12 The Radiation Chemistry of Persulfates
Radiation Chemistry Downloaded from pubs.acs.org by UNIV OF LIVERPOOL on 11/13/18. For personal use only.
M A R I A H E L E N A B. M A R I A N O Laboratório de Física e Engenharia Nucleares, Sacavém, Portugal
The effects of peroxydisulfate concentration, of pH, of time of irradiation and of added radical scavengers on the rate of decomposition of peroxydisulfuric acid, in the Co γ-radiolysis of this compound are reported. In dilute H SO solutions, the rate of S O 2- decomposition is very small. G(-S2O82-) decreases with pH and, at constant acidity, increases with [K S O ]. It is inferred that SO - radicals compete with peroxydisulfate for reaction with HO . The reactions 60
2
2
2
2
4
8
8
4
2
SO - + OH --> HSO - and SO - + SO - --> S O 24
5
4
4
2
8
do not take place in the bulk of the solutions. The proba bility of the dimerization of the sulfate radical within the spurs is small as compared with the probability of occurrence of the other SO - reactions. Peroxydisulfuric acid is pro tected against radical attack by added Br ions or peroxy monosulfate. 4
-
THor the correct interpretation of the phenomena taking place i n the radiolysis of sulfuric acid solutions, a good understanding of the radiolytic behavior of the peroxysulfuric acids, as well as of the ultimate fate of the sulfate radicals is necessary. In fact, the peroxyacids of sul furic acid are formed during radiolysis with yields which depend on the H S 0 concentration (1, 4). In aerated solutions this peroxyacid forma tion has been considered as the net result of Reactions 1 to 5: 2
4
OH + HS0 - ^ 4
S0 " + H 0
S0 - + S 0 S 0 4
4
2
4
8
(7,10,15)
2
~
2
(1,4)
(2)
S 0 " + O H -> H S 0 " ( 1 , 1 5 , 1 6 ) 4
(3)
5
S 0 " + H0
2
HS0 " + S0 " + 0
HS0 - + H 0
2
HS0 " + OH + 0
2
8
2
5
4
4
182
(1)
4
2
2
(3,8)
(4)
(3,11 )
(5)
12.
Persuifaies
MARIANO
183
A decomposition of S 0 8 " according to 2
2
S 0 " + H 0 - » H S 0 " + S 0 " (8) or (7) S 0 ~ + O H - » HSO-/ + S 0 " (8) 2
8
2
2
5
5
2
8
2
(6)
4
would also account for the production of peroxymonosulfate. Reactions 2 and 3 are believed to compete with S 0 " -)- H 0 —» H S 0 " + H 0 i n the bulk of the solutions and it is of interest to investi gate their relative importance and the possible existence of other mecha nisms involving the S 0 " radicals. Reaction 2 is still controversial: the yield of peroxydisulfate forma tion in aerated 0.4M H S 0 solutions has been found to be zero (J, JO) or very small ( 4 ) even in the presence of a scavenger of the H 0 radicals ( I ) despite the fact that, for this acid concentration, all the O H radicals are scavenged by the H S 0 ions, Reaction 1 being shifted to the right (10,15), O n the other hand, some authors have failed to observe isotopic exchange between sulfuric acid and peroxydisulfate in the radiolysis of aqueous solutions (3, 6, 12). However, such an isotopic exchange does occur during photolysis (15, 16) and, according to results obtained i n the flash photolysis of persulfate ions, the S 0 " radicals decay bimolecularly with a rate k(SOf + S 0 ) ~ = 3.7 X 1 0 M sec." (5). A reinvestigation of this problem seems to be desirable. The present paper reports the results we have obtained in the C o γ-radiolysis of aerated solutions of potassium peroxydisulfate. The effects of concen tration, of p H , of time of irradiation and of added radical scavengers are reported. 4
4
2
2
2
4
2
4
2
2
4
4
8
4
1
1
6 0
Experimental The irradiations were carried out at room temperature with doserates between 0.40 Χ 10 and 4.66 Χ 10 e.v./ml./hr. The absorbed doses were measured by the use of the Fricke dosimeter, taking G ( F e ) — 15.6 (9). Hydrogen peroxide was determined by cerate oxidimetry measuring the eerie ion absorption at 320 τημ after reduction of the peroxymono sulfate by arsenious oxide (11). A n excess of a concentrate and deaerated ferrous sulfate solution was then added to the deoxygenated sample and the number of F e ions produced was determined spectrophotometrically at 304 τημ (11). After correction for the reaction C e + F e -> C e + F e , the value obtained gives the peroxydisulfate concentration i n the sample. The peroxymonosulfate concentration was determined by dif ference between the total oxidizing power of the solution (obtained i n another aliquot after reaction with concentrated F e S 0 ) and the sum [H 0 ] + [H S 0 ]. The measurements were performed on a Zeiss P M Q II spectropho tometer equipped with a thermostated cell compartment. The molar extinction coefficients of the eerie and ferric ions were corrected for the acid concentration: in 0.4M H S 0 , e ( C e ) was taken as 5,580 liter M " 18
18
3 +
3+
4 +
2+
3 +
3+
4
2
2
2
2
8
2
4
4+
1
184
RADIATION
CHEMISTRY
1
c m ; (9) a n d e ( F e ) as 2,211 liter M c m ; a t 2 5 ° C . ( I I ) . In 5 M H S 0 , the values used were 6,800 liter M " c m ; and 2,874 liter M c m . respectively (2,13). A l l the G values quoted are based on the total quantity of energy absorbed in the system; thus, no attempt was made to calculate the fraction of energy absorbed by the sulfuric acid. The purification of the water has been previously described (10). W i t h the exception of the arsenious oxide (from Carlo Erba, Italy), the chemicals used were Merck analytical products. The peroxymonosulfate samples were obtained from hydrolysis of potassium peroxydisulfate solutions: a 1 0 M K S 0 solution in 1 M sulfuric acid was heated at 60°C. for 75 minutes and let cool overnight inside the thermostated bath (an ultrathermostat Colora was used). Under these conditions, the peroxydisulfate remaining in the solution, as well as the hydrogen peroxide formed in the simultaneous hydrolysis of the Caro's acid, are small when compared with the resulting peroxymonosulfate concentration. The solutions were prepared immediately before use with water (conductivity — 1 X 10" ohm" cm." at 25°C.) purified less than 24 hours before. 1
a
1
1
_2
2
2
1
2
1
1
4
-1
8
e
1
1
Results and Oiscussion The Effect of Peroxydisulfate Concentration on the H 0 Yield and on the Rate of Decomposition of the Peroxydisulfuric Acid. The results obtained in the radiolysis of a 10" M and a 5.2 X 10~ M K S O solutions in 0.4M sulfuric acid are plotted in Figures 1 and 2. The comparative analysis of a set of similar curves obtained with different peroxydisulfate concentrations shows that the hydrogen peroxide yield diminishes at a rate comparable with that of the increase in G( — S 0 " ) (see Figure 3), when [ K S 0 ] varies from 10" M to 10" M. Since no direct interaction between peroxydisulfate and hydrogen peroxide was observed, the results obtained give a strong support to the hypothesis of an interference of S O - with the mechanism of H 0 formation, through scavenging of the H 0 radicals according to Reaction 4. In fact it appears that Reactions 6 and 7 ( 8 ) have to be ruled out since when [ K S 0 ] increases, there is no detectable variation in the yield of peroxymonosulfate which, in each case, was found to be the H S 0 yield measured at zero perdisulfate concentration (Figure 3). If peroxydisulfate is indeed decomposed according to Reaction 4 then, from a similar reasoning, one concludes that Reaction 3 does not take place in the bulk of the solutions. Therefore, the measured H S 0 is a molecular product formed in the spurs. 2
4
4
2
2
2
2
5
8
s
8
2
2
2
s
2
3
2
2
2
2
2
2
8
5
5
O n the other hand, the very to conclude that, in 0.4M H S 0 disulfate formation is small not peroxyacid (15) but because its 2
2
4
low values of G ( —S 0 ") lead us also solutions, the measured yield of peroxyon account of a radical attack on the formation yield is low. In other words, 2
8
2
12.
185
Persuifat es
MARIANO
20
Dose (10 ey./mU
Figure 1. Concentration of H 0 , H SO , and H S 0 as functions of the absorbed dose in the Co y-radiolysis of an aerated I0~ ]Vi K S,0 solution: a) in water (dashed lines); b) in 0 AM H SO (full lines) 2
2
2
s
2
2
8
4
60
2
2
8
u
Dose rate = 0.40 X 10 e.O./ml./hr. Ο — X — peroxydisulfate φ — X — peroxymonosulfate Η X = hydrogen peroxide • — X = peroxydisulfate (in a solution which, at time zero, was 10~''M in 1S
K S,O , I 0 - ' M in KBr and OAM in Η SO,). 2
s
the probability of Reaction 2 taking place i n dilute sulfuric acid solutions seems to be very low. The following mechanism is believed to explain the results obtained: O H + S0 ~ (or H S 0 ) + H -> S 0 " (or H S 0 ) + Η,Ο
(8)
Η + 0 -> H O ,
(9)
4
2
4
4
2
H0
2
+ H 0 -> H 0 2
2
2
+ 0
2
4
(10)
186
RADIATION
S 0 " + H0 2
8
2
2
HS0 " + S0 " + 0 4
4
S 0 " + H 0 - » HS0 ~ + 0 4
2
4
2
CHEMISTRY
1
(4)
2
(3,16)
(11)
and, possibly, also S 0 - + H 0 -> H S 0 " + H 0 4
2
2
4
( 12)
2
From this mechanism one deduces a relationship G ( H 0 ) + G(-S 0 ') = GH O + i ( G " " — G O H ), the H 0 yield measured in 0.4M H S 0 solutions devoid of added solutes, i n accordance with the experi mental results (Figure 3 ) . ( G " " stands for G + G .) 2
2
8
2
2
2
2
2
H
2
2
4
H
H
e&a
Figure 2. Concentrations of the peroxysulfuric acids and of H 0 vs. absorbed dose in the Co y-radiolysis of an aerated 5.20 X 10~*M K S O -f 0.4M H SO^ solution 60
2
2
2
2
s
2
Dose rate = 0.40 Χ 10 e.v./ml./hr. Λ — X = total oxidizing power of the solution Ο — X = peroxydisulfate Η X = hydrogen peroxide φ — X = peroxymonosulfate 18
The Effects of p H and of Time of Irradiation. I n Figure 1 are given the results obtained in the radiolysis of a 10~ M K S 0 solution i n water. It is seen that the initial rate of decomposition of peroxydisulfate is very high: its value, inferred from the tangent to the curve at time zero, is about 6.0. The H 0 yield is zero for the small doses but it increases as the peroxydisulfate concentration diminishes and, once the 4
2
2
2
2
8
12.
MARIANO
Persuifat es
187
peroxyacid has been completely decomposed, regains the value corre sponding to pure water. N o production of peroxymonosulfate was observed. 1.4
[K S 0 ],M 2
2
8
Figure 3. Molecular yields of H 0 and H SO,~ and decomposition yield of S 0 ~ ions vs. peroxydisulfate concentration in aerated 0AM H SO solutions 2
2
2
2
2
8
2
h
Dose rate = 0A0 Χ 10 e.v./ml/hr. H X — H,O Ο X=-S Os ~ Φ - X = HS0 ~ 18
s
2
2
5
It seems, therefore, that, in the absence of the sulfuric ions, a 10~ M K S 0 solution inhibits completely the reaction H 0 - f H 0 —» H 0 + 0 and that the products of this decomposition can destroy the hydrogen peroxide diffusing from the spurs. The results obtained i n this aqueous solution, at very small doses may be explained by the following mechanism: 4
2
2
8
2
2
2
2
2
H + 0 ->H0 2
(9)
2
e-a + 0 - > 0 q
2
H 0 + 0 +
8
(13)
2
H0 + H 0
2
2
( 14 )
2
S 0 - + H 0 -» HS0 - + S0 " + 0 2
8
2
2
4
S0 - + H 0 - » H S 0 " + H 0 4
2
2
4
4
2
2
(4) ( 12)
188
RADIATION
CHEMISTRY
Figure 4. Concentrations of the peroxysulfuric acids and of H 0 as functions of the absorbed dose in the Co y-radiolysis of a 7.6 X 10~ M K S O + 2 X JO" M H SO solution in aerated 5M sulfuric acid 2
60
5
2
2
5
s
2
r>
Dose rate = 0.47 X 10 e.v./ml./hr. Λ — X = total oxidizing power of the solution Ο — X = peroxydisulfate Η X = hydrogen peroxide • — X = peroxymonosulfate 1H
2
1
12.
MARIANO
Fersuifaies
189
O H + H 0 -> H 0 + H 0 2
2
2
(15)
2
OH + H -» H 0 + H 2
(16)
2
(Note: The p K of the hydroperoxy radical i n Reaction 1 4 has a value between 4 . 0 and 4 . 8 (17) and the p H of a 1 0 " M K S 0 solution i n water is 5 . 3 5 . Thus, the concentrations at equilibrium of the basic and neutral forms of the radical are not very different. It may be that, as Reaction 4 proceeds, the equilibrium represented by Reaction 1 4 is shifted to the right till all the basic form, 0 ~ , is consumed. ) From this, one deduces a yield, G ( — S 0 ~ ) == G o + G - -f- G 4
2
2
8
2
2
+
G O H — 6.55 ( G
H 2
o
=
2
0.75, G E
A Q
+ G
H
=
2
8
H 2
3 . 2 0 and G
2
0
E
H
-
A Q
2.60)
H
(14),
a value to be compared with the experimental one, G ( — S 0 " ) — 6.0. The difference i n the rates of decomposition of the peroxydisulfate in water and i n acid solutions results, therefore, from the existence a competition between S 0 " and sulfate radicals for reaction with H 0 , this competition being evidenced by the rapid decay of G ( — S 0 ~ ) with the dose, i n the aqueous solution, its decay with the p H and its increase as [ K S 0 ] increases at constant sulfuric acid concentration. Amidst the various possible ways of reaction of the sulfate radicals in the bulk of dilute H S 0 solutions, the most probable seems therefore to be {
2
8
2
8
2
2
2
2
2
2
8
2
8
2
4
S 0 " + H 0 -> H S 0 " + 0 4
2
4
2
(3,16)
(11)
Reactions 2 and 3 have to be taken into account when the acidity of the solutions is increased over 0 . 4 M ; i n a 5 M H S 0 medium, peroxy disulfate builds up i n the solution and there is a fivefold increase i n the initial yield of formation of peroxymonosulfate (see Figure 5 ) . These variations are certainly the result of a direct effect of the radiation on the sulfuric acid ions leading to an intraspur formation of both peroxyacids. It must be pointed out that, i n concentrated sulfuric acid solutions, the radiolytic phenomena are masked by the high rate of hydrolysis of the peroxyacids (11) so that the interpretation of the experimental data is markedly dependent on the time factor. T o illustrate this, a mixture 2
4
of peroxydisulfate (/—'8.10~ Μ) and peroxymonosulfate Γ,
(2.10" M) 5
in a
5 M H S 0 solution was irradiated with dose-rates differing by a factor of ^~ 1 0 : the results have been plotted i n Figures 4 and 5 , showing a striking difference i n the pattern of the curves obtained. In Figure 5 are also given these results after correction for the effect of hydrolysis (by measuring the blank at the same time as the irradiated solution). From this figure it is inferred that the rate of peroxydisulfate formation is G ( S 0 ~ ) = 0 . 1 8 and that the initial yield of production of peroxymono sulfate equals 0 . 6 0 i n very good agreement with data of Boyle ( 1 ). The Effects of Added C e and Br" Ions. C e and Br~ ions are good scavengers of O H radicals and are likely to interfere with the phenomena 2
2
8
4
2
3+
3 +
190
RADIATION
CHEMISTRY
1
Figure 5. Concentration of the peroxysulfuric acids and hydrogen-peroxide, vs. absorbed dose, in the radiolysis of a 7.8 10-*M K S O + 2.10~ M H SO - solution in aerated 5M sulfuric acid: a) values actually measured (full lines); b) values obtained after correction for the hydrolysis of the peroxy sulfuric acids (dashed lines) 2
2
5
s
2
;
Dose rate = 4.47 Χ 10 e.v./ml./hr. Ο — X = peroxydisulfate φ — X — peroxymonosulfate Η X = hydrogen peroxide 18
taking place i n the radiolysis of potassium peroxydisulfate. T o test the validity of the hypothesis concerning the competition represented by Reactions 4 and 11 we have studied the effects of 10~ M C e and 10~ M Br" when added to 10~ M K S 0 solutions i n 0.4M sulfuric acid. ( A t the concentrations used both C e and B r " ions do not interfere with the phenomena taking place within the spurs.) In the samples containing cerous ions there was no variation i n the rate of decomposition of peroxy disulfate (neither on the yields of formation of the other products of radiolysis) as it was to be expected from the sequence of reactions 3
4
2
2
3+
4
8
3 +
Ce
3+
+ O H (or S 0 ) -> C e
Ce
4+
+ H0
4
2
Ce
3+
4+
+ O H " (or S0 ") and
+ H + 0 +
4
2
2
which has the same effect as that of Reaction 11. In what concerns the solution containing bromide ions a marked sensitization of the rate of decomposition of perdisulfate was expected to occur as a result of the well known reactions B r + O H - » Br + O H '
12.
Persuifates
MARIANO
191
and Br + H 0 - * B r + H + H 0 (18) 2
+
2
2
Rather surprisingly, however, we d i d verify that peroxydisulfate was efficiently protected by Br" ions against radical attack (Figure 1). This indicates that bromine atoms react via B r + HO2 ~» Br" + H + O2 rather than with hydrogen peroxide. +
The Effect of Added Peroxymonosulfate. Caro's acid also acts as a protecting agent against decomposition of peroxydisulfate by hydroperoxy radicals. Even at equimolecular concentrations, G ( — S 0 ~) drops to zero or very low values i n the presence of peroxymonosulfate. This, along with the fact that the peroxymonoacid presents a greater rate of decom position as compared with that of equal molar concentrations of S 0 ~ , indicates that H S 0 is a better H 0 scavenger than peroxydisulfate. 2
8
2
2
2
5
8
2
2
O n the other hand, the measured G ( H S 0 ~ ) values are greater than those corresponding to peroxydisulfate (Figure 5 and Reference 1). It appears, therefore, that the initial yield of peroxymonosulfate formation in sulfuric solutions far exceeds that of peroxydisulfate. These first conclusions drawn from the results obtained in the radioly sis of mixtures of both peroxyacids have to be completed with a more thorough study. In fact, the problems involved i n the radiolysis of per oxymonosulfate deserve further attention and w i l l be considered i n a later paper. 5
Acknowledgment W e are thankful to M . Coimbra for his skillful technical assistance. Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)
Boyle, J. W., Radiation Res. 17, 427 (1962). Boyle, J. W., Mahlman, Η. Α., Radiation Res. 16, 416 (1962). Buu, Α., Pucheault, J., J. Chim. Phys. 63, 1037 (1966). Daniels, M . , Lyon, J., Weiss, J., J. Chem. Soc. 1957, 4388. Dogliotti, L., Hayon, Ε., J. Phys. Chem. 71, 2511 (1967). Eager, R. L., McCallum, K. J., Can. J. Chem. 32, 692 (1954). Giuliano, R., Schwartz, N . , Wilmarth, W . K., J. Phys. Chem. 63, 353 (1959). Hart, E . J., J. Am. Chem. Soc. 83, 567 (1961). Hochanadel, C. J., Ghormley, J. Α., J. Chem. Phys. 21, 880 (1953). Mariano, M . H., Santos, M . L., Radiation Res. 32, 905 (1967). Mariano, M. H . , Anal. Chem. (in press, 1968). Riesebos, P. C., Aten, A. H. W., J. Am. Chem. Soc. 74, 2440 (1952). Scharf, K., Lee, R. M . , Radiation Res. 16, 115 (1962). Thomas, J. K., "Proceedings of the Third International Congress of Radia tion Research," p. 179, North-Holland Publishing Company, Amster dam, 1967. Tsao, M.-S., Wilmarth, W . K., J. Phys. Chem. 63, 346 (1959).
192
RADIATION
CHEMISTRY
(16) Tsao, M.-S., Wilmarth, W. K., Discussions Faraday Soc. 29, 137 (1960). (17) Czapski, G., Bielski, B. H. J., J. Phys. Chem. 67, 2180 (1963). (18) Sworski, T. J., J. Am. Chem. Soc. 76, 4687 (1954). RECEIVED
February 5, 1968.
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