The radiation-induced formation of NO3 in aqueous solutions

International Journal

ofAppliedRadiation

and Isotopes, 1967, Vol. 18, pp. 25-32.

The Radiation-Induced NO, in Aqueous

Pergamon Press Ltd. Printed in Northern Ireland

Formation Solutions*

of

R. K. BROSZKIEWICZ Institute

of Nuclear

Research,

Department

of Radiation

Chemistry,

Warsaw,

Poland

(Received 15 March 1966) The production of NO, by electron pulse radiolysis of nitric acid has been investigated over the concentration range from 10-s to 4.0 M HNO,. The NO, transient spectrum has been determined, peaks being found at 595, 635 and 672.5 rnp and probably in the U.V. below 300 m,e. NO, is formed by the reaction OH + HNO, + NO, + HsO and not by the reaction OH + NO, + NO, + OH- consequently, no NO, absorptian was recorded in neutral and alkaline solutions of KNO, or in nitric acid at a concentration below O-05 M HNO,. In neutral and alkaline solutions of KNO, a transient absorption at 430 rnp, due to the radical NO,, has been observed. NO, decays by a slow, first-order reaction the rate constant being k, = (7.59 & 021) X 103sec+. FORMATION

DE

NO,

EN

SOLUTION AQUEUSE RADIOLYTIQUE

PAR

EXCITATION

On a ttudit la formation de NO, au tours de la radiolyse electronique pulsatrice de I’acide nitrique dans le gradient de concentration de low3 a 4,0 M HNO,. On a determine le spectre passager de NO, et on a Ctabli que les maximum d’absorption ont lieu a 595,635 et 672,5 rnp et dans l’ultraviolet probablement en dessous de 300 rnp. NO, se forme par la reaction OH + HNO, -+ NO, + H,O et non par la reaction OH + NO,- + NO, + OH-, d’oh en consequence on n’a pas determine d’absorption de NO, dans les solutions de KNO, neutres et alcalines, ou dans les solutions de I’acide nitrique a concentration plus basse que 0,05 M HNO,. Dans les solutions de KNO, neutres ou alcalines on a Ctabli une formation passagere de l’absorption a 430 rnp, qu’on attribue au radical NO,. La disparution de NO, a lieu par une reaction lente du 1-er ordre avec une constante k, = (7,59 & 0,21) x lo3 see-r. PAAHAIJIIOHHO

BHjJYIIIlPOBAHHOE OBPA30BAIIME PACTBOPAX

NO8

B

BOAHbIX

&CneJ&OBaHO o6paaosanne NOs BO BpeMH eneKTpoHHor0 E~MII~JICHO~OpmrIonE3a pacTBOpOn a30TH08 KHCJIOTJ~ B npenenax KoHqeHTpaqnll 10+ go 4,0 MHNOs. Onpegeneno IIepeXOAHbIfiCneKTp NO8 II npll TOM HaIlAeHo a6cop6qnomrbre MaKCnMa npn 595,635 II 672,5 mp a BepoRTHo TaKXle B ynTpaf#HoneToBoti o6nacTn cneKTpa Home 300 mp. NOs o6pa3yeTcfi B peaKqIlH OH + l%No8 =NOs + Ha0 a He HBnneTCH pe3ynbTaToM peaKqw OH + NOa- =NOs + OH- II IIOTOM~ He 06HapymeHO a6cop6snA NOs B HeyTpanHux HJIH UeJIOYHbIX paCTBOpaX HNOs a TaKme B paCTBOpaXa30TH08KWIOTbICKOHIJeHTpa~EiettHH~e 0,05 MHNOs. B HeyTpaJIHJJX %I UenOYHbIX paCTBOpaX ICNOI 06HapymeHO BO3HkIKaHEle nepexonaoti a6cop6qan npn 430 rnp cBfl3aHHofi c paAHKanoM NO*. NO8 ycYe3aeT B MegJIeHHOtipeaKQHM IIepBOtiCTeIIeHIlC KOHCTaHTOtiks = (7,59 f 0,21) X 10' CeK-1 . STRALUNGSINDUZIERTE

NO,-BILDUNG

IN

WASSERIGEN

LOSUNGEN

Die Bildung von NO, wahrend der Radiolysevon Salpetersaure im gerulsten Elektronenstrom wurde fur Konzentrationen zwischen lo_3 bis 4,0 M HNO, bestimmt. Es wurde Ubergangsspektrum von NO, bestimmt, wobei Maxima der Absorption bei 595, 635 and 672,5 rnp und * Work performed at the British Empire Cancer Vernon Hospital, Northwood, Mid&., England.

Campaign, 25

Research

Unit

in Radiobiology,

Mount

26

R. K. Broszkia-wicz wahrscheinlich auchim UV-Bereich unter 300 rnp gefunden. NO, entsteht nach der Reaktion OH + HNO, --, NO, + H,O, und nicht nach der Reaktion OH + NO, -+ NO, + OH-; dementspechend wurde die NOs-Absorption weder in neutralen noch in alkalischen KNOsLosungen, noch in HNOs-Liisungen in Konzentrationen unter 0,05 M HNO,, festgestellt. In neutralen und alkalischen KNOs-Lbsungen wurde das Auftreten der vortibergehende Absorption bei 430 rnp festgestellt, was dem NO,-Radikal zugerechnet werden ~011. NO,-verschwindet in langsamer Reaktion der ersten Ordnung mit der Konstante k, = (7,59 f 0,2 1) x lo3 se+.

INTRODUCTION THE FORMATION and properties of the NO,radical have been recently extensively investigated. Its ESR spectrum has been observed by CUNNINGHAM,(~)GOLDING and HENCHMAN(~)and by ZELDES and LIVINGSTON~~)in y-radiolysis of solid nitrate; by KEVAN(~) in y-radiolysis of nitrate ices and by HAYON and SAITO’~) in u.v.-photolysis and y-radiolysis of nitric acid ice. The absorption spectrum of NO, has been determined by HUSAIN and NORRISH(~) after flash photolysis of HNO, vapour and by MARTIN et uZ.(~,*) in flash photolysis of ceric nitrate in nitric acid. A similar absorption spectrum was also observed by DEMORE and DAVIDSON(~) after photolysis of a film of frozen 0, in NO,. As some doubts have arisen as to whether or not the NO,-radical is formed by the electron transfer reaction it seemed desirable to investigate that problem using pulse radiolysis. EXPERIMENTAL The pulse radiolysis apparatus and technique have been described previously.(lO-12) In the present work the electron pulse was either the mean electron energy 0.2 or 2 psec long; was about 1.8 meV, and the peak current in the pulse was generally set to about 1.0 A. All solutions were prepared from reagents of the highest available purity and water triply distilled from a quartz still. RESULTS The deaerated samples containing nitric acid or KNO, in neutral or alkaline aqueous solutions were irradiated at room temperature. The concentration of nitric acid covered the range from 10m3 to 4-O M. The characteristic spectrum of NO,, consisting of peaks at 595, 635 and 672.5 m,u, was not observed in solutions containing less than 0.1 M HNO,. At 0.05 M HNO, the spectrum was changed

and the peaks were hardly distinguishable and below 0.01 M HNO, no transient absorption could be detected. Figure 1 illustrates the transient spectrum determined in 1.0 M HNO, and in Fig. 2 the dependence of the optical density at 635 rnp on the HNO, concentration is shown. The NO, spectrum was constant through-out the whole HNO, concentration range down to 0.1 M. Saturation of the solution with 0, had no effect on the spectrum, regardless of the HNO, concentration, but addition of O-1 M methanol to O-4 M HNO, removed the transient absorption completely. The addition of acetic acid notably decreased the absorption but not to such an extent as to remove it completely. No NO, spectrum was observed in the irradiated neutral or alkaline nitrate solutions. The transient absorption produced in irradiated deaerated 0.01 M KNO, is shown in Fig. 3. Similar spectra were observed when irradiating 1O-3 or 1.0 M KNO,. The effect of pH was studied further in O-01 or 1.0 M KNO, with KOH added up to a concentration of O-1 or O-5 M. The spectrum was virtually the same as in the neutral solutions. In all neutral and alkaline solutions the optical density measured 1 ,usec after the pulse decreases with increasing wavelength, being highest in the U.V. and approaching zero at 550-600 mp. The monochromator used did not allow one to check whether the spectrum observed is a tail of the peak existing deeper in the U.V. However, some 30 ,usec after the pulse a transient absorption appears with maximum at about 430 mp. In order to check the effects of NO,and HNO, concentration on NO, production, a few irradiations of KNO,-HNO, solutions were carried out. The results are shown in Fig. 4 (curve no. 1) which also shows the effect of increasing acidity on the NO, absorption. (curve no. 2). This latter experiment covered a series of solutions containing 0.4 M KNO, in

27

77~ radiation-inducedformation of NO, in aqueous solutions

Wavelength.

FIG.

mp

1. Transient absorption spectrum produced in the irradiated solution of deaerated

::01

0.2

0.4

0.6

0.6

[HNO,]

FIG. 2. The dependence

of the NO, absorption

2.0

I.0

1 *O M HNO,.

4.0

M/I

observed at 635 rnp on the concentration

of HNO,.

28

R. K. Broszkiewicz

14 /

I\

i I2 \ \

“0 ; IO1

---

I,usec

-

30 psec

\ \

350

400

4.50

Wavelength.

FIG. 3. The transient absorption

spectrum

produced

500

550

I

600

mp

in irradiated

solutions of deaerated

0.01 M KNO,.

CKWT] I.0

0.8

I 0 I

06

I

0.2

0.4

I

I

[“NO,]

0.4

0.2

I

I

I

0.6

0.8

I.0

I

I

0

I

t

bCl047

4. The dependence of the NO, absorption observed at 635 rnp on the concentration of HNO, and KNO,. l-the solution of HNO, and KNO,, 2-the solution of 0.4 M KNO, with addition of HClO, FIG.

7% radiation-inducedformation of NO, in aqueous solutions

I

0

I

IO

I

20

I

30

I

40

I I I

50

60

Time,

FIG. 5.

70

I I

80

90

I

IO0

I

110

I

120

P-C

Test for the first-order decay of NO, in deaerated

HClO* at concentrations rising from 0 to O-8 M HClO,. In a pure solution of 1.0 M HClO, no transient absorption observable at 635 rnp was produced. These results show that the contribution of NO,--ions to the NO, production is negligible, when aqueous solutions are investigated at the room temperature. The kinetics of NO, production were observed in O-1 M, 0.4 M and 1-O M HNO, and its decay in the concentration range from O-1 to 4.0 M HNO,. Formation of NO, proceeds by a pseudo first-order reaction with rate constants k’s.r M = 1.50 x IO5 see-l and k’0.4 M = k’r.,, M = 4.20 x However the bimolecular rate conlo5 see-l. stant could not be calculated due to the lack of TABLE 1. Analysis of the first-order NO,-decay data HNO, (M)

Half-life (iusec)

k x 10-s (set-l)

0.1 0.2 0.4 0.6 1.0 2.0 4.0

90 94 89 90 94 90 92

7.70 7.37 7.79 7-70 7.37 7.70 7.53 (7.59 f 0.21) x lOssec_l

1.0 M HNQ.

precise information on the concentrations of NO,and HNO, in the nitric acid at that concentration range. The decay of NO, proceeds by first-order In Fig. 5 the plot of the first-order kinetics. decay of NO, determined in a solution of 1.0 M HNO, is shown. Similar plots were obtained for other HNO, solutions and the results are collected in Table 1. DISCUSSION The’ transient absorption spectrum of NO, determined in this work and consisting of three peaks at 595, 635 and 672.5 rnp is very similar to those observed previously by other investigators. Two peaks in the red were established at 635 and 671 rnp by MARTIN”*@in a solution of nitric acid and at 624 and 662 rnp by HUSAIN and NORRISH(~) in nitric acid vapour and by DEMORE and DAVIDSON(~) in the frozen matrix of NO,. Less data are available on the peak in the green-yellow, Martin has reported its existence at 598 m,u but the other authors noticed only the increased absorption in that region without specifying the exact wavelength. Nevertheless the whole spectrum in the range 550-700 rnp is quite well established. The U.V. spectrum is uncertain, it has been observed only by CUNNINGHAM,(~)who noticed the absorption of NO,-radical at 246 rnp. It is

30

R. K. Broszkiewict

possible that the increased absorption at 32% 300mp, shown in Fig. 1, may be a shoulder of an unrecorded peak deeper in the U.V. Two alternative mechanisms of NO, formation and decay have been recently proposed. From investigations of the flash photolysis of HNO, vapours HUSAIN and NORRI~H~~) proposed the reaction mechanism HNO,

+ hv -+ OH + NO,

OH + HNO,

-+ HsO + NO,

(1) (2)

and, subsequently NO, -NO,

+ 40,

(3)

where the NO, formation is due to the abstraction of a hydrogen atom. From the flash photolysis study of reactions occurring in solutions of ceric nitrate nitric acid MARTIN postulated the following mechanism Ce(IV)NO,NO,

+ hv -+ Ce(II1)

+ Ce(II1)

-

+ NO,

Ce(IV)NO,-

(4) (5)

where NO, is produced and decays by an electron transfer reaction. The ESR study by HAYON and SAITO(~) on U.V. and y-irradiation of HNO, ices seems’ to support the mechanism proposed by Husain and Norrish. The results presented in the previous paragraph also support this mechanism. The NO, spectrum was observed only when the concentration of HNO, exceeds 0.05 M and the optical density approached the saturation level at about 1-O M HNO, (Fig. 2). Usually in the radiation chemistry of aqueous solutions a solute concentration of about 0.001 M is high enough to scavenge all radical products of radiolysis and the need for the application of a scavenger concentration about fifty times higher could be explained only on the assumption that the reaction (2) proceeds with a very low rate However, Husain and Norrish constant. have determined that ka= 1 x lo* set-l and the decrease if any, of this value in aqueous solutions in comparison to that in vapour can not be big enough to explain the considerable threshold concentration of HNO,. It is by far more probable that in the analysis of the kinetic data not a formal but the real concentration of

the undissociated HNO, should be considered. The absence of the NO, spectrum in the irradiated neutral and alkaline nitrate solutions as well as in the dilute HNO, solutions, in conditions when solely NO,--ions and not the HNOs-molecules are present, gives a further confirmation to this view. MAHLMAN~~) gave the evidence that the reaction between NO,- and OH NO,-

+ OH -

NO,

+ OH-

(6)

most probably does not occur, and this conclusion may be also drawn from other y-radiolysis data presented by SHARPATYI and OREKHOV.(~~) The existence of the undissociated HO-NO, form in HNO, has been proved by MINC and KECKI,(~~*~~) who observed the Raman spectra of nitric acid in water and acetic acid. This method is not accurate enough to measure the concentration of HO-NO, in less than 2.0 M in aqueous solution, but, as at that HNO, concentration [HO-NO,] = 0.19 M, it seems reasonable to expect that at lower HNO, concentrations the HO-NO, form still exists, even if in traces. The conclusion that the contribution of reaction (6) to the production of NO, is negligible is in keeping with the results shown in Fig. 4. The presence of NO,--ions seems somehow to retard the formation of NO,radicals as, comparing the results shown in Fig. 2 with those illustrated by curve 1 of Fig. 4, it is obvious that the saturation effect begins in the latter case at a lower level than in the pure HNO, solution. The rate constants of reactions of e,,- and H with NO,are very highu7*18) and one should expect that even the lowest concentration of HNO, or KNO,, investigated in this work, was sufficient to scavenge all these entities. The lack of NO, formation in neutral or alkaline solutions indicates not only that radical is formed in reaction (2) but also that e,, and H do not contribute to its production. Though the bimolecular rate constant of the NO, formation could not be established, it is quite likely that the second-order rate constant in aqueous solution is similar to that found by Husain and Norrish in vapour. It has been observed that addition of HNO,

The radiation-inducedformation of NO, in aqueous solutions to oxygenated solutions of 10Vs M KCNS does not effect the absorption of the radiationinduced CNS-radical, regardless of the applied HNO, concentration in the range from 5 x IOW to O-8 M. The measurements were made at 500 m,u, as described by ADAMS. As the CNS-radicals are produced in following reaction

CNS-

+ OH -+ CNS

+ OH-

(7)

it implies that NO, is as effective an oxidizing agent as OH and oxidizes the thiocyanide in the reaction NO,

+ CNS-

--+ CNS + NO,-.

(3)

It proves altogether that NO, was always formed in the duration of the pulse, so that the reaction (2) is very fast. The first-order decay of NO, is relatively slow and probably undergoes reaction (3). In the work reported here no evidence has been found in favour of a bimolecular decay NO,

+ NO,

-+ 2N0,

+ 0,

(9)

The transient absorption developing after 30 ,usec in the irradiated neutral or alkaline solutions (Fig. 3) is apparently that of NO,. A similar spectrum has been reported by CIJNNINGHAM,(~,~O) who irradiating solid KNO, found the NO, maximum at 410 rnp and established that this species is relatively stable at 300’K. ADAMS et uL.,(~~)in a pulse radiolysis study of neutral nitrite solutions observed the formation of a transient peak at ca. 400 m,u immediately following the pulse. They have attributed the formation of this peak to the NO,-radical arising from the reaction of OH with the nitrite ion. The spectrum observed in this work is probably due to the decay of the NOsV2 produced from the reaction of e,, with the NO,-. CONCLUSIONS The formation of the NO,-radical has been observed solely in irradiated solutions of nitric acid, at concentrations exceeding 0.05 M HNO,. It has a transient spectrum with well defined peaks at 595, 635 and 672.5 rnp and a possible peak in the U.V. below 300 m,u. No formation of NO, has been observed in neutral or alkaline solutions of KNO, and in solutions of HNO, at concentrations below that of O-05 M.

31

From the effects of scavengers such as methanol and oxygen it is possible to establish that NO, arises from the reaction of OH with the undissociated HO-NO, molecule. The effects of pH and of HNOs-KNO, concentrations on the formation of NO, indicate that the rate constant of reaction (6) is very small, if this reaction proceeds at all. These results are in keeping with those by HUSAIN and NORRISH(~) and HAYON and SAITO(~) but not with those Of MARTIN et ~1.‘~~~)It seems to prove that both the formation and decay of NO, in aqueous solutions at the room temperature proceed by reactions other than the electron transfer. NO, disappears in a slow, first-order reaction probably in accordance with equation (3) and not equation (9), the rate constant is k, = 7.59 x lo3 se+. It is a strong oxidizing agent, reacting with CNS- in a similar way to OH. In irradiated neutral and alkaline solutions of KNO,, the formation of a transient absorption at 430 rnp has been observed. This absorption is most probably due to the radical NO,. author is indebted to the International Atomic Energy Agency for the provision of a Fellowship, to the British Empire Cancer Campaign, Research Unit in Radiobiology for the use of the pulse radiolysis facilities and in particular to Dr. G. E. ADAMS and Mr. B. D. MICHAEL for all the assistance throughout the experiments and many valuable discussions. Thanks are also due to Professor J. W. BOAG for his interest and criticism.

Acknowledgments-The

REFERENCES 1. CUNNINGHAMJ. Phys. Chem. Solids 23, 843 (1962). 2. GOLDING R. M. and HENCHMAN M. J. them. Phys. 40, 1554 (1964). 3. ZELDES H. and LMNGSTON R. J. them. Phys. 41, 4011 (1964). 4. KEVAN L. J. phys. Chem., Ithaca 68, 2590 (1964). 5. HAYON E. and SAITO E. J. them. Phys. 43, 4314 (1965). 6. HUSAIN D. and NORRISH R. G. W. Proc. R. Sot.

273A, 165 (1963). 7. MARTIN T. W., HENSHALL A. and GROSS R. C. J. Am. them. Sot. 85, 113 (1963). 8. MARTIN T. W., R~MMEL R. E. and GROSS R. C. J. Am. them. Sot. 86, 2595 (1964). 9. DEMORE W. B. and DAVIDSON N. J. Am. them. Sot. 81, 5869 (1959). 10. HART E. J. and BOAG J. W. J. Am. them. Sot. 84,409O (1962).

32

R. K. Broszkiewicz

11. ADAMS G. E., BOAC J. W. and MICHAEL B. D. Trans. Faraday Sot. 61, 492 (1965). 12. ADAMS G. E., BOAG J. W. and MICHAEL B. D. Pulse Radiolysis, p. 13 1. Academic Press, London (1965). 13. MAHLMAN H. A. J. phys. Chem., Ithaca 64, 1598 (1960). 14. SHARPATYIW. A. and OREKHOVV. D. Nukleonika 4, 487 (1959). 15. KECKI Z. Prace Konferencji Elektrochemicznej (Pro-

ceedings of the Conference on Electrochemistry), Warsaw (1955), p. 687. PWN, Warsaw (1957). 16. MINC S. and KECKI Z. Roczn. Chem. 30,935 (1956).

17. THOMAS J. K., GORDON S. and HART E. J. J. phys. Chem., Ithaca 68, 1524 (1964). 18. ANBAR M. and MEYERSTEIN D. Unpublished results ct ANBAR M. and NETA P. Int. J. appl.

Radiat. Isotopes 16, 227 (1965). 19. ADAMS G. E., BOAG J. W., CURRANT J. and MICHAEL B. D. Pulse Radiolysis, p. 117. Academic Press, London (1965). 20. CUNNINGHAMJ. J. fihys. Chem., Ithaca 67, 1772 (1963). 21. ADAMS G. E., BOAC J. W. and MICHAEL B. D. Trans. Faraday Sot. 61, 1674 (1965).