CHEMICAL EFFECTS OF IONIZING RADIATIONS ON AQUEOUS INORGANIC SOLUTIONS' EDWIN I. HART Argonne National Laboratory, Lemont, Illinois
RADI.LTION chemistry is a rapidly expanding branch of chemistry. This growth is due in part to the postwar availability and development of powerful radiation sources and in part to its many applications to problems in chemistry, biology, and pile technology. The present paper deals specifically with the chemical changes induced in aqueous solutions under irradiation by a-rays, prays, X-rays, and y-rays. The mechanism of free radical formation, the principles of chemical dosimetry, the determination of primary yields of molecular products and free radicals, and the kinetics of some chemical reactions in aqueous solutions are discussed. The broad aspects of the interaction of ionizing radiations with matter have been treated previously in THIS JOURNAL by Burton's ''An Introduction to Radiation Chemistry" (1). Early work using a-particles and electrons is covered bv Lind (2). Recent detailed reviews on this subject m& also be found in Volumes 1-7 of the Annual Reviews of Physical Chemistry (5). Review papers by Lefort (4) on chemical effects in aqueous solutions, and by Miller (5) on dosimetry are also recommended for additional study.
The energy loss equation for heavy particles, usually employed at nonrelativistic energies in radiation chemistry studies, is:
where the definitions of symbols are identical to those used for electrons and z = charge on the particle. Since the velocity of heavy particles for a given energy is much less than that of electrons of this energy, the principal effect of heavy ions is to increase the energy loss parameter. Numerical values of these parameters taken from Lea (6) are given in Table 1 for electrons, protons, and a-particles. TABLE 1 Energy Dissipation by Ionizing Particles (6) Particle
Electron
Energy (Me".)
ev./i.
0.0001
3.32
PRIMARY PROCESSES
The ionization and dissociation processes which occur when electromagnetic radiation or charged particles interact with water have been described by Lea (6), Dainton (7), Allen (S), and Magee and Burton (9). X-rays and y-rays lead to the ejection of photo- and/or Compton recoil electrons capable of producing multiple ionizations of the water. The average energy loss per om. of track is given by Bethe and Ashkin (10) for electrons a t relativistic velocities:
+~1000
~.+NIOW
i.+
(A)
I.+-IO~DIAMETER IONlZATlON BY 0.5 MEV. ELECTRON
IONlZATlON BY A N (2-PARTICLE
where E = energy of the incident particle m = mass of the electron e = charge on the electron u = velocity of the particle c = velocity of light p = V/C N = number of atoms/cc. of matter irradiated Z = nuclear charge I = average excitation potential of the atom
For protons, or-rays, and other heavy particles, the rate of energy loss is much greater than for electrons. 1 ~~~~d on work performed under the auspices of the U, 'S, Atomic Energy Commission.
586
Pigun. 1.
Rdmtive Track D-Mities of E l e b r o w and n-Partial-
The distance between successive ionizations in the path of a primary recoil electron produced by Corn y-rays is of the order of 1000 A. Consequently, a rather sparsely ionized track is obtained. At each point of ionization, secondary electrons promote further ionization of the water (reaction 2) and form a group of ion pairs. These are represented by A in Figure 1, and are postulated to have an original diameter of the order of 10 to 20 A. (11, 12). or-Particles form an essentially continuous track of ionization ( B of Figure 1) resulting from overlapping spheres of JOURNAL OF CHEMICAL EDUCATION
ionization. The ions are converted to free radicals by the following processes:
H1O + (e, or e,,) = H
+ OH-(aq)
(4)
where e, = recoil electron, e,, = recoil electron after the nth ionization, e, = secondary electron, e,, = secondary electron after the nth ionization. Reaction (1) is the ~ r i m a r v ionization Drocess initiated by a'ricoil electron (e,)." Secondary electrons (e,) and the original slightly degraded recoil electron (e,,) continue ionization processes represented by reactions (2). Depending on the velocity of the ionizing particle, these ionization processes require of the order of 10-16 to 10-18 (13). ~h~ conversion of H ~ O +into a hydroxyl radical (reaction 3) about lo-" sec., the time of dielectric relaxation and orientation of dipoles. The thernialization and capture of electrons (reaction 4) forming a hydrogen atom also requires a time of this order of magnitude. The results of these ionization processes is a net dissociation of the water molecule according to reaction (5). (5)
-
Another im~ortantsource of free radicals for ionizin~ radiations is dissociation by excitation without ionization. Processes of this type, occurring within lo-" sec., take place in between the ionization spurs and add to the net formation of free hydrogen and hydroxyl radicals. I n view of the low local concentration of free radicals resulting from excitation, there is little likelihood of pair-wise recombination of these radicals to form hydrogen and hydrogen peroxide. In gases, the number of molecules dissociated by excitation is about equal to the number dissociated by ionization (16). It is likely that dissociation processes are of equal importance in liquids. SECONDARY PROCESSES
After conversion into free radicals but before appreciable diffusion has occurred, the original ionization spheres of Figure 1may contain several pairs of hydrogen and hydroxyl free radicals (Figure 2). Hydrogen and hydrogen peroxide are postulated to be formed by the pair-wise recombination of hydrogen and hydroxyl free radicals according to the reactions: H+H=H, OH
+ OH = H,Oz
(6)
(7)
On the basis of this theory, approximately one half of the free radicals react to re-form water,
+ OH = H 2 0
(8) .. The radical-diffusion model accounts satisfactorily for the production of hydrogen and hydrogen peroxide. This has been demonstrated by the mathematical treatments of Samuel and Magee (If), Fricke (I!?), and Schwarz (14). An elementary schematic representation of these diffusion reactions appears in Figure 2. The possibility of direct molecular dissociation of excited water molecules to give hydrogen and hydrogen
H
VOLUME 34, NO. 12, DECEMBER, 1957
i.4
ol-!
--
4H20=4H+40H
4 H 2 0 = H2
ri-
2 H Hz 2 0 H Hz02 HtOH = H 2 0
+ H 2 0 2 + H20 + H t OH
2. Di-ci.tion
.nd
Recombinstion in "Hot spot."
peroxide will be discussed later. Reactions (6)-(8) occur during diffusion and reactions of the radicals within lo-' sec. of the time of passage of the ionizing .. ,.-\ particle ( I S ) . If the solute species are present a t concentrations above lo-= M, hydrogen and hydroxyl radicals escaping reactions (6), (7), and (8) may react with the solute. These reactions occur in the volume of the liquid appreciably outside the original ionization sphere. Regardless of the genesis of the free radicals (H and OH) a n d molecular-species (Hz and HzOz), the basic radiation induced water decomposition equation is:
.
H 2 0 = aHl
+ bHIOn+ cH + d ( 0 H )
(9)
Equation (9) is important in explaining chemical phenomena associated with the irradiation of aqueous solutions. Since rate constants of radical-radical reactions are, in general, orders of magnitude greater than radical molecule reactions, hydrogen, hydrogen peroxide, hydrogen atoms, and hydroxyl radicals appear as primary products of the irradiation. Only a t solute concentrations above 0.01 M does the solute begin to interfere seriously with the yields of hydrogen and hydrogen peroxide assumed to be formed by reactions (6) and (7) (14). An important task undertaken by radiation chemists is to measure the yields of these molecules and radical species formed in water by a representative group of ionizing radiations. CHEMICAL DOSIMETRY
Dosimetry of the ionizing radiations is a prerequisite for quantitative studies in irradiated systems. Owing to the wide variation in ionization density between radiations of different quality, it is desired to determine energy absorption in the system rather than the radiation flux. Much of the work reported in the two previous decades has been in units of the roentgen (r.)2 or kiloroentgen (1000 r.). When low intensity X-ray tubes only were available as radiation sources, it was more convenient to measure the total energy absorbed in a given volume of air by ionization than by calorimetry. Upon irradiation, air becomes ionized and when a voltage sufficientto collect all the ions formed is applied across the irradiated zone, the so-called "saturaA roentgen is defined (1937) as the quantity of X- or 7radiation auch that the associated corpuscular emission per 0.001293 g. of air produces, in air, ions carrying 1 electrostatic unit of quantity of electricity of either sign.
587
tion" current through a given volume of irradiated air is a measure of energy absorption. The equation for energy absorption per roentgen per gram of air is given by: 'W
=
87.70 ergs/r.g., air
where 2.082 X lo0,number of ion pairs/roentgen 0.001293, weight in grams of 1 oc. of air under standard conditions WSi. = 34, energy in electron volts required to produce one ion pair in air = 1.602 X 10-IZ, factor for converting electron volts f into ergs n
to
Table 2 gives yields for a number of different - typical -radiations. Ferric ion yields for high energy electrons and y-rays have a maximum G(Fe+3) of 15.6. Densely ionizine narticles with a hieh linear enerav transfer have aUl&verlimit of G ( F ~ + of ~ ) about 3 0 which is obtained for U235fission fragments. Other particles listed in Table 1 have G(Fe13) values between these two extremes.
= =
As water has a much greater density than air, relatively more X-ray or y-ray energy is absorbed per unit volume in water than in air. In addition, water has more electrons per unit mass than air. Correcting for these factors, it is found that:
*
loo G(Fet')
=
38.6 X IOZ5C(for Coeoy-rays)
2 . 0 Mev.
X 6.02 X 10zaC
(10)
where C = the increase in Fe+3 concentration in moles/liter produced by the exposure Originally Frioke developed the ferrous sulfate do~imeterfor applications in X-ray therapy. For this use, it was essential that the chemical dosimeter and the standard air ionization chamber have identical relative responses to X-rays of different wave lengths. Fricke estimated that the necessary equivalence of the ferrous sulfate solution and air would be obtained if the aqueous medium was 0.8 N in sulfuric acid. Agreement to within ly0 wsa found experimentally between these dosimeters in the wave length range of 0.20 and 0.75 A. after corrections were made for the decrease in intensity of the radiations as they passed through the media. of the two dosimeters. While no special advantage is gained by the use of the 0.8 N sulfuric mid dosimeter in purely chemical studies, custom dictates its continued usage.
G(Fe +a)
15.45 =t0 . 3
Refe~.ences
(19)
4 . 5 5.69 kev. 21.16 Mev. 18.70 :, 12.0 6.2 " 3.47 " 40 "
In -
Ionization methods of dosimetry have largely been abandoned by chemists in favor of the widely accepted Fricke Dosimeter (17). This dosimeter, as currently used, consists of an air saturated solution containing 0.001 N ferrous sulfate, 0.001 M sodium chloride in 0.8 N sulfuric acid.8 Exposure of this solution to ionizing radiations results in the oxidation of the ferrous ion. The amount of chemical change is proportional to the total dosage, is independent of the dosage rate, and within wide limits is independent of ferrous ion, ferric ion, and oxygen concentrations. Chemical yields in aqueous solution are expressed in units of molecules formed/100 ev. (electron volts) of ionizing energy absorbed. The symbol, G(Fe+a), is used to denote the number of ferric ions formed/100 ev. Ferric ion concentrations are usually measured by spectrophotometry or by potentiometric titration. Hochanadel and Ghormley (IS),using calorimetry to measure Co60 y-ray energy absorbed in 0.8 N sulfuric acid. obtained a yield of G(Fe+a) of 15.6 0.3 Fe+a/lOO ev. Dosage is readily calculated for any type of radiation from G(Fe+3) and the amount of ferric ion formed, as follows : =
Energg
Radiation
Electrons cow y-Tay8 (1.1, 1.3 Mev.) Electrons Deuterons
He ions
ergs ergs Em = 9764 -- or 97.37 r.g. water r.ml. water
E(ev./l.)
TABLE 2 G(Fefa) for Ionizing. Radiations
~
5.3 " 1.99 " 0.98 " 5.3 " 2.35 " 4.78 162.0
Protons n-Particles Poa'@ Liyn, a)H8 Bl0(n, ar)Li7 U2" Fission
1:
4.8 8.0 7.2 5.0 0.2 5.69+0.12 4.22 + 0.08 3.0
(21 (21) (89) (24,ZO) (24,ZO) (MI
- --
The manner in which this ferrous sulfate system is used to calculate molecular product and radical pair yields is discussed in the next section. MOLECULAR PRODUCT AND FREE RADICAL YIELDS
With a sound basis for dosimetry established, the next step is t o determine the yields of hydrogen, hydrogen peroxide, hydrogen atoms, and hydroxyl radicals resulting from the decomposition of aqueous solutions (equation 9). Experience has shown that these yields, designated in the following discussion as g(Ht), g(H2O2),g(H), and g(OH), respectively, depend on the reactivity of the solute and on the solute concentration. I n this paper, lower case g's will be reserved for the yields of primary water decomposition products whereas capital G's will refer to the yields of products experimentally observed. g(H) and g(OH), it must be remembered, refer only to the yield of those radicals surviving the recombination reactions (6) to (8) and capable of reacting with the added solute molecules. These primary radiation yields are useful in determining the ability of a given radiation to form products and in deducing mechanisms of reactions. A brief outline of the method of calculating these primary yields follows: All chemical methods of measuring these yields are based on the assumption that the mechanism of the reaction between the solute species and the reactive fragments from the water radiolysis is known. A suitable system is the oxidation of air saturated ferrous sulfate since the mechanism of this reaction is now well understood. The principal reactions are: Fe++
+ HIOt = Fe++ OH + OH+ OH = Fe+' + OHH + Oe = H01
Fe++
(11) (12) (13)
JOURNAL OF CHEMICAL EDUCATION
From the above mechanism, it is noted that each peroxide molecule oxidizes two ferrous ions, each hydroxyl radical oxidizes one ferrous ion, and each hydrogen atom oxidizes three ferrous ions. Consequently, in the absence of any interfering reactions, we have
+
G(Fe+S) = 2g(H~02) 3 d H )
+ d0H)
(16)
highest for y-ray or electron irradiations. Thus, it is clear that X-rays and y-rays are best employed for studies of the effect of hydrogen atoms and hydroxyl radicals on chemical systems. Heavy particles, on the other hand, release hydrogen peroxide as the preponderant reactive species. Since the reactions of hydrogen peroxide with inorganic systems are well known, the major contributions from radiation chemistry lie in the use of y-rays and electrons for studying free radical reactions.
and g(H4
=
G(Hd
(17)
We further assume that hydrogen atoms and hydroxyl radicals are formed in equimolar amounts via reaction (5) and disappear via reactions (6) and (7) to form hydrogen and hydrogen peroxide. Then the equations of material balance are: g(-H*O) = d O H )
+ 2g(&On)
g(-HsO) = g(H)
+ 2g(Hd
(18) (19)
where g(-HnO) in these equations refers to the yield for the disappearance of water molecules. From (18) and (19) we have: Equations (16), (17), and (20) provide three equations for evaluating four unknown quantities. Consequently another reaction is necessary. A frequently employed reaction is the reaction of ceric sulfate. The established mechanism is (86):
+ H' Get'+ HO1 = Cet3 + H + + OP CetS + OH Cet4 + OHCef4
Ce+'
+H
=
CotS
+ H202 = Ce+8 + HO. + H + =
+ 2g(H202)- g(0H)
(21)
Combining (20) and (21),G(Ce+3)may be expressed as: G(CetJ) = 4g(HaOd - 2g(He)
(22)
From (17) and 22), g(H202)is obtained.
From (16) and (20) we may derive: g(H) =
G(FeCS)
G(H3) -2
(24)
Similarly using (21) g(OH) =
Radiation C O W 7-rays ( e ) X-rays ( e )
X-rays (e) X-rays ( e ) Ha &rays Deuterons a-Particles a-Particles
I n view of the complex array of reactive species, it is frequently desirable to emphasize the role played by either the hydrogen or hydroxyl radicals. This is accomplished under certain favorable conditions by the addition of hydrogen and hydrogen peroxide to the system exposed to the radiation. Hydrogen provides hydrogen atoms and hydrogen peroxide provides hydroxyl radicals in accordance with reactions (26) and (27). OH
From this mechanism we have: G(Ce+') = g(H)
TABLE 3 Molecular Product end Free Radical Yields i n 0.8 NSulfuric Acid
G(Hd G(Ce18) 4 +2 2 G(Fet8)
(25)
Therefore, from equations (17), (23), (24), and (25), g(Hn), g(H202)) g(H), and g(OH), respectively, may be calculated. Representative values found in 0.8 N sulfuric acid reported by Allen (26) appear in Table 3. Additional measurements employing hydrogen and oxygen, halide ions, and formic acid provide radiation yields essentially in agreement with those of Table 2 (87, $8). CHEMICAL REACTIONS
Ionizing particles leave an assortment of new molecules and split molecules along their tracks. Free hydrogen atom and hydroxyl radical yields are VOLUME 34, NO. 12, DECEMBER, 1951
H
+ Hn = H + HIO
+ H202= OH + H 2 0
(26) (27)
Consequently if the concentrations of these added compounds are properly selected, it is possible t o study the actions of hydrogen and hydroxyl radicals separately. As long as the concentration of a solute is below 0.1 molar, the radiolysis of this solute occurs by an "indirect" effect. Under these conditions the energy of the ionizing particles is absorbed by the water and chemical changes are subsequently brought about by the action of the free radicals on the solute. At concentrations above 1.0 molar, absorption of energy by solute becomes of importance and effects due to "direct" action may he observed. The following discussion deals only with the effects of "indirect" action and principally with Co60 y-rays a~ the ionizing radiation. DECOMPOSITION OF WATER
The radiolysis of water, when pure and free from organic and catalytic impurities, proceeds until a steady state is reached in which the concentrations of hydrogen, hydrogen peroxide, hydrogen atoms, and hydroxyl radicals remain constant. At y-ray dosage rates of the order of lo1%to 102Qev./l.min., the equilihrium concentrations of these species are found t o he very low and dependent on the conditions of irradiation. The theory of the radiation induced stability of water
was developed originally by Allen and co-workers (8, Z9). According t o this theory, the combination of hydrogen and hydrogen peroxide to re-form water occurs by free radical reactions (26) and (27) resulting in the net reaction (28).
+ HaOn= 2Hs0
HZ
(28)
However, if a large gas phase exists ahove the water, or if the water is agitated by boiling or shaking, then a continuous decomposition of water is observed (8, SO, Sf). Under these favorable conditions, the hydrogen is liberated with the molecular yield of g(H2) of 0.4 for Corn y-rays. One would also expect agitated water irradiated by Po210a-rays to undergo decomposition with a yield G(H,) of 1.6 equal to g(H2) for this radiation. (See Table 3.) Water decomposition increases with increasing ion density of the particle. As noted ahove, when the radiation induced steady state has been attained, disturbing the equilibrium by the removal of hydrogen gas will result in the further evolution of hydrogen. Since hydrogen can be remwed in~yield,g(Hz),.an.amount of hydrogen peroxide equivalent t o g(Hz) remains in solution in view of equation (20), and reactions (26) and (27). Oxygen arises as a result of the free radical initiated decomposition by hydrogen peroxide according t o the stoichiometric reaction: (Sf, 33) H*O*= Ha0
+
'/*02
(29)
Oxygen is therefore liberated under steady-state conditions with the yield G(Oz) = G(Hz)/2. The steady-state conceutration of hydrogen peroxide will therefore be dependent on the efficiency of removing hydrogen and oxygen from the irradiated solution. Bromide and iodide ions when present in low concentrations (10-e M) promote the decomposition of water in much the same manner as efficient shaking of the cells during irradiation (IS, $7, 34). Hydrogen is evolved with a yield, G(H,) = g(Hz), and peroxide is also liberated initially with the same yield. However, as the concentration of hydrogen peroxide increases, a free radical decomposition of peroxide proceeds as is shown by (20). The halide ions react according to the reactions: x + x = x 9 H+x*=H++x-+X
+ H*O = HD + HDO + 2H20 = Hl + 2HDO
DZ
(33)
Dz
(34)
The sum of the concentrations of D2, HD, and Hz equals the concentration of Dz initially dissolved in the water. Hydrogen peroxide is not a final product of the radiolysis. The hydrogen deuteride yield is dependent on pH in the range 0.5 to 13. The free radical reactions are: H+D2=HD+D OH
+ Da = HOD + D D+D=D1
(30) (31) (32)
The net over-all reaction is a catalytic recombination of hydrogen atoms and hydroxyl free radicals according to (8). The steady-stage concentration of halogen is very low unless the iodide ion concentration is increased. Then the equilibrium: prevails and reaction (27) decomposing hydrogen peroxide takes the place of (32). Under these conditions Xa- replaces hydrogen peroxide as a radiolysis product. DEUTERIUM-WATER REACTION
Water containing dissolved hydrogen undergoes no apparent reaction. However, if deuterium is used
(35) (36) (37)
Under conditions where reactions (35), (36), and (37) alone occur, G(HD) is a direct measure of g(H). G(H2) is independent of pH and is interpreted as equal to the molecular hydrogen yield, g(H,), originating by reaction (6) in the "spur" before reaction (35) takes place. The dependence of H D formation on pH is particularly marked in the range from 8 to 11.5. At high pH's, G(HD) approaches zero. This behavior is explained by an ionization of the hydroxyl radical in water according to: OH=O-+H+
(38)
The equilibrium constant for this reaction is about lo-". If the 0- radical ion is relatively nnreactive then its conceutration may increase until reaction (35) competes effectively with: 0 - + H =OH-
HALIDE ION EFECTS
OH+S=OH-+X
instead of hydrogen, hydrogen deuteride is formed (35). The following stoichiometric relations explain the experimental results:
(39)
Further evidence for ionization of the hydroxyl radical is provided in the study of the isotopic oxygen-water exchange reaction. DECOMPOSITION OF HYDROGEN PEROXIDE
The behavior of irradiated hydrogen peroxide solutions is complex (3.2, 33, 56, 57, 58). At concentrations below M, oxygen-free solutions display a fairly normal behavior since G(-H2OZ) is independent of concentration and dosage rate. At concentrations in the range from 0.01 t o 1.0 M, G(-HzOz) is inversely proportional to the square root of ionization intensity. At constant intensity, G(-H202) increases as (Hz02)"'. The dependence of the radiation yield on the square root of the intensity is similar to the photolysis of hydrogen peroxide and provides excellent kinetic evidence for a chain reaction. The propagation steps in this reaction are:
Reaction (40), consuming a hydroxyl radical, is continued by the reformation of a hydroxyl radical in (41). The lifetime of the kinetic chains for irradiations of 0.1 M hydrogen peroxide a t dosage rates decomposing 2 X 1 0 4 M peroxide/min. is of the order of one second, a factor of lo4 greater than the time necessary for interspur diffusion of radicals (33). Consequently a JOURNAL OF CHEMICAL EDUCATION
homogeneous distribution of hydroperoxy free radicals is attained in these hydrogen peroxide solutions. I n order to explain the dependence of the yield on (H202)"', a termolecular termination step is assumed (33). HOn
+ H01 + Hz02 = 2H201+ OP
(42)
which is equivalent t o a bimolecular reaction if the hydroperoxy radical forms a complex with hydrogen peroxide. The intermittent flash technique, used in photochemistry has been applied t o measurements of absolute rate constants of reactions (41) and (42). Two values of ku have been reported, namely: 3.7 (36) and 530 (33) 1. mole-' see.-'. It is difficult t o account for the large discrepancy in these two values. The lower value was obtained using peroxide concentrations greater than one molar, whereas the high value was obtained with 0.1 M peroxide. kd2 has the value of 2.6 X loT0La mole-2 set.-', indicating a high efficiency for reaction (42) if the complex HO2.H2O2 is not formed (33j. Dainton and Rowbottom (36) working a t peroxide concentrations above 1 M report a first order dependence on peroxide concentration and conclude that the termination step is HO?
+ HO? = H,0* + 0%
(42')
Their intermittent radiation studies lead to a value of k42' = 3.4 X 10' l./mole sec. This is a much lower termination rate constant than is cited above, (k4%)). More work is needed on this system to resolve these discrepancies. REDUCTION OF OXYGEN TO HYDROGEN PEROXIDE
Hydrogen peroxide is oxidized to oxygen and oxygen is reduced to hydrogen peroxide by the irradiation of oxygenated solutions. I n closed vessels, an equilibrium is reached containing hydrogen, hydrogen peroxide, and oxygen. With attention focused on the reaction of the radicals with oxygen, the initial peroxide yield is difficult to establish. G(H202) varies from 1.23 to about 3.0 for y-rays depending on a number of factors including purity of the water and the extent of the reaction. Even the lowest value G(H202) = 1.23 is substantially greater than g(HzOz). This result shows that hydrogen peroxide resulting from the reaction of hydrogen atoms with oxygen to form hydroperoxy radicals is an important source of the peroxide. The reactions are:
+ OX= HOz HOz + KOa = H*Oz H
(43) (44)
Hydrogen peroxide competes with oxygen for hydrogen radicals. For example, reaction (27) destroys hydrogen peroxide whereas reactions (43) and (44) form hydrogen peroxide. Studies using isotopically labeled oxygen in the form of 0 ' 8 0 ' 8 underline the importance of reactions (43) and (44). I n acid solutions, 70% of the hydrogen peroxide formed contains the heavy oxygen isotope. Since the thermal exchange between molecular oxygen VOLUME 34, NO. 12, DECEMBER, 1957
and hydrogen peroxide is slow, peroxide with oxygen-18 must have been formed via hydroperoxy radicals. I n alkaline solution, an extensive radiation induced reaction between O'BOISand H2Ol6 occurs (33). The reaction may be represented by the over-all equation: This reaction is sharply inhibited by hydrogen peroxide and catalyzed by hydroxide ions. G(OL601')is greater than 500 in strongly alkaline solutions. Yields of this order of magnitude are indicative of free radical chains since g(H) g(0H) is about 6 (see Table 3). These results are adequately explained by assuming an ionization of the hydroxyl radical (reaction 38) followed by the chain propagation steps, (018)- + 0 1 6 0 1 8 = (018)- + 01011
+
(0x6)-
+ 0"H-
=
(0")
+ OmH-
Termination of the chain sequence may occur by reaction of (0") - with a hydrogen atom or by the reactions (0'")-
+ (0'6)-
= (o'q")--
(OMOLB)-+ H+ = ( ~ 0 1 9 1 0 -
to form hydrogen peroxide. As the concentration of peroxide increases, (016)- radical ions may react as follows: (0'I)-
+ H*OI = 0"H- + H + + 0,-
This leads to termination at peroxide concentrations greater than lod M. OXIDATION OF FERROUS SULFATE
Ferrous sulfate, important in radiation chemistry as a dosimeter, has been thoroughly investigated. It is impossible to do justice to this subject in this short paper but a few of the important reactions involving ferrous and ferric ions will be discussed. Oxyga-Free Ferrous Sulfate in 0.8 N Sulfuric Acid. The mechanism of the aerated ferrous sulfate reaction is outlined in the reaction sequence (Q), (11)-(15). Of great interest has been the mechanism of the reaction in deoxygenated solutions. The ratio, G(Fe+3).i./G(Fe+3),., of the aerated yield totheair-free yield is now fairly well established as 1.88 + 0.04 (40). This low value leads to the conclusion that the hydrogen atom gives rise t o an oxidizing species in oxygen-free solutions. The reasoning is as follows: I n the absence of oxygen, the hydrogen atom might be expected to reduce ferric ions produced in reactions (11) and (12) by hydrogen peroxide and hydroxyl radicals. We have then the possible reaction Fe++
H = Fe++
+ H+
(45)
which, indeed, is observed a t pH's of 2 and higher. By regarding reactions (Q), (ll), (12), and (45) as the mechanism of the deaerated ferrous sulfate reaction, we find G(Fe+"),,. = 2dH90d
+ g(OH) - g(H)
(46)
Inserting the Co60 y-ray values for g(HZ02),g(OH), and g(H) from Tahle 2, we flnd G(Fet3),.,. = 0.96
G(Fe+8).i,/G(Fe+a)yw. then equals about 16 instead of 1.88.
The ratio of 1.88 may he obtained by assuming that the hydrogen atom is an oxidizing species in strong acid solutions. The reaction is Fe++
+ H + + H = Fe+' + H.
(47)
which accounts for the increased yield of ferric ion as well as for the yield of hydrogen. Now using the sequence (9), ( l l ) , (12), and (47), we have:
+
G(Fe+S)v,c.= 2g(M02) g(OH) G(Fe+a),.. = 8.24
+ g(H)
Insufficient data are available at present to distinguish between reactions (47) and (48). Ferr'ous Sulfate, Ozyga and Organic Compounds. Organic impurities from the water, from chemicals, or from glassware, have been the source of many problems associated with radiation dosimetry. Extensive investigations have now been made on the effects of hydrocarbons (dl), alcohols (48), and organic acids (45) on the oxidation of aerated ferrous sulfate solutions. The general effect is to increase G(Fe+a). This increase is accounted for by the generalized mechanism consisting of reactions (9), (11)-(15), plus the following reactions: OH + RH = HzO + R. (49)
+ OX= no2-
+ Fe++ Fe+" +OxR02- + H + = ROOH ROOH + Fet+ = RO- + FeCS+ OH ROT
=
(50) (51) (52)
(53)
It is observed now that the reaction of a hydroxyl radical with the organic molecule leads to a chain sequence capable of oxidizing many ferrous ions. Dewhurst (@) discovered that this chain oxidation of ferrous sulfate may be eliminated by the addition of chloride ion to the syztem. This is the purpose of sodinm chloride in the ferrous sulfate dosimeter. The reaction is altered in the following manner: OH
+ C L = OH- + CI
Fe++
+ C1 = Fef3 + C1-
(54) (55)
If the chloride ion is present in sufficiently high concentrations so that reaction (49) is negligible, the oxidation sequence (50)-(53) leading to high G(Fe+a)is avoided. Ferrous Sulfate-Cup& Sulfate. Interesting effects are observed by the addition of cupric sulfate to ferrous sulfate solutions. The yield is reduced from G(Fe+3) = 15.6 to 4.1 for aerated 0.8 N sulfuric acid solutions containing 0.01 M cupric sulfate. More striking is the difference in aerated 0.01 N sulfuric acid where G(Fe+J) is reduced from 13.8 to 0.66 on the addition of cupric sulfate (44, 45). A mechanism explaining the influence of added cupric ion is:
(12) (13) (56) (57) (58)
If reaction (14) can be prevented by reactions (56) or (57), then the ferric ion yield is reduced t o the value given by the equation, G(Fe+s, Cu++)
G(Fe+8),e/G(Fe+3)v,. = 1.89 in excellent agreement with the observed ratio of 1.88. Interesting is the suggestion of Weiss that the hydrogen molecule ion, H2+, is responsible for the oxidation of ferrous ion.
R.
+ OH = Fe+J+ OH' H + 0% = HO1 Cut+ + H0s = Cuf + H + + O2 Cu'+ + H = Cu+ + H + Cu+ + Fef8 = Cu++ + Fe++ Fet+
=
2g(H201)
+ g(0H) - g(H) = 0.96
(59)
Conditions in 0.8 N acid are not favorable for the elimination of reaction (14) compared to (56) by the addition of cupric sulfate. The agreement between the observed 0.66 and 0.96 is much better in 0.01 N sulfuric acid hut indicates that the radiation yields in Table 3 for 0.8 N sulfuric acidnolonger hold for solutions irradiated at pH's of 2.0. Reaction (15) appears to be very efficient since the molecular hydrogen yield may be reduced from g(Hz) = 0.45 to 0.33 at high cupric sulfate concentrations (45). This result demonstrates that reaction (57) interferes with hydrogen atom recombination reaction (6). REDUCTION OF CERIC SULFATE
In the discussion of molecular product and free radical yields, equation (21) was derived showing the ceric ion yield in terms of g(H), g(HzOz), and g(0H). Sworski (46) finds that the addition of thallous ion to an air-saturated ceric sulfate solution increases G(Ce+3), an effect, opposite to the influence of cupric ion on the oxidation of ferrous sulfate. The reduction of ceric ion in the presence of thallous ion is dependent on the ratio of cerous ion to thallous ion. The mechanism for the reduction of ceric ion with thallous ion is: Ce+'
+ HO1 = Cet' + H + + 0%
Ce+S
+ OH = Ce+' + OH-
+ OH = TI+++ OHCe+&+ Tl++= Ce+a + TI+" TI+
+ H202= Ce+' + H + + HOz
CeC4
(60) (61) (62) (63) (64)
Under conditions of (T1+)/Ce+a >> 1, reaction (61) is suppressed relative to (62) and ceric ion is no longer oxidized by hydroxyl radicals. Instead, ceric ion is reduced by TI++increasing the yield in accordance with the equation, G(Cea+) = g(H)
+ Zc(H.0.) + g(OH)
(65)
The experimental value of G(Ce+3)is 7.92, within 3% of the theoretical value given by equation (65). This small discrepancy can be explained by assuming that hydroperoxy radicals are produced in regions of high ionization density by the reaction OH
+ HnOs = HOI + H20
(66)
The occurrence of this reaction would decrease the Ce+3 yield in the presence of TI+ but would have no effect on the CefS or Fe+s yields in the ceric sulfate and ferrous sulfate dosimeters, respectively. Reaction (66) has also been postulated from oxygen formation using oxygen-free solutions of ferrous sulfate and cupric sulfate (46). This work on the ceric-thallous system also affordsan JOURNAL OF CHEMICAL EDUCATION
example of the use of radiation chemistry in measuring relative rate constants. The ratio of cerous ion to thallous ion has a pronounced influence on the ceric ion yield. By measuring radiation yields as a function of the Ce+3/T1+ ratio, it is possible to establish the relative rate constants of reactions (61) and (62).
This result shows that hydroxyl radicals are 38 times as reactive with TI+ as with Ce+S. SUMMARY A brief outline is presented of the primary and secondary processes taking place during the radiolysis of aqueous solutions. Ionization of the water leads to a non-homogeneous distribution of hydrogen atoms, hydroxyl free radicals, and the recombination products, molecular hydrogen, and hydrogen peroxide. These atoms and molecules initially distributed along the track of the ionizing particle are dispersed by diffusion and reaction with solute ions and molecules. The reactions of these species mith solute form a basis for radiation dosimetry and the measurement of the radiation yields of atomic hydrogen, hydroxyl radicals, hydrogen peroxide, and hydrogen. A short discussion is given of the mechanism of the decomposition of water, effect of halide ions, hydrogen, oxygen, hydrogen peroxide, oxidation of ferrous sulfate, and the reduction of ceric sulfate. LITERATURE CITED (1) BURTON, M., J. CHEM.EDUC.,28, 404 (1951). (2) LIND,S. C., "The Chemical Effects of Alpha-Particles and Electrons," The Chemical Catalog Co., Inc., New York, 10711 A , .
(3) Ann. Rev. Phys. Chem., Vols. 1-7 (1950-1956). (4) LEFORT,M., "Chimie des Radiations des Solutions Aqueuses, Actions Chimique et Biologiques des Radiations," M. HAISSINSKY, Editor, Maasan et Cie., Paris, 1955. (5) MILLER,N., "Introduction a. la Dosim6trie des Radiations, Actions Chimique et Biologiques des Radiations," M. HAISSINBKY, Editw, Ma880n et Cie., Paris, 1956. (6) LEA,D. E., "Actions of Radiations on Living Cells," Cambridge University Press, London, 1955. F. S., J. Phys. Chem., 52, 490 (1948). (7) DAINTON, (8) ALLEN,A. O., J. Phys. Chem., 52, 479 (1948). J . Am. Chem. Soe., 73, 523 (9) MAOEE,J . L., AND M. BURTON, (1951). (10) B&HE,'H. A,, AND J. ASHKIN,"Experimental Nuclear Physics," E. SEGRB,Editor, VOI. I, John Wiley and Sons, New York, 1953.
VOLUME 34, NO. 12, DECEMBER, 1957
SAMUEL, A. H.,
AND
J. L. MAGEE,J . Chem. P h p . , 21,1080
IlO'lrl~ ,A""",.
FRICKE,H., Ann. N. Y. Acad. ojSci., 59, 567 (1955). DEWHURST, H. A., A. H. SAMUEL, A N D J. L. MAGEE, RadiationResearch, l , 62 (1954). SCHWARZ, H. A,, J . Am. Chem. Soc., 77, 4960 (1955). HIRSCHFELDER, J. O., J. Phys. Chem., 52, 447 (1948). JESSE,W. P., A N D J . SADAUSKIS, Phw. Rar.. 97,1668 (1955). FRICKE,H., A N D S. MORSE,Phil. Mag., 7, 129 (1929). HOCHANADEL, C. J., A N D J. A. GHORMLEY, 3. Chem. Phys., 21, 880 (1953). SCHULER, R. H., AND A. 0. ALLEN,J . Chem. Phys., 24, 56 (1956). MCDONELL, W. R., A N D E. J . HART,J . Am. Chem. Soc., 76, 2121 - - (19541. , HART,E. J., W. J. RAMLER, AND S. R. ROCKLIN, Radiation Research, 4, 378 (1956). SCHULER, R. H., AND A. 0. ALLEN,J. Am. C h a . Soc., 79, 1565 (1957). TRUMBORE, C., private communioation. ~CHULER,R. H., AND N. F. BARR,J . Am. Chem. Sac., 78, 5756 (1956). EHRENBERG, L., AND E. SAELAND, Jener Rept. No. 8, 25 (18641. * - --,ALLEN,A. O., Radiation Research, 1, 85 (1954). HOCHANADEL, C. J., J. Phys. Chem., 56, 587 (1952). HART,E. J., Radiation Research, 1,53 (1954). ALLEN,A. O., J. Phys. Chem., 56, 575 (1952). TOULI~, W. J., U . oj Cal., Radiation Labwatories Rept. UCRL 583, 44 pp., February 10, 1950. GORDON, S., AND E. J . HART,Abstracts, April 7-12 (1957) Meeting of the American Chemical Society. FRrcKE, H., J. Chem. Phys., 3, 364 (1935). HART,E. J., A N D M. S. MATHESON, D i ~ m b s i m sof Fomday Soc., 12, 169 (1952). FRICKE,H., A N D E. J. HART,J. Chem. Phys., 3 , 596 (1935). GORDON. S.. A N D E. J. HART.J. Am. Chem. Soc., 77. 3981 (1955). DAINTON, F.S., AND J. ROWBOTTOM, Trans. Faraday Soe., 49. 1160 (1953). DOLIN,P. I . ~ AND'B. , V. ERSHLER, Natl. Conj. on the Peaeejul Uses o j Atomic Energy, 7, 564 (1955). DOLIN,P. I., Symp. on Radiation Chemistry, Acad. of the USSR, Div. of Chem. Soi., Moscow, 1955, p. 15 (Eng. Translation), Consultants Bureau, Inc., New York. HART,E. J., S. GORDON, AND D. A. HUTCAISON, J. Am. Chem. Soc., 75, 6165 (1953). BARR.N. F.. A N D C. G. KING. J. Am. Chem. Soc.. 76. 5565 ,
,
VERMEIL,C., M. COTTIN,AND M. HAISSINSKY, J . Chim. Phys., 49, 437 (1952). DEWHURST, H. A,, Trans. Faraday Soc., 48, 905 (1952). HART,E. J., J . Am. Chem. Soc., 74,4174 (1952). HART,E. J., AND P. WALSH,Radiation Research, 1, 498 (1954). HART,E. J., Radiation Research, 2, 33 (1955). SWOR~KI, T. J., Radiation Research, 4, 483 (1956).
