] E. M. BULEWICZ and P. J. PADLEY, Proc. Roy.Soc. Lendon A323, 377 (1971).
I. RUBEI&A
3OC’
I-’
OS’ M 0Fe &hi 4 Oco NiMO I 10 20
I 30
I 40
1 50
% depression Fig. 2. Correlation between catalytic efficiency on radical recombination and tin depressionin long path absorption tubes. Tin: 2 pg/ml. All interfereuts in 0.006 M solutions, added aa chlorides, only MO added as ammonium molybdate.
clear. BTJLEWICZ and PADLEY [13] were able to show, that with a high degree of probability, the Crst step of the catalytical mechanism of tin involves formation of tin hydroxide in an excited state SnO + H + SnOH* For the next step, they sssume a scheme which, according to conclusions based on kinetic considerations, is “the only scheme so far found to fit all essential features of the catalytic effect.” SnOH* (in state A) crosses to SnOH* (state B) and this then reacts with another hydrogen radical SnOH*(B) + H + SnO + H, Although this may be the prevailing reaction there might be a simultaneous reaction forming free tin atoms as sn intermediate product SnOH* +H+Sn
+H,O+SnO
+H,
The actual reaction path should depend on the probability of the formation of two different activated complexes [SnOH..
.H]*
or
ii: 1
SnOH z [
Unfortunately, &u&l knowledge at present available does not allow even a rough estimate of the particular heats of formation. Aokwx&dgeme~~ are due to Dr. Z. Herman from the Institute of Physical chemistry and Electrochemistry of the Czechoslovak Academy of Sciencea for several enlightening discussions.