752
Y.
Kobayashi, A. Barkatt, and J. Rabani
Yields of Radiation Products in Sodium Metaphosphate Glasses Yoshimitsu Kobayashi,' Aaron Barkatt,*lb and Joseph Rabani Department of Physical Chemistry, The Hebrew University, Jerusalem 9 1000, lsraei Received January 2, 1974)
(Received July 23, 1973; Revised Manuscript
Metaphosphate glasses have been irradiated and dissolved in aqueous solutions, where the trapped positive and negative species, MP+ and MP-, respectively, reacted with scavengers. I-, 0 2 , Fe(CN)s4-, Fe3+, and TMPD have been used to scavenge MP+ and MP-. From product analysis, G(MP+) = G(MP-) = 2.2 0.1 was calculated. Combining this with absorbance measurements in glasses yields ceoo(MP+) (the extinction coefficient of MP+ at 500 nm) = (1.92 f 0.17) X lo3 M - l cm-l. MP- reacts with aqueous 0 2 to produce peroxy radicals ( 0 2 - or MPOz-). Whatever peroxy radicals are formed, they decay in water to produce H202. MP- does not react with NzO and does not produce Hz upon reaction with alcohols in acid solutions. It is concluded that M P - does not form eaq- or H upon its interaction with water. Pulse radiolysis experiments show that aqueous solutions of metaphosphate are relatively inert toward ea,.- and OH. k(e,,MP) < 1 X loe and k(OH + MP) < 5 x 106M-lsec-l.
*
+
Introduction Upon irradiation of sodium metaphosphate (MP) glass by ionizing radiations, several absorption bands in the optical and esr spectra appear. These were assigned to electron-deficiency (hole) and electron-excess centers, respectively .za The processes leading to the formation of these products over an interval of up to sec after irradiation were previously investigated.2b It is the purpose of the present work to determine the G values of the stable products. The techniques employed here are based on analyzing the effects produced by the dissolution of the irradiated glasses in aqueous solutions. Similar effects have been investigated previously for solids, especially alkali halides, and they include detection of light emission,3 pH measurement^,^ Hz analysis, ti and investigations of redox reactions with solutes. These reactions include the oxidation of I - to 13-,5of Fez+ to Fe3+,6 and of tetramethyl-p-phenylenediamine(TMPD) to Wurster's blue.? Reduction processes investigated included the conversion of tetranitromethane (TNM) to nitroform7 and the formation of Nz from N20.8 Another system utilized in the present work was the reduction of Fe3+ to Fez+ in the presence of methanol, previously employed in the radiation chemistry of aqueous solutions .9 The use of suitable systems makes it possible to determine the yields of the stable radiation products and their reactivity in aqueous solutions. Comparison with the initial yields of their precursors enables us to test whether the change in optical absorption observed during the stabilization process involve annealing. Experimental Section Irradiations were carried out with a Radiation Machinery Corporation 137Cs y source. The dose rate was measured by means of the Fricke dosimeter taking G(Fe3+) = 15.6 molecule/100 eV. (The results agreed within 5% with results obtained by means of Coz+-doped 2-mm thick glass plates calibrated by the Soreq Nuclear Research Center, Israel.)lo The dose was 6.68 X l O l s eV hr-I g-l unless otherwise stated. The Journal of Physical Chemistry, Voi. 78, No. 7, 1974
The method of preparation of sodium metaphosphate (MP) glass disks was described earlier.11 The chemical composition of these samples was ( N ~ P O S where ) ~ , n cz 50, and with -OHterminal groups. The disks were 1.60-1.80-mm thick and weighed 1.0-1.5 g. All the reagents employed were of AR quality. Highpurity Ar, 0 2 , and NzO (Matheson Co.) were used. Ar and NzO were purified by bubbling through a trap of V2+ (prepared by shaking together NH4V03 and zinc amalgam in concentrated HC1)I2 and subsequently through traps of NaOH and of triply distilled water. Solutions for the dissolution of the irradiated glasses were prepared using triply distilled water. The optical densities of the dissolved irradiated glass samples were measured with the aid of a Beckman DB-G spectrophotometer against reference solutions containing the same reagents and the same amounts of unirradiated glass. All measurements were carried out at a temperature of 25 i 2". Results Determination of Yields. The yields of the oxidized and reduced products of the irradiation of M P glass were determined by dissolving the glass samples in the following solutions: air-saturated KI, air-saturated &Fe(CN)e, airsaturated TMPD, and Ar-saturated methanol-containing Fe(C104)3. In all cases, the following precautions had to be taken in order to obtain reproducible results. (1) The glass was crushed into fragments weighing less than 50 mg each before irradiation. During dissolution the water was vigorously stirred. This prevented recombination near the surface, so that the radicals reacted exclusively with the solutes. We have found that a further increase in the rate of stirring did not affect the results under our conditions. (2) The dissolution took place immediately after irradiation to minimize the effect of thermal decay. In all cases, 500-mg M P samples were dissolved in 25-ml scavenger solutions. Dissolution was usually complete within 20 min. Optical densities obtained with samples irradiated for long periods (in the KI system) were read after appropriate dilution.
753
Radiation Products in Sodium Metaphosphate Glasses DOSE ,10'8eVgr-'
IO
5
1
20
15
DOSE. ~ ~ " e ~ g r - ' 2
,
3 -1
0 1
01
I
I iJ i i
oo!
loa
5
10 15 IRRADIATION TIME, hrs
20
[ 13
25
30
Dissolution of irradiated MP in air-saturated 0.06 M KI (A 353 nm, I = 1 cm): ( 0 ) dose rate 6.68 X loi8 eV g-' hr-'; ( 0 )20-hr delay before dissolution; (El) 30-hr delay before dissolution; ( X ) dose rate 2.67 X 10l8 eV g - ' h r - ' , time Figure 1.
scale divided by 2.5.
( a ) Aerated KZ. The extent of the oxidation of KI to ioof M P was 2.1. The production of Fe(CN)03- was meadine was determined by reading the optical density D due A. 3E&nm, .&wrz ' q{,-J, ~ . ~ ~ ' C . r t ~ ~ ' C ) . M . - t e m . - t ,stuZr d 7.5-W mla after the. PALLof. the rlissohhion. by. read= ing the optical density at 420 nm. The absorption of Corrections were carried out for the equilibrium Fe(CN)s3- a t this wavelength was found to be independent of the presence of MP, with a molar extinction coefI, + II,- (yl = 774 M-9'4 (1) ficient of (1000 f 50) M - 1 cm-1 in agreement with previThe results obtained for the optical density at 353 nm as a ous data.l5 The dependence of Fe(CN)e3- absorption on function of irradiation time are given in Figure 1. Note irradiation time was linear in 0.01 M ferrocyanide up to 3 that D l h r ( l (KlB-1)-I) represents the optical density hr at least (see Figure 2), and the resulting yield of which would have been obtained had all the iodine been in the form of 13- irrespective of [I-]. Air-saturated aque[ F e ( c N ) ~ ~ was - l (17 f 1) X l o d 6 M/hr. We found that ous solutions acidified with HzS04 were employed, and using a constant dose (1-hr irradiation) D increased with the optical densities were read 10-15 min after the end of Fe(cN)c4-. Above 5 X M ferrocyanide, D became the dissolution. The results were independent of [H+]over independent of [ferrocyanide] (Figure 2). the range of p H 2-5. They were also not changed by satu( d ) Aerated TMPD. In the case of air-saturated TMPD the production of Wurster's blue was measured a t 600 nm. ration with 0 2 instead of air. No concentration effect on The introduction of M P did not change the absorption at the total iodine (I3- i- 12) can be observed within experithis wavelength and the value used for c was accordingly mental error at > l O - 3 M KI. From Figure 1 it can also be 10,100 M - l cm-1.16 The oxidation of TMPD by air over seen t h a t the total iodine production is linearly dependent on the irradiation time of the sample up to 3 hr (2 x 1019 the dissolution time ( 5 x (see Figure 5). It can be seen from both figures that irralo-* M (see Figure 3 ) . As in the case of I-, there was no diation at a high dose rate and subsequent delay gave effect of [OZ] which was varied from 2.6 x to 1.3 x practically the same results as irradiation with the same M . Replacing the 0 2 by Ar or NzO resulted in the dose at a slower rate over the same overall period of time. elimination of the product (Wurster's blue). ( b ) I - Saturated with 0 2 , N20, or A r at p H 2 and 4. ( e ) Ar-Saturated Methanol-Containing Fe( C104)3. The results obtained upon dissolving the irradiated glasses Aqueous 1,lO-phenanthroline (5 ml of 0.25% solution) was in 02-saturated solutions were identical with those obadded a t the end of the dissolution in a Fe3+-methanol tained in aerated solutions. When Ar or NzO were used aqueous solution. The optical density of the Fe2+-1,10instead of 0 2 , only about 10% of the Is- was obtained. phenanthroline complex was read at 515 nm. Although When IZ ( ~ X5 M) was initially present in the ArM P apparently forms a complex with Fe3+, suppressing or NzO-saturated 6 x M I - solution, no effect on its yellow color, it has no effect on the absorption of Fez+[I3- 1 could be observed upon dissolving the irradiated 1,lO-phenanthroline at pII 3, where it was found to have a glass. molar extinction coefficient of (11,000 f 300) M - l cm-I (c) Aerated K4Fe(CrV),. The solutions were prepared in in accordance with previous data.l7 The data (for a total 0.05 M HzS04, and the pH after introduction of 20 g 1.-1 volume of 30 ml) is presented in Figure 4, where a linear 1
+
The Journal ot Physical Chemistry, Vol. 78, No. 7, 1974
754
Y.
radicals are expected to oxidize the solute X (X = I-, Fe(CNh4-, TMPD), according to reaction 4 or 4’.2OJ1
DOSE, lO‘’eVQr-’ 04
-1
3,“,
I
IRRADIATION
TIME, hrs
Dissolution of irradiated MP in air-saturated TMPD ( A 600 n m , I = 1 cm). Figure 3.
-
+ 02- + X X+ + H202 2H’ + MP0,- + X X’ + H,O, + MP (x+is %(I2 + 13-), Fe(CN)e3-, or Wurster’s blue.) 2H’
I
I
M
DOSE, 10‘seVgr-’
--+
H202
I i I 1
,I
000
L , Iop[Fe)’]
000
I
2 3 IRRADIATION T I M E , hrs
4
5
Dissolution of irradiated MP in Ar-saturated 0.02 M Fe(C104)34- C H 3 0 H 4-o-phenanthroline (A 515 n m , I = 1 cm):
Figure 4.
(0)0.1 M C H 3 0 H ; ( 0 )0.2 M C H 3 0 H ; ( X ) 0.5 M C H 3 0 H ; ( A )
glasses doped with 0.1 m C d 2 + , 0.2 M CH30H.
dependence on irradiation time is observed a t 0.02 M Fe3+ and 0.2 M CH30H up to 3 hr. After normalization to 25 ml we obtain (9.8 k 0.5) x M FeZ+/hr. D became independent of [Fe3+] a t concentrations >1 x M. Varying the [CH30H] over the range 0.1-0.5 M does not have any significant effect on the results (see Figure 4). Mechanism. The results presented in the previous sections are in agreement with the following general mechanism.
-
-
metaphosphate glass intermediates M P + + M P (2) where MP- and MP+ represent the “trapped electron” and “trapped positive hole,” respectively. Note that MP+ and MP- are free radicals, the nature of which has been discussed.2 Upon dissolving the glass in 02-containing water, reaction 3 or 3’takes place. MPMP-
+
+ O2
0,
MP
-+
+ 02-
MP02-
(3) (3‘)
As we measure final products we are unable t o discriminate between reaction 3 and 3‘. If 0 2 - is produced, it is in equilibrium with HOz, pK(H02) = 4.8.18,19 The peroxy The Journal ot Physicai Chemistry, Val. 78, No. 7, 1974
(4) (4’)
Indeed, we have identified HzOz as a product in such solutions. When solid KI was added (up to 0.1 M) to the solution that contained the dissolved irradiated metaphosphate glass, 13- was produced, although a t a relatively low concentration (35% of the concentration obtained when I- was initially present). If the solution was treated for 20 min by M catalase a t pH 4.5, prior to the addition of KI, no oxidation of I- could be observed. This indicates that either reaction 3‘ is not important, or reaction 4’ proceeds as written, so that no peroxide other than HzOz is formed (catalase acts as a specific catalyst for the decomposition of HzOz).22The low yield (35%) can be rationalized since back reactions between MPOz - or 0 2 and M P + may take place when I- is absent. However, formation of some H202 may be expected from 02- produced in reaction 3 or 3’. MP+ is assumed to react with X according to reaction 5. We have previously shown that MP+ is a strong oxidizing agent.2b The yield of X + depends on the fate of HzOz. When X 3 I - , H2Oz oxidizes two equivalents of X, according to the general reaction MP+
O”’
Kobayashi,A . Barkatt, and J. Rabani,
+
+ 2X
-
-
X
MP 20H-
+
X+
(5)
+ 2X’
(6)
The results a t pH 2.1 (X = ferrocyanide) are in agreement with the proposed mechanism. We have confirmed that the oxidation of ferrocyanide by H202 a t pH 2.1 is not stoichiometric, but proceeds to 81 f 2%, in agreement with previous data.23 This oxidation is independent of the metaphosphate concentration (see Figure 2). Measurements were also carried out a t pH 4.5, where after 20 min two ferricyanide ions are produced per each Hz02 molecule, (The reaction is not stoichiometric, and the concentration of ferricyanide depends on the time.23 The results a t pH 2.1, where the optical density of ferricyanide does not change after 1 hr, are more reliable.) Indeed, higher ferricyanide yields were obtained a t pH 4.5. The kinetics and the material balance observed in the oxidation of ferrocyanide ions a t both pH 2 and 4.5 are in agreement with the assumption that HzO2 is produced upon the dissolution of the irradiated glasses. The oxidation of TMPD by H202 was found to be relatively slow, and reaction 6 could be neglected under our conditions. Thus, in iodide-02 solutions, G(I3-I = l/z[G(MPt) + 3G(MP-)I = 4.3 f 0.2, in ferrocyanide-02 a t pH 2.1, G(ferricyanide) = G(MP7) + 2.62G(MP-) = 7.8 & 0.3, in ferrocyanide-02 at pH 4.5, G(ferricyanide) = G(MP+) + 3G(MP-) = 8.6 f 0.6, and in TMPD-02, G(Wurster’s blue) = G(MP+) G(MP-) = 4.6 k 0.2. In the Fe3+, CH30H deaerated solutions, M P - reduces Fe3+ to Fez+. MP+ presumably reacts with methanol to produce radicals which reduce additional Fe3+.
+
--
+ Fe3+ MP + Fe” + CH,OH MP + CH,OH + H+ MP+ + CH,OH (MPCH~OH)+ CH,O + Fe2+ + H+ CH,OH + Fe3+ (MPCH,OH)+ + Fe3+ CHLO + Fe” + 2H’ +-MP MP-
MP+
--+
(7)
(8) (8‘) (9)
(9’)
Radiation Products in Sodium Metaphosphate Glasses DOSE ,10"eV gr 1.
5
7 1
10
-
IRRADIATION TIME, hrs
Spectrum and intensity (at 500 nm) of absorption in irradiated pure MP disks: (0) no delay; (a) 15-hr delay; (El) 25-hr delay. Figure 5.
+
G(Fe2+) = G(MP+) G(MP-) = 4.4 f 0.2. From the results obtained in all four systems one can calculate G(MP+) = G(MP-) = (2.2 f 0.1). The equivalence of G(MP+) and G(MP-) follows from the requirement for electrical balance. I t is in agreement with the results presented above, and with the lack of change in [I3-], when both I- and iodine were present before the dissolution of the glasses. NzO did not increase the yield of (I3- + 12) in comparison to Ar, as might have been expected if NzO could convert MP- into OH, as it does to eaq-. The low yield of (I3Iz) observed in Ar and NzO is probably due to the incomplete removal of adsorbed 0 2 . This conclusion is in agreement with gas chromatographic measurements which showed that the yield of N2 was less than 0.02 molecule/ 100 eV. Molar Extinction Coefficients of MP+ and MP-. Feldmann, Treinin, and Volterra reported the absorption spectrum of irradiated metaphosphate glass disks.ll We have carried out similar work and confirmed their results (see Figure 5). The absorbance a t 500 nm was linear with dose up to a t least 3 hr of irradiation. Moreover, combined pulse radiolysis and spectrophtometric techniques showed absorbance changes of less than 3% at 500 nm in the time range 30 msec-4 hr. Combining absorbance measurements with dosimetry yields G(MP+) x esoo(MP+) = 4.2 x lo3 M-l c m - l (100 eV)-l molecules, hence e500(MP+) = (1.92 i 0.17) x lo3 M-l cm-l. The spectrum in Figure 5 represents both M P f and M P - . Above 500 nm only M P + absorbs,ll and e = c(MP+). Properties of MP-. ( a ) Gas Chromatography. No H2 was detected upon dissolving the irradiated glasses in 1 M 2-propanol or 1 M methanol a t p H 1. This shows that MP- produces neither eaq- nor H atoms upon its interaction with water. This conclusion is in agreement with the NzO experiments which show that MP- does not react with NzO, and hence it does not react similarly to eaq-. ( b ) Light Emission. An RCA 1P28 photomultiplier, sensitive in the wavelength range 200-650 nm, showed no
+
755
light emission upon the dissolution of an irradiated glass sample (250 h r ) a t p H 4-6. Experiments with irradiated NaCl, under identical conditions, showed strong emission, in agreement with previous results.3b The dissolution of M P takes 20 min, as compared with 1 min for NaCl. However, surrounding the dissolution vessel by a Kodak ASA 400 film showed no darkening in the case of MP, and intense darkening in NaCl. These experiments demonstrate the different nature of products in NaCl and in MP. Had MP- produced eaq- and M P + produced OH upon reaction with water, the emission results would have been similar in both M P and NaCl. It has been proposed that the production of ea,- is responsible for the emission in irradiated NaC1.24 ( c ) Pulse Radiolysis of MP Solutions. The possibility of producing oxidized and reduced products of M P through reactions with the active irradiation products of water, uiz., e,,-, H, and OH, was investigated using pulse radiolytic techniques. The accelerator, the optical analyzing system and the handling of solutions were described previously.25 For all M P solutions used (in concentrations up to 1 M ) no formation of absorbing products other than those of the medium could be observed upon pulsing with 1.5 psec, 3 x lozo eV l.-I, 5-MeV electron pulses. Such negative results were obtained a t pH 5-5.5, 1-2 (HC104), and 11-12 (NHIOH, NaOH) in solutions saturated with Ar, NzO, air, and 0 2 , in the presence and in the absence of alcohols. These observations were in agreement with another series of measurements, where 1 M M P was found to have no noticeable effect on the yield or lifetime of ea,- as measured by direct observation, on the reduction of 5 x M KMn04,26 both in Ar-saturated solutions at pH 5-5.5 and in NzO-saturated solutions at pH 1-2 (HC104), and on the oxidation of aerated M K S C I V solutions a t p H 5-5.5. These results lead to the conclusion that the rate constants for the reactions of M P with ea,- and H are
