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The reaction of manganese dioxide with molten lithium-potassium nitrate eutectic
2870
Notes
J. inorg,nucl.Chem.,1968,Vol.30, pp. 2870to 2872, PergamonPress. Printedin Great Britain
The reaction of manganese dioxide with molten lithium-potassium nitrate eutectic (First received 3 August 1967; in revised form 30January 1968) THE REACTION of manganese dioxide with pure lithium-potassium nitrate eutectic at 160 ° has been studied and the results compared with those obtained when the oxide acceptor (Lux-Flood acid) potassium pyrosulphate was added to the melt and when the nitrogen dioxide/oxygen mixture, produced by the reaction of potassium pyrosulphate with a separate portion of melt, was passed through the reaction mixture. These results are now reported as a contribution to the current discussion on the nature of the active species in acidic fused nitrate solutions. The suggestion has recently been made that this species is nitrogen dioxide and not the nitryl ion[l]. This suggestion arose from a voltametric and chronopotentiometric study of solutions of acids and bases in sodium-potassium nitrate eutectic and the demonstration that nitrogen dioxide could produce the same reaction products as oxide acceptors such as potassium dichromate or pyrosulphate. This suggestion may be compared with the hypothesis of nitryl ion as an intermediate in the interpretation of e.m.f.[2] and kinetic studies[3-5], acid-base titrations [6] and a number of oxidation reactions [7-10]. The nitryl ion has also been postulated to be formed to a small but measurable extent by the selfionisation of the nitrate ion[2]. i.e.
NO3- ~ NO2++O 2-.
(1)
Thus on the second hypothesis oxide acceptors would act as acids by displacing equilibrium (1) to the right and the excess nitryl ions then reacting with nitrate ions[ 11], NO2++ NO3- ~ N20~ ~ 2NO~ +½0._,
(2)
while on the first hypothesis nitrogen dioxide and oxygen would be produced directly[l] and themselves act as the acidic species. EXPERIMENTAL The lithium-potassium nitrate eutectic was prepared and the reactions carried out as previously described[ I 0]. The manganese dioxides used were the hydrated y form prepared by aqueous oxidation of manganese(I I)[ 12], the fl form prepared by heating manganese(l I) nitrate [ 13] and B.D.H. precipitated grade which was heated at 150 ° for 24 hr. Titration with arsenious acid indicated these to contain 55.9, 62-4, and 54-7% Mn respectively (calc. for MnOe, 63.2% Mn). The reacted melts were analysed, after solution in water, by titration with EDTA at p i l l 0 using eriochrome black T as indicator[ 14]. The values given in Table I are the average of 2-4 determinations. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
L. E. Topol, R. A. Osteryoung and J. H. Christie,J. phys. Chem. 70, 2857 (1966). R.N. Kust and F. R. Duke,J.Am. chem. Soc. 85, 3338 (1963). I, Sl~ma, Colin Czech. chem. Commun. 28,985, 2810 (1963). F. R. Duke and M. L. lverson,Analyt. Chem. 31, 1233 (1959). F. R. Duke and S. Yamamoto, J. A m. chem. Soc. 81, 6378 (1959). A. M. Shams El Din and A. A. El Hosary,J. electroanal. Chem. 7,464 ( 1964): ibid, 9,349 (1965). J. D. Van Norman and R. A. Osteryoung,Analyt. Chem. 32, 398 (1960). R. M. Bennett and O. G, Holmes, Can. J. Chem. 41, 108 (1963). M. W. Y. Spink. Diss. A bstr. 26, 4274 (1966). B.J. Brough and D. H. Kerridge, lnorg. Chem4, 1353 (1965). F. R. Duke,Adv. Chem. Ser. 49,220 (1965). W.G. Palmer, Experimental Inorganic Chemistry. Cambridge University Press. (1954). J. Meyer and R. Kanters, Z. anorg. Chem. 185, 177 (1929). H. A. Flaschka, E.D.T.A. Titrations:An Introduction to Theory and Practice 2nd Edn. Pergamon Press, Oxford (1964).
Notes
287 I
Table 1. Percentage of m a n g a n e s e ( l l ) produced by the reaction of m a n g a n e s e dioxide in lithiump o t a s s i u m nitrate at 160 °
All reactants in melt
G a s e s from pyrosulphate/ nitrate
MnO,,
Concentration of MnO~ (m)
Concentration of K._,S2Or (m)
y 3' 3'
0.102 0.201 0.061
--0.71
y B.D.H. /3 y y 3'
0.062 0.060 0.066 0.061 0.102 0.101
2.4" 2.4 ~: 2.4* 4.8* 1.2" 1.2"
Concentration of K~SO4 (m)
T i m e for reaction (hr)
Mn(ll) (%)
3 3 4
I-7 1.5 37-~
--
4
6-6
--
4
6-3
--
4
6.6 9.2 3"0 2" I
m
0.88
4 3 3
*Concentration if K,_,S2Or had been dissolved in MnOe melt. P o t a s s i u m pyrosulphate was prepared by heating Analar potassium persulphate at 25/) ° for I day (98.3% K.2SeO7 by titration against alkali). T h e nitrogen dioxide/oxygen gas mixtures were made by heating p o t a s s i u m pyrosulphate with nitrate eutectic at 180 ° i.: a separate furnace and bubbled through the melts in a stream of nitrogen S._,O7'-'-+ 2NO3 --+ 2SO~e-+ 2NO., +½0,,,
(3)
RESULTS AND DISCUSSION M a n g a n e s e dioxide, in the y form, dissolved slowly in nitrate eutectic containing potassium pyrosulphate to give a colourless solution containing m a n g a n e s e ( l I) ions and brown f u m e s of nitrogen dioxide and oxygen, the reaction mixture being kept stirred by the passage of a stream of nitrogen. T h i s is analogous to the reaction of m a n g a n e s e dioxide with s o m e acids in a q u e o u s solution and may likewise occur by the initial formation of m a n g a n e s e ( I V ) ions which are subsequently reduced. A similar but m u c h slower reaction took place in pure nitrate eutectic, and one of intermediate rate when the gas mixture was passed through the melt. X-ray powder photography s h o w e d that the unreacted m a n g a n e s e dioxide remained in the y form, although m a n g a n e s e dioxide produced when manganese(I 1+ reacted with nitrate melt was the a n h y d r o u s form [ 15]. T h e percentage of reduced m a n g a n e s e produced by m a n g a n e s e dioxide reacted in 20 g portions of nitrate melt containing varying concentrations of m a n g a n e s e dioxide, potassium pyrosulphate and p o t a s s i u m sulphate are given in the Table. It can be seen that though the proportion of reduced manganese did increase with the quantity of gas mixture bubbled through the melt, reduction equivalent to that achieved by dissolved pyrosulphate, through which a similar a m o u n t of nitrogen was passed, would require very m u c h larger quantities of pyrosulphate to be reacted with nitrate in the separate gas production furnace. Sulphate ions produced in reaction (3) might have been responsible for the increased reaction with dissolved purosulphate but the results in the Table indicated that this was not so. Similarly, variation of the form of the m a n g a n e s e dioxide had relatively little effect, while other results, not listed in the Table, s h o w e d that very little change in the percentage of reduced m a n g a n e s e was produced through preheating the gases before bubbling (in order to dissociate any dinitrogen tetroxide). T h e r e was little difference in the a m o u n t of reaction with change in nitrogen dioxide/oxygen bubble size, for example exchanging a 3 m m i.d. delivery tube for a sintered glass disc (No. 3 grade, 3 cm 2 area) increased the m a n g a n e s e dioxide reacted from 6.6 to 7.5 per cent. T h e reaction of man15. D. H. Kerridge and S. A. Tariq, Unpublished.
2872
Notes
ganese dioxide with pure nitrate melt was not attributable to an impurity (e.g. manganese(llI) oxide) since similar results were obtained with manganese dioxide which had previously reacted to the extent of 10 per cent. These results show that, whatever the intermediate formed when pyrosulphate dissolves in nitrate melt, under comparable conditions this acidic melt is considerably more reactive towards manganese dioxide than the nitrogen dioxide/oxygen gas mixture and that the reactants are not, in this sense, equivalent (cf. Ref. [ 1]). Thus, on balance the nitryl ion hypothesis would seem to be favoured, though some of the reduced efficiency of nitrogen dioxide/oxygen mixture may well have been due to the less favourable situation of the gases when bubbled through, rather than being produced in, the melt and the possibility that dissolved pyrosulphate ions may attack manganese dioxide directly cannot be excluded.
Department of Chemistry The University Southampton
B. J. B R O U G H D. A. H A B B O U S H D. H. K E R R I D G E
Notes
J. inorg,nucl.Chem.,1968,Vol.30, pp. 2870to 2872, PergamonPress. Printedin Great Britain
The reaction of manganese dioxide with molten lithium-potassium nitrate eutectic (First received 3 August 1967; in revised form 30January 1968) THE REACTION of manganese dioxide with pure lithium-potassium nitrate eutectic at 160 ° has been studied and the results compared with those obtained when the oxide acceptor (Lux-Flood acid) potassium pyrosulphate was added to the melt and when the nitrogen dioxide/oxygen mixture, produced by the reaction of potassium pyrosulphate with a separate portion of melt, was passed through the reaction mixture. These results are now reported as a contribution to the current discussion on the nature of the active species in acidic fused nitrate solutions. The suggestion has recently been made that this species is nitrogen dioxide and not the nitryl ion[l]. This suggestion arose from a voltametric and chronopotentiometric study of solutions of acids and bases in sodium-potassium nitrate eutectic and the demonstration that nitrogen dioxide could produce the same reaction products as oxide acceptors such as potassium dichromate or pyrosulphate. This suggestion may be compared with the hypothesis of nitryl ion as an intermediate in the interpretation of e.m.f.[2] and kinetic studies[3-5], acid-base titrations [6] and a number of oxidation reactions [7-10]. The nitryl ion has also been postulated to be formed to a small but measurable extent by the selfionisation of the nitrate ion[2]. i.e.
NO3- ~ NO2++O 2-.
(1)
Thus on the second hypothesis oxide acceptors would act as acids by displacing equilibrium (1) to the right and the excess nitryl ions then reacting with nitrate ions[ 11], NO2++ NO3- ~ N20~ ~ 2NO~ +½0._,
(2)
while on the first hypothesis nitrogen dioxide and oxygen would be produced directly[l] and themselves act as the acidic species. EXPERIMENTAL The lithium-potassium nitrate eutectic was prepared and the reactions carried out as previously described[ I 0]. The manganese dioxides used were the hydrated y form prepared by aqueous oxidation of manganese(I I)[ 12], the fl form prepared by heating manganese(l I) nitrate [ 13] and B.D.H. precipitated grade which was heated at 150 ° for 24 hr. Titration with arsenious acid indicated these to contain 55.9, 62-4, and 54-7% Mn respectively (calc. for MnOe, 63.2% Mn). The reacted melts were analysed, after solution in water, by titration with EDTA at p i l l 0 using eriochrome black T as indicator[ 14]. The values given in Table I are the average of 2-4 determinations. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
L. E. Topol, R. A. Osteryoung and J. H. Christie,J. phys. Chem. 70, 2857 (1966). R.N. Kust and F. R. Duke,J.Am. chem. Soc. 85, 3338 (1963). I, Sl~ma, Colin Czech. chem. Commun. 28,985, 2810 (1963). F. R. Duke and M. L. lverson,Analyt. Chem. 31, 1233 (1959). F. R. Duke and S. Yamamoto, J. A m. chem. Soc. 81, 6378 (1959). A. M. Shams El Din and A. A. El Hosary,J. electroanal. Chem. 7,464 ( 1964): ibid, 9,349 (1965). J. D. Van Norman and R. A. Osteryoung,Analyt. Chem. 32, 398 (1960). R. M. Bennett and O. G, Holmes, Can. J. Chem. 41, 108 (1963). M. W. Y. Spink. Diss. A bstr. 26, 4274 (1966). B.J. Brough and D. H. Kerridge, lnorg. Chem4, 1353 (1965). F. R. Duke,Adv. Chem. Ser. 49,220 (1965). W.G. Palmer, Experimental Inorganic Chemistry. Cambridge University Press. (1954). J. Meyer and R. Kanters, Z. anorg. Chem. 185, 177 (1929). H. A. Flaschka, E.D.T.A. Titrations:An Introduction to Theory and Practice 2nd Edn. Pergamon Press, Oxford (1964).
Notes
287 I
Table 1. Percentage of m a n g a n e s e ( l l ) produced by the reaction of m a n g a n e s e dioxide in lithiump o t a s s i u m nitrate at 160 °
All reactants in melt
G a s e s from pyrosulphate/ nitrate
MnO,,
Concentration of MnO~ (m)
Concentration of K._,S2Or (m)
y 3' 3'
0.102 0.201 0.061
--0.71
y B.D.H. /3 y y 3'
0.062 0.060 0.066 0.061 0.102 0.101
2.4" 2.4 ~: 2.4* 4.8* 1.2" 1.2"
Concentration of K~SO4 (m)
T i m e for reaction (hr)
Mn(ll) (%)
3 3 4
I-7 1.5 37-~
--
4
6-6
--
4
6-3
--
4
6.6 9.2 3"0 2" I
m
0.88
4 3 3
*Concentration if K,_,S2Or had been dissolved in MnOe melt. P o t a s s i u m pyrosulphate was prepared by heating Analar potassium persulphate at 25/) ° for I day (98.3% K.2SeO7 by titration against alkali). T h e nitrogen dioxide/oxygen gas mixtures were made by heating p o t a s s i u m pyrosulphate with nitrate eutectic at 180 ° i.: a separate furnace and bubbled through the melts in a stream of nitrogen S._,O7'-'-+ 2NO3 --+ 2SO~e-+ 2NO., +½0,,,
(3)
RESULTS AND DISCUSSION M a n g a n e s e dioxide, in the y form, dissolved slowly in nitrate eutectic containing potassium pyrosulphate to give a colourless solution containing m a n g a n e s e ( l I) ions and brown f u m e s of nitrogen dioxide and oxygen, the reaction mixture being kept stirred by the passage of a stream of nitrogen. T h i s is analogous to the reaction of m a n g a n e s e dioxide with s o m e acids in a q u e o u s solution and may likewise occur by the initial formation of m a n g a n e s e ( I V ) ions which are subsequently reduced. A similar but m u c h slower reaction took place in pure nitrate eutectic, and one of intermediate rate when the gas mixture was passed through the melt. X-ray powder photography s h o w e d that the unreacted m a n g a n e s e dioxide remained in the y form, although m a n g a n e s e dioxide produced when manganese(I 1+ reacted with nitrate melt was the a n h y d r o u s form [ 15]. T h e percentage of reduced m a n g a n e s e produced by m a n g a n e s e dioxide reacted in 20 g portions of nitrate melt containing varying concentrations of m a n g a n e s e dioxide, potassium pyrosulphate and p o t a s s i u m sulphate are given in the Table. It can be seen that though the proportion of reduced manganese did increase with the quantity of gas mixture bubbled through the melt, reduction equivalent to that achieved by dissolved pyrosulphate, through which a similar a m o u n t of nitrogen was passed, would require very m u c h larger quantities of pyrosulphate to be reacted with nitrate in the separate gas production furnace. Sulphate ions produced in reaction (3) might have been responsible for the increased reaction with dissolved purosulphate but the results in the Table indicated that this was not so. Similarly, variation of the form of the m a n g a n e s e dioxide had relatively little effect, while other results, not listed in the Table, s h o w e d that very little change in the percentage of reduced m a n g a n e s e was produced through preheating the gases before bubbling (in order to dissociate any dinitrogen tetroxide). T h e r e was little difference in the a m o u n t of reaction with change in nitrogen dioxide/oxygen bubble size, for example exchanging a 3 m m i.d. delivery tube for a sintered glass disc (No. 3 grade, 3 cm 2 area) increased the m a n g a n e s e dioxide reacted from 6.6 to 7.5 per cent. T h e reaction of man15. D. H. Kerridge and S. A. Tariq, Unpublished.
2872
Notes
ganese dioxide with pure nitrate melt was not attributable to an impurity (e.g. manganese(llI) oxide) since similar results were obtained with manganese dioxide which had previously reacted to the extent of 10 per cent. These results show that, whatever the intermediate formed when pyrosulphate dissolves in nitrate melt, under comparable conditions this acidic melt is considerably more reactive towards manganese dioxide than the nitrogen dioxide/oxygen gas mixture and that the reactants are not, in this sense, equivalent (cf. Ref. [ 1]). Thus, on balance the nitryl ion hypothesis would seem to be favoured, though some of the reduced efficiency of nitrogen dioxide/oxygen mixture may well have been due to the less favourable situation of the gases when bubbled through, rather than being produced in, the melt and the possibility that dissolved pyrosulphate ions may attack manganese dioxide directly cannot be excluded.
Department of Chemistry The University Southampton
B. J. B R O U G H D. A. H A B B O U S H D. H. K E R R I D G E