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Heat of isomerization of peroxynitrite to nitrate and kinetics of isomerization of peroxynitrous acid to nitric acid
J. Inorg. Nucl. Chem., 1962, Vol. 24, pp. 1159 to 1162. Pergamon Press Ltd. Printed in England
H E A T OF I S O M E R I Z A T I O N OF P E R O X Y N I T R I T E TO N I T R A T E A N D K I N E T I C S OF I S O M E R I Z A T I O N OF P E R O X Y N I T R O U S A C I D TO N I T R I C ACID* J. D. RAy School of Chemistry, Georgia Institute of Technology, Atlanta 13, Georgia (Received 13 November 1959; in revised form 22 February 1962) Abstract--The reaction" between hydrogen peroxide and nitrous acid at 0 ° results in high yields of the rapidly formed intermediate peroxynitrous acid which isomerizes to nitric acid at a slower rate. By stopping the reaction with the addition of base at various times and measuring the total heat evolved in a calorimeter it was possible to determine the rate constant for isomerization to be 0-099 sec- 1 at 1°. The heat of isomerization of the peroxynitrite ion to the nitrate ion was deduced to be - 38.8 +_2 kcal/mole in dilute aqueous solution at 1°. KINETIC studies o f the reaction between nitrous acid and hydrogen peroxideO, 2, 3) at low concentrations of the latter reagents are in agreement that the rate expression is predominately expressed b y : --d(H202) k(H202)(HONO) (H+). (1) dt -
-
Results of the present study are in agreementwith (1) and the followingmechanism: H ++HONO+H202 ---- H O O N O + H 2 0 + H + kl H O O N O -----H O N ° k2 NO2 q- H O O N O = H O N ° - b N O ~ k3 HOONO ~ HO+NO2 k4 The present study differs from the kinetic studies in having concentrations o f reagents adjusted so as to obtain a concentration of the intermediate peroxynitrous acid that is nearly quantitative with respect to conversion of nitrous acid to peroxynitrous acid. The present experiments thus are similar to those carried out by GLEU and HUBOLD(4). Materials and methods. "Baker's Analysed" 31.9 per cent hydrogen peroxide was used in all experiments. Du Pont nitric acid of 70 per cent concentration was used. The sodium hydroxide was obtained from a bottle of its concentrated aqueous solution in which it had been standing for several days so as to settle the carbonate. All of these reagents were pipetted from their concentrated solutions. The potassium nitrite was part of a preparation previously used351 *Presented at the Southeastern Regional Meeting of the American Chemical Society, Birmingham, Ala., Nov. 3-5, 1960. I1) E. HALFPENNYand P. L. ROBINSON, J. Chem. Soc. 928 (1952). tz) M. ANBAg and H. TAUBE,J. Amer. Chem. Soc. 76, 6243 (1954). (3) E. A . SRILOV, A . A . RYBAKOW and H. A. PAL, Chem. Zbl. II, 377 (1931). E. A. SaILOV and Z. S. STEPANOVA, Zh. Fiz. Khim. 24, 820 (1950); Chem. Abstr. 413, 4121 (1951). E. A. Snmov, Zh. Fiz. Khim. 27, 1103 (1953); Chem. Abstr. 49, 5937a. {4) K. GLEU and R. HUBOLD,Z. Anorg. Chem. 223, 305 (1935). ~5) j. D. RAY and R. A. Oaa, JR., J. Phys. Chem. 60, 1599 (1956). 1159
1160
J . D . RAY
The calorimeter solutions after reaction were analysed by potentiometric titration of excess H202 with hypochlorite at pH 12 using platinum and normal calomel electrodes. The nitrite was determined with permanganate. The calorimeter and its calibration by using the heat of water vapourization as the energy standard have been described previously36) The calorimeter was placed in a container of crushed ice so as to minimise temperature drift. All calorimeter reactions reported in this article were carried out with all of the reagents at about 0 °. The defined thermochemical calorie was used for calculations. Heat o f isomerization ofperoxynitrite to nitrate. Fig. 1 is a piot of observed temperature rise of the calorimeter as a function of the number of seconds between addition of acid followed by base to the calorimeter. Inasmuch as the rate of decomposition of peroxynitrite ion is very slow, it is possible to trap peroxynitrous acid in the f o r m of peroxynitrite by the addition of a strong base. Since the reactions
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40 and
50
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Fie. 1.--Observed temperature rise of calorimeter as a function of the number of seconds between addition of acid and base. The dashed line intersects the temperature axis to indicate the temperature rise associated with quantitative peroxynitrite formation. The shape of the curve is due to the first order reaction HOONO = HON °. being studied are much too rapid to be followed by means of the temperature change as registered on a Beckman thermometer, the observed temperature changes shown on Fig. 1 correspond to those involving both acid and subsequent base addition. In all cases represented by plotted points except that at zero time the calorimeter contained 500 ml of a solution containing 3.000 g of potassium nitrite and 5.00 ml of 31"9 per cent hydrogen peroxide. The point at zero time involved only the acid and base but no nitrite or peroxide. Solutions of 4.00 ml of 70 per cent nitric acid and 5.00 ml of 46 per cent sodium hydroxide both of which had been diluted to 50 ml were added to the calorimeter in a manner to effect rapid mixing. The acid and base solutions were put in 100 ml beakers which were placed in a container of crushed ice next to the calorimeter. The beakers of acid and base were stirred with thermometers having (6) j. D. RAY, Rev. Sci. Instrum. 26, 863 (1956).
Heat of isomerization of peroxynitrite to nitrate and kinetics
1161
0.1 ° graduations so that temperature drifts could be followed. The acid and base solutions were not just poured in but rather literally "thrown" downward out of the beakers into the calorimeter so that mixing was effected in approximately one second. The two low points near zero time on Fig. 1 indicate some time lag in mixing. The stirring paddle was very effective so that the limiting factor in mixing time was simply getting the solutions out of the beakers. In the present study the reaction of peroxynitrous acid to form nitric acid can be represented as a first order process. Thus, after the first 2 or 3 sec, all of the nitrite ion has been consumed in making peroxynitrous acid. Therefore, step k 3 is not active beyond the first short period of time that it takes nitrous acid to be formed from nitrite. SHILOVand STEPANOVA(3) suggest that the isomerization may proceed through an acid catalysed step. But the data of Fig. 1 were plotted as the logarithm of the calorimeter temperature rise at infinite time between addition of acid and base minus the temperature rise corresponding to actual time intervals to obtain a straight line between about 31- sec and 50 sec. The derived rate constant may actually be pseudo first order and include the hydrogen ion concentration. When the data of Fig. 1 are plotted on log paper as described above, the data extrapolate as indicated by the dashed line of Fig. 1 with intercept 2-03 c' rather than extending down to intersect the temperature rise axis at 1.35 °. The 1.35 ° intercept represents the observed temperature rise associated with just the heat of acid-base neutralization and dilution effects in the absence of KNO2 and H202. The calculated value for this rise, 1.38 °, based on the energy equivalent of the calorimeter, 640 cal/°C is within the precision of the calorimeter and reflects a small amount of acid retained in the beaker. The difference between the 1-35 ° intercept and the extrapolated intercept at 2.03 ° represents the temperature change associated with the formation of peroxynitrite plus hydroperoxide ion and negligible dilution effects. The temperature rise, 0.22 °, due to hydroperoxide formation was calculated from JOYNER'S(7) data. The temperature rise, 0.46 ° due to the reaction HaO2(soln)+NO2(soln) ~- OONO-(soln)-~H20(soln) corresponds to a molar heat of reaction of --8.3 kcal and a heat of formation of peroxynitrite ion from the elements in the solution at 1° of --11-4 kcal, and a heat of isomerization of peroxynitrite to nitrate of --38-1 kcal. The difference between the intercept at 2.03 ° and the temperature change 4.2l ° associated with infinite time elapsed between addition of acid and base is proportional to the heat of isomerization of peroxynitrite ion to nitrate ion, --39-4 kcal. This value corresponds to --10.1 kcal for the heat of formation of peroxynitrite ion from the elements at 1°. The difference in the two values for the heat of formation of peroxynitrite ion, --11.4 and --10.1 reflects neglect of concentration effects on standard heats of formation of the reaction species, uncertainty in the heats of formation of the species, and experimental precision. The average values are AH°274 OONO-~--10.8 kcal and AH°274OONO _ =NO3 ~ - - 3 8 . 8 k c a l . The precision is estimated as ~z 2 kcal for both values. Rate of isomerization of peroxynitrous acid to nitric acid. The half-life of the isomerization of peroxynitrous acid to nitric acid deduced from Fig. 1 and from a plot of the data on log paper is 7.0 sec which corresponds to a rate constant of 0.099 sec- 1 at a temperature of about 1°. (7)
R. A. JOYNER, Z. Anorg. Chem. 77, 103 (1912).
1162
J . D . RAy
Acknowledgements--This research was supported in part by the United States Air Force under Contract No. AF 49(638)-286 monitored by the AF Office of Seientitic Research of the Air Research and Development Command, the Research Corporation and the National Science Foundation (NSF G-15560).
H E A T OF I S O M E R I Z A T I O N OF P E R O X Y N I T R I T E TO N I T R A T E A N D K I N E T I C S OF I S O M E R I Z A T I O N OF P E R O X Y N I T R O U S A C I D TO N I T R I C ACID* J. D. RAy School of Chemistry, Georgia Institute of Technology, Atlanta 13, Georgia (Received 13 November 1959; in revised form 22 February 1962) Abstract--The reaction" between hydrogen peroxide and nitrous acid at 0 ° results in high yields of the rapidly formed intermediate peroxynitrous acid which isomerizes to nitric acid at a slower rate. By stopping the reaction with the addition of base at various times and measuring the total heat evolved in a calorimeter it was possible to determine the rate constant for isomerization to be 0-099 sec- 1 at 1°. The heat of isomerization of the peroxynitrite ion to the nitrate ion was deduced to be - 38.8 +_2 kcal/mole in dilute aqueous solution at 1°. KINETIC studies o f the reaction between nitrous acid and hydrogen peroxideO, 2, 3) at low concentrations of the latter reagents are in agreement that the rate expression is predominately expressed b y : --d(H202) k(H202)(HONO) (H+). (1) dt -
-
Results of the present study are in agreementwith (1) and the followingmechanism: H ++HONO+H202 ---- H O O N O + H 2 0 + H + kl H O O N O -----H O N ° k2 NO2 q- H O O N O = H O N ° - b N O ~ k3 HOONO ~ HO+NO2 k4 The present study differs from the kinetic studies in having concentrations o f reagents adjusted so as to obtain a concentration of the intermediate peroxynitrous acid that is nearly quantitative with respect to conversion of nitrous acid to peroxynitrous acid. The present experiments thus are similar to those carried out by GLEU and HUBOLD(4). Materials and methods. "Baker's Analysed" 31.9 per cent hydrogen peroxide was used in all experiments. Du Pont nitric acid of 70 per cent concentration was used. The sodium hydroxide was obtained from a bottle of its concentrated aqueous solution in which it had been standing for several days so as to settle the carbonate. All of these reagents were pipetted from their concentrated solutions. The potassium nitrite was part of a preparation previously used351 *Presented at the Southeastern Regional Meeting of the American Chemical Society, Birmingham, Ala., Nov. 3-5, 1960. I1) E. HALFPENNYand P. L. ROBINSON, J. Chem. Soc. 928 (1952). tz) M. ANBAg and H. TAUBE,J. Amer. Chem. Soc. 76, 6243 (1954). (3) E. A . SRILOV, A . A . RYBAKOW and H. A. PAL, Chem. Zbl. II, 377 (1931). E. A. SaILOV and Z. S. STEPANOVA, Zh. Fiz. Khim. 24, 820 (1950); Chem. Abstr. 413, 4121 (1951). E. A. Snmov, Zh. Fiz. Khim. 27, 1103 (1953); Chem. Abstr. 49, 5937a. {4) K. GLEU and R. HUBOLD,Z. Anorg. Chem. 223, 305 (1935). ~5) j. D. RAY and R. A. Oaa, JR., J. Phys. Chem. 60, 1599 (1956). 1159
1160
J . D . RAY
The calorimeter solutions after reaction were analysed by potentiometric titration of excess H202 with hypochlorite at pH 12 using platinum and normal calomel electrodes. The nitrite was determined with permanganate. The calorimeter and its calibration by using the heat of water vapourization as the energy standard have been described previously36) The calorimeter was placed in a container of crushed ice so as to minimise temperature drift. All calorimeter reactions reported in this article were carried out with all of the reagents at about 0 °. The defined thermochemical calorie was used for calculations. Heat o f isomerization ofperoxynitrite to nitrate. Fig. 1 is a piot of observed temperature rise of the calorimeter as a function of the number of seconds between addition of acid followed by base to the calorimeter. Inasmuch as the rate of decomposition of peroxynitrite ion is very slow, it is possible to trap peroxynitrous acid in the f o r m of peroxynitrite by the addition of a strong base. Since the reactions
i I
! o
i
i
.E_ o
I
(.)
0
20
tO Seconds
between
, cddifion
L
30 of
ocid
40 and
50
bose
Fie. 1.--Observed temperature rise of calorimeter as a function of the number of seconds between addition of acid and base. The dashed line intersects the temperature axis to indicate the temperature rise associated with quantitative peroxynitrite formation. The shape of the curve is due to the first order reaction HOONO = HON °. being studied are much too rapid to be followed by means of the temperature change as registered on a Beckman thermometer, the observed temperature changes shown on Fig. 1 correspond to those involving both acid and subsequent base addition. In all cases represented by plotted points except that at zero time the calorimeter contained 500 ml of a solution containing 3.000 g of potassium nitrite and 5.00 ml of 31"9 per cent hydrogen peroxide. The point at zero time involved only the acid and base but no nitrite or peroxide. Solutions of 4.00 ml of 70 per cent nitric acid and 5.00 ml of 46 per cent sodium hydroxide both of which had been diluted to 50 ml were added to the calorimeter in a manner to effect rapid mixing. The acid and base solutions were put in 100 ml beakers which were placed in a container of crushed ice next to the calorimeter. The beakers of acid and base were stirred with thermometers having (6) j. D. RAY, Rev. Sci. Instrum. 26, 863 (1956).
Heat of isomerization of peroxynitrite to nitrate and kinetics
1161
0.1 ° graduations so that temperature drifts could be followed. The acid and base solutions were not just poured in but rather literally "thrown" downward out of the beakers into the calorimeter so that mixing was effected in approximately one second. The two low points near zero time on Fig. 1 indicate some time lag in mixing. The stirring paddle was very effective so that the limiting factor in mixing time was simply getting the solutions out of the beakers. In the present study the reaction of peroxynitrous acid to form nitric acid can be represented as a first order process. Thus, after the first 2 or 3 sec, all of the nitrite ion has been consumed in making peroxynitrous acid. Therefore, step k 3 is not active beyond the first short period of time that it takes nitrous acid to be formed from nitrite. SHILOVand STEPANOVA(3) suggest that the isomerization may proceed through an acid catalysed step. But the data of Fig. 1 were plotted as the logarithm of the calorimeter temperature rise at infinite time between addition of acid and base minus the temperature rise corresponding to actual time intervals to obtain a straight line between about 31- sec and 50 sec. The derived rate constant may actually be pseudo first order and include the hydrogen ion concentration. When the data of Fig. 1 are plotted on log paper as described above, the data extrapolate as indicated by the dashed line of Fig. 1 with intercept 2-03 c' rather than extending down to intersect the temperature rise axis at 1.35 °. The 1.35 ° intercept represents the observed temperature rise associated with just the heat of acid-base neutralization and dilution effects in the absence of KNO2 and H202. The calculated value for this rise, 1.38 °, based on the energy equivalent of the calorimeter, 640 cal/°C is within the precision of the calorimeter and reflects a small amount of acid retained in the beaker. The difference between the 1-35 ° intercept and the extrapolated intercept at 2.03 ° represents the temperature change associated with the formation of peroxynitrite plus hydroperoxide ion and negligible dilution effects. The temperature rise, 0.22 °, due to hydroperoxide formation was calculated from JOYNER'S(7) data. The temperature rise, 0.46 ° due to the reaction HaO2(soln)+NO2(soln) ~- OONO-(soln)-~H20(soln) corresponds to a molar heat of reaction of --8.3 kcal and a heat of formation of peroxynitrite ion from the elements in the solution at 1° of --11-4 kcal, and a heat of isomerization of peroxynitrite to nitrate of --38-1 kcal. The difference between the intercept at 2.03 ° and the temperature change 4.2l ° associated with infinite time elapsed between addition of acid and base is proportional to the heat of isomerization of peroxynitrite ion to nitrate ion, --39-4 kcal. This value corresponds to --10.1 kcal for the heat of formation of peroxynitrite ion from the elements at 1°. The difference in the two values for the heat of formation of peroxynitrite ion, --11.4 and --10.1 reflects neglect of concentration effects on standard heats of formation of the reaction species, uncertainty in the heats of formation of the species, and experimental precision. The average values are AH°274 OONO-~--10.8 kcal and AH°274OONO _ =NO3 ~ - - 3 8 . 8 k c a l . The precision is estimated as ~z 2 kcal for both values. Rate of isomerization of peroxynitrous acid to nitric acid. The half-life of the isomerization of peroxynitrous acid to nitric acid deduced from Fig. 1 and from a plot of the data on log paper is 7.0 sec which corresponds to a rate constant of 0.099 sec- 1 at a temperature of about 1°. (7)
R. A. JOYNER, Z. Anorg. Chem. 77, 103 (1912).
1162
J . D . RAy
Acknowledgements--This research was supported in part by the United States Air Force under Contract No. AF 49(638)-286 monitored by the AF Office of Seientitic Research of the Air Research and Development Command, the Research Corporation and the National Science Foundation (NSF G-15560).