THE JOURNAL OF
PHYSICAL CHEMISTRY Regiaked in U . S. Patent O$ce
@ Copyright, 1967, by tlic Ameriwn Chemiwl SocieC
~~
VOLUME 71, NUMBER 13 DECEMBER 15, 1967
The Formation of Hydrated Electrons by the Reaction of Hydrogen Atoms with Fluoride Ions1
by M. Anbar2 and Edwin J. Hart Argonne Natwnal Laboratory, hgonne, Illinois
60&0
(Received July 18, 1067)
+
+
Spectroscopic Confirmation of the reaction, H F- + eaqHF, is presented. I n potassium fluoride solution containing 0.016 M Hz, the decay of the eaq- absorption band is delayed relative to that in similar solutions containing 0.0006 M Hz. The rate constant of this reaction is 1.0 0.5 X lo4 M-I sec-'.
*
Introduction Among the hundreds of hydrated electron reactions known, only one is unquestionably reversible. This reaction
+ H JJ eaq- + HzO
OH-
(l)
first discovered by competition kinetics, was later confirmed spectroscopically by the optical absorption of Now another reaction
F-
+ H )r esq- + H F
(2)
has recently been added.4 Again, this reaction was first discovered by competition kinetics and from the behavior of the eaq- decay curve in electron pulse irradiated, Hzsaturated fluoride solutions, we now verify it. By this procedure hydrogen converts the radiolytically formed hydroxyl radicals, which are inert t o F-, into H atoms via
+ Hz = H + HzO (3) As a result, hydrated electrons, formed via reactions (3) + (1) or (3) + (2) are added to the radiolytOH
ically produced ones. These reactions taking place during a l m - ~ s e cPeriod after the Pulse lengthen the decay of esq-. BY comparing the half-lives of ens- in fluoride-containingsolutions in the presence and absence of appreciable Hz, we find the half-life longer in the former case, thereby confirming reaction 2.
Experimental Section The reaction of H atoms with F- was studied by following the decay of eaq- spectrophotometrically after irradiation of H2saturated 1.0 M KF by single pulses of 1PMev electrons. The irradiations were carried out in a cylindrical high-pressure Pyrex cell, 5 cm long and 2.5 cm in internal diameter.5 A connecting stem formed a T with the cell. The two windows of the cell were made of Suprasil silica (Amer(1) Based on work performed under the auspices of the U.S. Atomic Energy Commission. (2) Weizmann Institute of Science, Rehovoth, Israel. (3) M. 8. Matheson, W. A. Mulac, and J. Rabani, J. Phys. Chem., 67, 2613 (1963). (4) M.Anbar and P. Neta, Trans. Faraday SOC.,63, 141 (1967). (5) E. M. Fielden and E. J. Hart, ibid., in press.
4163
4164
M. ANBARAND EDWIN J. HART
si1 Inc., Hillside N. J.). They were silvered except for two 0.25411. openings at one end. Through these openings the analyzing laser beam of 6328 entered and emerged, and by altering the axis of the cell, the angle of incidence of the beam was changed, thereby enabling one to obtain from 2 up to 14 light passes. Impurities were minimized by attaching the windows to the cell without gaskets. I n order to do this, the cell ends and their windows consisted of polished optical flats clamped together mechanically. If care was taken to avoid overheating during irradiation, this cell was leak-proof, even under 500 psi internal pressure. The 1.0 M KF solution was degassed, saturated with hydrogen, and stored in 100-ml syringes. This solution was then put in the H2-purged cell by attaching a capillary extension to the syringe. After filling, an assembly consisting of a head with a pressure gauge and fittings was mechanically attached to the stem of the cell and sealed with a silicone gasket. After sealing, the gas space above the solution was purged by alternately pressurizing to 400 psi hydrogen and releasing to atmospheric pressure several times. Finally, the solution was equilibrated at the desired pressure by shaking for 5 min. Thereafter, oxygen and impurities inherent in the K F were "cleaned up" to a steady state of eaq- decay by preirradiation with a train of %rad pulses of X-rays6 Since there was no trace of the eaq- decay curve initially, this cleanup operation was essential. Usually after about 2000 pulses, cleanup was complete.
Results A resume of experimental conditions for two representative irradiations is given in Table I. It was found that 5300 X-ray pulses were required to clean up the 1 M KF solution initially, although fewer pulses were needed for subsequent experiments. The decay curves for the irradiations of Table I were followed for more than 2 half-lives and analyzed by CHLOE, as previously described.' The data are presented in Figure 1 as a plot of log (optical density) vs. time in microseconds. The plus signs (+) are the experimental points; the solid lines are the calculated decay curves. Figure 1 provides unmistakable proof that the decay of eaq- is delayed in hydrogen-saturated K F solutions. The addition of 0.016 M hydrogen (410 psi) to 1.0 M K F increases the half-life by nearly a factor of 2. Compare the decay of eaq- in the 0.016 M H1solution, designated by the plus signs, with the decay in 0.00059 M H2designated hy the circles. We ascribe this result to reaction 2, followed by reaction 3. The reaction Of atoms with F- to form is a slow one. For this reason, a reliable rate constant is
Table I : Irradiations of Hydrogen-Saturated Potassium FluorideSolutions High hydrogen
Low
hydrogen
Exptl Conditions
KF, M PH HZpressure, psi (Hd, mM" Pulse lengths, psec Dose rate, ev/l. sec X 10-94 Optical path length, cm Initial ea,- concentration,
1 .o 8.3 410 16.0 0.4 13.1
1.0 8.3 15 0.59 0.4 8.35
70.0 0.150
60.0 0.235
PM
Initial optical density No. of cleanup pulses Impurity effects kC, sec-1 k H + F -,k p M sec-'
0.148 1000 2 . 5 x 104 7 . 5 x 103
0.166 500 2 . 5 x 104 1 . 5 x 104
a Hydrogen concentration based on experimental determinations in H9-saturated 1 M KF = 0.586 mM H2 at 760 mm pressure and 23.5'.
Table I1 : Reaction Mechanism in Irradiated Hydrogen-Saturated Potassium Fluoride Solutions" No.
1 2 3 4 5 6 7 8 9
10 11 12 13 14 15
-
Reaction-
+ OH- = ea,+ F - = eaq- + HF OH + HI = HpO + H eaq- + X = Xeaq- + H2OZ= OH- + OH ea,- + ens- - 20H- + Ha H' + OH- = HzO Ha0 = H + +OHea,- + H = H2 + OHH +H =H2 H + OH = HzO OH + OH = HzOz ea,- + H + = H ea,- + HzO = H + OHeaq- + OH = OHH
H
M-1 set-1
2.0 x 107 Variable 4 . 5 x 107 Variable 1.20 x 1010 6 . 0 x 109 1.43 X 10" 2.64 x 3 . 0 X 1O1O 1 . 0 x 10" 2 . 5 X 10'O 5 . 0 X lo* 2 . 0 x 1010 16 3 . 0 X 1Olo
' Assumed values for yields are aa follows: G(H+) = 3.90; G(OH-) = 1.00; G(Hz) = 0.45; G(eaq-) = 2.7; G(0H) = 2.60; G(H20z) = 0.80; G(H) = 0.54. difficult to derive, but utilizing the rate constant data and reactions of Table I, we compute a rate constant consistent with the data of Figure 1. Unfortunately, the impurities inherent in the K F even after cleanup contribute importantly to the decay. Consequently,
~
The Journal of Physical Chemistry
(6) E. J. Hart and E. M. Fielden, Advances in Chemistry Series, No. 50, American Chemical Society, Washington, D. C., 1965, p 253. (7) E. J. Hart, E. M. Fielden, and M. Anbar, J. Phys. Chem., in
press.
REACTION OF HYDROC~EN ATOMSWITH FLUORIDE IONS
4165
confirm reaction 1. These results, less reliable than those described above, gave a kz of 5 X loa M-’ sec-1.
Discussion -0.95 -
From our results, it is evident that the lifetime of eaq- is extended in F- solutions in the presence of H2. A quantitative evaluation shows the specific rate of the F- reaction to be the same as was derived by H competition kinetic^,^ namely 1.0 =t 0.5 X lo4 M-’ sec-’. The relatively slow rate of this reaction and the large contributions of the competing reactions do not allow a more accurate determination of k H + F - . Our pulse-radiolysis experiments have, however, offered an independent proof of this unique reaction. While the F- reaction is evidently “driving force” of the H the the H-F bond energy (D(H-F) = 134 kcal/rn~le),~ F- $ HF eaq- has a positive over-all reaction H AF0.4 The reaction H OH- & eaq- HzO, on the other hand, which has a negative AFol10 is facilitated by the lower electron affinity of the OH radical as compared with F, and by the substantially lower A F O of HzOcompared with that of OH-.I1 In general, the reaction R YRY eaq-
+
>r
z
-1.05-
c
iY-
.0
-1.15-
0 -I
+
-
.-1.25
+
-1.35-
-
-1.45 I
1 20
\
I 40
60
Time in Microseconds
Figure 1. Effect of hydrogen concentration on the decay of eaq- in 1.0 M potassium fluoride at pH 8.3: 0.0160 M Ha; dose rate = 1.31 X IOas ev/l. sec; 0, 0.00059 M Hz; dose rate = 8.35 X loa4ev/l. sec; -, kc = 2.5 X IO4 sec-1, k~ F- = 1.5 X lo4 M-1 sec-1, 0.016 M Ha; ---, kc = 2.5 X IO4 sec-1, k~ + F- = 7.5 X loa M-’ sec-1, 0.00059 M Hz; - -, kc = 2.5 X lo4sec-1, KH + F- = 5.0 X 103 M-1 sec-’, 0.00059 M H2.
+,
+
impurity reaction 4 must be programmed into the regular set of radiolysis equations given in Table 11. By assuming that these reactions proceed with the indicated rate constants, reactions 2 and 4 may be fitted with rate constants consistent with the observed decays. The fit with the 0.016 M H2 upper curve is exceptionally good for k2 = 1.5 X lo4 M-l sec-’ when the impurity reaction has a k4C = 2.5 X lo4 sec-’. The solid line through the +’s is the theoretical one derived from the mechanism of Table I1 by the Schmidt CDC computer program.s With the same impurity effect, the low-pressure H2 curve is spanned by k i s of 5.0 X lo3 and 7.5 X lo3 M-1 sec-l. While more weight should be given to the high-pressure hydrogen experiments, we feel that kz is not more reliable than (1.0 f 0.5) x lo4M-’ sec-1. Similar experiments, carried out in 2.5 M KF, also
+
+
+
+
+
will proceed from left to right at an appreciable rate if the bond energy R-Y gained in this reaction is considerably higher (AH > 30 kcal/mole) than the electron affinity of Y. This is a necessary but not sufficient condition, as the energies of hydration of the four species involved may affect the equilibrium unfavorably, thereby making the reaction too slow to be detected. From the practical standpoint, Y- should be stable in aqueous solution and not react with water to give HY OH-. Other possible eaq- forming reactions are CF3 FCF4 eaq- and c&’5 F- F? C6F6 eaq-. The CF3-F and CsF5-F bond dissociation energies are 121 and 145 kcal/mole, respectively. Although the solvation energy of the electron in alcohols is close to that in water, the reactions, H RO- and H F-, in alcoholic solutions are expected t o be more favorable than in water since the anions have lower solvation energies in the alcohols.
+
+
+
+
+
+
+
(8) K.H.Schmidt, Argonne National Laboratory Report No. ANL7199 (TID-4500), April 1966. (9) T. L. Cottrell, “The Strength of Chemical Bonds,” Butterworth and Co. Ltd., London, 1958,p 188. (10) J. H.Baxendale, Radiation Res. Sup& 4, 139 (1964). (11) W. M. Latimer. “The Oxidation States of the Elements and Their Potentials in Aqueous Solution,” Prentice-Hall, Inc., Englewood Cliffs, N. J., 1952.
Volume 71. Number 13 Decembm 1967
