.Draganid and I. G.Draganlk" Boris KidriF Institute of Nuclear Sciences, 11001 Belgrade, Yugoslavia
(Received February 15, 1972)
'The formation of GH was studied in 7-irradiated, neutral, aqueous solutions cont'aining selected mixtures of scavengers for hydrated electron and hydroxyl rad.ical. Experimental verification wa,s niade of various assumptions based on the free-radical model of water radi.olysis. It was confirmed that .the formation of M: atoms through the reaction eaq- H30sq.+ -+I 3 HzO can be strongly influenced by scavenging hydrated electrons; a cation (cuz'),an anion (N03-), and neutral species (&Oz and acetone) were used for scavenging hydrsted electrons. It has been shown that the depression of Gn could be quantita,tively correlated with the renetivity toward eaa-, Also, it has been demonstrated t h a t an eircient removal of O H radicals leads to an increase in Ga, by depressing the water re-forming reaction (OH and leaving more ea,a-availabie for increased B-atom formation in the reaction with hydronium ion. These effects were also observed in solutions containing simultaneously larger amounts of scavengers for QH and e%q-. Experimental curves agree reasonably well with Kuppermann's predictions based on a diffusion-kinetic model of water radiolysis. Experimeatal results point to 0.68 as probably the best value for GH. It was estimated that two-thirds of the primary hydrogen atom yield originates in the reaction of eaQ-with &OaQi-. It mas also found that, iodide and bromide ions react efficiently with hydrogen atoms in neutral sohitions: k(N $- I-) = 4 X 1 0 7 nczo?-' sec-l Eimd k ( H Br-) = 3.3 x 107 mol-1 sec-1. The corresponding value Cor chloride ion was estimat,ed as fihcrw that this fall can be well corre0.8 1 I lat)ecl with l,he re9 e t i vily toward the hydrated electron . regsrdless of (,he cliemical nature of the scavenger 0.7 (neultml spec' ~e,4~ p o d i v e or negative ion) The trends G 0.6- _________-___-----of both experinmicnla8 curves confirm the assumption 0.5. on &heimpoxtarwe of the reaction 2 for t)he origin oi hydragerl tam. They are in a fair agreement with 0.4 1 .I theoretical predictions represented with the dotted lines. 107 108 io9 10'0 These theoretical eurves were calculated after KupperkoH+SIS1rset+. M ~ I Ifrom ~ , ~ R tliffusion free-radical model of water Figure 3. Dependence of Ca: on the OH radical reactivity in radiolysis w$~~i:li1.8 using the same parameters that deaerated, neutral solutions containing simultaneously larger furnish R good agreement with other observed effects. amounts of esq- scavengers: 0, 0.25 M NO,- + EXCOO-; I, The d a h for F'igurc 3 are taken from Table I. They 0.2 iM H20~+ HCOO-; A, 0.1 M CU'+ $- HCOO-; show tlmt the priniary W-atom yields are increasing M NO,- + (CH3)tCHOH; X, (CHs)zCO (CJ%)zCHOI4. with the reacl,iv~tymward OH radicals also in soluThe dotted line is the best line through the dat)a obtained from solutions containing low eaq-scavenger concentrations, t i o m contaroiorg sirviultaneously larger amounts of e,;- ~ ~ ' ~ ~ For e n comparison, ~ e ~ s ~ the upper curve shows the beat Pine tirough the data obtained from soluforming reaction (eq 3), the primary H-atom yields will tro11e coxlhainmg 0 1 scavengers as above but only low e,,- concerhatio The trends of the best lines are be larger than in pure water. If e,,- are efficiently scavenged, GH value is lower. somewhat diffrrerat, the effect being more pronounced The lowering of GH with increasing reactivity in the preseim of larger amounts of scavengers for toward ea,- enables an estimate of the relative eontrie,,-, This nughd 1363 an indication that the reaction bution of the ractions 1 and 2 to the formation of primechanism is sornewhttt changed because of secondary mary H-atom yields. Comparing the lower curve in reaclione, which BPC 01 herwise absent. Figure 2 in the present work with the Figure 5 in ref O n the biripor.tcmce of Reactions 1 and 2 for the For2, which shows the dependence of G H on ~ the reactivity rnczliwn OJ Q,. 'The mfluence of reactivity toward OH toward ea,- and was obtained with the same scavengers and eaq- on the psiniary yields of H atoms (Figure 2 ) under the same conditions, we can see that the scavengthe significance of reaction 2 and of ing of hydrated electrons influences less efficiently GH mns for the formation of GH. If eaqthan GH,. The trends of these curves point out that are wailable in exce3s because of an efficient removal about two-thirds of GE originates in reaction 2 and of the OiIh radic an sxrd the depression of the water re-
+
+
+
I
+
The Journal
o~fPhy~icalChemistry,
Vol. 76, N o . 19, 1972
that reaction 1 c;m be only the source of the rcmaining one-third of primary yield. A support that W P indecd have the H atoms, and not the long-livta!! cwitcd watw tnolecules in reaction I a n d l or long-livctl, IT30 intermedial cs in reaction 2 , can b r fouiid 1x1 the f:tct that th(a rate constants determ i n d in c~oricc~rrt rated solirtions, in this work, agree rc~~sonab1y n ( h l i \\ i t i t the published va1uc.s which werc o%t:t.incdin dilutci ,ohat ions (Tablc IV). ‘P’ht inrrwsc. of ionic strerigth should itifluence reaction 2 by d w r ~ a ~ i i ii tgs rate constant. Since at tht. samc time also the rmction paqeaq- becomes slightly fastcr, on(’ would i x p t ~ ?smaller contribution of rcaction 2 to thc !;I, i’ixpcriments were made to check this usrumption rlrr:ilitittive!y. As sw11 in Tab!c V, the prewncc of largc. amounts of inert salts does not infbucncc ncnsured (111 vnhitls in our working conditions. Absolute i‘ulue qr’ the Primary A i m Yzeld. Unlike thr primary 2;icld of niolccu1;tr hydrogen, the published va1uc.s of v:zry i onsid~rably,from 0.45 to 0.7ri.”6112-2r> T h r rn(%zsurrrnmt,iin thih htudy show that (;a = 0.38 rt; 0 05 in dihitcb, nc*utrttlaqueous solutions. RI:tximal devi:ition is w m w h a t hrgrr than the possible. cxpertmcntal error. T4ic WUBC‘ of this is not clear; it swms, ntwrthcltw, that i t ~houldbe rather sought in the choiw of HR ( ompound, and some additional sourw of millcdar. hydrogrti, thnri in reactions 1 and 2. Iigclrogen il foin Rc ms with Halide Ions. Competition plolh in rig 1 show that thri rc:iction of hy31’ogm nt om\ L** it 11 fcrm:Ltc ions
+
1% 4 HCOO- -+ E-JL -C l’? k, = 2.j
x
108 mol-’ sec-l
(4)
in competition with IC-atom reactions with halide ions
IS
I%+ 1- + (HJ)--, IC,
f-I + 13r- +( H k ) , k6
=
SPC-’
(5)
107 mo1-l sec-’
(6)
4.0 X lo7 mol-’
= 3.3
x
+
whilti k(H Cl -) was estimated to bc less than 10j sec-I. I’rwiously, the reaction with I- was observed only in acid solutions where H2+ species, or :L terniolecular rmction (H H+ I-), were proposed.’O 26 Present study shows that a direct reaction b e h e e n hydrogen atoms arid halide ions is involved. The fate of (131)- or (HBr)- could be in an cl trunsfw rraction or, as proposed in the case of fluoridc in the formation of hydrated electron, (HE‘)- + caq- HI’. O n the Eventual Scavenging of Positive Ion-Precursors of El A t o m s . The data obtained in the prtwnt work mablc somt’ rcmarl
