L. DOGLIOTTI AND E. HAYOS
3802
Transient Species Produced in the Photochemical Decomposition of Ceric Salts in Aqueous Solution. Reactivity of NO, and HSO, Free Radicals
by L. Dogliotti and E. Hayon Pioneering Research Division, U . S. A r m y Natick Laboratories, .$latick, Massachusetts
(Received A p r i l 3, 1967)
The flash photolysis of ceric sulfate and ceric nitrate in aqueous solutions was studied. Ceric sulfate produced a transient species with an optical absorption maximum at 4550 A which decays bimolecularly with k = 6.5 f 1.3 X lo8 M - l sec-I arid is assigned to the HSOc free radical. Ceric nitrate gave rise to an optical spectrum with maxima at 5950, 6400, and 6750 A. This intermediate was identified as the NO3 radical by its characteristic absorption spectrum in the visible region. It was found to decay by a first-order process with k = 9.5 X lo2 sec-’. In addition, a new ultraviolet band with maxima at 3350 -4 is reported. The main primary photolytic process in both sulfate and nitrate salts is explained on the basis of a charge transfer from the hydration sphere to the Ce(1T’) ion: Ce(IV).HzO Ce(II1) OH H f , followed by reactions OH HS04- + HSO, OH- and OH HN03 + ?\To3 HzO. Evidence for this mechanism was derived from the extrapolation to “zero” time of the pseudo-first-order decay plots in presence of various additives. The decrease of the “initial” concentration of HSO, and S O 3 produced mas found to be proportional to the k(OH S) X C, values of the solutes used. The rate constants for the reaction of HSOd and X03with added solutes-formate, acetate, methanol, ethanol, 2-propanol, thallous ions, and cerous ions- were obtained. HSO, is found to be a stronger oxidizing agent than NOs although HSO, is not as strong as OH radicals.
2
+
+
+ +
+
+
+
Introduction The action of ultraviolet light on ceric salts, particularly sulfate and perchlorate and also nitrate in aqueous solutions, has been studied considerably.’-’ Weiss and Porret’ interpreted the primary photochemical process due to the transfer of an electron from the hydration sphere to the excited ceric ion
2
C ~ ( I V ) . ( H ~ O ) Ce(II1)
+ OH + H+
(1)
+ (HzO)+
(1’)
sulfate and perchlorate, and in the photolysis of ceric sulfate in the presence of OH-radical scavenger^.^ On photolysis of frozen aqueous solutions of 0.01 ill ceric perchlorate in 6 M HC104, JIoorthy and Weiss6 have observed an esr absorption spectrum. which has been assigned to the positive “hole” (H20)+.in accord with reaction 1’. Indirect support for the over-all reaction mechanism was also provided from the study of the radiation chem-
or Ce(IV).(H20) 2 Ce(II1)
followed by the back reaction (2) to account for the low net photochemical reduction of Ce(1V) ions in solutions free of impurities Ce(II1)
+ OH -+-Ce(1V) + OH-
(2)
This reaction scheme provided a satisfactory interpretation of the results obtained in the photolysis of ceric T h e Journal of Physical Chemistry
(1) J. J. Weiss and D. Porret, S a t u r e , 139, 1019 (1937). (2) E. Rabinovitch, Rev. Mod. Phys.. 14, 112 (1942). (3) L. J. Heidt and 13.E. Smith, J . Am. Chem Soc., 70, 2476 (1948); L. J. Heidt and A. F. McRlillan, Science, 117, 75 (1953). (4) T. J. Sworski, J . Am. Chem. Soc., 77,1074 (1955); 79,3655 (1957). (5) T. W. Martin, A. Henshall, and R. C. Gross. J . Am. Chem. SOC., 85, 113 (1963); T. W. Martin, R. E. Kummel, and R . C. Gross, ibid., 86, 2595 (1964); T. U’. Martin, J. 31. Burk, nnd A . Henshnll, ibid., 88, 1097 (1966). (6) P. N. Moorthy and J. J. Weiss, J . Chem. Phys., 42, 3127 (1965). (7) E. Hayon and E. Saito, ibid.,43, 4314 (1965).
TRANSIENTS PRODUCED IN THE PHOTOLYSIS OF CERICSALTS
istry of aqueous solutions of ceric salts.8 There were indications, however, that a t least in sulfuric acid solutions, OH radicals react with HSO4- to form HSO4 radicals as intermediates
OH
+ HSOI- +HSO, + OH+ HKOa +KO3 + HzO
+ NO3
-
O.D. 0.10
(4)
Martin, et al.,z studied the flash photolysis and photochemistry of ceric nitrate, and have interpreted their observation of the optical spectrum of KO3 radicals due to an intramolecular electron transfer from the nitrate to the ceric ion ce(IV).SO3- hv_ Ce(II1)
0.15
(3)
A similar indication was provided' for the reaction of OH radicals with H x o 3 OH
3803
-
0.05
0 1
350
(5)
This paper reports a study, using the technique of flash photolysis, of the transients produced in the photolysis of ceric sulfate, nitrate, and perchlorate. The effect of some added solutes has also been examined.
Experimental Section The flash photolysis lamp, optical cells, optical detection system, and calculation of rate constants have been d e ~ c r i b e d . ~Solutions were prepared using water purified by distillation, radiolysis, and photoly~is.~ Reagents supplied by G. F. Smith and Baker and Adamson were the best research grades available and were used without further purification. The ceric solutions, particularly potassium ceric nitrate, K2Ce(SO&, were prepared daily and the addition of other solutes carried out, just before photolysis. Particular attention was given to minimize exposure of the solutions to the monitoring light beam from the Osram XBO 4.50-w xenon lamp. The bimolecular rate constant values given here refer to 2k. Potassium ceric nitrate in aqueous solutions is acidic due to hydrolysis of the salt. The pH values of lo-' A4 and 5 X -11 K2Ce(N0J8 are 0.65 and 1.20, respectively. These solutions were photolyzed unbuffered at room temperature, 24 f 1". The nitrate, sulfate, and perchloric salts of ceric ions form various complexes in solution. The nature of these complexes is also known to depend on the hydrogen ion concentration and to a smaller extent on the ceric ion concentration. S o attempt was made to characterize these complexes under the experimental conditions used in this work. Results The flash photolysis of ceric sulfate gave rise to a r ansient species in the wavelength region 400-560 mp,
I
1
I
I
400
4 50
500
550
1
Amp Figure 1. Absorption spectrum of HS04 transient produced on flash photolysis of aerated aqueous solutions of M ceric sulfate in 1.0 M H2SOa. The optical density was measured a t 40 psec after the start of the flash. Identical spectrum was obtained in absence of 0 2 .
with a maximum a t about 455 mp (see Figure 1). The extent of formation of this intermediate was independent of the presence of oxygen, slightly dependent on the concentration of ceric sulfate in the range studied, 1-2.5 X M , and very dependent on the concentration of sulfuric acid added. The amount of transient formed increased with increase in HzS04 in the concentration range to 1.0 M. This increase was considerably greater than the increase in the absorption coefficient of ceric ions with increase in HzS04 concentration.'O This transient decays bimolecularly, with a decay constant which is independent of 0 2 , Ce(IV), and HzS04 concentration. The optical absorption spectra and bimolecular decay constant for this transient are quite similar to those which have been observed on flash photolysis of persulfate ions9 and sulfate ions," and assigned to the so,radical anions. Since there appears to be essentially no difference in the absorption spectra and decay constants for SO,- and HS04,formed in acid solutions, there is no way of distinguishing between them or obtaining the pK value for this species (see Table I). It was not possible in this work to obtain the complete absorption spectra (8) A. 0. Allen, "The Radiation Chemistry of Water and Aqueous Solutions," D. Van Nostrand Company, Inc., Princeton, N. J., 1961, p 38. (9) L. Dogliotti and E. Hayon, J . Phys. Chem., 71, 2511 (1967). (10) C . M.Henderson and N. Miller, Radiation Res., 13, 641 (1960). (11) E.Hayon and J. J. McGarvey, J. Phys. Chem., 71, 1472 (1967).
Volume 71, Number 18 November 1967
3804
L. DOGLIOTTI AND E. HAYON
Table I : Bimolecular Decay of HSOa Radicals Formed on Flash Photolysis of Aerated Aqueous Solutions of 2.5 X lo-* M Ceric Sulfate in 1.0 M H2SOr
a
e
k(HSO4 f HSOr),
usedo
M-1 sec-1
455
1.55 f 0 . 4 X lo6
460
475 425 400
1.61 f 0.3 X IOs 1.36 f 0 . 3 X lo6 1 . 9 5 rt 0 . 3 X lo6
410 415 330
Molar extinction coefficient obtained from ref 9, assuming e:&?'
of HSOl down to 290 mp, as was done in the flash photolysis studies of S20.2+ and SOP'-, due to the strong initial absorption of ceric sulfate itself in the wavelength range from 400 down to 290 mp. Reduction in the ceric sulfate concentration below M resulted in an appreciable decrease in the amount of transient observed. The reactivity of HSOl radicals produced in the photolysis of ceric sulfate has been studied. A few selected organic and inorganic solutes were added initially, and changes in the decay of the transient species followed a t 455 mp. The HSO4 radical was found to follow a pseudo-first-order decay in the presence of these additives. These first-order decay constants were plotted as a function of added solute concentration, and from the slope of this linear dependence the second-order rate constants for L(HSO4 S) were obtained. These are given in Table 11. The flash photolysis of potassium ceric nitrate gave rise to one transient species having an optical absorption spectrum stretching from the far-ultraviolet to the far-visible region of the spectrum (see Figure 2). The three bands in the visible region with maxima a t about 595, 640, and 675 mp (in the liquid phase) are characteristic of the NOSfree radical. Very similar spectra for this radical have been obtained in the gas phase,12 on photolysis and radiolysis of HN03 ices,' on flash photolysis of ceric nitrate solution^,^ and on pulse radiolysis of H N 0 3 and NOa- ion^.'^-'^ In addition, a new absorption band has been observed in the ultraviolet region, Amax 335 mp (Figure 2), which has not been previously reported. This finding agrees with the esr observations' of the photolytic decomposition in the ultraviolet of NO3 radicals trapped in H N 0 3 ices. The transient produced in the flash photolysis of ceric nitrate was found to be independent of the presence of oxygen, but the amount of transient formed was dependent on the concentration of E(zCe(N03)6,pH, and added H N 0 3or KN03. The effect of adding HX03is to
+
The Journal of Physical Chemistry
Published values, M - 1
7.14 f 1.8 X 108 6.60 f 1 . 2 X 5.65 f 1 . 2 X 6 . 4 5 =t1 . 0 >( Mean 6.46 f 1 . 3 X
8ec - 1
4 . 0 X 108 9 a t pH 4.8 3 . 6 X IO8 a t pH 0.1 4 . 2 X 1081latpH5.5
IO8 108 lo8 lo8
= 460 M-1 em-*.
=::::e
0 250
300
350
400 450
500
550
600
650
700
A, mp.
Figure 2. Absorption spectrum of NOI transient produced on flash photolysis of aerated aqueous solutions of 0.1 M potassium ceric nitrate. The optical density was measured a t 200 psec after the start of the flash. Identical spectrum was obtained in absence of 02.
increase the extinction coefficient of K2Ce(N03)6and to shift the over-all absorption spectrum to higher wavelengths. Consequently, all the work carried out here was done using 0.1 M K2Ce(X03)Bconcentration. The NO3 radical was found to decay by a first-order process, with a rate constant which is essentially independent of the presence of 02,HN03, or K S 0 3 concentrations in the concentration ranges studied, and is the same over the whole wavelength range 300-655 mp, see Table 111, indicating the presence of only one intermediate. The reactivity of NO3 radicals with a number of added organic and inorganic solutes was studied. In all cases, the plot of the pseudorfirst-order decay constant in presence of additives vs. the concentration of additive (12) D. Hussain and R. G. W. Norrish, Proc. Roy. SOC.(London), A273, 165 (1963); G. Schott and N. Davidson, J . A m . Chem. SOC., 80, 1841 (1958); E. J. Jones and T. R. Wulf, J . Chem. Phys., 5 , 873 (1937). (13) M. Daniels, J . Phys. Chem., 7 0 , 3022 (1966). (14) R. K. Brosskiewics, Intern. J . Radiation Isotopes, 18, 25 (1967).
TRANSIENTS PRODUCED IN THE PHOTOLYSIS OF CERIC SALTS
3805
Table 11: Reactivity of HSO4 Radicals with Added Solutes. Transient Formed on Flash Photolysis of M Ceric Sulfate in 1.0 Jf HzSOI, and Monitored a t 455 mp Aerated Aqueous Solutions of 2.5 X
HCOOH CHsCOOH CH3OH
0.4-5 x 10-3 0.4-2 X lo-* 0.3-2 X
CHaCH2OH
0.6-5
i-PrOH
1-6 x 10-4 0.5-2.5 X lo-' 0.5-2 x 10-4
T1 +a Ce(III)a Q
Sulfate salts.
x
1 . 3 5 i: 0 . 2 X lo6 8.80 i 0 . 2 x 104 1.10 0 . 2 x 107
... ...
3.44 rt 0 . 3
10-4
x
2 . 5 It 0 . 6 X 1070 2 . 0 x 10715 6 . 2 i: 1 . 4 X 3 . 0 x 10715 9.1 i 2.8 X
107
4.60 i 0 . 2 x 107 1 . 7 0 4 0 . 2 X lo9 1.43 i:0 . 3 X lo8
... ...
The k values in ref 9 were obtained in neutral solutions.
Ce(IV).HzO -%- Ce(II1)
Effect of 0%
Acid concn
0 65 0.65 0 65 0 65 0 65 0 65 0.65
In presence of 5 X M KzCe(N03)6.
Air-satd
O2 free Air-satd Air-satd Air-qatd Air-sat d Air-satd Air-satd Air-satd Air-satd Air-satd Air-satd
monitored,
600 600 300 350 450 640 685 600 600 600 600 600
(HzO)+
k, see-1
m p
0.91 i 0.06 x 0.95 i 0.05 x 1.07 i:0.10 x 1.08i0.10 x 0.90 i 0.06 x 0.99 i 0.10 x 1.07 =t0.10 x 0.94 i 0.02 x 0.97 i 0 . 0 3 X 1.30 i0.20 x 0.91 =t 0.10 x 0.50 =t 0.10 x
M K2Ce(N03)6.
+ (HzO)+
(1')
In aqueous solutions, (HzO)+ reacts very rapidly with water to give an OH radical
x
0
Published values, ~ - sec-lb 1
=
Table 111: First-Order Decay Constants for NO3 Radicals Produced on Flash Photolysis of 0.1 3f K2Ce(NO&
Unbuffered, pH Unbuffered, p H Unbuffered, pH Unbuffered, pH Unbuffered, pH Unbuffered, pH Unbuffered, pH 0 1 M "03 2 "03 3 $f "03 3.14 HNOP 6 M HNOab
k ( H S O 4 f s), M-1 sec-1
Concn range, *%f
Solute
103 103 103 103 103 103 103 103 lo3 103 103 103
I n presence of
10-3
was a straight line xith an ordinate intercept equal to the first-order decay constant, of the SOs radical itself in the absence of added solutes. From the slope of these plots the second-order rate constant for Ic(X03 S) was determined and the values are given in Table IV. The flash photolysis of aerated aqueous solutions of 2.5 X ill ceric perchlorate in 0.6 M perchloric acid gave rise to a very weak absorption in the wavelength region 350-600 mp. Due to this very weak absorption it was not possible to follow or study this transient.
+
Discussion The photochemically induced charge-transfer reaction mechanism for the photolysis of ceric sulfate and ceric perchlorate in aqueous solution appears to explain adequately all the experimental facts known about these salts.
+ HzO +OH + H30+
(6)
so that the oxidizing species reacting in solution at room temperature is the OH radical. In ceric perchlorateperchloric acid ices irradiated at 77°K an esr spectrum due to trapped (HzO)+ has, however, been reported.6
Table IV: Rate Constants of SO3 Radicals with Added Solutes, Obtained from the Photolysis of Aerated Solutions of 0.1 Jf K2Ce(NOs)6. NO3 Transient Followed at 600 m p Concn range, M
Solute
HCOOH CHICOOH CHSOH CH3CH,0H i-PrOH
T1+ a Ce(III)a a
10-3-10-2 10-'-4 X lo-' 2 x 10-3-10-2 1 . 2 5 x 10-4-l.5 X 2 . 5 x 10-4-1.0 x io-ab 10-4-6 x 10-4 10+-8 X 2 x 10-3-2 x 10-2
Kitrate salts.
k(NOs M-1
+ s), sec-1
2.06 rt 0 . 1 X 4 . 6 i:0 . 4 X 1 . 0 i0.1 x 3.85 i.0 . 3 X 3.90 rt 0 . 4 x 3.60 2c 0 . 2 X 3.46 i 0 . 1 x 3.70 rt 0 . 1 X
lo6 lo4 106 lo6 106
lo6 107 lo6
' In presence of 0.1 M and 0.01 M HxO3.
Further support for this primary photolytic process comes from the observation that in flash photolysis of ceric sulfate, the formation of the HSO, transient is strongly dependent on the HzS04 concentration. This is due to reaction 3 competing with reaction 2. The rate of reaction 3 is relatively low, with k3 = 8 X J4-l sec-l.l5 (15) E. Heckel, A. Henglein, and G. Beck, Ber. Bunsengs. Physik. Chem., 70, 149 (1966).
Voliime 7 1 , ATzimber 12
Sovember 1967
L. DOGLIOTTI AND E. HAYON
3806
c\
4
1.0
log(0.0.) x ioo
0.5
I
t
\, h
't 'r
t 50
0
IO0
I
1 I50
t, psec Figure 3 . Pseudo-first-order decay plots of HSOl formed Al ceric sulfate and 1.0 -44 in the photolysis of 2.5 x H2S04in presence of additives. Transient monitored a t S) X C,. 453 nip. Values in parentheses are k(OH -If cerous sulfate (4.4X lo4 sec-l); A, 2 X X, 2 X M thallous sulfate 12 X lo5 sec-I); ,. 2.5 X M thallous sulfate (2.5 X IO5 sec-l); 0, 3 X 1O-l N ethanol (3.3 x lo5 sec-1): e, ,5 x IO-' . W ethanol (S.5 X lo5 sec-I).
+
On addition of various organic and inorganic solutes to ceric sulfate solution previous to photolysis, it was possible to deterniine the reactivity of HS04 radicals with these solutes (see Table 11). A plot of the pseudofirst-order decay in the presence of these additives is shown in Figure 3. On extrapolation of the pseudofirst-order plots to zero time (the maximum intensity of the flash, and hence the highest HSO, concentration, is at about 5 psec), one can note that the straight lines do not extrapolate to a common point. Indeed, the log (OD) values are dependent on the solute used and its concentration. This dependence can be seen to follow the product of k(OH S) X C,, and not k(HS04 S) x C, values for the different solutes examined. The k(OH S) values used are 2 . 2 x 108 M - I ~ e c - ' , loio ~
+
+
The Journal of Physical Chemistry
+
M--l sec-1,4,16and 1.1 X lo9 M-' see-117 for Ce(III), TI+,and ethanol, respectively. This observation shows that the solutes compete with HS04- for the OH radicals produced in the primary photolytic process, in addition to their reaction with the formed HSO, radicals. This is in agreement with the results obtained in steadystate photolysis of ceric sulfate. Addition of OH radical scavengers like Tlf and Ce(II1) was shown by Sworski4 from competition kinetics measurements to compete with each other for the primary-produced OH radicals. The flash photolysis of ceric nitrate solutions gives rise to the formation of NO3radicals. A new ultraviolet band has been observed for this radical, in addition to the characteristic three bands in the visible region of the spectrum (Figure 2 ) . KO3 decays by a first-order process, with a rate constant of about 9.5 X lo2sec-1 which is essentially independent of 0 2 and nitric acid concentration (see Table 111). That SOs does not decay by reaction with Ce(II1) ions formed during the flash as a result of reduction of ceric ions was shown by adding cerous nitrate previous to irradiation. ilplot of the pseudo-first-order rate constant as a function of [Ce(III)] extrapolates to a k = 9.5 X lo2sec-l, in agreement with the first-order k values for S O 3 itself. Martin, et u E . , ~ have reported that S O 3 decays bimolecularly on flash photolysis of M K&e(SO& in 6.0 M Hxo3. The formation of X03 in the systeni was accounted as due to an intramolecular electron transfer from the nitrate to the ceric ion
2Ce(II1) +
C~(IV).SO~-
NO^
(5)
We have repeated these conditions and find that KO3 decays by a first-order process with k = 5 X 102 sec-l (see Table 111). This value is lower than the rate coristant obtained in ceric nitrate solutions containing up to 3.0 M H S 0 3 . In pulse radiolysis studies of HSOa and NO3- in aqueous solutions, Daniels13 and Rroszkiewiczi4 also find that X03 follows a first-order decay, with k = 7.6 X lo3 sec-'.14 The formation of SO3 is dependent partly upon the concentration of ceric nitrate and to a much larger extent on the hydrogen ion concentration. Increasing the pH of 0.1 1%' K2Ce(N03)6above 0.65 decreases rapidly the amount of transient formed. Siniilarly, addition of HS03 increases considerably the concentration of S O 3 . This observat,ion plus the effect of added solutes known to be good OH-radical scavengers indicates that so3 is formed according to reaction 4. (16) E. Hayon, Trans. Faraday SOC.,61, 723 (1965). (17) G. E. Bdams, J. W-.Boag, and B. D. Michael, ibid., 61, 1417 (1965).
TRANSIENTS PRODUCED IN THE PHOTOLYSIS OF CERICSALTS
OH
+ HK03 +No3 + H20
(4)
This dependence on hydrogen ion concentration (in the case of sulfuric acid and nitric acid) does not seem to be related to any change in the nature of the ceric complex with pH, since a very similar pH dependence for reactions 3 and 4 has been observed13-14 in pulse radiolysis studies. The possibility still remains that in 6.0 M H N 0 3 the ceric ion is present entirely as the hexanitratocerate ion, Ce(N03)62-,while at low [HSO3], Ce(0H)(N03)3is also present, and one could explain the above results as follows
c ~ ( x o ~hv_ ) * c- ~ ~ ( K o ~ )+ ~ -NO^ ~
+ OH
Ce(OH)(iY03)3-% Ce(N03)3
3807
0
(B)
+
-----f
products
a
03
(7)
The OH radicals produced in 0.1 M ceric nitrate solutions (unbuffered pH of solutions is 0.65) react with "OB present to form NO3 radicals. A similar mech-
I
0 4
I 05
t ,msec Figure 4. Pseudo-first-order decay plots of NO3 formed in the photolysis of 0.1 M KzCe(N03)ein presence of additives. Transient monitored a t 600 mfi. Values in parentheses are k(OH S) X CB: (a) no additive; (b) 4 X M Tl+ (4 X lo6sec-1); (c) 8 X lo-+ $1 TI+ (8 X lo6 sec-1); (d) 8X M Ce(II1) (1.76 X lo6 sec-1); (e) 2 X M Ce(II1) (4.4X lo6 sec-l); (f) 4 x 10-3 -11ethanol (4.4X lo6 sec-1). Experimental points have been left out.
+
anism has been proposed to account for the formation of SO3 in the esr study' of photolyzed H I 0 3 and HSO3 plus ceric nitrate glasses at 77"K, and in the pulse radiolysis of nitrate and nitric acid solution^.'^ The addition of solutes (except Ce(II1) ions) and their reaction with HSO, or NO3 leads to a final photoreduction of ceric ions in solution. This was observed in flash photolysis when events were followed at a wavelength where Ce(1V) ions absorb. The following reactions in addition to reaction 7 can explain the net Ce(1V) reduction: in the case of Tl+
+ T1+ +HSOd- + T12+ T12+ + Ce(1V) +T13+ + Ce(II1) HSOi
+
+S
01
(A)
However, though the results obtained above cannot entirely disprove reaction A in the photolysis of ceric nitrate in 6.0 M H N 0 3 where the competition kinetics in the presence of added solutes cannot be followed, it can be excluded in solutions up to about 1 M "03. In view of the results reported above, reactions A or B are considered unlikely and the work of Martin, et al.,5 does not provide any evidence in its support. The addition of solutes allows one to study the rewtivity of 503 radicals in solution. A few organic and inorganic additives were examined and the rate constants k ( X 0 3 S) determined (see Table IV). The pseudo-first-order decays were plotted and are shown in Figure 4.Again one finds on extrapolation of these linear plots to zero time that the *.initial" amount of transient formed is not constant, but decreases wit'h increase in the concentration of the solutes present in solution. As in the case of the ceric sulfate photolysis, the decrease in [NO3J at "zero" time is proportional to the product k(OH S) X C, of the solutes added. From this one must conclude that the primary photolytic process in ceric nitrate solutions is the same as that in ceric sulfate, namely an electron transfer to the ceric ion with the subsequent formation of OH radicals. On flash photolysis of 0.1 M ceric nitrate in the presence of 3.0 M (or 6.0 M ) HN03,and the same additives, one finds that the first-order decay plots extrapolate a t ii zero" time to almost a common poirit. This provides strong support to the mechanism proposed, and indicates that in 3.0 M or in 6.0 M H N 0 3most of the OH radicals react via reaction 4 and that there is essentially no competition from reaction 7 for the OH radicals. OH
I
02
I
0
In the case of organic solutes HSO,
+ CH3CH20H +HSOI- + CH3CHOH + H + CHSCHOH
+
0 2
+CHsCHO
+ HO,
+ Ce(1V) --+CH3CH0 + Ce(II1) + H f HOz + Ce(1V) Ce(II1) + H + + 02
CH3CHOH
4
I n the presence of Ce(II1) ions, produced in the primary act or added previous to photolysis, the following reaction occurs HSO,
+ Ce(II1)
4
Ce(IV)
+ HS0,-
Similar reactions could explain the reduction of Ce(1V) ions in nitrate solutions. The role and reactivity of the HSO1, Sod-, and NO3 free radicals have been largely ignored in explaining the Volume 7 1 , Sumber 12
Sovember 1967
3808
M. E. LANGMUIR AND E. HAYON
chemical kinetics of react,ions taking place in photochemistry and radiation chemistry. These radicals can be formed by reactlion of S04'-, H2S04, NOa-, or HN03 with OH radicals, or by direct phot'olytic or radiolytic decomposition of these molecules. I n addi-
tion, these radicals once formed can react with other solutes present in solution. A comparison of IC values in Tables I1 and IV shows that HS04 or SO4- is a much stronger oxidizing species than NO3, though it is not as strong as hydroxyl radical.
Flash Photolysis Study of Mercury(I1) Halide Complexes in Aqueous Solution. Rates of Reaction of X; Radical Anions
by M. E. Langmuir and E. Hayon Pioneering Research Dinision, U . S. Army h'atick Laboratories, aVatick, Massachusetts (Receised M a y 5, 1967)
The photochemistry of aqueous solutions of mercury(I1) halide complexes has been studied. The optical spectra of the transient species formed have been observed using the technique of flash photolysis. The complexes HgC12, HgC12-, HgBr2, HgBr42-, and HgT,'- all give rise to the corresponding radical anions C12-, Br2-, and 12-. HgI, produced an intermediate with a maximum at 330 mfi. The decay kinetics of the transient species formed from the different polyhalide complexes have been examined in neutral and acid solutions. The rates of reaction of Cl2- and Brz- with added solutes (methanol, ethanol, isopropyl alcohol, oxalate, and H202)have also been determined in neutral and acid sdutions. 12radical anions are found to be relatively unreactive (ks 10' IM-' sec-' with alcohols). A comparison of the rates of reaction of the inorganic oxidizing radicals OH, SO4-, X03, C12-, Br2-, and 12- with some compounds indicates that the oxidizing power qf these radicals follows the order OH > SO4- > Clz- > YO3 > Br2- >> I,-.
