Photolysis of Periodate and Periodic Acid in Aqueous Solution BY ULRIK K. KLANING" Department of Chemistry, Aarhus University, 140 Langelandsgade, DK-8000 Aarhus C, Denmark KNUD SEHESTED AND Accelerator Department, Risar National Laboratory, DK-4000 Roskilde, Denmark Received 30th January, 1978 The photochemistry of periodate and periodic acid in aqueous solution was studied (i) by quantum yield measurements at low light intensity (ii) by flash photolysis, and (iii) by photolysis of glassy samples at 77 K. The photochemical studies were supplemented with pulse radiolysis studies of aqueous periodate solutions and with kinetic studies using stopped-flow technique. In strongly alkaline solution the photodecomposition of periodate proceeds via formation of 0- and I v I . At pH < 12 an additional primary process is the formation of IV and H202.In neutral solution 03P is formed in a small yield. The energetics of the reaction of O'D with HzO with formation of H20z is discussed. It is suggested that oxygen atoms are formed only from 10; and not from other IVII species. Mechanisms for the secondary processes involving IvIII and IVI are given. IVIII and its relatively stable complex with IVII both form IV. IVII and 02. Depending on pH and concentration, IVI either disproportionates to IV and P I , reacts with IVII with formation of IV and P I 1 1 or dissociates into 0-(OH) and IV. Investigations of photodecomposition of the oxyanions XO,, n = 1-3 of chlorine, bromine and iodine in aqueous solution indicate that the primary reactions are one or more of reactions (1)-(4) ?l v xo, -+ xo,*- 1 + 0- xo,- -+ XO~--~ + 0 3 ~ xs, + X0,Y-I + UID xo,; -+ xo,z+o2 hv EL V k v where the formation of 0 3 P and of OID depends on the energy of the exciting light.lm4 A recent investigation of the photochemistry of perbromate, Br04 in aqueous solution has shown that the photochemistry of Br0: resembles that of other oxyanions of halogens, the primary reactions being reactions (3) and (4).5 Periodates are exceptional in the sense that several periodate species (in the following denoted col- lectively by IV") can exist in aqueous solution.We have studied the photochemistry of periodate in aqueous solution emphasizing the possible photochemical indi- vidualities that may arise from the different structures of the various IV1I species.The following IV"' species can exist in aqueous solution : H510s, H410g, 104, H310%-, H,I,O$C and H,IO2-. The equilibria among these IVI1 species are shown in (5)-(9) 281 8U. K. KLANING AND K . SEHESTED 2819 T = 298 K HSI06 = H410; +H+ H410; = H310%- +H+ pK1 = 3.3 pK2 = 6.7 (5) H410; = 10:+2H20 K, = 40 (7) (8) HJOg- = H210%-+H+ pK3 = 12.2'. (9) 2H310;- = H21204; +2H20 K, = 141 mol-1 dm3 ' Previous investigations have shown that irradiation of IVI1 in near neutral solution with ultraviolet light gives oxygen and iodate.8 It has been suggested that the photolysis proceeds via a free-radical chain mechanism, in which hydroxyl radicals are formed in a primary process. Irradiation of IV" dissolved in aqueous phosphate- borate glasses at liquid N2 temperature does not lead to decomposition of the Iv" species," a result which is not inconsistent with a free-radical chain mechanism at room temperature.In the present investigation we have studied the photochemistry of the various 1'" species at room temperature (21 & 1 ) O C at low light intensity under conditions of steady state and at high light intensity in flash photolysis. We have also studied the photochemistry of IV1I dissolved in various aqueous glasses at 77 K. We have supplemented the photochemical studies with pulse radiolysis studies of aqueous IV" solutions and with kinetic studies using the stopped-flow technique. EXPERIMENTAL MATERIALS All solutions were prepared from water doubly distilled in a Heraeus Bi 4 all-quartz still. N2 and Ar were AGA Special gases. NaOH, HzS04 and HC104 were Merck Suprapur.NaI04 and KI03 were Merck p.a. KBr04 and aqueous HBr04 was supplied by E. H. Appelman, Argonne National Laboratory, U.S.A. Carrier-free NaIl in aqueous NaOH was obtained from Institutt for Atomenergi, Kjeller, Norway. U02(C00)2 was prepared from AnalaR uranyl nitrate and oxalic acid (Merck p.a.) and recrystallized from water. Aqueous H202 was prepared from Merck unstabilized perhydrol and standardized against KMn04. Na3H2106 isotopically tagged with was prepared by oxidation of NaI containing Il3l with sodium hypochlorite in alkaline solution. To 5 x mol NaI containing 10 mC 1131 in 1 cm3 dilute aqueous NaOH solution was added 2 cm3 solution containing 1.5 x lo-" rnol NaOH and 4x rnol C12.The solution was heated to boiling for 2min. The precipitate of Na3H2106 formed was washed 4 times with water at 0°C. After each washing the precipitate was separated from the supernatant by centrifugation. ANALYSIS I-, 13, PHI [I-] and pH were measured on a Radiometer PHM 64 Research pH meter, fitted with a F 1032 I Selectrode and a K 701 reference electrode for [I-] measurements and fitted with the electrode set G K 2301C for pH measurements. [I;] was measured spectrophotometrically at 350 nm taking &I,- as 25 700 dm3 mo1-1 crn-l. 10,- 10, formed by photolysis of aqueous IvIr was measured by an isotope dilution technique. To aliquots (5 cm3) of solutions prepared from In1 isotopically tagged with 1131 was added subsequently : NaOH until pH > 13, 2 cm3 0.5 mol dm-3 BaCI2 and 2 cm3 0.2 mol dm-3 NaIO,. The precipitates of Ba(I03)2 were washed three times with a solution containing 0.05 mol dm-3 BaCI, and 0.1 rnol dm-3 HC104.The precipitate and supernatant were separated by centrifugation. The activities C and Co (in counts s-l) of Ba(103)2 precipitated2820 PHOTOLYSIS OF PERIODATE AND PERIODIC ACID from aliquots of irradiated and non-irradiated solutions were measured on a Harshaw gamma scintillation spectrometer. The total activity C, in the aliquot of the solution was determined by reducing all Ivrl to iodate with ethylene glycol prior to the precipitation of IOF. The extent of photolysis ~ 1 0 , - is given by a10,- = [IO~]/([IO;]+ [Iv1']) = (C- C O ) ~ (Ct- Co). It was found necessary to make the solution alkaline before the precipitation of Ba(103)2.The separation of IVT1 and 10, was poor in neutral and acid solution. IW' The decrease in [Ivr1] during photolysis was measured spectrophotometrically using a Beckman DU or a Cary 14 spectrophotometer. Denoting the absorbance before and after irradiation by go and E, respectively, and the extinction coefficients of Ivrl and 10; by EIVII and ~10,- , the extent of photolysis ap11 is given by ~ I V I I = (1 -e/so)/(l - ~ 0 ; /&PI). 0 3 mol dnr3 acetic acid as stabilizer was measured spectrophotometrically from the decrease in absorbance observed after flushing the solution with Ar, taking E O ~ at 253.7 nm equal to 3300 dm3 mol-l cm-l 11 [O,] was also measured iodometrically. 40cm3 solution was transferred with an all- glass syringe from the reaction cell to a gas-washing flask.Ar was passed through the solution and subsequently through two other gas-washing flasks each containing 20 cm3 0.1 mol dm-3 aqueous KI solution. [I;] formed in the two KI-containing gas-washing flasks was measured spectrophotometrically. Under the conditions employed 95 % of the ozone was absorbed in the first flask. [03] formed on photolysis of IvJJ solutions containing 10-3-5 x H202 In acid solution H202 reacts very slowly with IV1I. H202 was determined after removal of 03. In irradiated solutions containing 10-3-5 x lov3 mol dm-3 acetic acid and 2x rnol dmP3 [Iw1], H202 was identified and measured polarographically on a Metrom E 505 polarography stand attached to a E 506 Polarecord using the method of phase-selective AC polarography with a dropping Hg-electrode with controlled drop times and drop-synchronized current integration.To 25 cm3 of the irradiated solution was added 2 cm3 saturated KNOB solution. After flushing with N2 an a.c. polarogram was recorded between - 0.8 and - 1.3 V against s.c.e. H202 in known concentrations was then added and a polarogram was recorded after each addition of H202. The procedure was repeated with the non-irradiated solution. [H202] in the irradiated solution was determined by linear interpolation. In alkaline solution H202 reacts fast with IVI1 with formation of 10;. Analysis for 10: after the solution is made alkaline indicates that [H202] = [IO;]-A~vlr], where AIIvrr] is the decrease in [Ivr1] on irradiation of an acid solution of P I , in which H202 is stable.0-, OH saturated with O2 at 1 atm pressure [OF] was measured spectrophotometrically on the flash photolysis apparatus at 430 nm just after the flash irradiation, taking goo,- equal to 1900 dm3 mol-1 cm-l.13 [O-]+[OH] were found from eqn (111, which corrects for the reactions (12) and (13) (1 1) (12) IwI+OH+ IvIII (1 3) OH f H++O- (14) At 12.3 < pH < 14 0- was detected as 0; l2 by the observation of 0; in solutions 0 - + 0 2 + 0,. (10) i0-1 + [OH] = [o,]{ki 0[021~14 f [IvI11(ki 2Ki4 -k ki 3[Hfl)) /(kl 0[021K14) IVII+ 0- -+ IVITIu. K. KLANING AND K. SEHESTED 2821. where klo = 2 . 5 ~ lo9 dm3 mol-'~--~ ; l4 klz = 3 x lo7 dm3 mol-' s-l (this work), kls = 2 x lo8 dm3 mol-l s-l (this work), and in solutions containing 5 x 1 0 - ~ < [Cog-] < 0.1 mol dm-3 [O-]+ [OH] were taken equal to [COT], which was measured spectrophotometrically at 600 nm just after the flash irradiation, taking 8~0,- at this wavelength equal to 1860 mol-1 dm3 crn-l.l6 The decay of COT is of second order; the rate of decay increases with the ionic strength and is for [IV1I] ,< mol dm-3 independent of [Iw1] We find k16 = 5 x lo6 dm3 mol-I s-l at zero ionic strength in fair agreement with the value determined in pulse radiolysis 6 x lo6 dm3 mol-I At 0 < pH < 7, OH was detected by the formation of the hydroxyl adduct of benzene = 1.3 x mol dm3.IJ At 10 < pH < 13, OH+O- was detected by the formation of C o g CO$-+OH = CO$+OH-.(1 5 ) 2CO5-3 P. (1 6 ) formed on flashing a solution of IVI1 containing benzene.The concentration of the hydroxyl adduct was measured spectrophotometrically, extrapolating the logarithm of the absorbance at 312 nm to 20ps after the flash, and taking the extinction coefficient equal to 2100 dm3 mol-I CM-~.~' Due to the fact that benzene absorbs light from the flash, the concentration of hydroxyl adduct decreases with increasing benzene concentration. This effect was corrected for by extrapolating the absorbance at 312 nm to zero benzene concentration. APPARATUS FLASH PHOTOLYSIS The flash photolysis apparatus, reaction cell and the experimental procedure have previously been de~cribed.~~ The sensitivity at long wavelengths of absorbance measure- ments was enhanced by interchanging the original RCAlP28 photomultiplier with a RCA484-0 photomultiplier.In some experiments the energy E of the flash was attenuated by wrapping brass nets around the flash lamp. The transmittance of the brass nets were 0.43 at 250 nm. In other experiments E was varied by varying the voltage V and/or the capacity C of the condenser battery, It was found for 800 < E < 3200 J that the shape of the light pulse emitted at 300nm depends solely on E. With increasing E, the decay of the light pulse becomes slower, whereas the rate of growth of the light pulse is largely independent of E. The decay of the light pulse is exponential with a half life proportional to E*. At E = 3200 J the half life is -lops. LASER FLASH PHOTOLYSIS The laser flash photolysis experiments were made at the Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel.The instrument used was a frequency quadrupled neodymium laser flash photolysis system constructed by Ch. R. Goldschmidt.' To get a high energy per pulse at 265 nm the laser was adjusted to give multimode pulses. The intensity of the 3 x s pulse was determined by measuring the absorbance at 415 nm of naphthalene triplet formed on pulsing a solution of naphthalene in cyclohexane. The absorbance of the triplet formed was - 0.5. Taking the quantum yield for triplet formation equal to 0.75 and the extinction coefficient at 415 nm equal to 25 000 dm3 mol-I cm-', a light intensity I in the irradiated volume of the 1 cm reaction cell equal to -2 x einstein dm-3 pulse-' results.20 At this intensity and with a limit of detection of transient absorbance of 0.005, a transient species is observable provided the extinction coefficient is greater than -300/@, where 0 is the quantum yield for formation of the species.PULSE R AD IOLY SIS The pulse radiolysis experiments were carried out at the Riso HRC-linac, using an optical detection system similar to that described previously,21 but modified by moving the detection2822 PHOTOLYSIS OF PERIODATE AND PERIODIC ACID system outside the radiation field. The essential features of the set up are a linear accelerator delivering 0.2-4 ps single pulses of 10 MeV electrons with a peak current of 800 mA, a Varian VIX 150 U.V. lamp, a Zeiss MM12 double quartz prism monochromator, an EM1 95584 photomultiplier and a Tektronix 555 double beam oscilloscope.The irradiation cell is cylindrical with two Suprasil windows and a double lightpass of 5.1 cm. The electron pulse current, recorded in every experiment by monitoring the current induced in a coil surrounding the electron beam, was used for relative dosimetry. The absolute dose was measured with the hexacyanoferrate (11) dosimeter,22 using G(e&+ OH) = 5.3 and &420nm 1000 moP1 cm-l. The dose used in the experiments was varied from 1 to 20 h a d in a 1 ps-pulse and up to 80 h a d in a 4 ps-pulse. The solutions were prepared in 100 cm3 syringes and deaerated by bubbling either N20 or Ar through the solution for 15 min. The pH of the solutions was adjusted with sodium hydroxide and was measured on a Radiometer digital pH meter, PHM52. All the chemicals were of p.a.quality and used without further purification. The water was triply distilled and all glassware was prebaked at 450OC. STEADY STATE PHOTOLYSIS The light source used in the steady state experiments was a flat spiral-shaped 100 W low pressure Hg lamp made of Spectrosil fitted with an optical filter, which consisted of 1 atm chlorine gas in a 4 cm long cylindrical Spectrosil cell. The filter limited the emission in ultraviolet to the Hg 253.7 nm resonance line. The 5 cm long cylindrical reaction celi with a volume of 66cm3 was placed in contact with the chlorine filter, which in turn was placed a few m i from the Hg lamp. During irradiation the solution in the reaction cell was stirred. The light intensity was determined by means of uranyl a~tinometer.~~ The actinometer solution was irradiated for 15-45 rnin in the reaction cell.The light intensity at 100 % absorption was 1.43 x einstein dm-3 s-l. The extent a of the photolysis of IV1I was <40 %. Quantum yields @ were determined in solutions containing 10-4-2x mol dm-3 IVI1. The light absorption at 253.7 nm in the solution was >85 %. The decrease in light intensity during the irradiation was so small (< 5 %) that the light intensity could be treated as a constant. 10; does not photodecompose to a measurable extent under the conditions of the present experiments. The quantum yield @VII for the photo- decomposition of IVI1 in alkaline solution is therefore equal to the quantum yield for formation of 10;. @p is given by eqn (1 8) (1 8) where [IVIr], is [IvJ1] before irradiation and t is the time. In acid solution, where H202 and O3 can be stabilized, the corresponding quantum yields ( D H ~ O ~ , (Do3, and (DIo,- are given by = @)IVII[X]/~[I~~I]~. @)IVII is given by eqn (18), and X stands for H202, O3 or 10,.When [H202] was not determined separately, ( D ) N ~ o ~ was taken equal @o; -QVIT. @IvII = [lvII]O[(l -EIO,- /cIvII)a- €10, /EIV~I In (1 - a)]/(tI) LOW TEMPERATURE PHOTOLYSIS The light sources in the low temperature photolysis experiment were a Philips 93106E Zn lamp and the lamp used in the steady state photolysis experiments fitted with a 0.2 cm Schott filter UG5 to cut off the visible light. The irradiation of the IV1l containing glasses was made in Spectrosil tubes of 0.2-0.3 em internal diameter submerged in liquid NZ in a clear Spectrosil dewar.The glasses consisted of 15 mol dm-3 aqueous LiCl, 10 mol drr3 aqueous NaC104, and 1 mol dmW3 HCI04+ 10 mol dm-3 aqueous NaCI04. E.s.r. spectra were recorded at the X-band on a Varian E-15 spectrometer. After heating to morn temperature 0.5-1 cm3 samples of the irradiated glass were analysed spectrophotometrically for IVI1 and by isotope dilution for 10;. STOPPED -FLOW hl E A S UR E Ivl E N TS Kinetic measurements of the reaction of H2Q2 with IVL1 in alkaline solution wcre carried The changes in absorbance at 270 nm The output from the out on an Aminco-Marrow stopped-flow apparatus. were monitored with a modified Beckman DU spectrophotometer.u. K. KLANING AND K . SEHESTED 2823 photometer was recorded on a Tectronix 5103 N storage oscilloscope fitted with a C-59 oscilloscope camera.All measurements were made at ambient temperature (21 & 1)"C. COMPUTATIONS The numerical integration of differential equations was performed using a second order predictor-corrector method combined with a fourth order Runge-Kutta method. RESULTS AND DISCUSSION FLASH PHOTOLYSIS AND PULSE RADIOLYSIS Besides 10; and unstable iodine-containing intermediates, the photo-products formed on flash irradiation of IvI1 solutions are: OH and H,O, in 1 mol dm-3 HC104 ; OH, O3 and H202 in 10-3-5 x acetic acid saturated with 1 atm O2 ; OH and H202 at 10.5 < pH < 11.5 (see below) and 0- in 0.2 mol dm-3 NaQH. Table 1 lists yields, Y, of the products and Y(IV1') of reduction of IV". Y is independent of the presence of 10, at pH < 11.However, on addition of 10: at pH > 12, Y(IV") decreases, whereas KO, increases. TRANSIENT ABSORBANCE AT 320 < 1 < 800nm The transient changes in absorbance observed in pulse radiolysis and in flash photolysis of IV" solutions depend on pM. In 1 mol d n r 3 HC104 no transient change in absorbance is observed. At 5 5 pH < 14, a transient increase in absor- bance is observed at 310 < 3, < 800 nm consisting of two overlapping bands 24 one at 360nm and another at 510-540nm depending on [Iv"] (see fig. 1). At [Iv"] - mol dm-3 the band maximum is at 540 nm ; at [Iv"] - lo-' mol dm-3 it is at 510nm. In the following we shall designate the band by its average position at 525 nm. At pH 5-7 the band at 360 nm decays in a pseudo first order process with a rate constant proportional to [Iv1'] which is too fast for observation on the present flash photolysis apparatus and which leaves a small slowly decaying absorbance belonging to the band centred at 525 nm.At pH > 9 the band at 360 nm is observed in flash photolysis decaying in a first order process at a rate which is independent of pH and [Iv"]. In pulse radiolysis at high doses (-20 krad) the decay at 360 nm starts off as a second order reaction and terminates as a first order reaction with a rate constant which equals the constant found in flash photolysis experiments and in pulse radiolysis of low dose (-2 krad). mol dm-3 butan-2-01 or of 10-3-10-1 rnol dm-3 CO,"-, substances which react fast with 0- and OH, the band at 525 nm disappears, whereas the band at 360 nm remains with unchanged decay kinetics.The species responsible for the absorbance at 360 nm are, however, not formed in the same process in flash photolysis and in pulse radiolysis. The 360 nm band does not appear on pulse radiolysis of N,O-containing IV" solutions, whereas no effect of adding N20 can be observed in flash photolysis. Also the absorbance at 360 nm, OD36o, is in pulse radiolysis proportional to the dose P of the electron pulse, whereas we find in flash photolysis d2(OD360)/dE2 < 0, indicating that the species absorbing at 360 nm is not formed in a primary process. At pH < 12.7 the band at 525 nin displays complex decay kinetics. At low IV" concentrations ([Iv1'] < 5 x msl dm-3) and high P or at high E the observed decay is a second order process.At higher IV" concentrations ( 5 x < [IVIr] < 5 x mol dnr3) and low P or E, the decay of the 525 nni band takes place in two steps. Finally at high Iv" concentrations and at low values o f P or of E, the decay is again of second order. However, the rate is much smaller than the one which is measured at low IV" concentrations and high values of P and E, fig. 1. The On addition of 10-3-5 xcd TABLE EXTENT OF PHOTODECOMPOSITION OF Y(Ivl') AND YIELDS, Y, OF PHOTOPRODUCTS ON FLASHING 45 Cm3 1"" SOLUTIONS WITH A 3240 J FLASH AT 21 & 1°C 4 0 [IVII] /lO-3 mol dm-3 0.2 0.9 1 .o 0.05 0.2 1 .o 10.0 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 1 .o 0.0s [IO;] added /lO-3 rnol dm-3 pH 0 0 0 0 0 3-5 0 10.5 0 10.5 0 10.5 0 10.5 0 12 0 12.3 0 12.7 0 13.3 0.25 10.5 0.25 13.3 0.05 13.3 0 13.3 Y ( P ' ) 0.7 /lo+ rnol dm-3 - 21 0.7 1.7 - 1.2 4.4 0.9 1 .o Y(O-+OH) /lO-5 mol dm-3 -0.1 0.8 2.9 0.4-0.5 0.78 1.2 1.5 1.2 1.2 1.2 1.3 - 0.5 yIo - /lo- m d dm-3 1.46 7.9 1.26 3.1 4.6 33 - 5 4.1 3.3 2.1 1.3 4.6 1.5 1.3 0.4 0.76 0 0 13 0.5 0.6 0 1.4 0 0 - - 0 U Y 0 t3 Y c3U.K. KLANING AND K. SBHESTED 2825 rate of decrease in absorbance varies with pH. The rate of decrease in absorbance in the fast step increases with decreasing pH. The rate of decrease in absorbance in the slow step varies with pH attaining its smallest value at pH - 12. At pH > 12.7 the kinetics of decay in absorbance at 525 nm changes. The two steps of decay observed at 7 < pH < 12.7 merge to a single second order step, the rate constant of which decreases with increasing [Iv"].0.01 lo 1 I I I 1 300 4G0 500 600 700 nm FIG. 1 .-Spectra of the transient absorbance OD ; x , after electron pulse irradiation (dose 1.6 had) of a 5 x mol dm-3 IVII solution at pH 11.3 saturated with Ar, and 0, after flash irradiation (energy 3240 J) of a mol dm-3 I m I solution at pH 13.3 saturated with Ar normalized to the same OD at 550 nm. Inset : Oscillograms from which transient absorbances after flash irradiation are calculated, (a) at 360 nm, horizontal 1 ms/div, vertical 1 V/div ; 100 % light corresponds to 6.8 V ; (b) at 525 nm, horizontal 10 msldiv, vertical 2 V/div ; 100 % light corresponds to 12.8 V. (c) at 525 nm ; pH = 12.3 ; horizontal upper trace 5 ms/div, lower trace 20 ms/div ; vertical 1 V/div, 100 % light corresponds to 7.0 V.At pH > 9 we observe in pulse radiolysis an increase in absorbance at 525 nm on adding N20. This effect is not observed in flash photolysis or in pulse radiolysis of solutions in which pH < 9. ASSIGNMENTS IV"' The above observations lead to following asbignments : the transient absorbance at 525 nm is assigned to the hydroxyl adducts of IV1I species, IV1I1 and the relatively stable dinuclear species, (IvllIvlll), formed in reactions (12), (1 3) and (1 9) (Ivlll)z + OH -+ (IvllIvlll). (19)cd X 0 0 F TABLE 2.-kACTIONS INVOLVING IV1" SPECIES, 0- AND OH. EIVIJI = 1000 dm3 mOl-' Cm-' AT 540 nIll; E ~ I I V I I I ) =: 1200 dm3 I1101-l cm-' AT 510 nin (21 3- 1)"C. 2 12 IVII+O- + IVIII 13.3 lo-2 H2IOZ- 3x 10' 0 10.5 2~ 10-4 H310;- 2 .4 ~ lo8 cd 13 p + O H -+ P I 1 13 IV"+OH -+ IVI" -7 10-4-1 0-3 10, 4.5 x lo8 m z 1.5 x lo8 0 20 2IV'I' 3 IV+ IWI+ 0 2 -7 10-4-1 0-3 10, 20 21VIII -+ I V S P I + 0 2 10.5 < 2 x 10-4 H3@- 5 x 10' U * 4 20 2 F ' -+ IV+ P I + 0 2 13.3 < 2 x 10-4 H,IO: - 3.5 x 107 21 IVII + IVIII -+ (IVIIIVIIT) 10.5 10-3 HsIOZ-, H2120:; 4~ 105 m - 21 (IVIIIVIII) -+ p I + IVIII 10.5 10-3 H3IO;-, H2120:; 1 . 5 ~ 102 c + Z tl 24 2(Iv111v111) 3 Iv+ 3IV1I+ O2 7-12 10-3-10-2 IO,, HJOi-, H2120:; 3 x lo6 5 x 10-5-10-2 H2IOZ- 2 . 4 ~ lo6 cd 24 2(IV"IvT") 3 Iv+ 3IV1I+ 0 2 13.3 28 IVI+ IVII + IVIII+ IV 5-7 5 x 10-4-10-3 10, 1.3 x lo8 m z 0 - 6 x lo9 25 a 20H -+ H202 <11 - tl d - 9 x lo8 26 a 20- + H202 > 12 I a Ref. (15). b Estimated uncertainties 10 %-50 %.C s-'. 9 c U rate constant b reaction no. reaction PH [IvlI]/mol d i r 3 IVII species /mol-1 dm3 s-1 v1 H20 H UU . K . KLANING AND K . SEHESTED 2827 The kinetics of decay of the 525 nm transient absorbance is assigned to reactions such as 2 IV"' + IV + IV" + 0 2 (20) (21), (-21) (22) (23) (24) IV" +IVIII 2 - (Iv"Ivrll) IV"' + (1V"IV"') + I V + 2IV" + o2 2(IV1IIv1I1) 3 Iv + 3IV" + 02. IVIII + (IVII)~ + IVII + (IVIIIVIII) Table 2 lists the values of the extinction coefficients E ~ ~ ~ ~ I at 540 nm and E(IVIIIVIII) at 510 nm determined by pulse radiolysis and by flash photolysis at pH 13.3. Table 2 also shows which of the IV1' species 104, H,IOg-, H2120Lf;, H2102- dominates at the various listed pH values. The value for EIVII is found from &pII = ~ ~ 5 4 0 / ~ [ I v " ' ] and qIWr lvI1l) from qIvII IvIII) = 0 D5 lo/Z[(IvrrIvrrl)].At high concentration of IVIr, [(IvrlIvrrl)] was taken to equal [OH]+[O-1. At low [Iv"] it is necessary to correct for the reactions and 20H + H202 (25) 20- + H202. (26) (27) Eqn (27) is derived by integration of equations for d[IV"']/dt derived from H20 In this case [Iv"'] is calculated from eqn (27) where for pH > 13, x = k12[IV1r]/2k26 and y = [o-]/x, k26 = 9 x 10' mol-' dm3 s-l . 15 eqn (12) and (26) and eqn (13) and (25). At pH < 11, x = k13[IVr1]/2k25 and y = [OH]/x, k25 = 6 x lo9 mol-l dm3 s-l.15 The rate constants k12, k13, k20, k z r , k-21 and k,, are given in table 2. k20, k21, k-21 and k24 were determined by trial and error from the numerically integrated rate equation corresponding to reactions (20), (21) and (24) neglecting reactions (22) and (23) and simulating corresponding values of and time t.Fig. 2 shows OD525 plotted against t and a graph determined by the simulation procedure. k12 and kI3 were determined in pulse radiolysis of Ivxl solutions saturated with 1 atm N20. The observation that the decay of the transient absorbance at 525 nm is of second order at pH > 12.7 is ascribed to an increase in the rate constants of reactions (21) and (- 21) such that equilibrium between the species IV1I, IV"' and (IvllIvlrl) is maintained during the decay. Fig. 3 shows the second order constant for the decay at pH 13.3 of the transient absorbance at 525 nm, 2kapp, plotted against [Iv"] and a graph of eqn(24a) which may be derived by assuming equilibrium between IVrr, Ivrrl and [Ivr1'] = x In (y+ 1) (IV"IV1' '1 2kapp = (2k20 + 2k24[IV1r]2k~1/k~ 21)/(1+ [IV"]k21/k-2 (244 where the values of k20 and are taken from table 2 and k21/k-21 is taken equal to 2000 mol-1 dm3.~ 1 ~ 1 ~ 1 and E ( I V I I I V I I I ) at 525 nm are taken equal to 1000 mo1-I dm3 cm-l. IV' The absorbance at 360 nm is assigned to a species containing iodine in the oxida- tion state six, Ivl, which in pulse radiolysis stems from the electron adduct of IV" and in flash photolysis from a species formed in a primary reaction analogous to reaction (1).2828 PHOTOLYSIS OF PBRIODATE A N D PERIODIC ACID The pseudo first order decay observed in electron pulse irradiated IV" solutions at pH 5-7 is assigned to reaction (28) k2,q= 1.3 x lo8 mol-l dm3 s-l, fig.4. This assignment is based on the fact that addition of N20 has no effect on the yield of IV1". This means that Ivl and OH both react with IV" forming the hydroxyl adduct IV"' since in N20 saturated solutions e&+N,O 4 OH+N2. (28) IWI + IV' + IV + p i ' N2O 0.1 5b n 1 I t I 1 1 2 3 4 s FIG. 2.-Transient absorbance OD at 525 nm after flashing (energy 3240 J) a mol dm-3 IVII solution containing 2 x lW3 mol dm-3 NaOH plotted against time. Continuous curve is calculated by numerical integration of rate equations corresponding to reactions (20), (21), (-21) and (24). Inset : Oscillograms (a) and (b) from which absorbance data ( x ) are calculated. Horizontal (a) 20 msl div, (b) 5 ms/div vertical ; (a) and (b) 1 V/div 100 % light corresponds to 6.8 V.10-3[I~I]/mo1 dm-3 F'Io. 3.4tcond order rate constant 2kapp for the decay at pH 13.3 of transient absorbance at 525 nrn plotted against [IVII]. Continuous curve is calculated from eqn (244.U. K. KLANING AND K. SBHESTED 2829 The following observations indicate that the rate at which various Ivl species attain the most stable configuration by reaction with the solvent may be comparable to the rate at which the Ivl species disproportionate. An Ivl species, Irl, is formed in pulse radiolysis as the 0- adduct of 10; Iv+O- 3 IF; k29 = 3 x lo9 mol-1 dm3 s-1.25 (29) It has an absorption band at 360 nm [ E ~ ~ ~ ( I ~ I ) = 26001 25 decaying in a second order process at a rate which strongly depends on pH. We assign the decay to reaction (30) (at pH = 13.3, k30 = 7 x lo7 mol-1 dm3 s - ' ) .~ ~ 21r 3 IV + IV" (30) 'Ot 91- WL 71 + t - t I .I- 1 2 3 2 10-5 s FIG. 4.-First order plots of change in absorbance (OD- OD,) at 360 nm after electron pulse irradiation (dose 2.2 krad). a, [IVII] = 5 x lW3 mol dm-3 ; 0, [In11 = loA3 mol drr3 ; and $, [IvII] = 5 x mol dm-3 (pH = 5-7). ODm absorbance of IVIII after IVI has reacted. Inset : Oscillograms from which the plots are made (a) [IVII] = 5 x mol dm-3 ; horizontal 0.2 ps/div, vertical 0.01 V/div, 100 % light corresponds to 0.239 V; (b) BVII] = rnol d r r 3 ; horizontal 0.5 s/div, vertical 0.01 V/div, 100 % light corresponds to 0.243 V ; (c) [IVII] = 5 x mol d~n-~; horizontal 2 ,us/div, vertical 0.01 V/div, 100 % light corresponds to 0.225 V. Another Iv* species, If", is observed in flash photolysis at 360 nm, fig.I. Its decay is a first order process with a rate constant equal to 3.3 x lo3 s-l. A third Ivl species, IJI, formed as the electron adduct of IV1I, has an absorption band also centred at 360nm, with an extinction coefficient ranging, however, from 3400 to 4000 dm3 mol-' cm-l dependent on [Iv1']. At high dose the decay of Iyl is of second order The rate constant has the value kS1 = 4.5 x lo8 mol-1 dm3 s-1 independent of pH. At small dose the decay of I:1 is of first order process with a rate constant equal to The similarity of spectra and in decay kinetics suggests that reaction of I:* with 2111 3 IV+IV". (31) 4 x 103 s-1.2830 PHOTOLYSIS OF PERIODATE AND PERIODIC ACID water forming Iyl competes with reaction (31).We assign the decay of I:' to the reaction Further, we assume that I," is identical with If"'. Accordingly, we denote in the following IT' and 1:' by Ivl. Ivr observed in flash photolysis at 360 nm is not formed in a primary process. At pH = 13.3 we find for the yield Y,vI that d2YFI/dE2 < 0 and YIVI c Y(o-+oH). These observations may be rationalized by assuming that I:', formed in the primary reaction (1) either disproportionates or undergoes hydrolysis 1 1 + IV+0-. (32) 2Iy 3 IV+IV" and (33) (34) k34/(=33 q0- +OH)) FIG. 5.- XVI / Y(o- +OH) plotted against k34/(2k33 Y(o- +OH)). mol df~1-~, 0 [IVII] = rnol dm-3, * [IVII] = mol dm-3. 9, 0 and * [NaOW = 0.2 mol dnr3 ; A [NaOH] = 0.5 mol dm-3. k34/2k~3 = mol dm-3. Continuous curve is calculated from eqn (35).A and @ [IVII] = 2 x By assuming that 0-(OH) is formed in reaction (1) only and by neglecting the sub- sequent decay of Ivl in which reaction 0-(OH) also is formed [reaction (32)] we obtain eqn (35) from which equation we may estimate a lower limit for k34/k33. Fig. 5 shows that, when k34/2k33 is taken as mol dm-3, the corresponding values of YIV, Y(0- + OH) and k34/(2k33 Y(o- +OH)) fit the graph of eqn (35). The growing-inof the absorbance at 360 nm is too fast to be observed on the present apparatus, indicating that k34 > lo4 s-l and thereby implying that k33 must have a value close to the upper limit for a rate constant of a bimolecular reaction in aqueous solution. Direct observation of 1;' was attempted using the laser flash irradiation with a flash duration of 3 x s.However, no transient absorbance was detected at 300 < A < 850 nm. Taking the quantum yield for (0-+OH) formation to be 0.1, this means that ~~g~ < 3000 dm3 mol-l cm-l at 300 < A < 850 nm. In table 3 are summarized the reactions of Ivl mentioned above. -%/Y(O-+OH) = k34/[2k33y(O-+OH)I In [1+2k33 Y(O-+OH)/k341 (35)TABLE 3.-REACTIONS INVOLVING Ivl SPECIES (21 1)"c reaction PH [Ivlrllmol dm-3 1V" species rate constant b/mol-l dm3 s-1 remarks IV'I+IV' j IV+IV"I 5-7 5 x 10-4-5 x 10, 1.3 x lo8 pulse radiolysis 2 krad, 10, + 0- + 1:' > 12 0 3 x lo9 pulse radiolysis of 10; a 2 y + 1'+ IV" 13.3 0 7 x lo7 $" 2600 mol-l a 21y -+ I'+I~' 11.5-13.3 5x 10-4-5x H310g-, H2120i; 4.5 x lo8 &ifo 3400-4000 mol-1 1 PS dm3 cm-l s H2IO;- dm3 cm-1 pulse radiolysis 20 had, 1 P S reaction no.28 29 30 31 32 33 34 4 x lo3 C 3.3 x lo3 C pulse radiolysis 1.6 krad, flash photolysis 1 P Ivl -+ IV+O-(OH) 11.5-13.3 5 x 10-4-5x H,IOg-, Hz1204, H2IO;- k34/k33 > 2~ flash photolysis i 21x1 + IV+ IV" 11.5-13.3 5 x 10-4-5 x H310i -, H2130Lf; 1;' + I"' 11.5-13.3 5 x 10-4-5x H3IOg-, H2120f; k34 > lo4 H210, H2O H2IO;- a Ref. (25). b Estimated uncertainties 10 %-50 %. C s-' . dmol dm-3. c P 7f r *: z u Z 0 * z U h, 00 w c2832 PHOTOLYSIS OF PERIODATE A N D PERIODIC ACID The assignment of reaction (32) to the observed first order decay of Ivl is based on the observation that the concentration of 0 3 formed in reaction (10) on flashing an O2 containing IV" solution increases after the flash in a first order process, which matches the decay of Ivr. The assignment is further substantiated by the following observations.The transient absorbance at 360 nm observed on flashing a IV" solution, increases at pH 2 12 on addition of 1 0 3 to the solution. Furthermore, we observe a change h ""%\ 0 1 2 3 10-3 s FIG. 6.-Transient absorbance OD at 360 nm after flashing (energy 3240 J) rnol dm-3 IVII solutions containing 0.2 mol dm-3 NaOH and 10; in concentrations 0, 5 x mol dm-3 ; *, 2.5 x rnol dm-3 ; V , 6.25 x lW5 mol dm-3 and A, nil. Continuous curves are calculated from eqn (36). Inset : Oscillograms (a) and (b) from which absorbance data and A are calculated. Horizontal 500 ps/div, vertical (a) 1 V/div, (b) 0.5 Vldiv. 100 % light corresponds to in (a) 5.6 and in (b) 3.15 V.Oscillograms c1 and c2 show transient absorption at 525nm after flashing, in c1 [IO;] is nil, in c2 [IO;] = 2.5 x rnol dm-3 ; horizontal 20 ms/div, vertical 1 V/div, 100 % light in c1 and c2 corresponds to 6.8 V. in the kinetics of the decay of the transient at 360 nm. No effect on addition of 103' is observed at pH < 11, where 0- is protonated to OH which reacts slowly with 10; 2 5 or in the presence of substances which react with 0-, such as Cog- or butan- 2-01. We assign the increased absorbance to reaction (29), the rate constant of which is 30-40 times greater than the rate constant of the reaction of 0- with IV1I [reaction (12)]. However, due to reaction (32) the addition of 103 inhibits the photolysis of IV1I only to a relatively small extent since 0- formed in eqn (32) sub- sequently reacts with Iv" forming IV1*' [reaction (12)] (table 1) (cf.tables 2 and 3).U. K . KLANING AND K . SEHBSTED 2833 Fig. 6 shows agreement between the calculated and measured corresponding values of absorbance OD (at 360 nm) and time after the flash irradiation of a mol dm-3 IV1I solution containing 0.2 mol dm-3 NaOH and 103 in varying concentrations. OD as a function of t, OD(t), is calculated in the following way. Equations for d[Ivl]/dt and d[IV"'1/dt may be derived from eqn (12), (29), (30) and (32). By neglecting the decay of IV1I1 and applying the condition of steady state for [0-1, these equations may be integrated to give expressions for [I"] and [Ivr1'] as a function of the time t (cf. tables 2 and 3). As E ~ I and EPII are known, OD(t) may be calculated from these expressions [eqn (36)l oD(t) = [IV1]o(eFIZexp (-At)/[(2k,,[IV1~,/A)C1 -exp (-At))+ I]+ In [(2k30[Iv1]0/4(I -exp ( - A t ) ) + I])+ [IvIII]O&IvIII I (36) where Z is the opticaI length of the flash cell, EIVIII is taken equal to 350 mol-l dm3 cm-l at 360 nm, and [Iv1l0 and [Ivrlqo are concentrations at t = 0.A = k1zk32[1V1r]/ (k39[IV1 +k12[IV"I). I FIG. 7.-First order plots of change in absorbance OD at 270 nm ; 0 and * after flash irradiation (energy 3240 J) of IvIIsolutions, 0, [IVIIl = 4.5 x mol dm-3 ; 0, after mixing equal vdumes of 4 x Iwl and of 4 x rnol ~ I r n - ~ HzOz in stopped- flow apparatus (PH = 10.5). Inset: Oscillograms from which the plots are made. (a) and (b) flash irradiations, (c) stapped-flow measurement.(a) [IrVII] = 1.85 x lW4 rnol dm-3 ; horizontal 2 s/div ; vertical 1 V/div. (b) [ I 9 = 4.5 x mol dm-3 ; horizontal 5 s/div ; vertical 1 V/div. (c) [JW] = 2 x mol dm-3 ; [HzOz] - 2 x moI dm-3 ; horizontal 1 s/div ; vertical 0.1 V/div. mol dm-3 ; *, [IVII] = 1.85 x rnol Owing to the limited time resolution of the flash apparatus, [Ivqo and were calculated in the following way where ODo is the absorbance at 360 nm just after the flash. [Iv1] s t t = 2 x s is taken to be equal to OD(t=2xlo-4J(~P Z)-[Ivlll]o, and [Iv1], is then found by extrapolating to t = 0 using eqn(36). Thus we have in the calculation of [Iv'lO neglected the formation of IV1I1 in the first 2 x [IV"']0 = ODok12[IV"]/(ErVI Z)(k,,[IV] + k,,[IV1i]), s.2834 PHOTOLYSIS OF PERIODATE AND PERIODIC ACID TRANSIENT ABSORBANCE AT 270 < 3, < 310 STOPPED-FLOW EXPERIMENTS On flash irradiation of a 5 x 10-5-2 x mol dm-3 IV1I solution at pH - 11 with a 3240 J fiash, a decrease in absorbance occurring in two steps is observed.The decrease in absorbance in the initial fast step was roughly equal to the decrease in absorbance in the subsequent slow step. In the slow step the change in absorbance decreases exponentially with time with a rate constant which is proportional to [Iv"] and independent of pH at 10 5 pH 5 11. The fast step is assigned to a decrease in [Iv"] mainly taking place in the primary reaction. The slow step is assigned to reaction (37) of IV" with H202 formed in the fast step in a primary reaction We base the latter assignment on stopped-flow kinetic measurements of the reaction of IV" with H20z at 10.4 < pH < 11.1, [Iv"] N 2 x mol dm-3 and [H20,] 2: 2 x mol dm-3.Fig. 7 shows first order plots of the absorbance changes measured in the stopped- flow experiment and in the flash photolysis experiments. From the measured pseudo first order constants and the initial concentration [Ivlr]O we find, within the accuracy of the experiments, the same value for the second order constant in the stopped-flow measurement (2.6 x lo3 mol-l dm3 s-l) and in the flash photolysis experiment (2.7 x lo3 mol-1 dm3 s-l). The yields of H2Q2, Y H ~ o ~ , are shown in table 1. IV1r+H,02 -+ IO,+H,O++O,. (37) STEADY STATE A N D L O W TEMPERATURE PHOTOLYSIS It has been shown that complete reduction of periodate to iodate may be carried out, when aqueous 1'" solutions are irradiated with U.V.light, without significant photolysis of iodate due to the very small quantum yield of photolysis of iodate at wavelength 2 253.7 nm.26 In the present investigation the extent of the photolysis of IV" at 253.7 nm did not exceed 40 % and, accordingly, we did not detect I-, which is a photoproduct of iodate. @IO,-, ( 9 ~ ~ 0 ~ AND @03. IRRADIATION WITH LIGHT OF WAVELENGTH 253.7 nm, LIGHT INTENSITY TABLE 4.-QUANTUM YIELDS @ FOR REDUCTION OF IvI1, &I, AND FOR THE PHOTOPRODUCTS, I = 1.43 x einstein drn-3 s-l, [IVII] = 2x mol d ~ n - ~ (21 & 1)"C PH 1"" species qv11 OHzOz %OF % 3 0 ~ ~ 1 0 ~ 0.07 0.07 0.14 0 3-5 10, 0.28 0.20 0.56 0.e 3-5 10, 0.26 0.18 0.48 0 . 0 4 5 O - 0.17 - c 0.08 - a Saturated with 1 atm.02. * 02-free. 10.5 H3IOz- 0.15 13.3 HZIO2- 0.06 Table 4 lists the quantum yields for reduction of Ivrr, QDIVII, and the quantum yields for production of 103, H202 and O3 at varying pH. Also shown in table 4 are the 1''" species which dominate at the actual pH values. The above measurements do not exclude the possibility of an additional primary process (38) h v IV1I + Iv+20- or (OH) (38) the analogue of which plays an important role in the photolysis of Br02.5 In caseU. K . KLANING A N D K . SEHESTED 2835 of Br0; the formation of 0- was detected by e.s.r. measurements in aqueous NaOH glasses containing Br04 after irradiation with 253.7 nm light at 77 K. Irradiation at 77 K of IV" in aqueous glasses containing borate and phosphate or phosphoric acid alone does not lead to decomposition of IVr1.l0 In the present investigation glasses were prepared from 15 mol dm-3 aqueous LiC1, of aqueous 10 mol dm-3 NaC10, and of aqueous 10 mol dm-3 NaClO, containing 1 mol dm-3 HClO,.Irradiation for 2 h, with light of wavelength 253.7 nm at 77 K, of glasses containing mol dm-3 IVIr leads to 20-30 % decomposition in the LiCl and the NaClO, glass and 10-15 % decomposition of IV" in the NaClO, glass containing 1 mol dm-3 HClO,. No e.s.r. signals were detected by irradiation with light of wavelength 253.7 and 213.9 nm. KBrO, and HBrO, irradiated under the same conditions gave e.s.r. signals whereas no signals were detected from glasses containing the photoproduct H202 in the concentration 2 x lo-, mol dm-2.These measure- ments suggest that reaction (38) does not take place at 77 K. Formation of H202 and of OH radicals in a primary process is usually visualized as a process subsequent to formation of OID.ll This model originates in the com- parison of the photolysis of O3 in gas phase, where OID is formed at short wavelengths, with the photolysis of O3 in aqueous solution, where at the same wavelength H202 is forrned.ll The reactions leading to H2O2 are 20H 7 01D+H20 .+ (20H),,,, -+ H202 (39) (40) where two OH radicals formed within the solvent cage may combine to form Hz02 or break out of the solvent cage, depending on the excess kinetic energy liberated.27 However, the above findings suggest that O'D is formed only from the species 102 and not from other Ivrr species.The photolysis of halogen oxyanions usually results in the formation of 0 3 P when the irradiation is carried out of a wavelength longer than that required to produce H202. The only known exception is BrOz which does not absorb light at the wavelength which corresponds in energy to the threshold of formation of 03P.5 In the photolysis of aqueous IV1I, however, we observe 0 3 P formation only from solutions in which the predominant IV" species is 104 (table 1 and 4) despite the fact that the energy required to form O'D from 104 is much less than from other Ivrr species. The long wavelength edges for processes (41) and (42) h V 10, + 10, + 0% (41) BrO, 3 BrO, +OID (42) h v calculated from the enthalpies of formation of OID and of crystalline KI04, KBr0, and KBr03 are 301 and 329 nm, respectively.These values are expected to be close to the actual long wavelength edges, since the configuration of the initially formed Iv and BrV cannot be very different from the equilibrium configurations of 10: and Br0;. The long wavelength edges for reactions (43)-(45) h v h v h v HSI06 -P I0;+2H2O+O1D+H+ (43) H,IO; .+ 10:+2H20+01D (44) H3102- + I0~+HzO+OH-+O1D (45) calculated from the enthalpy of reaction (41), the standard enthalpy change in2836 PHOTOLYSIS OF PERIODATE A N D PERIODIC ACID reactions (5), (6) and (7) 6* ' and the enthalpy of ionization of water, are 270 nm for reactions (43) and (44) and 234nm for reaction (45). For reactions (43)-(45) the actual long wavelength edges may be much smaller than the values given above and are calculated from equilibrium data, since the configuration of the initially formed Iv can be different from the equilibrium configuration of 103.On the basis of these considerations, as well as the observation that H202 is the main photoproduct from photolysis of H5106 and H,IO%-, we suggest that H202 formed in the photolysis of H5106 and H,IO;- is formed directly in a primary process whereas oxygen atoms, whether in the state 3P or ID, are formed in the photolysis of 104 only.* These ideas may also explain the fact that OH radicals are formed from Br04 and not from IV" by irradiation at 77 K. The 1'" species present in the glasses at 77 K are not known. However, since the enthalpy of reaction (7) is positive '* ' the relative concentration of the species 104 will probably be negligible at 77 K.SUMMARY The primary photochemical reactions may be summarized as follows : (I) pH 13.3 (11) pH 11 (111) pH 11 (IV) pH 3-8 H,IOg- Ix'+O- 1:' + OH 7 L HJOZ- Iv + H202 1;1+ OH 7 4 H2IzOfo (Iv + H202) ? (Igl + OH) ? The secondary reactions in (I)-(IV) are summarized below (see tables 2 and 3). suggested that 10; is hydrated in aqueous solution Accordingly, the direct formation of H20z may be visualized as * Note added in proof: M. Anbar and S . Guttmann (J. Amer. Chem. Soc., 1961, 83, 781) have IOsfH2O = HzIOZ. hv h v hv H510s + H3104+H202 HJO; +- H2IO,+ H202 H3IOg- + HIO:-+ €3202.u. K . KLANING AND K . SEHESTED 2837 (1) The secondary reactions subsequent to (I) depend on the light intensity. Under steady state conditions, where intermediate-intermediate reactions may be neglected, the secondary reactions in I of 0- are reactions (12), (29) and (32), followed by some or all of reactions (20)-(24).The secondary reactions of 1;' are (34), (29) and (32). The mechanism is compatible with the observation that the quantum yield for reduction of IVI1, OIvII, is independent of the concentration of added 103, since a reaction of 0- with 10, does not lead to a formation of IV". In flash photolysis, however, reaction (33) is an important reaction of 1:' in addition to reaction (34), (29) and (32). This is in agreement with the finding that Y(IV") is close to Yo- at high flash energies (table 1). In reaction (30), Iv" is partly reformed. Therefore, Y(IV1I) decreases on addition of 103 to the solution.0- formed in reaction (1) and (32) reacts in eqn (29) with the added 10; with formation of Ivl, which again forms IVx1 in reaction (30) (table 1). However, we find that addition of 10.: causes a small increase in yIos, see table 1. KO, is determined as the yield of isotopically tagged I*O; formed from isotopically tagged I*v11. We suggest that the small increase observed on addition of " cold " 10; is due to the isotopic exchange reaction I*vl+Iv + I*v + IV1. (11) AND (111) The secondary reactions of OH with IV1I and (Iv11)2 are reactions (13) and (19) followed by some or all of reactions (20)-(24). The reaction of H202 is eqn (37). 1;' reacts in eqn (34) and (29), and further in (33) after flash photolysis. At pH 11, 0- is protonated to OH, which reacts slowly with 10; 25 and we observe no effect of adding 10; (table 1).YH202 was not measured at [Iv"] > 2 x 10-4mol dm-3; however, since KO, / Yo, is largely independent of [Iv"], we assume that the primary processes of H21204; are similar. (IV) The secondary reaction of Ivl is eqn (28). The secondary reaction of OH with IV1I is (13) followed by some or all of reactions (20)-(24). This means that we have the following relations among the yields Y and among the quantum yields 4 @ 10,- = @H202 + @ IV" + QO, (47) The values given for Y and O in tables 1 and 4 agree with reactions (46) and (47) within the estimated error. No secondary reactions are observed. The authors thank Henrik Loft Nielsen of the Physical Institute, this University, for help in the preparation of isotopically tagged periodate and in carrying out gamma spectrometric measurements. We thank Jarrgen Jakobsen of the Biochemical Institute, this University, for making the stopped-flow measurements, Shmouel R. Goldschmidt of the Department of Physical Chemistry, Hebrew University, Jerusalem, Israel, for making the laser flash photolysis measurements? James Norris of the Argonne National Laboratory? and Jsrgen Byberg of this Institute, for making the e.s.r measurements. The computer program employed to determine rate constants was2838 PHOTOLYSIS OF PERIODATE A N D PERIODIC ACID devised by Nis Bjerre. Evan H. Appelman of the Argonne National Laboratory, U.S.A., is thanked for supplying KBrO, and HBrO,. G. V. Buxton and M. S. Subhani, J.C.S. Furuduy I, 1972,68,958. ' G. V. Buxton and M. S. Subhani, J.C.S. Furudzy I, 1972, 68,970. F. Barat, L. Gilles, B. Hichel and B. Lesigne, J. Phys. Chem., 1971, 75, 2177. A. Treinin, Israel J. Chem., 1970,8, 103. U.K. Klaning, K. J. Olsen and E. H. Appelman, J.C.S. Furuduy I, 1975, 71,473. C. E. Crouthamel, A. M. Hayes and D. S. Martin, J. Amer. Chem. Soc., 1951, 83, 82. F. S. Head and H. A. Standing, J. Chem. SOC., 1952, 1457. M. C. R. Symons, J. Chem. Soc., 1955,2794. lo U. K. Klaning and M. C. R. Symons, J. Chem. Soc., 1960,977. l1 H. Taube, Trans. Faraduy SOC., 1957,53,656. l2 G. Czapski and L. M. Dorfmann, J. Phys. Chem., 1964,68,1169. l3 F. S. Dainton and P. Fowles, Proc. Roy. SOC. A, 1965,287,295. l4 G. B. Adams, J. W. Boag and B. D. Michael, Proc. Roy. Soc. A, 1966,289,321. l5 J. Rabani and M. S . Matheson, J. Amer. Chem. Soc., 1964, 86, 3175. l6 J. L. Weeks and J. Rabani, J. Phys. Chem., 1966,70,2100. l7 S . Gordon, K. H. Schmidt and E. J. Hart, J. Phys. Chem., 1977, 81, 104. l8 U. K. Klaning, J.C.S. Furuduy I, 1977, 73,434. l9 U. Lachish, A. Shafferman and G. Stein, J. Chem. Phys., 1976, 64,4205. 2o Ch. R. Goldschmidt, personal communication. 21 H. C. Christensen, G. Nielsson, P. Pagsberg and S. 0. Nielsen, Rev. Sci. Znstr., 1969, 40, 786. 22 R. Rabani and M. S. Matheson, J. Phys. Chem., 1966,70,761. 23 W . A. Noyes and P. A. Leighton, The Photochemistry of Gases (Reinhold, New York, 1941), 24 F. Barat, L. Gilles, B. Hichel and B. Lesigne, Chem. Comrn., 1971, 847. 2 5 F. Barat, L. Gilles, B. Hichel and B. Lesigne, J. Phys. Chem., 1972,76, 302. 26 L. Farkas and F. S. Klein, J. Chem. Phys., 1948,16,886. 27 E. Rabinowitch, Trans. Furuday SOC., 1937, 33, 1225. ' G. J. Buist, W. C. P. Hipperson and J. D. Lewis, J. Chern. Sac. (A), 1969, 307. p. 82. (PAPER 8 1162)