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React. Kinet. Catal. Lett., Vol. 27, No. 2, 379--385 (1985) OBSERVATIONS ON THE ACETONE-MALONIC ACID MIXED SUBSTRATE HYDROGEN PEROXIDE-IODATE-MANGANESE(II) OSCILLATING SYSTEM D. O. Cooke Fire Service College, Moreton In Marsh, Glos. GL56 ORH, United Kingdom Received February 8, 1984 Accepted ,July 16, 1984 The acetone-malonic acid mixed substratc hydrogen peroxide-iodate-man- ganese(Ii) oscillating system has been examined under fixed conditions where two oscillatory phases are separated by a lengthy iodine production and con- smnption period. The intermediate period is considered to result from oxida- tion of malonie aeid-iodomalonic acid. Oc~HaaHpymmafl CHCTeMa cnemaHHoro cy6cTpaTa a~eTOH-~a~a0HOBa~ KHC~0Ta -- nepeKHcb BoA0p0Aa -- H0~laT MapraHaa(I I) 6b~;aa gcc~e2t0BaHa npH HOCT0fIH- HNX yCJ]0BHflX, rxe ~Be 0cuuanHpymume ~)a3H pa3;ieJ]ff~OTCfl np0RoJ]~HTeJ]b- HbIMH nep~0AaMH np0RyKtI~r~ ~ pacxoAa Ho~a. [lpoMe)KyT0qHbL~ nepH)0R Hp0Hcx0- RI4T BcJIeRCTBHe 0KI4CaeHI4fl MaJIOHOBa~I KI~iCJIOTa-HOROMaJIOHOBaFI KHCJIOTa. An acetone-malonic acid (MA) mixed substrate Briggs-Rauscher type oscil- lating system which exhibits two oscillatory regimes separated by a lengthy phase was recently described [1]. Further observations are reported here and a brief explanation of the phenomenon is attempted. EXPERIMENTAL G. P. R. Ma and "A" grade acetone were used without further purification The hydrogen peroxide was stabilizer free. Other materials were of "Analar" grade. Iodide ion concentrations were measured using an E. I. L. iodide ion selective electrode calibrated with iodide solutions at the appropriate acidity. Calibration may only be considered approximate [2]. Absorbance measure- ments were made using a Cecil CE373 speetrophotometer with thermostated cell compartment. Iodomalonic acid (IMA) and idioacetone for calibration were prepared in solution from potassium iodide-MA-hydrogen peroxide and potassium iodide-acetone-hydrogen peroxide mixtures. Absorbanee measure- ments were obtained against appropriate hydrogen peroxide and/or acetone blanks*. Kinetic data are at 25 _+ 0.1 ~ * Acetone-hydrogen peroxide mixtures can be very hazardous [3]; on no account should these solutions be evaporated to dryness. 379

Observations on the acetone-malonic acid mixed substrate hydrogen peroxide-iodate-manganese(II) oscillating system

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React. Kinet. Catal. Lett., Vol. 27, No. 2, 379--385 (1985)

OBSERVATIONS ON THE ACETONE-MALONIC ACID MIXED SUBSTRATE HYDROGEN

PEROXIDE-IODATE-MANGANESE(II) OSCILLATING SYSTEM

D. O. Cooke

Fire Service College, Moreton In Marsh, Glos. GL56 ORH, United Kingdom

Received February 8, 1984 Accepted ,July 16, 1984

The acetone-malonic acid mixed substratc hydrogen peroxide-iodate-man- ganese(Ii) oscillating system has been examined under fixed conditions where two oscillatory phases are separated by a lengthy iodine production and con- smnption period. The intermediate period is considered to result from oxida- tion of malonie aeid-iodomalonic acid.

Oc~HaaHpymmafl CHCTeMa c n e m a H H o r o cy6cTpaTa a~eTOH-~a~a0HOBa~ KHC~0Ta - - nepeKHcb BoA0p0Aa - - H0~laT M a p r a H a a ( I I) 6b~;aa gcc~e2t0BaHa npH HOCT0fIH- HNX yCJ]0BHflX, r xe ~Be 0cuuanHpymume ~)a3H pa3;ieJ]ff~OTCfl np0RoJ]~HTeJ]b- HbIMH nep~0AaMH np0RyKtI~r~ ~ p a c x o A a Ho~a. [lpoMe)KyT0qHbL~ nepH)0R Hp0Hcx0- RI4T BcJIeRCTBHe 0KI4CaeHI4fl MaJIOHOBa~I KI~iCJIOTa-HOROMaJIOHOBaFI KHCJIOTa.

An ace tone-malonic acid (MA) mixed subs t r a t e Br iggs-Rauscher t y p e oscil- la t ing sy s t em which exhibi ts two osci l la tory regimes sepa ra t ed b y a l eng thy phase was recent ly descr ibed [1]. F u r t h e r observa t ions are r epor t ed here a n d a br ie f exp lana t ion of the phenomenon is a t t e m p t e d .

E X P E R I M E N T A L

G. P. R. Ma and " A " grade ace tone were used wi thou t fu r the r pur i f ica t ion The hyd rogen peroxide was s tabi l izer free. Other mater ia l s were of " A n a l a r " grade. Iod ide ion concent ra t ions were measured using an E. I. L. iodide ion select ive e lectrode ca l ib ra ted wi th iodide solutions a t the app rop r i a t e acidi ty . Ca l ibra t ion m a y only be considered a p p r o x i m a t e [2]. Absorbance measure - men t s were made using a Cecil CE373 s p e e t r o p h o t o m e t e r wi th t h e r m o s t a t e d cell c o m p a r t m e n t . Iodomalon ic acid (IMA) and idioacetone for ca l ibra t ion were p r epa red in solut ion f rom po tass ium iodide-MA-hydrogen peroxide and po t a s s ium iod ide-ace tone-hydrogen peroxide mixtures . Absorbanee measure - men t s were ob t a ined aga ins t a p p r o p r i a t e hydrogen peroxide and /or a c e t o n e blanks*.

Kine t i c d a t a are a t 25 _+ 0.1 ~

* Acetone-hydrogen peroxide mixtures can be very hazardous [3]; on no account should these solutions be evaporated to dryness.

379

COOKE: OSCILLATING SYSTEMS

R E S U L T S AND DISCUSSION

Conditions for observat ion, in this system, of two oscil latory regimes separat- ed by a len thy non-osci l la tory phase and the iodine concentra t ion behavior o f the reaction under a va r ie ty of conditions have been repor ted previously [1]. In the present work we examine the sys tem under a par t icu lar set of condi- t ions; [KIO3]=0.175 M, [H202]=0.726 M, (MnS04]=0.0112 M, [H~SO4]= = 0.086 M, [MA] = 0.0067 M, [(CH3)2CO ] = 2.7 M.

Iodide ion concent ra t ion vs. t ime curves for this sys tem and for the cor- responding single subs t ra te systems are shown in Fig. 1.

-I 0 -s

-10 -6

A

_10 -s

60 sec L_J

~I 0 =s

_i0 -S

-I 0 -7

Fig. 1. Iodide ion concentration (molarity) of single and mixed substrate systems (T =25 ~ [KIO3]=0.0175M [Mn([I)SO4]=0.0112M [H~O2]=0.726 ~I [H2SO4] =0.0858 M A) [MA] =0.0067 M [(CH.~)2CO ] =2.7 M B) [MAJ =0.0067 M no acetone C) [(CH:~)2CO ] -2 .7 M- no malonie acid

Absorbanee measurements in the near u l t ravio le t enable some comment on the accumulat ion of iodinated organic material . Below 320 nm hydrogen per- oxide and acetone absorb strongly. At 325 nm iodoacetone (e~ 77), IMA (e~ 57) and tr i iodide ion ( s~ 1.66 • 104) all show significant absorbance. At 350 n m tri- iodide ion (e~ 2 .6• 104) absorbs s t rongly and iodoacetone ( e~ 15) and IMA (e~ 14.8) less so. Absorbanee versus t ime behavior for the acetone single

380

COOKE: OSCILLATING SYSTEMS

E t -

O

*4t

E

t r3 0 3

E c 1.0

04 n m o3

0.8 0.4 r o

0 .6 -0 .3 o

x l

< o. z, . o.2 1 o J ,__o-' 5x10 -3 10 -4

s 6

350 /.,60

Time --~ Fig. 2. Absorbanee versus time curves for an acetone o~cillator (T -26 ~

[KIO3]=0.0175 M[Mn(II)SQ]=0.0112 m [H20~]=0.726 M [H2SO~]-0.0858 3I [(CH3).~CO ]=2.7 M A) /. = 325 nm monitoring iodoaeetone }. = 350 nm monitoring triodide-iodoaee-

tone combined B) 460 nm monitoring iodine.

subst ra te sys tem at 325 nm, 350 mn and 460 mn a r e shown in Fig. 2. The triiodide ion concentra t ion in this system is low enough tha t the absorpt ion a t 325 mn gives a very good measure of the product ion of iodoaeetone. Oscilla- tions continue almost to complete conversion of the iodate to iodoacetone. The stepwise accumula t ion of iodoacetone reflects the availabil i ty of iodine and resembles the accumula t ion of bromomalonie acid in the Belousov- Zhabotinski i reaction [4]. For the MA single substrate system under these con- ditions the tri iodide ion concentra t ion is much higher and, in addition, Ere - cipi tat ion of iodine beyond the first 120 see or so renders absorbance measure- ments of dubious value. However, it is observed tha t the absorbance in the range 320--325 nm can fall before tha t at 350 nm does. This is taken as indica- t ive of the dest ruct ion of iodinated organic material. Absorbance vs. t ime curves for the mixed subst ra te oscillator at 320 mn and 350 nm are shown in Fig. 3. For this sys tem at commencement of the second oscil latory phase ca. 50% of the initial iodate remains. F r o m this point onwards the behavior is r emarkab ly close to t ha t for an identical sys tem which had contained no MA.

The following discussion implies famil iar i ty with current views on the single subst ra te (MA or acetone) reaction [5--7]:

11 381

COOKE: OSCILLATING SYSTEMS

1.2

E 1.0

g o.8

u c

o 0.6 ...,o

S m n < 0.4

02

' '~350 nm

325 nm

60sec L---J

0.3

E c

O

t~

o.2 s 0

L_ 0 l J

. 0

0.1

Time -,--

Fig. 3. Absorbanee versus time curves for an aeetone-matonie acid mixed substrate oscillator (T = 25 ~ [KIOa]=0.0175N [Mn(II)SO~]=0.0112M [H~O~]=0.726M EH~SO~?=0.0Sss N [(0~3)~00] =2.7 M E~A] =0.00~7 M A) k = 350 nm B) ,t=325 mn

Bo th acetone and MA are iod ina ted b y an 'eno l iza t ion mechanism. The ra t e of iodinat ion of MA(1 ), is ve ry much fas ter t h a n t h a t for acetone(2).

CH2(CO2H)2 + I 2 - - ~ ICH(CO2H)., + H + + I - (1}

(CHa)2CO + I 2 - - ~ I C H C O C I t 3 + t t + + I - (2)

Although di iodinat ion is possible, evidence so far ob ta ined suggests t h a t this is not of par t i cu la r impor tance . Kine t ic d a t a for these react ions unde r oscil- l a to ry condit ions are given in references [5- -8] ; the MA sys t em is ev iden t ly more complex t h a n a s imple enolizat ion [9]. The ac tua l va lue of the ra te of iodinat ion MA: ace tone under condit ions encounte red within a typ ica l single subs t ra t e oscil lator appears to be abou t 300--600. Values can be calcula ted f rom avai lable da ta . I f we t ake the best represen ta t ion of the iodinat ion of MA in the presence of iodate and a t 0.1 M H + as [6]:

- d [ I2 ]_ 16[MA][I2] d t 1 + 10~[I2]

382

COOKE: OSCILLATING SYSTEMS

and the corresponding iodination of acetone, considered independent, of iodine to well below 10 -5 M, to be represented [8] at 0.1 M H + ;

-d[I2] dt

-- 1.2 X 10-6[(CH3)2CO]

for iodine concentrations in the range 10-a _ 3 • 10 -~ M the ratio changes from 121 to 1000. Under typical conditions for observation of two oscillatory phases - - say those of Fig. 1 in this study with [acetone] ~ 400 [MA] the ratio iodina- tion rate-MA : iodination rate-acetone would change from 0.3 to 2.5 as the iodine concentration changes from 10 -5 to 3 • 10 -a M. Clearly it is seen tha t MA under these conditions can compete favorably for iodine produced in the manganese(II) catalyzed hydrogen peroxide-iodate reaction. The system in the early stages appears truly a mixed substrate system, both MA and acetone are expected to make a significant contribution to the removal of iodine. The initial induction period* generally observed in the MA system [5] and absent in the acetone system [8] is almost vanishingly small in the MA system at the low MA concentration and low acidity employed here (Fig. 1). I t is absen~ in the mixed substrate system.

Stoichiometrieally in the mixed substrate system the only significant dif- ferences between the initial concentrations and those on completion of the first oscillatory phase are that the MA concentration has fallen significantly and the IMA and iodoaeetone concentrations have increased. From observations on the single substrate systems (Fig. 1) it is clear that the onset of iodine pro- duction marking completion of the first oscillatory phase is a function of the behavior of MAriNA and not acetone/iodoacetone. On completio~a of the first oscillatory phase, iodine production is proceeding with the iodide ion concen- tration being high (Fig. 1). Under these conditions it is not considered that iodous acid removed through (3) is present at high enough concentration to permit iodine pr0duetion through the manganese(II) catalyzed hydrogen peroxide-iodate reaction initiated via (4)

I t I O 2 + H + + I---~-2 HIO (3)

HIO2+ I t + + IOY--~-2 I0., + I-I20 (40)

An additional source of iodine capable of proceeding at higher iodide ion concentration is required. I t is considered that oxidation of IMA is the factor responsible. Oxidation of IMA in this type of system has been noted previously [5] though computer calculations modelling the MA oscillating system have indicated tha t the oxidation of MA/IMA is not essential to the oscillatory behavior [6, 7].

* It is suggested that the induction period in thi~ system serves to adjust the con- centrations of various species not, reactions which build up required intermediates as with the Belousov-Zhabotinskii reaction (10). IMA does, however, exert an influence on this period; see discussion below..

11" 383

COOKE: OSCILLATING SYSTI~MS

Organic material is required to be oxidized while the iodine in IMA is re- duced to free iodine; we represent the overall process by 5.

ICH(CO2H)2--~nI 2 + products (5)

For the mixed substrate acetone-MA system it is considered that the initial oscillatory phase continues until the IMA concentration is high enough for (5) to dominate the combined effect of 1 and 2. Oxidation of IMA then proceeds with the non oscillatory phase involving the cycle 5, 6 and 7.

5 CFL(CO.~H)~ + 2 12 + IO7 + H +--~5 ICH(CO.~H)2 + 3 H20 (6)

5 (CHa)2CO + 2 12 + IO:~- d- H ~---,5 ICH2COCH a + 3 H20 (7)

This situation continues with accumulation of iodoacetone until sufficient IMA--MA has been removed to enable 7 to dominate and the iodine concentra- tion to fall. The iodide ion concentration now falls to the lex-el required to a]tow initiation of the manganese(II) catalyzed hydrogen peroxide-iodate reaction and the system is effectively though not completely an acetone oscillator. The effect of MA concentration on the rate of iodine eonsmnption at the close of the intermediate non oscillatory phase and the observation of eonsmnption rates higher than those observed for acetone alone [1 ] indicates that some MA (proportional to [MA]0 ) remMns on completion of this phase.

We now consider the process whereby iodine may be liberated from I~A. Investigations have been carried out using hydrogen peroxide-IMA solutions

obtained by adding potassium iodide to hydrogen peroxide-MA-acid mixtures and allowing the iodine consumption to proceed to completion 8--9

2 I - +H.>O.)+2 H + - - ~ I 2 + 2 H._,O (8)

2 CH2(CO2H)2 + 12 + H202--~-2 ICH(CO2H)2 + 2 [-[2 ~ (9)

These solutions contained some MA and possibly some diiodinated material, they are, however, an accurate representation of the sort of conditions within the system under discussion.

Addition of iodate to a solution, obtained by the above method, in the ab- sence of manganese(iI) did not yield iodine. Iodine production in these systems (absence of manganese(II)) is however induced by copper(I) and vanadium(iV). This leads to hydroxy and perhydroxy radicals as candidates for the oxidation of IMA. Addition of manganese(III) or cerium(IV) (0.05 M) to IMA-H20 ~ solutions does not yield iodine. Hydroxy radicals are not expected in the reac- tion of these metal ions with hydrogen peroxide under these conditions though perhydroxy radicals are [11]. This leads to the conclusion that hydroxy radicals can oxidize IMA and that under oscillatory conditions IMA can be expected to compete with hydrogen peroxide for the hydroxy radical. With hydroxy radical considered as oxidant for IMA, the most plausible at tack on IMA would appear 10

384

COOKE: OSCILLATING SYSTEMS

ICH(CO.,H)., + �9 OH > IC(CO.~H)., + H.,O (10)

Hydrolys is of the ensuing free radical via 11 would lead to iodide ion and ac- count for the exper imenta l ly observed format ion of iodine.

Id(CO2tt)., + H.,O--,-tt + + I - + nOd(CO.~n)., (11)

Wi th 12 responsible for iodine produc t ion this yields a value of 0.6 for n in equa t ion 5

[O..;- + 5 I - + 6 H + - - ~ 3 H 2 0 + 3 I., (12)

Sustaining the process in the presence of high iodide ion concent ra t ion requires a radical source o ther t han 13 responsible for radical ini t ia t ion in the manga- nese(II) ca ta lyzed hydrogen peroxide- iodate react ion as this is inhibi ted 14 at the high iodide ion concent ra t ion present in the in te rmedia te non-osci l la tory phase.

HI0 . , + H + + IO~---~2 I0, ,- + H.,O (13)

H I O 2 + I - + H + - - ~ 2 H I O (14)

Radical propagat ion within the system is to be expec ted th rough react ion of the ensuing organic radicals with hydrogen peroxide or iodate ion. In agree- ment with these observat ions it is no ted t ha t the initial presence of IMA in an MA oscillator decreases the initial induct ion period and the t ime to onset of f inal iodine product ion.

Acknowledgements . I. t h ank The Roya l Society for a Scientific Invest iga- t ions Grant , Lapo r t e Indust r ies Ltd . , Warr ing ton , U. K., for a gift of unsta- bilised hydrogen peroxide and Fire Service College, Moreton-in-Marsh U. K. for provision of l abora to ry facilities.

R E F E R E N C E S

1. D. O. Cooke: Int. J. Chem. Kinet., 14, 1047 (1982). 2. S. D. Furrow: J. Phys. Chem., 85,. 2026 (1981) and Z. Noszticzius, E. Nosztiezius

and Z. A. Schelly: J. Am. Chem. Soe., 104, 6194 (1982). 3. A. D. Brewer: Chem. Brit., 11, 353 (1975). 4. M. Burger and E. K6r6s: J. Phys. Chem., 84, 496 (1980). 5. D. O. Cooke: Inorg. Chim. Acta, 37, 259 (1979). 6. S. D. Furrow, 1~. M. Noyes: J. Am. Chem. Soc., 104, 38 (1982). 7. P. De Kepper, I. R. Epstein: J. Am. Chem. Soc. 104, 49 (1982). 8. D. O. Cooke: Int. J. Chem. Kinet., 12, 683 (1980). 9. K. R. Leopold, A. Haim: Int. J. Chem. Kinet., 9, 83 (1977).

10. G. Czapski, B. H. J. Bielski, N. Sutin: J. Phys. Chem., 67, 201 (1963). 11. G. Davies, L. J. Kirsehenbaum, K. Kustin: 'Inorg. Chem. 7, 146 (1968).

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