FLASH PHOTOLYSIS OF PERSULFATE IONS
2511
Flash Photolysis of Persulfate Ions in Aqueous Solutions. Study of the Sulfate and Ozonide Radical Anions
by L. Dogliotti and E. Hayon Pioneering Research Division, U.S. Army Natick Laboratories, Natick, Massachusetts (Received December 19, 1966)
The primary species produced in the photolysis of persulfate ion in aqueous solution is shown to be the sulfate radical SO4- formed from SzOs2--% 2s04-. The optical absorption spectrum of this transient has a maximum at 4550 A, a half-life of about 300 psec, molar extinction coefficient e s ~ , =- 460 ~ ~&~25 M-’ cm-l, and decays bimolecularly with rC(S0,SO4-) = 3.7 X lo8 M-l sec-l. It is found to be stable in presence or absence of oxygen and in neutral and acid solutions. At pH >8.5 it starts decaying rapidly and has completely disappeared at pH 10.7-10.8. I n alkaline solutions, the SOI- radical is apparently H 2 0 e OH S042- H+. A transient spectrum converted to an OH radical SO4assigned to the ozonide ion 03-is observed on flash photolysis of aerated alkaline persulfate, formed from reactions OH OH- --+ 0 . H20 and 0 . O2 + 03-. This transient has a maximum a t 4300 A and its decay is dependent on pH and concentration of 03-ions in agreement with previous observations for this transient species. The SO4anion reacts, probably by H atom abstraction, with methanol, ethanol, 2-propanol, and bicarbonate ions with second-order rate constants of 2.5 X lo7, 7.0 X lo7, 8.8 X lo’, COS-) = 1.7 X and 9.1 X lo6 M-l sec-l, respectively. The rate constant k(C03lo7M-I sec-l was obtained on photolysis of aerated persulfate-bicarbonate solutions.
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+ +
+
+
+
+
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Introduction Persulfate ion is an efficient oxidant for many types of organic and inorganic molecules. Considered to be an important source of free radicals, it has been frequently used as an initiator of free-radical reactions in solution.’ The radical-forming reaction has been considered to be a simple 0-0 bond scission
thermal decomposition of persulfate ions. More recently, Tsao and Wilmarth6 studied the photolytic decomposition of aqueous persulfate ions at 2537 A. They suggested that the SO4- radical anion formed is
(1) C. Walling, “Free Radicals in Solution,” John Wiley and Sons, Ino., New York, N.Y.,1957. (2) J. H. Mertz and W. A. Waters, Discussions Faraday SOC.,2 , 179 --f 2504(1) (1947); J. W. L. Fordham and H. L. Williams, J. Am. Chem. SOC., 73, 1634, 4855 (1951); I. M. Kolthoff, A. I. Medalia, and H. P. to form a sulfate free radical SO4-. The reactions of Raaen, ibid., 73, 1733 (1951); L. J. Csanyi, Discussions Faraday the radical produced in p h o t o c h e m i ~ a l , ~ ~SOC., ~ 29, 146 (1960). (3) C. R. Giuliano, N. Schwartc, and W. K. Wilmarth, J. Phys. and radiation chemical? studies of persulfate ion have Chem., 63,353 (1959);M-8. Tsao and W. K. Wilmarth, Advances in been considerably studied, but no evidence as to its Chemistry Series, No. 36, American Chemical Society, Washington, D. C., 1962,p 113. nature has been provided. I n thermal reactions, the (4) W. V. Smith, J. Am. Chem. SOC.,71,4077(1949);I. M. Kolthoff, free radical formed has been shown to be able to induce P. R. O’Connor, and J. L. Hanson, J. Polymer Sci., 15, 459 (1955). oxidations and polymerization in organic substances2 (5) (a) R. H. Crist, J . Am. Chem. SOC.,54, 3939 (1932); (b) L. J. and its presence confirmed by the incorporation of s5S, Heidt, J . Chem. Phys., 10, 297 (1942); (c) L.J. Heidt, J. B. Mann, and H. R. Schneider, J. Am. Chem. Soc., 7 0 , 3011 (1948); (d) L. J. from s5S-labeled persulfate, into polymer chain^.^ Heidt and V. R. Landi, J. Chem. Phys., 41, 176 (1964). Isotopic oxygen atom exchange between dissolved oxy(6) M-S. Tsao and W. K. Wilmarth, J . Phys. Chem., 63,346 (1959). gen and water has also been brought about by the (7) E.J. Hart, J. Am. Chem. SOC.,83, 567 (1961).
-o~s-o-o-so~-
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July 1967
L. DOGLIOTTI AND E. HAYON
2512
rapidly converted, over the entire pH range 0-13.3 studied, to OH radicals
SO4-
+ HzO
OH
+ 504’- + H +
(2)
Reaction 2 has also been suggested by other workers.2Js8 Heidt and Landisd studied the flash photolysis of persulfate ions in aerated solutions of 0.1 M NaOH and observed a transient absorption spectra with A, 4300 A which they identified as the ozonide ion 0 3 - . I n this paper we present the results obtained in the flash photolysis of persulfate ions, in the absence and presence of oxygen, over the pH range 0-14. The effect of some added solutes is also reported.
Experimental Section The absorption spectra of persulfate ion have been measured9 and an extinction coefficient at 2540 A equal to 22 has been reported.5b Solutions were prepared using water purified‘O by distillation, radiolysis, and photolysis. All reagents, supplied by Baker and Adamson and Mallinckrodt, were best research grades available and were used without further purification. The pH was changed using HzS04and “low carbonate” KaOH and measured on a Beckman Model G glass electrode pH meter, calibrated with appropriate standard buffers. No buffers were employed since they were found to react with the transients produced on photolysis. Flash Photolysis Setup. The flash photolysis lamp used in this work is identical with that described by Lindqvist.” Four flash tubes of 20-cm length, 15mm i.d., and 3-mm wall thickness were connected in series for easy triggering. Tungsten electrodes were used. The tubes were filled with oxygen a t 4 cm pressure and the flash discharge was produced from a 9-pf capacitor bank charged to 20 kv, corresponding to total flash energies of 1800 joules. The characteristics of the flash produced are: duration at “l/e time” 5 psec, total flash duration 70 psec. The lamps were mounted symmetrically alongside the photolysis cell a t an axis-to-axis distance of 4.5 cm from the cell. Two types of quartz cells were used with optically flat quartz windows sealed a t each end. One, 20 cm long, 12 mm i.d., was used mainly for experiments carried out in the presence of air. The other, 10 cm long, 13 mm i.d., was used for runs done in absence of air. I n the latter cell, solutions were deaerated by the syringe technique developed for use in pulse radiolysis work by Hart and co-workers12 a t Argonne National Laboratories. Optical Detection System. A high-pressure xenon arc lamp (Osram XBO 450 w) with a regulated power supply provided from a 60-v battery bank was used The Journal of Physical Chemistry
as the continuous light source for monitoring transient species. Collimated light from the arc passed once through the photolysis cell into the entrance slit of a monochromator (Bausch and Lomb 250-mm plane grating with 1200 grooves/mm). The emerging light beam was monitored by an EM1 9552 B photomultiplier tube using a Tektronix 5358 oscilloscope via a cathode follower. Traces were recorded on Polaroid film (3000 speed, Type 47) and the optical absorption spectra were obtained by the point-by-point method. The absorption spectrum of the 0 3 - ion was obtained using the small Bausch and Lomb grating monochromator with appropriate gratings. EM1 9558 Q photomultiplier tubes for the wavelength region 2000-8000 A were employed. Calculation of Rate Constants. Traces were read on a Gerber scanner (Model S-10-C), the data punched directly into cards, and rate constants were calculated using the method of least squares on a GE 225 computer, programmed for first- and second-order decay kinetics. In recombination reactions, k values given refer to 2k.
Results Fresh solutions of potassium persulfate were prepared daily and addition of other solutes to it was done just previous to exposing the solutions to the flash of radiation. The flash photolysis of aerated aqueous solutions of lo-’ M potassium persulfate a t pH 4.8 (unbuffered) gave rise to the formation of a transient species with a half-life of about 300 psec, absorbing in the wavelength region 2800-5700 A (see Figure 1). Except for variations in the amount of transient formed, the same optical absorption was found a t all persulfate concentrations studied up to 5 X M. A concentration of M SzOs2-was chosen for most of the experiments. A similar spectrum was obtained on flashing the same solution in the absence of oxygen (Figure 1). The effect of acid does not seem to change the transient spectrum, as shown on flash photolysis of lo-’ M S2082-a t pH 1.03 (Figure 1). This transient absorption spectrum, assigned to the sulfate radical SO4-, decays bimolecularly a t pH 4.8 and 1.03, as ~
~~~
~~~~
(8) R. Woods, I. M. Kolthoff, and E. J. Meehan, J. Am. Chem. SOC.,85, 2385 (1963). (9) R. P. Buck, S. Singhadeja, and L. B. Rogers, Anal. Chem., 26,
1240 (1954). (10) E.Hayon, Trans. Faraday SOC.,60, 1059 (1964). (11) L. Lindqvist, Rev. Sci. Insir., 35, 993 (1964). (12) J. K.Thomas, S. Gordon, and E. J. Hart, J. Phys. Chem., 68, 1524 (1964).
FLASH PHOTOLYSIS OF PERSULFATE IONS
2000
2513
e000
3000
WAVELENGTH
Figure 1. Optical absorption spectra of transients produced in the flash photolysis of aqueous solutions of 10-2 M potassium persulfate, in aerated solutions a t pH 4.8 (0) and p H 1.03 (O), and in deaerated solutions a t pH 4.8 (a) and p H 10.4 ( 0 ) . The first three spectra were obtained using a 20-cm optical path and the last spectrum using a 10-cm optical path cell. Optical density measured 40 psec after start of flash.
shown on plotting l/(OD) vs. t according to the secondorder rate law
1/(OD)t
k
= ;t
+ [l/(OD)o]
(3)
where OD is the optical density at times equal to zero and t , respectively, k is the rate constant, I the optical path of cell used, and B the molar extinction coefficient of transient. From the slopes one can calculate k/e. These values are given in Table I, column 2, for different pH’s. Within experimental variations these values are considered to be independent of pH in the range 0.1-4.8 in the presence or absence of oxygen. Table I: Rate Constants for the Bimolecular Decay of SO4- in Aqueous l o + M Persulfate Solutions k / e X 106
useda
k X 108, M -1 sec -1
7.80 It 0 . 7 0 6.66 f 0.23 8 . 5 0 f 0.92 9.0 f 0 . 9 2
460 460 460 460
3.58 f 0.34 3.06 f 0 . 0 9 3.91 f 0.40 4 . 1 4 zk 0 . 4 0
€
PH
0.1 1.0 4.8 4.8b
See text for determination of this value. sulfate solutions. a
PH -
Figure 2. Variation of optical density of SO4- transient a t 455 mp with pH in the flash photolysis of deaerated 10-2 M persulfate solutions using 10-cm cells. Optical density measured 40 psec after start of flash.
with further increase in pH until pH 10.8 when no transient absorption can be seen. The absorption spectrum of the transient a t pH 10.4 is shown in Figure 1 and the change in optical density at 455 mp with pH shown in Figure 2. In this alkaline region, the sodradical anion when monitored a t 455 mp is found to follow a first-order decay (OD)
kit
(4)
These first-order rate “constants” are given in Table 11. Aerated Alkaline Solutions. On flash photolysis of alkaline persulfate solutions in the presence of oxygen, a new transient species is formed in place of so4which decays more slowly. The optical absorption spectrum of this new species with A,,, 4300 A is shown in Figure 3 for persulfate solutions photolyzed a t pH 11.9. The formation of this transient species is dependent upon pH, as shown in Figure 4. Table 11: Rate Constants for the Decay of sod- Transient a t 455 mp in Aqueous 10-2 M Persulfate Solutions First-order rate “constant,”
I n deaerated per-
The formation and decay kinetics of the so4- transient species are, however, dependent on pH in the alkaline region. On flash photolysis of deaerated M persulfate, the amount of transient formed and its decay rate constant remain unchanged up to about pH 8.5. Above pH 8.5, the optical absorption of the transient monitored at 455 mp starts decreasing rapidly
=
PH
sec -1
9.76
5.74 x 103 5.92 x 103
9.96
7 . 1 4 x 103 7.37 x 103
10.36
1.35 x 104 1.35 x 104
10.41
1.34 x 104 1.47 x 104
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July 1967
L. DOGLIOTTI AND E. HAYON
2514
''2a] by dividing with the appropriate concentration of solute. These second-order rate constants are given in Table 111.
1.0
O
. 2000
j
,
),
sow
i
4000
Table III: Rate Constants for Reaction of SO,- with Various Reactants on Photolysis of Aerated Aqueous Solutions of 10-2 M Potassium Persulfate Concentration M range,
Reactant
,L,i .mp
6000
WAVELENGTH A
Figure 3. Optical absorption spectrum of 0 8 - transient produced on flash photolysis of aerated persulfate solutions a t p H 11.9. Optical density measured 80-100 psec after start of flash.
0.3-1.0 0.3-1.0 1.5-6.0 3.0-8.0 4.0-8.0 4.0-8.0 0.4-2.0
Methanol Methanol Ethanol Ethanol 2-Propanol 2-Propanol HCOa-
X X X X X X X
PH
4.8 1.0 4.8 1.0 4.4 1.0 7.5-8.5
10-3 lo-' lo-' lo-' lo-' 10-3
M - 1 8ec - 1 k,
2.5 f0.4 X 2 . 5 f 0.6 X 7.7 & 2.2 X 6.2 f 1.4 X 8.5 3.0 X 9 . 1 3~ 2 . 8 X 9 . 1 =k 0 . 4 X
lo7 lo7
10' 10' lo7 10'
10'
Discussion Almost all earlier studies on the photochemistry of persulfate ions postulated the rupture of an 0-0 bond and the formation of sulfate radicals SO4- according to
5202-
h', 2504-
(1)
The sod- radical anion was considered,2ma,6J'however, to be unstable and to hydrolyze in aqueous solutions, giving rise to a hydroxyl radical
Sod-
PH Figure 4, Change in optical density a t 430 mp 2)s. p H in the flash photolysis of aerated lo-* M potassium persulfate solutions using 10-cm optical path cells.
+ H2O
were found to react very fast thermally with persulfate ions or with S04- and could, therefore, not be used to study the chemistry of S04- anions with them, e.g., acetic acid, formic acid, and arsenite. The reactivity of S 0 4 - with methanol, ethanol, 2-propanol, and bicarbonate was studied. In the presence of these solutes, SO4- follows pseudo-first-order decay kinetics. From these slopes the first-order decay constants obtained were converted to second-order rate constants The Journal of Physical Chemistry
+ sod2-+ H+
(2)
Evidence for reaction 1 is now obtained from the absorption spectrum of the transient species produced on flash photolysis of persulfate ion (Figure 1). This transient, with A,, 4550 A, is formed on flashing deaerated, aerated, neutral, or acid persulfate solutions. Support for the assignment of this spectrum to the Sod- radical anion was obtained from the flash photolysis of sulfate ions in the vacuum ~ltraviolet.'~The absorption spectrum of so42-has been described as an electron-transfer spectrum hv
S042-(aq)
Reaction of SO4- with Added Solutes. Many solutes
OH
~
so,- + cap-
(5)
Onflash photolysP in thevacuum ultraviolet of aerated solutions of sulfate ions, an absorption spectrum with A,, 455 mp identical with the one shown in Figure 1 was obtained and another absorption with Xmax 240 mp corresponding to the 0 2 - ion formed from the reaction esp~
+
0 2
-+0 2 -
(6)
~~~
(13) E. Hayon and J. J. McGarvey, J . Phys. Chem., 71, 1472 (1967).
FLASH PHOTOLYSIS OF PERSULFATE IONS
2515
was also found. This observation is taken as strong evidence for the identity of the absorption with ,A, 4550 A. Furthermore, in the course of this work, Heckel, Henglein, and Beck14 published a paper on the pulse radiolysis of aqueous solutions of sulfuric acid where they report a spectrum with A,, 4500 A, but without the broad absorption in the 300-350-mp region, which they assign to the Sod- ion formed from the reaction OH
0.10
1.0
-
0.8
0.D.
0.4
+ HSO4- +Sod- + HzO
The S04- transients were found to follow a secondorder decay (see Table I).
SO4-
+ SO4- --+
(7)
products
In order to determine the bimolecular rate constant for the decay of Sod-, the extinction coefficient is required (see eq 3). This was derived from the photolysis of an aerated solution of persulfate in presence of bicarbonate ions. This system converts the so4radical anions to COS- radical anions, according to
Figure 5. Flash photolysis of aerated lo-* M persulfate in presence of bicarbonate ions in the pH range 7.5-8.5. Decrease of Soh- transient a t 330 mp (measured 40 Nsec after start of flash) and formation of COatransient a t 600 ma as a function of [HCOa-] (measured 80-100 Nsec after start of flash).
1.25 X 107 M-’ sec-1 obtained in the pulse radiolysis of NzO solutions of carbonate ions.15C Thus, the observation and identification up to pH 8.5 of the 804- species in the flash photolysis of persoh- HCOa- --t CO3- 504’- H+ (8) sulfate indicate the relative stability of SO4- with respect to hydrolysis in aqueous solutions. Above pH with ICs = 9.1 f 0.4 X lo6 M-l sec-l. Since the ab8.5, this transient absorption decreases rapidly. It 6000 A) and the extinction cosorption spectrum (A, can be seen from Figure 2 in the photolysis of deaerated efficient ( ~ c o1830 ~ - f~ 30) ~ ~ of COa- are known115this persulfate that at pH 10.7-10.8,the SO4- absorption allows one to determine ego4-. The decrease of so4has completely disappeared. Over the pH range 9.5radical anion monitored at 330 mp (COS- has a negli10.5,the SO4- radical disappears by a first-order progibly small absorption at this wavelength) and the forcess. From the data available it is not possible to mation of C03- a t 600 mp was studied as a function of distinguish categorically between a first-order decay bicarbonate concentration. Bicarbonate ions were reaction and a pseudo-first-order decay dependent used instead of carbonate ions so that the pH of the on [OH-]. It is thought, however, that the reaction solution remains 8.5,thus avoiding the formation SO4OH- -t S042OH is not very likely to of ozonide ions. The results are shown in Figure 5. take place in view of the high electron affinity of OH-. The ratio (optical density a t 330 mp of SO4- transient)/ Further work is needed to clarify the mechanism leading (optical density at 600 mp of COB- transient) = 0.132 to the conversion of SO4- into OH radicals in alkaline f 5%. Taking15bscC C O ~ 1830 - ~ f 30, one obtains solutions. e ~ 242 f ~ 5%. ~ Since - optical ~ density ~ at, 455 mp/ On photolysis in the presence of oxygen, a new longer optical density at 330 mp = 1.9 for the 804- transient, lived species is formed, with a maximum at 4300 A one therefore derives an extinction coefficient for (Figure 3). The initial optical density of this transient SO4- at its maximum absorption of ~ ~ ~ , -f~25. ~ ~ at 4 6 0 A is also found to increase with an increase in 4300 This value is in good agreement with a value of egg4-4M)o pH (Figure 4). This transient species is considered 450 f 45 derived independently in the vacuum ultrato be the ozonide ion 03-,formed according t o the violet flash photolysis of sulfate ions.la Taking this reactions value of the extinction coefficient, k . ~was calculated. Table I, column 4 shows these values and they appear to be almost independent of pH in neutral or acid solu(14) E. Heckel, A. Henglein, and G. Beck, Ber. Bunsenges. Physik. Chem., 70, 149 (1966). tions. (15) (a) J. P.Keene, Y. Raef, and A. J. Swallow, “Pulse Radiolysis,” The COa- formed in aerated solutions of persulfate M. Ebert, J. P. Keene, A. J. Swallow, and J. H. Baxendale, Ed., in presence of bicarbonate ions was found to follow Academic Preess Inc., New York, N. Y., 1965,p 99; (b) G.E.Adams, J. W. Boag, and B. D. Michael, Proc. Roy. SOC.(London), A287, COa-) = 1.7 X a second-order decay, with Ic(COa321 (1965); (c) J. L. Weeks and J. Rabani, J . Phys. Chem., 70,2100 lo7 M - l sec-I. This value is to be compared with (1966).
+
+
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