C. Gopinathan, P. S. Darnle, and E. Hart
3694
tron is little influenced by the pressure whereas the slight decrease of A might be accounted for by the increased hindrance of the rotation of dipoles in the bulk which suppresses the polarity of the solvent. The temperature effect on the spectrum is much more conspicuous as shown in circles of Figure 5.13 The coefficient A decreases rapidly with temperature and at the extreme temperature of 390" it becomes as low as 0.15. Since a t this supercritical temperature most of the water molecule may be in the monomeric form and be vigorouslv agitated,14 the effective potential for the optical electron would not be of a long-range type as shown by Figure 1A or B in the preceding paper and the situation for the electron may be similar to that in paraffins. The decrease of the ground-state energy at higher temperatures is explicable by the temperature dependence of the terms in (-p/3hTRz) eq I; above all, the decrease in (cos and N and the increase in R will be effective for the de-
stabilization. It is emphasized that the change of the absorption line shape from symmetric to asymmetric upon the temperature rise is automatically explained by the present theory in terms of the decrease in the value of A which means the gradual change of the major component of the absorption from the 1s-2p bound-bound to the bound-free transitions.
Concluding Remark Considering that the hydrogenic model is coarse graining in the sense that it ignores the microscopic structure of the solvent, it is gratifying that the model can account for the difference of the solvent molecule to a considerable extent and extract some information proper to the electron in the solvent. (13) B. D. Michael, E. J. Hart, and K. H. Schmidt, J. Phys. Chem., 75, 2798 (1971). (14) W. A. P. Luck, Ber. Bunsenges. Phys. Chem., 69, 626 (1965); J. Phys. Chem., 74,4006 (1970).
y-Ray irradiated Sodium Chloride as a Source of Hydrated Electrons' opinathan, P. S. Damle, and Edwin J. Hart* Chemistry Division, Argonne National Laboratory, Argonne, lllinois 60439 (Received March 75, 7972)
Publmtion costs assisted by Argonne National Laborafory
Hydrated electrons form when y-ray irradiated sodium chloride dissolves in water. By dissolving the salt in aqueous solutions with different N2O and 0 2 concentrations, N2 evolves in amounts expected for eaqNzO], of 2.3 f 0.1 is found for disBelow 0.0004 M 0 2 the ratio of rate constants, h[eaq- + Oz]/h[e,,solving irradiated NaCl compound with a ratio of 2.4 f 0.1 in electron or y-ray irradiated solutions. At higher 0 2 concentrations, 0 2 is a more effective scavenger of the precursor of eaq- and the rate constant ratio for this species reaches 14.8. In 02-free solutions, the N2 yield is independent of the NzO concentration in the range 0.0003-0.010 A4 N2O and approximates the F center yield. The mechanism of eaq- formation in dissolving irradiated NaCl is briefly discussed.
+
Introduction
For nearly 2 decades it has been known that irradiated sodium chloride induces chemical effects when it dissolves in ~ a t e r . ~Hydrogen -~ and iodine form when solution takes place in aqueous i ~ d i d e Light .~ e m i ~ s i o nand ~ , ~the oxidation of ferrous ion8 were discovered a little later. Since then, other s t u d i e ~ ~reveal - ~ ~ that fluorescent solutes intensify the emission, whereas other solutes such as 0 2 , NOa-, and reducing agents quench this emission. While the possibility of direct excitation of water has been considered, these phenomena have generally been attributed to the release and reaction of hydrated electrons. There has been B revival of interest in this field. A new explanation has been published,12 postulating the release of triplet excitons when irradiated sodium chloride dissolves in water. If this is indeed the case, then the appliThe Journal of Physical Chemistry, Vol. 76, No. 25, 7972
cations to aqueous radiation chemistry are obvious since evidence for exciton reactions in the radiation chemistry of frozen aqueous systems has been reported r e ~ e n t 1 y . A l~ (1) Work performed under the auspices of the U. S. Atomic Energy Commission, (2) M .Hacskaylo and D. Otterson, J. Chem. Phys., 21, 552 (1953). (3) M. Hacskaylo and D. Otterson, J. Chem. Phys., 21, 1434 (1953). (4) W. G. Burns and T. F. Williams, Nature (London), 175, 1043 (1955). (5) J. G. Rabe, B. Rabe, and A. 0. Allen, J . Phys. Chem.. 7 0 , 1098 (1966). (6) T. Westermark and 5. Grapengiesser, Nature (London), 188, 395 (1960). (7) T. Westerrnark and B. Grapengiesser, Ark. Kemi, $ 7 , 139 (1961). (8) T. Westermark, 6. Grapengiesser, and N . Biesert, Ark. Kemi. 17, 151 (1961). (9) G. Ahnstrbm, Acta Chem. Scand., 19, 300 (1965). (IO) L. E. Eriksson,Acta Chem. Scand., 16, 2113 (1962). (11) B. Lelierere and -1. P.Adioff, J. Phys. (Paris), 25, 789 (1964). (12) J. P. Mittal, Nature (London), A230, 160 (1971). (13) P. N. Moorthy, C. Gopinathan, and K. N . Rao, Radiat. E f f . . 2, 175 (1970).
y-Ray Irradiated Sodium Chloride model of aqueous radiolysis involving excitons has also been proposed.14 l5 However, a later group of workers16 support the earlier suggestion that hydrated electrons are involved in the light emission process. We attempt to establish beyond controversy the nature of the chemical reducing species released into water when irradiated salt dissolves, as this may have important consequences for the radiation chemistry of water and of the structure of the hydrated electron I
Experimental Section For all experiments where salt was irradiated, Merck Reagent Grade NaCl (Batch 7407) was used without further modification. Where aqueous solutions of salt were irradiated, analytical reagent grade NaCl or the above batch of NaCl was used. The water used was triple distilled in the normal way. All other chemicals used were of analytical reagent grade. The gases used were Matheson Research Grade. The hydrogen yields were measured by taking a known weight (-3 g) oE irradiated NaCl in a syringe, flushing it for 15 min with a stream of argon gas, and then pushing a sufficient amount of argon-purged 0.4 M H2S04 into the syringe until the salt dissolved. The resultant solution was drawn into a Van Slyke apparatus, and the hydrogen gas equilibrated by stirring and analyzed on a n attached gas chroma tograp h. A more elaborate procedure was followed for determining Nz yields from N20 solutions. Water adjusted to pH 11 with CO&-free NaOW was carefully degassed on a vacuum line and saturated with N2O that had been purified by freezing and pumping. 02-saturated pH 11 solutions were similarly made. Mixing these two solutions in the proper ratios in syringes gave solutions containing the A known amount (6.14 g) desired amounts of N2CI and 0%. of irradiated NaCl was taken in a Pyrex cell provided with a fritted glass disk and a three-way stopcock. This dissolver unit was attached to a vacuum line and pumped for about 15 min until the residual pressure was less than lo-* mm. Helium was then introduced and allowed to flow throughout the salt at atmospheric pressure. With helium flowing, the cell was removed from the vacuum line and fitted into the 5/20 cone of the Van Slyke apparatus containing 20 ml of the solution to be used for dissolving the salt. 'This solution was forced up through the salt, effecting complete solution. The resultant solution and evolved gas were replaced by Hg and moved into a syringe that was then detached from the dissolving unit. This solution and gas mixture was next drawn back into the Van Slyke apparatus and equilibrated and the evolved Na was measured on the molecular sieve column used for the Hz analysis. All unirradiated NaCl controls gave a small amount of Nz. We have subtracted this amount from the Nz yields shown in Table I and in the figures. Whereas this correction was insignificant at the higher N2 measurements, it amounted to about 30% for the lower N2 yields. For this reason experimental fluctuations become greater in solutions containing the higher concentrations of K20 and 0 2 . The light emission was measured with a 1P28 photomultiplier. A Pyrex tube containing water or the required aqueous solution was clamped directly onto the window of the tube. T h e irradiated sodium chloride was dropped in through a funnel-shaped tube. The photomultiplier output was fed into an oscilloscope through a resis-
3695
tor-capacitor (RC) coupling and the trace was photographed. Any distortion produced by the RC coupling is unimportant for the present work as only relative emissions for various solutions have been given. 60Co y rays were used for all irradiations except those described below in which emission from Linac-irradiated NaCl was sought. The dose delivered to the NaCl was calculated from the dose measured in the Fricke dosimeter (G(Fe3f) = 15.6) multiplied by the ratio of densities of NaCl to 0.8 N H2S04 (2.165/1.024). Results 1. Light Emission. In agreement with earlier workers, we find that when irradiated salt dissolves in water, light emission takes place. We also find that the addition of potassium nitrate efficiently quenches this emission. With 0.0001 M NO3- the emission falls to about 50% cbmpared with the emission from a similar sample added to heliumpurged, solute-free water. The presence of 0.001 M NO3reduces the emission by 90 f 10%. The intensity of the initial light pulse obtained in the absence of solute was used to calculate the percentage decrease in emission. A similar quenching by N o s - of irradiated NaCl fluorescence in fluorescein solutions has already been reportedg so we did not continue our emission measurements. 2. A Search for Fluorescence by Pulse Radiolysis Technique.17 A saturated chlorine water solution (pH l.l), a dilute chlorine water solution (pH 3.3), and sodium bromide solutions in the concentration range 10-*-1 M (neutral pH) were irradiated with 3-psec electron pulses (6.0 MeV) from a linear accelerator.18 Under the same conditions no emission could be detected after the end of the electron pulse in the wavelength region 400-600 nm apart from the fluorescence emitted from a similar wa,ter filled cell. 3. Hydrogen Yields. Dissolving the irradiated salt in 0.4 M H2S04 (argon-purged) produces hydrogen, in agreement with earlier ~ o r k . The ~ . ~hydrogen yield is given as a function of the time of irradiation of NaCl in Figure 1. The reason for the bad scatter of points at higher doses is not known, but the actual yields are not of much significance since small and irreproducible amounts of oxygen remain or are adsorbed on the surface of the salt even after argon purging. Not only is 0%a quencher of light emission but it also lowers the Hz yields by forming H 0 2 from the H atoms liberated by the reaction of the eaqwith Hf.However, the yields are significant inasmuch as they show that the amount of H2 increases with dose establishing that the H2 yield arises from the defect centers in the irradiated salt as was earlier ~ o n c l u d e d The . ~ smali yield of H2 occasionally found in our unirradiated salt is difficult to explain. This may come from impurities or from a small concentration of defects already present in the salt. 4 . Nz Yields from N2O. Dissolution of irradiated NaCl in alkaline N2O solutions releases nitrogen with a yield that increases with dose. The effect of N2O concen(14) C. Gopinathan, Proc. Depf. Atomic Energy Syrnp. Chem., Chandi-
garh, India, 2, 196 (1960). (15) C. Gopinathan, Proc. Symp. Radiat. Chem,. Prombay, India, Bhabha Atomic Research Center Report 489, 28 (1970). (16) H. J, Arnikar, P. S. Damle, and 6. D. Chaure. Radiochem. Radioanai. Lett. 5, 25 (1970);J. Chem. Phys., 55, 3668 (1971). (17) These experiments were suggested by Professor H. J. Arnikar of the University of Poona, Poona, India. (18) C.Gopinathan, P. S. Damle, and E. J. Hart, unpublished work. The Journal of Physical Chemistry, Yo/. 76, No. 25, 1972
C. Gopinathan, P. S.Damle, and E. Hart
3696 I20
I00 0-
so
x
0
i) 0
z cn
60
ol
a N
*
40
I I 20
Minutes, irradiation 2.0
~ j g 1. ~ ~ Hydrogen e
evolution from dissolved y-ray irradiated crystalline NaCl : dose rate = 49.7 i 0.6 kradslmin. Lration on Nz yield is shown in Table I for NaCl irradiated to a dose of 4.5 Mrads. These data reveal that the N2 produced by the irradiated salt is independent of the NzO concentration in the range 0.0001-0.003 M N20. Furthermore, if we assume that the dose given in the present work saturates the NaCl with F centers, then a t least 70% of the F centers reported by Ahnstrom9 for coarse NaCl produces T\J2 accordink to the reactions
---+
4.0
6.0
8.0
[02j 104 Figure 2. Effect of oxygen concentration on nitrogen evolution during dissolution of y-ray irradiated NaCl in 0.001 M N 2 0 sohtions: y-ray dose = 4.5 Mrads.
t cn 0.06t
F centers %eaqeaq--
+
N,O
N2
I
0-
M , the mean separation At a Concentration of 3 x of PJZO molecules is 821 A and the species produced from F centers must travel about half this distance on the average to react with NaO. Since there is no increase in the Na yields over a 30-fold increase in NzO concentration we conclude that there is close to a 1:1 conversion of F centers into eaq- and that short-range neutralization of eaqby the “hole species” does not occur in these NzO solutions. ‘The quenching of a major fraction of the light emission by 0,0001-0.001 M N o s - also suggests that this process, too, involves eaq- . LE I: Effect of N 2 0 Concentration on Nz Formation from iesolved NaCl Irradiated to a y-Ray Dose of 4.5 X l o 6 Rads [NzOj, M
,umoi of N2/ g ot NaCl
[NzOI, M
pmol of N2/ g of NaCl
1.0 x 10-4 3.0 X
0.058 0.071
1.0 x 10-3 3.0 x 10-3
0.057 0.067
5. NzO-02 Competition. In order to establish the nature of the species reacting with NzO, we investigated the effect of added 0 2 . The NaCl in all cases was irradiated to a dose of 4.5 Mrads. The results are shown in Figures 2 and 3. Figure 2 shows the effect of changing oxygen concentration in a 10-3 M NzO solution at pH 11. Figure 3 shows the effect of changing NzO concentration in a pH i l solution containing 5 x M 0 2 . It is clear from these figures that 0 2 a t higher concentrations reacts much faster with the “hydrated electron” in these systems than the ,$[e,,02]/k[e,,- f N201 ratio of 2.30 obtained f r o m the rate constants.*g 2 0 x 101* and 8.7 x 109 M - l
+
X e Journal of Physical Chemistry, Vol. 76,No. 25, 1972
[NzO]
X
!03
Effect of nitrous oxide concentration on nitrogen evolution during dissolution of y-ray irradiated NaCf in 0.0005 M 02 solutions: y-ray dose = 4.5 Mrads; pH 11. Figure 3.
sec-l, for the reactions eaq- with 0 2 and N20, respectively. The kinetic plot of Nz yields in y-ray irradiated pH 11 solutions containing NzQ and 0 2 shown in Figure 4 confirms this ratio. This plot gives a h[e,,- +- OZ]/k[e,,- +NzO] value of 2.54 in excellent agreement with the published ratio cited above. A similar ratio was obtained in y-ray irradiated 5 M NaCl solutions containing 0.0001 M 0 2 with variable N20.
Discussion As its concentration increases 0 2 becomes increasingly more effective than NzO in scavenging eaq- generated upon the dissolution of irradiated NaCl crystals. This unexpected result is clearly brought out in the kinetic plots of Figure 5 . The lower curve of this figure is for the 0.001 M N20, variable 02 data of Figure 2. Instead of the -t normal linear relationship, with a h(e,,- f O,)/k(e,,NzO) ratio of 2.3 the slope of this plot increases as the [02]/[NzO] ratio increases. From the initial slope of this curve we calculate a rate constant ratio, k(e,,- f 0 2 ) / (19) E. J. Hart and M. Anbar, “The Hydrated Electron,” Wiley-lnterscience, New York N. Y., 1970.
y-Ray Irradiated Sodium Chloride
--
3697 20 o
1 -
15.0
Figure 4. Kinetic plot of nitrogen yield in y-ray irradiated solutions containing 0 2 and 0.001 M N20: y-ray dose = 4.5 Mrads;
ptl11.
+
hie,,NzC)), of 2.3 Sr 0.2. On the other hand a rate constant ratio of 14.8 is calculated from the upper curve of Figure 5 obtained from a kinetic plot of the high 0 2 concentration data of Figure 3. This ratio is about six times the normal value of 2.30. Note too that the slope of the lower curve approaches that of the upper curve at high [ 0 2 ] / [ N 2 0 ] ratios thereby supporting our conclusion that the effectiveness of 0 2 in scavenging e,,, - (or its precursor) increases with increasing concentration. The solubilities of 320 and 0 2 were determined in a saturated NaCl solution at room temperature in order to prove that the changing solubility of the gases as the NaCl dissolves does not affect cine rate constant ratios. They were, respectively, 5.8 x and 2.1 x M. Comparing these values with the known solubilities of N 2 0 and 0 2 , 2.5 X 10-2 and 1.25 X 10-8 M , respectively, in water it is seen that the 0 2 concentration is, in fact, diminished more than that of NzO.Therefore, the lowered solubility of these gases cannot be the cause of the higher rate constant ratio for i.rradiated salt. In actual practice, of course, it is likely that the reactions take place under supersaturated conditions without sufficient time being available for the release of gases. Since 0 2 and KzO are neutral moleculee kinetic sa?t effects may be ruled out as was experimenta!.ly d.emonstrated above by carrying out Nz0 and 0 2 competition measurements in y-ray irradiated 5 A4 NaC1. Both as shown in the present work as well as in earlier work, hydrated electron scavengers are able to quench light emission at very low concentrations as has been shown above a4 well as in earlier work.9-11-16This result together with the fact that F centers are converted into eaq- very efficiently suggests that the F center release is essentially R bulk process. That the variation of NzO concentration has no effect on the yield confirms this view, and also suggests the unimportance of any “short-range” neutralization process. Our NzQ-02 competition data confirm eaq- formation but also support the existence of a second species which reacts much faster with 0 2 than with NzQ. According to Figure 5 it seems that the reaction with this second species becomes more important at higher 0 2 concentrations. However, it is difficult to say whether the rate constant ratio at low 0 2 concentrations is identical with the normal ratio of 2.30 or not. At the moment, therefore, it is possible only to speculate about the nature of this second species of unusually high 0 2 re-
E02l/[N4
Figure 5. Kinetic plots of nitrogen yield [N20] ratio in solutions prepared from
as 2 function ot [02]/ irradiated NaCi (data
from Figures 2 ( 0 )and 3 ( 0 ) .
activity. Among the possibilities to be considered are an exciton and a “hydrated electron” (“eaq-’l) still under the electrostatic influence at the “hole,” which must be released into the solution, along with ‘‘eaU-.” While the exciton alternative looks very attractive, since it is easy to imagine trapped energy being directly transferred to water, there are certain drawbacks. A Frenkel exciton is defined as a quantum of delocalized excitation in an ordered matrix.20’21 The structure in water, being essentially short range, will therefore not allow the movement of the excitons to the extent required. It is interesting to imagine this new species as a different form of eaq-. When the NaCl structure breaks up, the trapped electron is probably directly transferred from its defect site in the NaCl lattice to a corresponding site in the water structure, assuring instantaneous hydration. At the moment we can only speculate why there is a different “primordial” form of eaq- which reacts very much faster with 0 2 . The trapped positive holes in the NaCl lattice, whatever form they are in, must also be released into the water along with the electron. If “eaq-’’ comes under the electrostatic influence of a hole during release into water, it will form a species which is strongly paramagnetic, thereby explaining our fast reaction with oxygen. The form in which the hole is released does not matter. Only a certain amount of mobility of the hole-e,,pair is required. Disappearance of the hole will liberate eaq-. This hole-e,,pair is similar to the excitonic species (HzOf-..e-) already postulated.22 On the basis of the oxidation of tetramethyl-p-phenylenediamine (TMPD) to Wursters blue by dissolving irradiated NaC1, a positive charge, presumably HzQf, has rebeen suggested as the oxidant. The reducing species is unquestionably eaq- as has been demonstrated by our work and by the reduction of tetranitromethane22 and by other eaq- scavenger^.^^ These studies may contribute little to an understanding of the radiolysis of pure water or dilute aqueous solutions. (20) D. L. Uexter and R . S. Knox, “Excitons,” Interscience, New York.
N. Y., 1965. (21) J . Frenkel, Phys. Rev., 1276 (1931). (22) J. P. Mittai and J, Shankar, Radiochem. Radioanal. Left.. 6, 115 (1971).
(23) J. P. Mittai to C. Gopinathan, private communication. The Journal of Physical Chemistry, Vol. 76, No. 25, 1972
C. Gopinathan, G. Stevens, and E. J. Hart
3698
However, irradiated salt may be looked upon as a source of low-energy electrons and when dissolving in water as a source of eaq- .24 But whether the hole-e,, - pair postulated as responsible for our "oxygen" effect forms in pure water remains an unresolved question. Predictions that oxygen at 0.001 Ad would lower the eaq- yield in subnanosecond time periods could not be confirmed by our pre-
liminary experiments with Argonne's 50-psec electron pulse.
Acknowledgment. C. G. wishes to thank Dr. J. Shankar for helpful discussions and encouragement and Dr. J. P. Mittal for discussions on recent unpublished data. (24) n. J. Arnikar to E. J. Hart, private communication.
Fluorescence sf the Uranyl Ion in Electron-Irradiated Sulfuric Acid Solutions'
. Gopinathan, G , Stevens, and Edwin J. Hart* Chemistry Division, Argonne Nationai Laboratory, Argonne, lllinois 60439 (Received March 15, 7972) Publication costs assisted by Argonne National Laboratory
The characteristic fluorescence of the UOz2+ ion is obtained in sulfuric acid solutions by irradiation with 6.3 MeV electrons and by tritium 0 rays from 3HOH. The effect of U0z2+ concentration in the range from 0.007 to 0.50 M UOzS04 in 0.9 M sulfuric acid is reported. Although Cerenkov light emission may contribute to the fluorescence, it is not the primary source of excitation. Energy transfer from water to the U0z2+ ion and excitation by water subexcitation electrons are briefly discussed as possible mechanisms.
Introduction Although the phenomenon of excitation energy transfer is well established in organic systems consisting of complex molecules, direct energy transfer in aqueous systems remains an open question. We have studied the fluorescence of the aqueous UOz2+ ion in an attempt to elucidate the role of excitation energy transfer as a primary process in the irradiation of liquid water. The U0z2+ ion fluoresces by direct photoactivation in aqueous solution.2 Its emission spectrum centered a t 515 nm is assigned to a partly forbidden transition. A number of studi e ~ 3 of - ~ the light emission produced when water or ice is irradiated with ionizing radiation have appeared. In most of these studies the light emission is attributed to causes other than the fluorescence of HzO and since water is a simple low molecular weight molecule, it is not an ideally fluorescent materiai. Therefore, studying the light emission process in aqueous solutions containing well-known fluorescent ions seems to be a good way of establishing possible energy transfer processes in water. Our work was carried out with solutions of 0.90 M or higher in HzSQ4 in order to intensify the fluorescent spectrum and to eliminate reactions of en,-- with U0z2+ E x ~ e ~ i ~Section e n t ~ ~ Purified uranyl sulfate (recrystallized reagent grade) was dried in a vacuum desiccator before dissolving in HzSO4. The other chemicals (to be listed) were of analytical reagent grade, and were used without further purification. Pure tritiated water of 3000 Ci/ml activity was obtained by the oxidation of tritium gas by 0 2 at the surface of a palladium thimble using a procedure already described .9 The Journal of Physical Chemistry, Vol. 76, No. 25, 1972
This water was diluted with deaerated triply distilled water to an activity of about 10 Ci/ml. From this water we prepared solutions of the desired radioactivity. The faint emission from the tritiated water solutions was measured by an integrating device consisting of a light chopper, a monochromator, a phototube, a picoammeter, and a 400 channel analyzer. The light chopper disk had 6 equidistant symmetrical slots and rotated at exactly 1 rpm. The tritiated solutions, approximately 1 ml in volume, were contained in sealed Pyrex tubes. The emitted light was focussed by a quartz lens onto the slits of the monochromator with the light chopper between the monochromator and the lens. The light emerging from the monochromator was collected by a phototube whose output was read on a picoammeter and its signal stored in the 400 channel analyzer and displayed on a cathode ray oscilloscope. The sweep time was adjusted so that the 400 channels were covered in exactly the same time that one light and one opaque portion of the chopper moved across slit. By this arrangement it was possible to add the Work performed under the auspices of !he U. S. Atomic Energy Commission. C. A. Parker, "Photoluminescence of Solutions," Elsevier, New York, N. Y., 1968, p 472. L. I . Grossweiner and M. S. Matheson, J. Chern. Phys., 20, 1654 (1952). L. I. Grossweiner and M. S . Matheson, J , Chern. Phys.. 22, 1514 (1954). W. M. Jones, J. Chern. Phys., 20,1974 (1952). J. A. Ghormley and A. 0. Allen, Oak Ridge National Laboratory Report ORNL-128 (1958). D. N. Sittratamarao and D. F. Duncan, J. Phys. Chem., 67, 2126 (1963). G. Czapski and K. Katakis, J. Phys. Chern., 70, 637 (1966). E. J. Hart, J. Phys. Chem., 56, 594 (1952)
