2846
KLAUSH. SCHMIDT AND SABINEM. ANDER
Formation and Recombination of H,O+ and Hydroxide in Irradiated Water' by Klaus H. Schmidt and Sabine M. Ander Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60489 (Received December 17, 1968)
Measurements of transient conductivity changes in aqueous solutions during and after irradiation with 15-MeV electron pulses were carried out to study the formation and recombination of hydrogen and hydroxide ions. The hydrogen ion yield is G,+ = 2.9 f 0.2 (ions/100 eV), independent of the pulse dose and the pH between 5 and 9. On the basis of G,- = 2.8, GOH- = 0.1. The apparent neutralization rate constant k(H++OH)varies between 4.4 X 1Oloand 1.5 X 10l1 M-1 sec-1, depending on pulse dose and pH. Lower rate constants were obtained by increasing the number of ions formed by radiation and also by decreasing the concentrations of ions present before irradiation. The experimentaldata, including those of other authors, are consistent with the hypothesis that H* and OH- ions produced by radiation recombine more slowly than normal ions at equilibrium,possibly due to a different structure of their hydration shells. However, their mobilities must be the same as those of normal ions, according to experimental evidence. It was further found that NzO, but not HzOz, converts hydrated electrons into hydroxide ions within less than 0.2 p sec. With HZOZ,a negatively charged intermediate forms which has a half-life of 0.3 sec in alkaline solution.
Introduction If water is subjected to ionizing radiation, one observes the formation of a number of products, namely esq-, H30aq+,OH,,-, H, OH, Hz, H202, and a small quantity of HOz. The yields of most of these species have been determined experimentally by chemical or optical methods, and except for H30aq+ and OH,,-, their values can be considered well established. At the time of this writing, no experimental values for these yields have been reported. G H +and ~ GoH-are related by the equation GH+ = G,,,- GOH- with G,,,- = 2.8.a-10 "Molecular" hydrogen is generally assumed to be the product of hydrated electron reactions in the spurs, in particular reaction 1
+
eaq-
+ eaq- -+
Hz
+ 20H-
(1)
For each H2molecule, two OH- ions are produced. As will be discussed later, the formation of H atoms from electrons should ah0 be accompanied by OH- formation. Anbar and Meyerstein, l1 on the other hand, con,,lude, in a reviewof isotopic studies, that H and H~cannot be the products of electron reactions. They suggest several other possible mechanisms, but only one of them would lead to the formation of OH-. SwOrSki12 SUEgests excited water or H3O-OH pairs as precursors of molecular hydrogen. This mechanism also would not produce OH-. It was therefore hoped that a direct measurement of GH+ would help in deciding among these mechanisms. The determination of GH+ and GOH- is closely connected with the measurement of the rate constant of reaction 2. IC2 has been measured by Eigen and DeMaeyer13 using the field-dissociation effect and by H+
+ OH- +HzO
(2) Ertl and Gerischer14 by means of a temperature-jump method. These authors obtained rate constants of 1.3 The Journal of Physical Chemistry
X 10" and 1.43 X 10'l M-' sec-1, respectively, at 25" Barker and Sammon,16 applying a transient conductivity method, measured a significantly smaller value, ' a t 15", in comparison with Ertl and namely 7.3 X 1OO Gerischer's value of 1.16 X 10" at this temperature. It was the purpose of the present work to determine reliable values for GH+ and kz by the conductivity method. We expected to convert ea,- into OH- by reaction 3 or 4. In this way the only charged species
eaqe,,-
-
+ H20z
+ NzO
+ OHOH + OH- + N2 OH
(3) (4)
would be H+ and OH-, These ions with well-known mobilities would then neutralize each other by reaction 2* (1) Based on work performed under the auspices of the U. S. Atomic Energy Commission. (2) For the sake of brevity we shall henceforth omit the subscript "aqlv from ~ ~+ and0 OH,^ %-. ~ H +shall be synonymous with ~ $+, 0 (3) For all further calculations, we shall use this value for 0,-, which is an average of recent experimental determinations.4-@ This value was also obtained by Czapski'o by applying corrections to a number of smaller yields determined by other authors. (4) K. D. Asmus and J. H. Fendler, J . Phys. Chem., 72,4286 (1968). (5) F.M. Fielden and E. J. Hart, Radiat. Res., 32,664 (1967). (6) G.E.Adams, J. W. Boag, and B. D. Michael, Trans. Faraday SOC., 61,492 (1965). (7) G.V. Buxton and F. S. Dainton, Proc. Roy. Soc., Ser. A , 287,427 (1966). (8) F. S. Dainton and R. Rumfeldt, ibid., 287,444 (1966). (9) E. J. Hart, B. D. Michael, and K. H. Schmidt, unpublished results. (10) G. Czapski, Advances in Chemistry Series, No. 81, American Chemical Society, Washington, D. C., 1968,p 106. (11) M. Anbar and D. Meyerstein, Proceedings of the 19th Farkas Memorial Symposium, Jerusalem, Dec 1967. (12) T. J. Sworski, Proceedings, 5th Informal Conference on the Radiation Chemistry of Water, Notre Dame, Ind. 1966,p 28. (13) M. Eigen and L. DeMaeyer, Z . Elektrochem., 59, 986 (1956). (14) G. Ertl and H. Gerischer, ibid., 65, 629 (1961). (15) G.C. Barker and D. C. Sammon, Nature, 213,66 (1967).
FORMATION AND RECOMBINATION OF n30+ AND OH-
2847
+-
,
T E
. U
. . .. . . .
0
i t
Experimental Technique Eleclronic Circuitry. We used an improved version of an apparatus described in an earlier paper.In The principal features of the method are outlined in Figure 1. The cell voltage is a square Wave Of about 25 and 300 Hz. This avoids the distorting effects of electrolysis and electrode polarization one would have with a dc voltage. The signal is derived from two symmetrical voltage sources consisting of a Tektronix Type 106 square wave generator, a phase splitter circuit," and two identical power amplifiers, with an output impedance small compared to R1. The two resistors R1on the left match the 185-ohm coaxial cables connecting the conductivity cell in the irradiation vault with the rest of the apparatus. The other two resistors RI, together with the four resistors R2 (680 ohms), and with R3, form a compensating network. If the Helipot R3isadjusted to thevalueof thecell resistance, the points 3 and 4 have the same square wave potential as 1 and 2, and therefore either of the two points 5 and 6 has zero potential to ground, Le., no square wave signal appears on the input of the amplifier. The cell is irradiated with single 15-3IeV electron pulses, triggered18 during the flat portion of the square wave as indicated in the loiver portion of Figure 1. An induced transient conductivity changes the voltage across 1 and 2, and the corresponding signal across 5 and 6 is amplified and displayed on the oscilloscope. Owing to the absorption of beam electrons in the cell, negative potentials at 5 and 6 are superimposed on the conductivity signal. They are cancelled by adjusting the two channels of the Tektronix Type 1Al plug-in unit19 that serves as a differential amplifier. This adjustment is performed by pulse-irradiating the cell with no voltage applied. Since complete compensation is not possible because of the sliehtlv ._ " different shaues of the Dulses coming- from the tjvo cell electrodes, two oscilloscope pictures with opposite polarities of the square wave voltage are taken for each measurement. The algebraic difference of
Figure 2, Osrilicirccqir 11.iiws cii,liiiiiwl w i l l ) U ~ ~ X I - I I ,v,l i,, lqCllre 6.
these two signals is the "pure" conductivity signal. Figure 2 shows a typical pair of oscillograms thus obtained. The difference signal can he scen in Figure 6 (larger signal). For calibration, resistors of various values can he connected in parallel to the cell, thus simulating known conductance changes. Cell and Mechanical Arrangemeiil. The Conductivity cell, shown in Figure 3, is built of quartz and hasO.1-mm platinum electrodes. The irradiating beam is confined to a diameter of 2.5 cm by means of a triple collimator for minimum conversion into X-rays (see Figure 3). The beam penetrates the cell perpendicularly to the electrodes. In the irradiated volume hetween the electrodes, the electrical field is practically homogeneous; so is the radiation dose, as was determined by studying the dose distribution in a dummy cell consisting of quartz and Plexiglas disks. The Faraday cup shown in Figure 3, which stopped all remaining electrons, was used for dosimetry. Possible Sources of Error. The space charge deposited in the cell by beam electrons is not equally distributed along the path of the beam. Since this asymmetry must not distort the conductivity signals, a detailed calculation of the charge distribution in the cell (16) K. H. Schmidt and W. L. Buck. Schnce. 151.70 (1966). (17) The signals from the and ' I - ' ' output3 of the generator
"+"
were not sufficiently symmetrical. (18) The authors are srnteful to Mr.B. E. CliBt for designing the wellfunctioning triggering arrangement. (19) This plug-in unit is used separately from the oscilloscope and is operated from a Tektronix T Y P ~127 preamplifier power BUPPIY.
2848
KLAUSH. SCHMIDT AND SABINE M. ANDER
n
COLLlM ATOR
CELL
FARADAY CUP
EPOXY CEMENT CM-
1 0
Figure 3.
Conductivity cell and irradiation set-up.
and the potentials at the cell terminals as functions of time was carried out. We could show that the spacecharge induced signals at both cell terminals were indeed independent of the applied source (square wave) voltage, even with conductivity as a function of time, so that they would cancel if two conductivity signals with opposite polarities of the source voltage were subtracted. Quantitatively, the result can be expressed by eq El and E2. ABlf
- AVi-
=
(1
AG,(1 +2VoR 2RG2)2
r =
(El)
2RC, 1 2RG2
+
with the conditionz0 AGO > [OH-f]; (3) kf, from our measurements in alkaline solutions, where [H+,I >> [ H f f ] and [OH-,] < [OH-f]; (4) kf, from our measurements in acid solutions, where [H+,] < [H+f] and [OH-,] >> [OH-!]. Using kff = 1.43 X loll, kf, = 1.4 X loll, k,f = 9.5 X lolo, and k,, = 4 X 101O M-’ sec-’, we calculated an apparent rate constant
k,
=
+ kfs[H+fl[OH-,l + k,t[H+,] [OH-f] + k,,[H+,l [OH-,] [H+r][OH-r] + [H+i][OH-,l + [H+sl[OH-fl + [H+s1 [OH-, 1
kff[H+f][OH-r]
(E3) the ion concentrations being those after ca. 30% decay32 of the transient conductivity signal. I n Figure 8 the results are plotted against the measured values kz. Considering the relatively coarse method of calculation and the experimental errors of 10-25%, the calculated k, agree sufficiently well with the measured k2, so that we regard our proposed kinetical model as consistent with the experimental results. the rate of reaction According to Debye’s between two particles is a function of their diffusion coefficients and the reaction radius. The following experimental evidence shows that our “slow” ions have the same mobilities and therefore diffusion coefficients as normal ions: It was demonstrated earlier that, in alkaline solution, the formation of a negative species with a mobility lower. than that of OH- gives a negative The Journal of Physical Chemistry
I
U
I
I
5 IO 15 Apparent r a t e constant, measured (IO” M“ sec-I)
__
Figure 8. Amarent rate constant k, of neutralization, as calculated according to model described in the text, us. measured values kz (B & S = Barker & Sammon,16 E & G = Ertl and Gerischer14). I
conductivity signal after the pulse. For analogous reasons, the same effect would be caused in acid solution by the formation of a positive species with a mobility smaller than that of H+. Computer calculations assuming the formation of positive and/or negative “precursor” ions with mobilities significantly smaller than those of normal H+ and OH- yielded deviations from the measured signals that were far beyond experimental errors. It is conceivable that radiation-produced ions differ from normal ions in the structure of their hydration shells, which could result in a smaller reaction radius or perhaps in the establishment of a potential bar1ier.3~ If this is true, it will be difficult to explain the long life~ ~ special structure in contrast to time (>10-6 S ~ C of) this the short time of 10-l1 sec for the rearrangement of water and the fact that the “slow” ions have the same diffusion coefficients as the “fast” ions. So far, we can regard our kinetical model as no more than a formal interpretation of our measurements.
Acknowledgments. We are indebted to Dr. E. J. Hart for his valuable suggestions and discussions and his critical advice in preparing the manuscript. We thank Messrs. R. C. Sadler, J. C. Hodur, E. C. Yoder, and E. E. Klocek for building the conductivity cell, and Mr. S. G. Petrek for building the electronic equipment. (31) Perhaps even by any process other than thermal dissociation. (32) This is approximately the center of the curve section used for our
fitting procedure. We furthermore introduced a first-order conversion of the “slow” ions into the “fast” ions with a ratJeconstant IC(‘) = 6 X 104 sec-1, as this improved our model with respect to the values obtained in neutral solution. However, we can only conclude that 0 < IC(’) < 106sec-1. (33) P. Debye, Trans. Electrochem. Soc., 82,266 (1942). (34) To test the latter possibility, experiments to determine the aotivation energy of ICss are in preparation. (35) E. Wicke, Angew. Chem., 78,1(1966).
