3809
RADIOLYSIS OF CRYSTALLINE LITHIUMBROMATE
hydrogen ion concentrations expressed in molarity units, whereas our PUD value is referred to molality units. The two scales of PUD differ by log do, where do is the density of the solvent. In deuterium oxide, D the molality scale is 0.043 unit higher therefore, ~ U on than that on the molarity scale, and the appropriate c~rrection‘~becomes 0.451, in excellent agreement with 0.447 unit which me have found. There is evidence that the glass electrode responds to deuterium ion as efficiently as it responds to hydrogen i0n.~2,~4It is appropriate, therefore, to define the operational pD of a solution X in the same way as the operational pH is daefined
where EX and Es are the e.m.f. values of the pH cell containing the “unknown” and the pD standard, respectively. The conventional ~ u Dvalues of selected reference solutions are identified with pD(S)
for the experimental determination of pD by eq. 10. (14) P. R. Hammond, Chem. I n d . (London), 311 (1962).
Radiolysis of Crystalline Lithium Bromate by Lithium-6 Fission Recoil Particles’
by G. E. Boyd and
T. G. Ward, Jr.
Oak Ridge National Laboratory, Oak Ridge, Tennessee
(Received J u n e 2g71964)
Measurements were made of the decomposition of Br03- ion and the production of oxidizing fragments in crystalline LiBrOa by energetic tritons and a-particles released following neutron capture by 6Li in the Oak Ridge graphite reactor. A strong dependence of the radiolysis on the linear energy transfer (LET) was indicated by the initial 100-e.v. yield for bromate decomposition, Go(-Br03-) = 1.48, which was five times larger than that obtained with 6OCo y-rays. A correspondingly large yield of oxidizing fragnients also wat; observed; this yield and the ease with which these fragments could be removed from the crystals by mild thermal annealing was interpreted as being inconsistent with a “thermal spike” radiolysis mechanism. The observation that ihe yields, Go(- BrOa-) , Go(“Ox”), and Go(Br-), all were approximately five times those observed with y-rays also suggested1 that the mechanism for the radiolysis did not change with increasing LET. A tenfold increase in the dose rate caused no change in the yields either for broniate deconiposition or oxidizing fragment production.
The dependence of radiolytic yields on the linear energy transfer (LET) is well known for liquids and gases although, 8s Yet, there is comparatively little inforlllation on track &‘ects in crystalline, inorganic solids. The alkali metal nitrates have been studied
most extensively in this connection, and yields have (1) Presented before the Division of Physical Chemistry, 146th National Meeting of the American Chemical Society, Denver, Colo., Jan. 19-24, 1964. Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide ~ o r p .
V o l u m e 68,N u m b e r 12 December, 1964
G. E. BOYDAND T. G. WARD,JR.
3810
been reported for the production of nitrite by the action of “CO y-rays (-0.06 e.v./&), 44.5-ke.v. X-rays (-0.8 e.v./8.), 3.4-fi1e.v. a-particles (-34 e.v./8.), and uranium fission recoil particles (-730 e.-~./i.).~ The initial 100-e.v. yields of nitrite, Go (KO2-), in K S 0 3 at 2 5 O , for example, were 1.5, 2.0, 2.2, and 6.0, respectively. The increase in nitrite ion yield with the LET is roughly parallel to the increase produced when the temperature is raised in the radiolysis with low LET radiations. This analogy was led to the suggestion that the high temperature in heavy particle tracks may be an important factor in the dependence of yield on LET.2 Observations on the chloride ion yields in the radiolysis of NaCIOa by yrays and a-particles also have been interpreted in terms of the “thermal spike” hypothesis. An LET dependence has been noted4 in our own recent work on the radiolysis of LiBrO3 in the Oak Ridge graphite reactor (ORGR). The yield for bromate decomposition by energetic tritons and aparticles from the thermal neutron fission of 6Li was Go(- Br03-) = 1.4, which is significantly larger than the yield (0.31) observed with 60Co y - r a y ~ . ~To study this effect further, a series of neutron-irradiated crystalline LiBr03 preparations containing small, predetermined amounts of 6Li was examined. An estimate of the decomposition caused by reactor y-rays, energetic neutrons, and thermal neutron capture in bromine was obtained by extrapolating the observed bromate decompositions to zero 6Li content. The gross radiolysis therefore could be corrected to give the decomposition by the 6Li(n,c1)~H reaction. Actually, most of the bromate radiolysis resulted from this reaction because of its large cross section and the large energy release per 6Li fission (4.787 Illev.) and the fact that the energetic triton and a-particles were stopped entirely within the crystals. The use of isotopically labeled LiBr03 crystals also permitted an investigation of the role of the dose rate.
Experimental Preparation of Anhydrous Compounds. Crystalline LiBr03 preparations with 0.01 to 1.0 atomic % 6Li were synthesized by mixing aqueous solutions of 6LiBrO, (99.374 6Li) and 7LiBr03(99.99% 7Li) and evaporating them to dryness a t 60-70’ under vacuum to give the anhydrous salt. These preparations were stored away from light over PZ05 in a vacuum desiccator. The 6LiBr03solution was prepared by dissolving 6Li metal in water to form LiOH which was neutralized to ca. pH 4 with HBrO3 prepared from reagent grade KBr03 by cation exchange. A slightly acid solution BrOowas employed to permit the reaction: 5Br-
+
The Journal of Physical Chemistry
+
-
+
6H+ 3Brz, 3HzO. Subsequent evaporation of the solution removed excess Br- from the system. A 6LiBr03 preparation containing ea. 7 p.p.m. of Brwas separated by crystallization and made anhydrous by heating at 60” under vacuum. An accurately weighed quantity of this salt was dissolved in a weighed amount of water to give a solution of known concentration. The ’LiBrOj solution was prepared from 7Li metal as before, except that the initial solution was not evaporated and the 7LiBr03crystallized from it; instead, its concentration was determined by evaporating aliquots of known volume to dryness. This more expedient method was followed because the atomic percent of 6Li in the desired isotopic preparations was insensitive to slight variations or inaccuracies in the concentration of the 7LiBr03 solution. The 7LiBr03 contaiiied 48 p.p.m. of Br- ion. Flame spectrophotometric analyses on the 6LiBr03 and 7LiBr03 solutions showed less than 20 p.p.m. of the other alkali metals, except with the former where 297 p.p.m. of Xa+ was found. Irradiation of Samples. Aliquots (3 g.) of each of the ten isotopic LiBrO3 mixtures were irradiated i n the active lattice of the ORGR for 2-, 4-, and 6-hr. periods, respectively. These samples were placed in sealed, 12 x 20 inm., cylindrical, polystyrene capsules of 1mm. wall thickness and were irradiated together with thermal and fast neutron flux monitors. The thermal and fast fluxes were 4 t h = 6.75 X l o l l , 6.72 X lo”, and 6.97 x lo1* and 4f = 9.5 x lo9, 10.0 x lo9, and 9.2 x lo9 cm.-* sec.-l, respectively. The methods for flux estimation and the irradiation facility are described elsewhere. Analytical Methods for Radiolytic Products. The irradiated salts were analyzed first for their oxidizing power. A dilute, alkaline solution of sodium arsenite was employed because it does not reduce bromate. The complete reduction of the oxidizing fragments was slow, and it was necessary to adopt a standardized procedure to obtain reproducible results. Enough salt to contain ca. 0.01 mequiv. of oxidizing power was weighed into 9 ml. of 0.2 M XaHC03 containing 0.; nil. of 0.1 N KaAsOz solution. After a time (ea. 6 hr.) sufficient for complete reaction, this mixture was back-titrated with standardized 1 2 solution. Spectrophotometric studies of the kinetics of (2) For a summary of these investigations, see C. J. Hochanadel, Radzation Res., 16, 286 (1962). (3) C. J. Hochanadel, J . Phys. Chem., 67, 2229 (1963). (4) G. E. Boyd and Q. V. Larson,‘ibid., 68, 2627 (1964). ( 5 ) G. E. Boyd, E. W. Graham, and Q. V. Larson, ibid., 66, 300
(1962).
3811
RADIOLYSIS O F CRYSTALLIXE LITHIUM BROMATE
.-
the oxidation-reduction reactions indicated that the reduction of BrO- and BrOz- was first order and pH dependent. The irradiated crystals also were analyzed for total bromate decomposition by weighing out 0.2-0.5-g. amounts and dissolving them in water containing bicarbonate and excess (20%) arsenite. Aliquots of these solutions were titrated with 0.01 N AgNO3 solution 12-18 hr. after preparation. Blank corrections determined by bromide ion titrations on aliquots of the unirradiated salts were applied to the foregoing results. It was assumed that the net Br- ion found was equivalent to the number of BrOa- ions deconiposed by radiation Significant quantities of gas (presumably oxygen) were evolved when the irradiated crystals were dissolved prior to analysis. Dosimetry. The radiation dose per mole of BrOaions imparted by the 6Li fission recoil particles was estimated from
D,(e.v. mole-I)
= $thtueffN&erfc
(1) where 4th is the thermal neutron flux, 2 is the t h e in seconds, u,ff is the “effective” neutron capture cross section of “i, N A is the Avogadro number, E , is the energy release per fieision (4.787 hlev.), and e, is the fraction of the energy of the recoil particles absorbed by the crystals, The quantity f is a self-shielding factor to correct for the flux depression in the interior of the samples by the strong absorption of neutrons at the surface, and c is the atomic per cent of 6Li in the preparations. Because of the very short range of the tritons (29.2 p ) and a-particles (5.8 p), it was assumed that 6, = 1.0. The “effective” neutron capture cross section of 6Li for the energy spectrum of neutrons existing in the ORGR was estimated with Ueff
= gua
+ - Io’ +r
+th
where
(2)
uo is the
2200-m. sec.-l cross section (945 barns), is the ratio of the resonance neutron flux, I&, to the thermal flux, 4 t h ) and Io’ is the resonance integral defined by +r/Gth
1Mev.
Io’=
l~~
u(E)dE ___ E
(3)
where E , = 0.0431 e.v. for the ORGR. When u(E) varies as l / v , a,s is the case for 6Li,Io’ = 0 . 9 0 and ~ g= 1 The ratio &/&h may be estimated from cadmium ratio (Cd R ) measurements on the thermal neutron flux monitor: using Cd R = 9.8, oo = 37 barns, and Io= 75 barns for Co when present at high dilution in pure
aluminum, we find +r/+th = 0.057; accordingly, teff = 993 barns for 6Li. Neutron self-shielding corrections were estimated with equations derived from neutron diffusion theory.’ These were small; their maximum value was only 6.8% for the most highly enriched LiBrOB preparation employed. Thermal Decomposition of LiBrOa during Reaclor Irradiation. The possibility that part of the bromate decomposition was caused by generalized heating of the LiBrOs preparations by the 6Li fission process was examined. Crystalline LiBrOs (n1.p. 254’) showed dlecomposition rates of 3.1 and 10.2 p.p.m. hr.-I whien heated for extended periods at 175 and 203’, respectively. The maximum temperature rise possible, that for adiabatic conditions, was calculated for the greatcst dose received by any sample employed (i.e., the 1% 2Li sample irradiated for 6 hr.). Assuming the specific heat of LiBrOs to be independent of temperature and the same as for LiKOa (0.387 cal. deg.-l g,-I), an increase of only 28’ was estimated from the energy balance. The temperature a t the irradiation position was approximately 40°, so that the highest temperature reached would not have exceeded 70’. Thermal decomposition of LiBrOs during irradiation, therefore, was considered as negligible. Thermal Annealzng of Irradiated LiBrOa. Measurements also were made of the changes in the oxidizing power of the irradiated LiBrOa crystals brought about, by heating them for periods up to 24 hr. a t teinperatures as high as 228’. In these experiments, aliquots of a preparation containing 3.78 atom % 6Li which ha,d been irradiated for 1 hr. were placed in a porcelain boat located at the center of an electric tube furnace whose temperature was controlled to f2’. Periodically, small samples were withdrawn and analyzed. The fraction of oxidizing;power removed, &, for a given time was computed from [(Ox), - (Ox)t]/(Ox),
(4) where (Ox), and ((Ox)$are the oxidizing powers in mequiv./g. initially and after heating for time t. cbt =
Results and Discussion The observed gross bromate decompositions and oxidizing power yields are plotted as a function of the atom per cent of for each of the three reactor bonibardments in Fig. 1 and 2, respectively. Extrapoltttion of the yields to zero 6Li content gives the contribution of the generalized reactor radiations (i.e., 7R.W. Stoughton and J. Halperin, Nucl. Sci. Eng., 6, 100 (1959). (7) P . F . Zweifel, Nucleonics, 18, 174 (1960). (6)
Volume 68, Number 12 December, 1964
3812
G. E. BOYDA N D T. G. WARD,JR.
l i I DEPENDENCE OF BROMATE RADIOLYSIS ON L I CONCENTRATION ~ IN L I ' B ~ O ~A N D ON IRRADIATION T I M E
600OI ~
I
- io
i
RADIOLYSIS OF CRYSTALLINE L i B r o 3 BY L i 6 F I S S I O N R E C O I L PARTICLES
/ / /
A
2 HOUR IRRADIATION
0 4
8 I
0 6
I,
,
DOSE (ev/moie B r 0 3 x 10")
i.', ,
0
I
02 03
C1
,
1
,
04
05 06
1
I
I
I
Figure 3. Radiolysis of crystalline LiBrOa by 8Li fission recoil particles.
0 7 0 8 0 9 10
ATOM PERCENT L I ~ I NL i B r o 3
Figure 1. Dependence of bromate radiolysis on eLi concentration in 7LiBr(& and on irradiation time.
oeoor
I1
1
l
l
,
,
I
l
l
1
-
-9
20-
m
-
IPg
-
PRODUCTION OF OXIDIZING FRAGMENTS IN THE RADIOLYSIS OF CRYSTALLINE L I B r o 3 BY L I FISSION ~ RECOIL PARTICLES
/
//
/'
/ Gz(O,l
,'
= 3.67
-
-
//
DEPENDENCE OF OXIDIZING FRAGMENT YIELD ON L i 6 CONCENTRATION A N D ON IRRADIATION T I M E
.
,/
/'
/
' ' '
'1;'
' '
/ o
'lit
'
!2bl
'
'
'215'
1
' 30
35
40
45
DOSE I ev/mole BrO; x ! O z z )
'0
01
02
03
0 4 0 5 06 07 08 ATOM PERCENT L i 6
09
10
11
Figure 2. Dependence of oxidizing fragment yield on 6Li concentration and on irradiation time.
rays, fast neutrons, and neutron capture in bromine) ; the net decomposition or oxidizing power yield may be identified with that caused by the 6Li(n,a)3H reaction. The linearity of the deconiposition with 6Li content up to 0.9 atom yo for the 2-hr. irradiation (Fig. 1) showed that the radiolysis was independent of the dose rate; the fact that all of the data of Fig. 1 may be plotted against the dose absorbed, estimated with eq. 1, to give a single curve (Fig. 3) suggests that the decomposition mas dose rate independent. The yields of oxidizing power (Fig. 2 ) when plotted against dose also gave a single curve (Fig. 4). This latter result may indicate that the oxidizing fragments themselves were not radiolyzed perceptibly. Support for this view was given by the observation that the oxidizing power yields, y , were related to the bromate deT h e Journal of Physical Chemistry
Figure 4. Production of oxidizing fragments in the radiolysis of crystalline LiBrOI by 6Li fission recoil particles.
+
compositions, J : , by the simple relation: y = 2 . 1 7 9 ~ 0.274. The slopes of the initial parts of the curves in Fig. 3 and 4 were employed to compute 100-e.v. yields for bromate decomposition,Go(- BrOa-) = 1.48,and oxidizing power, Goe("Ox'J) = 3.7. Earlier work4 has shown that the initial average oxidation number of the fragments in LiBrOa was 2.5. If this number is assumed to apply to the results from this investigation, as seems reasonable, a value of GO("Ox") = 1.1 may be computed for the initial oxidizing fragment yield. The yield for bromide ion production is then Go(Br-) = 0.38. The values for Go(-BrOs-), Go("Ox"), and Go,~ LET is ca. 0.06 (Br-) found with 6OCo y - r a y ~whose e.v./8., were 0.31, 0.21, and 0.10, respectively. These values are approximately one-fifth as large as those obtained with fission recoil particles whoseoLET values are 9.4 e.v./8. (tritons) and 34.7 e.v./A. (aparticles) , respectively. Clearly a pronounced LET
3813
RADIOLYSIS OF CRYSTALLINE LITHIUMBROMATE
dependence exists for the radiolysis of bromate ion; however, the observation that all the Go values were approximately five times those for y-rays suggests that the initial decomposition mechanism did not change with increasing LET. As noted earlier in this paper, increases in Go with LET have been interpreted in terms of the “thermal spike” hypothesis which assumed that the high temperature in heavy particle tracks may be an important factor. Accordingly, the thermal annealing experiments are of particular interest (Fig. 5)) because they reveal that most of the oxidizing power of the radiolyzed LiBrO3 crystals, was removed quite readily by heating them a t relatively low temperatures. The oxidizing fragments produced by 6oCo y-rays in LiBrOs also have been found to show changes on heating quite similar to those in Fig. 5. The production of large yields of decomposition products which can be removed by mild thermal treatment is difficult to reconcile with the hypothecris that a “thermal spike” is produced during the passage of the energetically charged particles through crystalline LiBrOs. Tracks with intense local heating would be expected to give only relatively stable end products which would not decompose on heating a t temperatures well below the melting point (254’). The formation of thermally unstable products also has been observed in the y-radiolysis of crystalline KC103.8 Values of G(C102-) = 1.2 were found which could be reduced to 0.8 on heating the irradiated salt a t 200’; the reduction in chlorite ion yield was attributed to decomposition to give chloride and oxygen gas. Moreover, later workg with NaCIOI has shown that G(C1-) increased on heating the salt a t various temperatures up to 210’ after irradiation. After the same compound was irradiated with 3.3-Mev. CYparticles, however, G(Cl-) was reported3 to have remained unchanged on heating at 185’. This result has been interpreted as supporting the view that thermal effects in a-particle tra,cks are a significant factor in the radiolysis of molecular ions in crystals. We would point out, however, that the manner in which energy was deposited in the crystals was radically different than in our work where fission tracks were produced a t random throughout the body of the solid. In the former experiments a-particles impinged on the external surfaces of the NaC103 crystals and penetrated to a depth of (10 p ; accordingly, only the surface region was affected, and this was decomposed extensively ( i . e ~ , 18%). Under these circumstances the unstable intermediates (ClOz--, ClOz, C10-, etc.) would have reached a steady-state concentration which was quite small compared with the amounts of C1- ion formcd. Ac-
i
T H E R M A L ANNEALING OF OXIDIZING FRAGMENTS I N PILE RADIOLYZED L i B r 0 3
0 20 0 10 I
2
3
4
‘5
6 7 8 9 IO 14 12 TIME OF HEATING (hours1
13 14 15 16 17 t8
Figure 5 . Thermal annealing of oxidizing fragments in pile radiolyzed LiBrOa.
cordingly, only a very small increase in G(C1-) wouljd have been produced by heating after the irradiation. Differences in the rapidity of distribution of the ionization and excitation energy deposited in the crystals by y-rays or by tritons and a-particles, respectively, may explain the large LET dependence observed in this research. With y-rays the energy will be dissipated by diffusion from widely separated, essential1,y spherical regions of excitation; with 6Li fission recoil particles, energy is deposited along a cylindrical track extending through many unit cells in the crystal lattice. The energy density in the 6Li fission track is several hundred times greater than for y-rays, and this energy will be dissipated less rapidly because of the smaller surface-to-volume ratio for the track. The critical excitation or dissociation energy of a bromate ion therefore is exceeded for a longer time resulting in a localization of radiolytic products along the track. Back reactions reconstituting BrOs- ion are expected to be more important than those with y-rays because of the higher density of decomposition products. The evidence for a back reaction, however, is only indirect: with 6Li fission fragments the decomposition became nonlinear after ca. 0.2% radiolysis of Br03- (Fig. 3); with 6oCoy-rays15in contrast, the radiolysis increased linearly with dose up to the highest decomposition,s which were above 0.4y0. If a back reaction is assumed in the mechanism for Br03- ion radiolysis, a quantitative treatment of the data of Fig. 3 may be obtained. Thus, formally ~~
~
( 8 ) A . S. Baberkin, “The Action of Ionizing Radiation on Inorganic!
and Organic Systems,” Academy of Science of the USSR Press, Moscow, 1958, p. 187. (9) P. F. Patrick and K. J. McCallum, N a t u r e , 194, 766 (1962).
Volume 68, N u m b e r 1.9 December, 1964
K. R. BROWER, R. L. ERNST, AND J. S.CHEK
3814
kzi
Br3-%"-l
Br03-
Akl2 + Br-
+ nO + ve-
30
where n is an integer with values 0, 1, or 2, and v 2 0. The species Br3-;-' represents the oxidizing fragments which may be either free radicals and/or bro-
mite plus hypobromite and/or positively charged bromine-containing ions. A least-squares fit of the data of Fig. 3 to the equation for the decomposition of &Osgives:klz = (1.96 f 0.84) X 10-23m~lee.v.-1,k21= (1.11 f 0.26) x mole e.v.-l, and ICBl = (1.88 i 0.05) X 10-26 mole e.v.-'. To reproduce the nonlinearity of the radiolysis with dose which sets in at ca. 0.2% decomposition, therefore, a relatively important reverse reaction (klz = 1000k21)would seem to be required.
The Volume of Activation in the Alkylation of Ambident Anions
by K. R. Brower, Robert L. Ernst, and Jean S. Chen Department of Chemistry, iVew Mexico Institute of M i n i n g and Technology, Socorro, X e w Mexico (Received J u l y 1 , 1564)
Determinations of relative and absolute volumes of activation were performed on several alkylation reactions of sodium nitrite and the sodium salts of 2,4-pentanedione, methyl acetoacetate, phenol, and 2-pyridinol. I n all reactions but one, the application of pressure up to 1360 atm. has no effect on the proportions of isomeric products, and the isomeric transition states therefore have nearly identical molar volumes. Since the products usually differ in volume by several ml./mole, it is inferred that the branching of the reaction pathways occurs a t or beyond the transition state. I n the benzylation of phenol the activation volumes for the three major products differ substantially, and a special explanation is given.
Introduction I n recent years the alkylation of ambident anions has been intensively studied in order to discover and explain the effects of reactant structure and reaction conditions on the proportions of isomeric products. The practical result of this work is that syntheses which utilize such reactions can be designed to increase the yield of the desired isomer, but it also has theoretical importance insofar as the ratio of isomers is a clue to certain mechanistic details of nucleophilic substitution reactions. The measurement of product proportions has been applied to many classes of reactions in order to find the difference in free energy of isomeric transition states formed from identical reactants under identical conditions. I n most of T h e Journal of Physical Chemistry
such studies the isomerism has been positional, but the alkylation of ambident anions involves the more interesting case in which different elements are being bonded. Aside from gross constitutional variations, the factors which are reported to affect the proportions of products are solvent polarity,lS2 h e t e r ~ g e n e i t y , ~ ,steric ~ hindrance,5 and hydrostatic pressure.6 All of these (1) N . Kornblum, P. Berrigan, and W. le Noble, J . Am. Chem. Soc., 85, 1141 (1963). (2) N. Kornblum. R. Seltzer, and P. Haberfield, ibid., 85, 1148 (1963). (3) N. Kornblum and A. P. Lurie, ibid., 81, 2706 (1959). (4) D. Curtin and D. Dybvig, ibid., 84, 225 (1962). (5) N. Kornblum and R. Seltzer, ibid., 83, 3668 (1961).
