17 The Interconvertibility of e¯ and H JOSEPH RABANI* Argonne National Laboratory, Argonne, Ill.
The conversion of e¯ into H atoms and the re action of H atoms with OH are reviewed. ¯
e¯ reacts with acids to form H atoms. The reactivity is correlated with the dissociation constants of the acids. The relation is in ac cordance with a general acid catalysis for the Only one conversion of e¯ into H atoms. base, OH , has been found as yet to be effec tive in converting H atoms into e¯ . The rate constant of 2.2 Χ 10 M sec. can be cal ¯
7
¯1
culated from different results. and k
¯1
From
k
¯
H+OH
, pK = 9.6 is calculated for the dis
e+H2O
sociation constant of the hydrogen atom.
ionizing radiation provides the most convenient method of obtaining e'aq and H atoms i n water. It is believed that homogeneously dis tributed e'aq, H , O H , H 0 +, O H " , H 0 , and H are formed within 10 " sec. as a result of the absorption of the ionizing radiation. O f these so-called " p r i m a r y products," the last four are stable species and their properties are well known. U n t i l 1960, the reactions of e~ , H , and O H had been studied only indirectly. B y adding various radical scavengers, rates of disappearance of these scavenger solutes could be studied, or the nature and rates of formation of stable products. In many cases, the immediate products of the scavenging reactions are also radicals. T h e mechanism may be very complicated and the final products only of little value understanding i n the mechanism. W i t h the development of very high intensity radiation sources, the reactions of the primary species can now be followed directly (26). B o t h optical spectra and electrical conductivity have been used to follow chem ical reactions induced b y pulse radiolysis. e~ and O H radicals have optical absorptions that can be easily followed. H atoms cannot be 8
2
2
8
2
ag
aq
* Permanent Address:
Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel.
242
17.
RABANI
243
Interconvertibility
observed directly, but very often the product of their reaction with a scavenger can be followed spectroscopicaUy. Among other methods of forming eâq or H atoms i n aqueous solutions, the more important are: (a) generation of H atoms by a discharge in H gas. T h e partially dissociated gas is then bubbled through water. T h i s method has been developed by Czapski and Stein (13) and has the advantage of forming initially only one type of reactive species—i.E., H atoms. T h e greatest disadvantage of this method is the inhomogeneous radical distribution i n the water. There are higher concentrations near the surface of a bubble; (b) eâq can easily be obtained photochemically. Jortner, Ottolenghi and Stein (32) have shown that eâq is obtained i n the photochemistry of halide and O H ~ ions. Matheson, M u l a c , and Rabani (38), and Grossweiner, Swenson, and Zwicker (22) confirmed this observation using a flash photolysis apparatus. C N S ~ and F e ( C N ) " also formed eâq upon absorption of light. 2
e
The Radiolytic Production of the Reducing
4
Radicals
In the 1940's and early 1950's, H atoms were believed to be the only reducing radicals produced by irradiation (41\ 54). T h e possibility that eâ may take part i n a chemical reaction was also considered (19, 51, 55), but no experimental evidence was used to support this view until 1958. Baxendale and Hughes (7) observed i n aqueous methanol solutions a decrease i n G H going from 0.12V to neutral solutions. T h e y concluded that at the higher p H a reducing species which is unable to dehydrogenate is produced, and suggested an equilibrium between eâq and H , shifted towards H i n the acid. H a y o n and Weiss (29) used chloroacetic acid as a scavenger for the reducing radicals i n irradiated water. T h e y found that dehydrogenation of chloroacetic acid was the major reaction i n the very acid region, while in less acid solutions dechlorination took place at the expense of the dehydrogenation. T h e sum of dehydrogenation and dechlorination was constant throughout the region. T h i s indicated two interconvertible species wherein one could be converted to the other by H 0 . Barr and Allen (6) measured the effect of 0 on the chain reaction induced by ionizing radiation to combine H and H 0 to water. H 0 can be reduced by both H and e~ to O H . q
H 2 S O 4
3
+
2
2
2
2
2
2
aq
H 0 2
H 0 2
+ e
— OH + O H -
aq
2
2
(1) (2)
+ H — OH + H 0 2
O H radicals are converted to H atoms by their reaction with H . 2
OH + H
2
H 0 + H 2
(3)
In this system, Reaction 3 supplies the reducing radicals which carry on the chain. Barr and Allen found that 0 was very efficient, compared to H 0 , i n scavenging the reducing radicals formed by 3. T h i s was i n 2
2
2
244
SOLVATED ELECTRON
contradiction with a ratio of 1.85 (3) between the scavenging efficiency of 0 and H 0 towards the reducing radicals produced directly by the radiation in neutral solutions. T h e result forced Barr and Allen (6) to conclude that the reducing species formed by Reaction 3 were different than the primary species formed by the radiation i n neutral p H . Allan and Scholes (2) obtained quantitative evidence that one form of the reducing radicals (they assumed it was e' ) was converted to the other by reaction with H 0 + . Their competition experiments showed that acetone could be reduced by e~ i n competition with Reaction 4. 2
2
2
aq
8
ag
eâq + H O + s
H + H 0
(4)
2
M a n y others confirmed later the existence of two different species, stoichiometrically equivalent to H atoms, but different in reactivity. A n unequivocal proof, however, that the neutral form is e q has been obtained by Czapski and Schwarz (12). T h e y showed that the ionic strength effect on reactions of e~ was indeed as expected with unit negatively charged species. T h i s result was verified by other workers (11). Finally, the development of pulse radiolysis enabled a direct observation of eâq, and a direct distinction between e~ and H could easily be made. Matheson (37) (with spectroscopic data obtained by Keene) suggested that e has optical absorption i n the visible. H a r t and Boag (26) used spectrographs plates and studied this absorption. T h e effect of solutes, which were known as electron scavengers led to the conclusion that the absorption was due to eâg- It was confirmed later, that the absorption belonged to unit negatively charged species by means of a salt effect (20) as well as by conductivity measurements (49). M a n y more papers on the absorption spectrum and rate constants of the hydrated electron have since appeared (16). a
9
aq
ag
aQ
y
Evidence for an Independent Yield of H Atoms Allan and Scholes (2) suggested an independent yield of H atoms in neutral solutions. A s will be discussed later, eâg can be converted to H atoms by reaction with H 0 + and other scavengers. In the acid p H region, hydrogen atoms will dehydrogenate an appropriate organic solute, R H , according to Reaction 5. 3
2
H
+ RH
2
— H
2
+ RH
(5)
T h e experimental yield of H is then, G ( H ) equal to the sum of the radical yield of eâg G , of H , G and of the molecular yield of H , G H . Allan and Scholes (2) investigated 2-propanol solutions and have shown that an electron scavenger, acetone, could compete with H + for eâ . A decrease of G ( H ) was observed at lower [H +]/[acetone] ratios. However, under conditions that all eâg were supposed to react with acetone without the formation of H or H , G ( H ) = 1.05 was obtained. Allan and Scholes concluded G H = G ( H ) - G = 1.05 - 0.45 = 0.6 molecules/(100 E.v.). 2
f
e
2
H
2
2
e
2
2
2
2
H L
17.
RABANI
245
Interconvertibility
Rabani and Stein (43, 46) measured the relative rate constants of H atoms i n neutral solutions with various scavengers and compared them to acid solutions. T h e relative reactivity of H atoms towards ferricyanide, 2-propanol, glucose, glycerol, formate, and methanol was the same in neutral and acid p H . (Formate results have been corrected for the dissociation of the acid.) Scholes and Simic (48) confirmed these observations and extended them to additional scavengers. There is now little doubt that G H (the neutral yield of H atoms) c~ 0 . 5 , but there is no agreement about the precursor of these H atoms (4, 28, 35). Conversion
of e~ to H aq
It has been mentioned that eâq can be converted to H atoms b y Reaction 4. Lifshitz (36) has found that dihydrogen phosphate ions increased G ( H ) i n neutral formate solutions and suggested Reaction 6 followed b y 5. H e r results were 2
eâq + H P 0 2
H + HPO4-
4
(6)
2
confirmed by others (31), who used both ionizing radiation and ultraviolet light to produce eâq» Other acids, were also reported to convert eâq to H atoms. In Table I we present rate constants for the reaction of eâq with an acid, as measured by different techniques. We used for Table I the rate constants (16), / ^ N O , - = 3 . 9 X 1 0 , 9
k^rNO,-
=
9.0
X
1 0 , ke+mo+ =
2.0
9
Χ
10
1 0
and
=
5.2
X
1 0 M ~ sec. - (slightly corrected [E.g. see (45) ] for other parallel reactions, when such corrections had been neglected (17,20, 27, 52)). T h e values (16), Of ke+ ferricyanide needed no correction of that type. These rate constants have been used i n combination with competition work to cal culate the rate constants i n both columns 4 and 5. In column 5 , the results have been calculated for zero ionic strength using the expression: μ l°g (ku/ko) = —Z-—•—— where k is the rate constant for e q with an 1 + μ ion of a charge Z, at the ionic strength μ. Although at high ionic strengths, the correction to μ = 0 is less certain, the corrected results should be preferred to the uncorrected values. T h e results based on ferricyanide were calculated with ke+ f e r r l o y a n i d e — 1.1 χ 1 0 M sec. (16), which can be obtained by an extrapolation from μ = 0 . 2 0 to μ = 0 . 2 5 . T h e photochemical results differ somewhat from the radiation chem ical, probably because of the approximations in the photochemical model. T h e phosphate results based on (31) were obtained at p H = 6 . 2 5 (HP0 ~ present does not react with eâq)- If K H P 0 alone was present, the H i n equilibrium with it is supposed to scavenge eâq- T h e high result (52) for k = 1.5 X 1 0 M~ sec. -\ obtained in 1 0 - A f K H P 0 alone (53) is surprising. We were unable to advance a logical explanation for the discrepancy, which would be i n agreement with all the experimental data. Further investigation of eâq decay i n H P 0 ~ is desired. Recent pulse radiolysis experiments i n which the decay of eâq was fol9
1
1
112
u
a
112
1 0
_1
_1
2
4
2
4
+
e + H , P O 4 -
9
1
3
2
4
2
4
246
SOLVATED ELECTRON
Table I. Acid (HA) H,0
+
H,0
+
H,0
+
Rate Constants for Conversion of m
aq
Ionic Strength ~o
pH
