The radiolysis at room temperature and 118°C of aqueous solutions containing sodium nitrate and either sodium formate or 2-propanol

ALLEN JOHN ELLIOT A N D FREDERICK CHARLES S ~ P C H Y S H Y N Atomic Ellergy c,$ Cor~nrlrr Litnited Resetrr-ch Con~prrt~y, Cholk River- Nrrcleor- Lrrbor-ntor-ies. Physicnl Cllernistr~l Br-nrzclz,

Chnlk River-, O I I ~ . , Cnnotki KOJ IJO

Rcccived November 19. 1982

ALLEN JOHN ELLIOT and FREDERICK CHARLES SOPCHYSHYN. Can. J . Chcm. 61, 1578 (1983) Dcoxygenatcd sodium nitrate solutions (5 2.5 x I O ' M ) containing sodium formate ( 5 0 . 5 M) have been irradiated at room

tcmperaturc and 118°C. With formatc concentrations 5 5 x lo-' M, G(NO2F) increased -25% ovcr the temperature range. Calculations suggest part of this increase in G(N0,-) may arisc from an incrcasc in thc overall solvated elcctron yield. With 0 .5 M sodium formate solutions containing 2.5 x 1 0 ' M sodium nitratc, G ( N O I ) rosc from 3.6 at room temperature to - 14 at 118°C. suggesting thc occurrence of a chain rcaction at higher formate concentrations. G(HzOz) and G(H,) were not functions of temperature for all the above solutions.

The pH dependence of G ( N O Z ) was determined at room temperature for sodium nitrate solutions containing cithcr sodium formate or 2-propanol. The G(N0,-) as a function of pH could only be modelled satisfactorily with rate constants from the literature in the sodium formate case.

I t is concluded that the solvated elcctron yield in irradiated aqueous solutions at elevated temperatures cannot be confidently deduced from measurements made with nitratc as an elcctron scavenger in thc presence of formatc ions or alcohols.

ALLEN JOHN ELLIOT et FREDERICK CHARLES SOPCHYSHYN. Can. J. Chem. 61, 1578 (1983). On a irradiC des solutions dCsoxygtnCes de nitrate de sodium ( 5 2,5 X 10-'M) contcnant du formiate dc sodium ( 5 0 . 5

M ) B la temptrature dc la pikce ct 6 I lS°C. Dans cet intcrvalle de temperaturc ct pour des concentrations en formiate 5 5 x lo-", G(NOZ-) augmente de 25% environ. Les calculs suggercnt qu'une partie dc cette augmentation dc G(NOz-) peut &tre due iI I'augmentation du rendement global en Clectrons solvatCs. Pour dcs solutions dc formiate de sodium 0.5 M contenant du nitrate de sodium B une concentration de 2,5 X 10.' M, G(NO2-) passe de 3,6 6 la tempCrature de la pikce iI 14 environ B 118°C ce qui suggere la prCsence de rkaction cn chaine lorsque les concentrations en formiate sont ClevCcs. Dans toutes les solutions ttudiCes G(H202) ct G(HZ) ne dCpendent pas de la tempCrature.

On a dttermink I'influencc du pH sur G(NOz-) pour des solutions de nitrate de sodium contenant soit du formiate de sodium, soit du propanol-2 6 la tempkrature de la pikce. Les constantes de vitessc relevies dans la littkrature, dans le cas du formiate de sodium, permettent d'ttablir un modele satisfaisant uniquement pour G(N02- ) cn fonction du pH.

On conclut donc qu'on ne peut deduire avec cxactitude le rendement en Clectron solvat6 dans les solutions aqueuses irradiCes 6 dcs temptratures tlevCes B partir des mesures prises avec les nitrates commc pieges B Clcctrons en prtsencc d'ions formiates ou d'alcools.

[Traduit par le journal]

Introduction In nuclear reactors, water, which can be used as a moderator

(70- 120°C) or a heat transfer medium (-300°C), is subjected to intense fields of ionizing radiation. While the radiation chemistry of aqueous solutions at room temperature is, in gen- eral, well understood ( I ) , the very little data published (2-4) on water-radiolysis at higher temperatures is often confusing. As an example, Freeman and co-workers (3) report that the G-value (number of species formed per 100 eV of radiation energy absorbed) of the solvated electron, e-,,,, increases from 2.7 at room temperatures to 4.8 at 300°C yet Burns and Marsh (4) report a G(e-,,) of 0.4 at 400°C.

The yields of e-:,,,, hydroxyl radicals, hydrogen atoms, mo- lecular hydrogen, and hydrogen peroxide in radiolysed water are measured by adding chemical scavengers and analyzing for products (1-4). The measured yield is dependent on the re- activity (R) of the scavenger (5) present, where R is defined in eq. 111,

and k x ks is the rate constant for reaction of a primary radical ( X = e-;,,, OH, H) with the scavenger (5). For small values of R (< lo7 s-I), only those primary radicals which escape the spur will be scavenged but as R increases, primary radicals will be scavenged from the spur itself (1, 5). DraganiC and DraganiC (5) have systematically documented this effect at room tem- perature. The effect of a moderate rise in temperature

(< 150°C) will reflect the change in k x , relative to geminate recombination and diffusion out of the spur for the primary species plus any increase or decrease in overall radical yield.

Interpretation of scavenger experiments require an under- standing of the underlying chemistry. In this paper we report results obtained from the radiolysis of solutions at room tem- perature and I 18°C which contain the nitrate ion, which is often used as an electron scavenger, and the formate ion or 2-propanol molecule, both of which are used as hydroxyl rad- ical scavengers.

Experimental The solutc chemicals (purchased from Fishcr Scientific Ltd. or

Anachcm Chemicals Ltd.) were used as supplied. Thc water was triply distilled. In experiments whcre thc yields at room temperature werc compared to those at 118OC, the solutions were irradiated in cylindri- cal Pyrex cclls. The samples were degassed. and - I0 mL of solution transferred to thc -13 mL cell which was then sealed with a flame. The samples werc irradiated in a thermostatcd oil bath in an AECL Gammacell, the dose rate of which was -2 x 10" cV kg-' s-. ' . Sarnplcs irradiated at 118 * 3°C were rapidly cooled immediately after irradiation to room temperature (25 * 3°C). In other room tcmperaturc experiments, solutions wcrc irradiated in syringes after being saturated with thc appropriate UHP gas. Nitrous oxidc (Mathe- son) was first passed through a RlDOX (Fishcr) column to remove oxygen.

Thc nitrite ion concentration was determined by the diazo-method (6) using an extinction cocfficient at 540 nm of 52 860 M - ' cm- ' . Hydrogen peroxide yields werc assayed by the tri-iodide ion method

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ELLIOT AN[) SOI'CI-IYSt-IYN 1579

(7 ) utilizing an extinction coefficient at 350 nm of 25 500 M ' em ' . A McLcod gauge was uscd to measure thc pa5 yields volumctrically and thc fraction of the gas in the sample was determined on n CEC- 2 1-610 mass spectrometer.

Since hydrogcn pcroxidc was to be assayctl from solutions irra- diated at 118°C. the thermal stability of -2 x 10 ' M hydrogen pcroxitlc solutions was followed at 25. 70. 100. and I 18°C. Thcsc samples which wcrc prepared from a 30% hydrogen pcroxidc solution (unstabilizcd) wcrc degassed and scaled in Pyrcx cells as uscd for the irradiations. The hydrogcn pcroxidc and oxygen concentrations wcrc followed with time. The pcroxidc dccomposctl at mcasurablc rates only at 100°C and 118°C and here the half-lives wcrc 5-6 h and 0.75- 1 h. respectively. The sum of the pcroxidc conccntration plus twice the oxygcn concentration equalled the initial pcroxidc concen- tration. As no irradiation was longer than 60 s anti since the solutions wcrc cooled immediately. the thermal decomposition of pcroxidc will not be a source of el-ror in the experiments at I18'C.

Results The G-values of nitrite. hydrogen peroxide. and hydrogen

are summarized in Figs. I and 2 and Table I for degassed sodium nitrate solutions containing sodium formate irradiated at room temperature and 118°C. The hydrogen peroxide and hydrogen concentrations were linearly dependent on the dose received; the G-values given in Table 1 are the mean of at least five determinations and the errors are one standard deviation.

As can be seen in Fig. I , G (N02- ) decreased with increased dose and when extrapolated to zero dose, G ( N Q - ) obtained at room temperature agreed with those previously reported by DraganiC and DraganiC (5) whose results are given by the triangles in this figure. At 118"C, G ( N 0 2 - ) was always greater than the room temperature results: however, for 0.5 M sodium formate solutions. G(N0 , - ) (Fig. 2) was higher than expected (see later) so a number of further experinlents were carried out. Firstly, to see if ionic strength was a factor. a 2.5 x lo-' M sodium nitrate. 5 x 10-'M sodium fornlate solution was irra- diated with and without 0.2 M sodium sulphate present. At best, G(NOI-) was marginally higher for the solution con- taining the sodium sulphate as can be seen in Fig. 1 for both irradiation temperatures. Secondly, to see if a stable end- product formed during the irradiation was capable of reducing nitrate ions to nitrite at 1 lg°C, solutions containing sodium nitrate and 0.5 M sodium formate were first irradiated at room temperature and then heated at 118'C for longer than I0 min. There was always a small increase in G(NO?-) . as shown by the diamond symbols in Fig. 2, but the increases were probably within experimental error and therefore not considered to be significant.

Solutions of sodium nitrate with the 0.5 M sodium formate replaced by 0.5 M 2-propanol were also irradiated. For a nitrate concentration of 2.5 X lo-' M G(N0'-) was 3.0 and 3.4 at room temperature and I 18°C respectively, while for a nitrate concentration of 2.5 x lo-' M, the corresponding G(N02- ) were 2.6 and 2.8. As with the formate samples. the G(N0,- ) decreased with increasing dose so the quoted values are those extrapolated to zero dose.

No nitrite was detectable in an uniradiated sodium nitrate solution containing either formate or 2-propanol which had been preheated to I lg0C. However, care had to be taken in sealing the cells during sample preparation to ensure no nitrate was left in the constriction as this thermally decomposed to form nitrite.

The effect of pH on G(NO?-) is shown in Fig. 30 for N2- saturated 5.0 x lo-' M sodium formate solutions containing

'I I Oo I I I I I 2.0 4.0 6.0 8.0 0 2.0 4.0 6.0 8.0 10.0

DOSE (eV/kg)

DOSE ( e V / kg) ,-.,O 2.0 4.0 6.0 8.0 0 4.0 8.0 12.0 16.0 20.0

FIG. I . The yield of nitritc at room tcrnpcraturc (0) and I 18°C (0) as a function of dosc from degassed solutions containing: ( ( I ) 2.5 x 10 ' M sodium nitratc ancl 5 .0 X I0 ' M sodium formate: ( h ) 2.5 x 10 ' M sodium nitratc and 5.0 X 10 ' iL1 sotlium formate: ( ( . ) 2.5 x 10 ' M sodium nitratc ancl 5 .0 x 10 ' M ~otliurn formate (open points), containing 0.2 M sodium sulphntc (solitl points): ( ( 1 ) 2.9 x 10 ' M sodium nitratc and 5 x 10 ' M sodium formatc. The trianslcs ( A ) arc the room temperature values born ref. 5.

I I I I a

0.00 1 2.0 I I I I I I I I I 4.0 6.0 8.0 10.0

DOSE (eV/ kg1

I I I I b I

FIG. 2. The yield of nitrite at room temperature (0) and 118°C (0) as a function of dosc from degassed solutions containing: ( ( I ) 2.5 x 10 -' M sodium nitratc and 5.0 X 10 ' iLI sodium formatc: ( h ) 2.5 X

M sodium nitratc and 5 .0 X 10 ' iL1 sodium forniatc. The diamond (0) symbols arc for solutions irradiated at room temperature and heated to I18'C before analysis. The triangle ( A ) syn~bol is the room temperature result from ref. 5.

either 2.5 x lo-' M or 2.5 x lo-' M sodium nitrate irradiated at room temperature. Shown in Fig. 36 are the results when the formate was replaced by 5 x lo-' M 2-propanol.

To investigate the decrease of G(NO?-) with dose for both formate ion and 2-propanol solutions, the reactions of CO?' and ( cH ' ) ?~oH with nitrite ions were followed at room tem- perature. Firstly, nitrous oxide saturated solutions containing

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CAN. J . Ct-[EM. V O L , hl. ICJX.7

T,IBLE I . GIH202] and GIH,I fl.orii irrndiatcd solutions containing sodium formatc on sodiurii nitrate

G(H202) G(H')

(NaNO,]/M (Na forniate]/M 25°C I 18°C 25°C I 18°C

"'Also contains 0.2 M Na,SO,

Reaction 121 indicates the species present in irradiated neu- tral water.

When nitrate ions and one of the above hydroxyl radical scav- engers are present, the generally accepted mechanism is reac- tions 131 and 141 followed by [9] - [ I 1 l in the case of formate and [17]-[I91 for 2-propanol (5 , 9- 13) (see Table 2) . The other alcohols react in a rnanner similar to 2-propanol. Pro- vided the formate or 2-propanol concentration is twice the nitrate concentration so that reaction 1101 or 1181 scavenges greater than 95% of the hydrogen atorns in competition with [4 ] then G ( N 0 2 - ) can be equated to G(e-,,I scavenged. However, for this last relationship to hold it must be assumed that reac- tions [ I 11 and 1191 are sufficiently fast so that no nitrogen dioxide is converted to nitrite via reactions 151-(71.

The p H clepr1lr1e1zc.e of' G ( N 0 - - ) crt r ' oo~?~ tcr~zperrrf~rr-e In the present study, G ( N 0 2 - ) was used as a measure of

G(e-,,) scavenged for irradiations at room temperature and I 18°C. The choice of which hydroxyl radical scavenger, for- mate or 2-propanol, was based on how well the pH de- pendence, at room temperature, of G(N0, - ) (Fig. 3) could be modelled since the only rate constants not known are [ I I ] and

FIG. 3. The yield of nitritc as a function of pH at room temperature from 2.5 x IO- ' M sodium nitratc (0) and 2.5 X lo-' M sodiu~ii [ l o ] (Table 2).

nitrate (n) solutions containing: ( ( I ) 5.0 X 10 ' M sodium forniatc: By comparing Fig. 3rr to 3b , it is obvious that for similar ( b ) 5.0 x I O ~ - ' M 2-propanol. The solid and dashcd lints are coniputcr r e ~ t i v i t y (eq. 11 1 ) of S O ~ U ~ ~ S . G(NOI-) for the 2-propanol calculated yields (see tcxt). solutions was lower than that of the corresponding formate

solutions for pH > 5 .5 where reaction [8] does not compete 0 .5 M sodium formate and 3-20 x 10-'M sodium nitrite and, secondly, nitrogen saturated solution containing 0.26 M ace- tone, 0.26 M 2-propanol, and 3-20 x lo-' M sodium nitrite were irradiated in syringes. In both cases G ( N 0 2 - loss) was - 1.9.

Discussion One of the most widely used methods to determine the scav-

engable yield of e-,, has been to measure G(NO?-) in deoxy- genated solutions containing nitrate ions as an electron scav- enger and either formate (5, 9 , lo) , 2-propanol (5, 1 1 - 13), ethanol (5 , 8, 14), or methanol (13) as a hydroxyl radical scavenger.

significantly. In fact, this is generally true of all nitrate solu- tions where alcohols are used as a hydroxyl scavenger ( 5 , 9- 14). This suggests that [I91 is either too slow to compete with [5]-[7] and [20] or that reaction of the alcohol-radicals with nitrogen dioxide does not form nitrite exclusively. A pos- sible reaction could be [2 I ] .

/ NO:

12 1 I (CH,),COH + NO: + (cH,)?c \ OH

The pH dependence of G ( N 0 2 - ) for nitrate-formate solu- tions can be satisfactorily modelled (using the computer pro- gram MACKSIMA CHEMIST (22)) with the reactions and rate

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T A B L E 2. Rcactions with ratc constants for irradiated nitratc solutions

No. Rcaction Ratc constant Refcrcnce

NO1- + c;,,, H+ NO, + OH

NO< + H --, NO, + OH NO, + NO, --, NIOl N20, --, NO, + NO? N2O4 + H20 + N O , + N O , + 2H' H + + e,,,- + H H C O O + OH + CO,' + H 2 0 H C O O + H + CO,' + H, NO, + CO,' + NOz-. + CO? coz' + co, I + (coz-), HCOOH -. HCOO- + H + H C O O + H + + HCOOH HCOOH + OH + CO,' + H20 + H' HCOOH + H + CO?' + Hz + H' (CH,)CHOH + OH + (cH.,),COH + HzO (CH,)ZCHOH + H + (cH,),COH + H Z NO? + (cH,),~oH + NOz- + (CH,),C=O (cH,) ,~oH + (cH~),COH + (CH,)zC=O

+ (CH,),CHOH or (CH,),C(OH)-C(OH)(CH.&

16 18 18 17 15 17 16

Sec tcxt 19 20 20 17 16 17 16

See text

constants in Table 2 provided the rate constant k l l was equal to or greater than 6 x 10" M I s-I. This value of k , , is greater than ( 2 k x 2kIZ)"' as would be expected for a redox reaction (23) . The initial G-values for the species formed in reaction 121 were taken from the reactivity plots in ref. 5 . Although both reac- tions 131 and 181 are ionic strength dependent. the actual map- nitude of the dependence is ambiguous from the literature (24-27). The values used (Table 2) were a conservative attempt to correct for ionic strength and the calculated results are given by the solid lines in Fig. 30.

The pH dependences of G ( N 0 , - ) for the nitrate-2-propanol solutions (Fig. 3 0 ) are similar to those reported by Sawai ( I I ) . As noted earlier, G(NO?-) for this system is lower than that for the corresponding formate solution of the same reactivity. A fit between the experimental results and those calculated using the rate constants in Table 2 could not be obtained using G-values for the products in 121 calculated from reactivities (5) (solid line - Fig. 3 0 ) . An attempt to fit the data with the scavenged G(e-,,) equated to G(NOI-) for pH > 5.5 (and the G-values for the other species formed in 16) scaled down to ensure a mass balance) was also unsuccessful as shown by the dashed lines in Fig. 30 . The rate constant k,, was fixed at 6 x 10" M-I s - ' for these calculations. From this it was apparent that the chemistry in irradiated nitrate-2-propanol is not well understood, so for the majority of the high temperature studies the nitrate-formate system was used.

Cotnprrrisotl qf the rodiol~ses of tlitrrrte .sol~rtiotls crt root71 te171- perntlrre ntlcl 11 8°C

At room temperature, the results for G(NOZ-) when extrap- olated to zero dose agree reasonably with those of DraganiC and DraganiC (5) (Fig. I); however, the decrease of G ( N 0 , - ) with dose cannot be due to depletion of the initial solutes. Since it was found that both CO.' and (CH,),COH could react with nitrite ions in sodium nitrite solutions, the same reaction [22]

[ I I ] o r 119) as the nitrite concentration builds up during the irradiation.

When the nitrate-formate solutions were irradiated at 118"C, G(NOI- ) was always greater than that at room tem- perature (Fig. I ) . For formate concentrations of 5 x lo-, M and 5 x 10-'M. the increase in G(NOZ-) of 25% over the room temperature value is compatible with most of the published G(e-,,,) data. Freeman and co-workers (3) used sulphur hexa- fluoride (SF(,) and nitrous oxide (NIO) as electron scavengers and found for SF6 over the temperature range 23°C to 106OC a 20% increase in G(F-16) while for N'O, G(NZ) increased 19% over the range 23°C to 80°C. Kalecinski ( 8 ) who irradiated deoxygenated 0 . I M potassium nitrate and 0.1 M ethanol solu- tions found G ( N 0 , - ) increased 16% over the temperature range 20°C to 130°C. However, above 130°C, Kalecinski found G(NO?-) decreased to zero by - 195°C.

When the formate was replaced by 0 .5 M 2-propanol (which has OH reactivity similar to the forrnate) in our experiments, the increases in G(NOZ-) were 8% and 15% for the 2.5 x lo-" M and 2.5 x lo-' M sodium nitrate respectively. Again these results are consistent with earlier published results (3, 8) and the present results with formate concentrations less than 0.5 M. However, with the 0.5 M formate solutions, when extrapolated to zero dose, the increase in G ( N 0 , - ) of 55% and 280% for G(NO2-) from 2.5 x lo-' M and 2.5 x lo-' M nitrate concen- trations respectively appears high when compared to the above results.

Figure I c shows that raising the ionic strength from 7.5 x 10-3 M to 3 x lo-' M with 0 . 2 M Na,SO, did not affect G(NO?-) significantly. The hydrogen and hydrogen peroxide yields for all the nitrate-formate solutions were temperature independent within experimental error (Table I). Heating to I 18"C, solutions previously irradiated at room temperature did not affect G(N0, - ) (Fig. 2). Taking these results in total indi- cates that for the 0.5 M formate solutions, nitrate is not being

1221 NO,- + CO,' or (CH,),COH -. products reduced to nitrite by a stable end product of the radiation at 118"C, and it is not high ionic strength giving rise to the

might be occurring in the nitrate solutions in competition with increase in C ( N 0 2 - ) . The results suggest that the high nitrite

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1582 C/\N J CHEM L 'OL. 6 1 . I ux3

yields in 0.5 M formate solutions arise from a chain reaction. possibly initiated by the direct action of the radiation on the forniate ion, which reduces nitrate to nitrite.

C ~ ~ I c ~ i ~ l ~ i ( i o / ~ of' G(c)-<,,,) .f i.o/)~ G(N0.-) CI( / /<Y0C Leaving aside the 0.5 M formate solutions, the question

arises as to whether the increase of 25% in G ( N O ? - ) found for the solutions with lower forniate concentrations reflects a real increase in the overall e-;,,, yield; an increase in the reactivity of the nitrate; or simply a chain reaction as postulated in the 0 . 5 M forniate case. Based on an activation energy of 2-4 kcal/niol for reaction (31 (24) . the reactivity of the nitrate ion will increase by a factor of two to five. Presunlably the reac- tivity of formate (reaction 191) will increase by a similar amount. This should lead to an increase in e-;,,, scavenged for the low solute concentrations and. hence. an increased G ( N 0 , - ) ( 5 ) .

One approach to determine whether. in addition to the in- creased reactivity of the solutions at 118°C. there is an increase in the overall G(e-,,,,) is the following. With the nitrate - 5 x lo-' M forniate solution, assume in going from 25°C to 118°C that: the yield of the products in 161 do not change; the reac-

t tivity plots in ref. 5 are still valid; and the rate constants for 131 and 191 increase a factor of five. The predicted increases in G ( N 0 2 - ) for 2.9 X 10-'M, 2.5 X 10-'M, and 2.5 X lo-' M nitrate solutions are 10%. 17%. and 204. . respectively. The

i corresponding calculated decreases in G(HzO,) are 1 9 7 ~ ~ 17%, and 14%. However, G(H,02) is not a function of temperature

I in the current experinients (Table 1 ). The predicted decrease in G(H,) of - 10% is within the experimental error of our experi-

i ments (Table 1). Furthermore, the predicted increases in

G ( N 0 2 - ) do not account for the observed 25% increase ! observed at ill nitrate concentrations. This indicates that re-

activity alone under these conditions cannot account for the results. However, if the reactivity increase is coupled with an increase in the yield of the products of reaction [6] at 118°C then this may account for the results.

The constancy of the 25% increase in G ( N 0 , - ) for all solu- tions with forniate less than 0.5 M suggests that the processes occun.ing in the 0.5 M forniate solutions are not occurring in the dilute solutions. In the 0 . 5 M formate case the percentage increase in G(NO?-) was nitrate concentration dependent (Fig. 2) .

Conclusions The use of nitrate as an electron scavenger in the presence of

forniate or 2-propanol is not a good method to use in evaluat~ng G(e-,,) at teriiperatures above room temperature. The chem- lstry of the solutions is conlplex and not completely under- stood. The weakness of the scheme lies, In part, In expecting 100% efficiency from ~eact lons [ I I ] and [ 191. In princ~ple, the pest scavengers are those w h ~ c h are converted by the e-,,, (or O H ) directly Into a stable product or to an intermed~ate which decays by a f~rs t order process to a stable product. Studies towards this end are under way in these laboratones now.

Acknowledgements The authors wish to thank Dr. J . W. Fletcher for valuable

discussions during the course of this work. The technical SLIP- pol? of E. B . Selkirk is also greatly appreciated.

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