Reaction Kinetics and Catalysis Letters, Vol. 4, No. 3, 329-396 (1976)

O B S E R V A T I O N S ON THE H Y D R O G E N P E R O X I D E - I O D I C A C I D - M A N G A N E S E ( I I ) - O R G A N I C

SPECIES O S C I L L A T I N G SYSTEM

D. O. Cooke

Hastings College of Further Education, Sussex TN38 OHX England

Received February 25, 1976

Accepted April 12, 1976

The effect of acidity and manganese (II) concentration on the title reaction has been examined for acetone and malonic acid as the organic species. Cerium (III) has been shown to replace manganese (II) without loss of the oscillatory behavior. Some mechanistic details are given.

B]bI~ RCC/IeBOBaH 9~@eKT KHC/IOTHOCTH H KOHUeHTpaUHM

MapFaHlla (II) B 3ar';laBHOH peaKr/HH Ha npHMepe al/eToHa H MaaOHOBOI~ KI4CnOTbl, KaK opr'aHHqeCKHX KOMnOHeHTOB. BblnO o6Hapy)KeHO, qTO 3aMeHa Mapr'aHua (If) Ha ue- pH~i (Ill) He npMBOBHT K Hc'4eSHOBeHHIO OCI~Hn/IMpyIomeFo noBeaeHHs. IqpHBOaSTCH HeKOTOpble [~eTaaH MexaHHCTH'Iec-

KOfi Mo~enR.

As part of the cominued investigation of the hydrogen peroxide-iodic acid

-manganese(II)-malonic acid oscillation system/1, 2/the effect of acidity and the

one electron reductant has been examined. Some mechanistic interpretations are

given. It has been shown in the present work that manganese(If) may be replaced

by cerium(ill) and that sulfuric acid in the source of iodic acid/1/may be replaced

by phosphoric acid, nitric acid and perchloric acid in both instances. Precipitation

of cerium(Ill) iodate and phosphate prevents examination of the cerium catalyzed

reaction at high cerium, phosphate or iodate concentrations. To facilitate

examination of certain mechanistic aspects of the reaction malonic acid has been

3* 329

COOKE: OSCILLATING SYSTEM

replaced by acetone/2/. ~ The acetone system exhibits oscillatory behavior over

a much wider concentration range. Dependent on the concentrations oscillations o

of period 1-600 sec may be obtained at 25 C. The effect of acidity on the

malonic acid system differs considerably from the acetone system as a result of

the behavior of the iodinated malonic acid,

EXPERJMENTAL

Oscillations in redox potential were monitored using a bright platinum

elect rode/1/ . Iodide ion concentration was monitored using a AgI/Ag2S ion

selective electrode. Oscillations in iodine and tri-iodide ion were observed

spectrophotometrically. The potential oscillations were observed in total darkness

for the malonic acid system/1/ . The acetone system which is considerably less

photosensitive was examined in diffuse room light. Laminar magnetic stirring was

used throughout. There was no significant difference in behavior between hydrogen

peroxide obtained from Hopkin and Williams (containing an undisclosed inhibitor)

and hydrogen peroxide containing no inhibitor (Laporte Industries Ltd. ).

RESULTS

It is observed that the acidity affects the behavior throughout the cycle. The

effect appears to be largely that of pH but results using phosphoric acid suggest

that this may not be entirely so. For the acetone system a decrease in pH decreased

the oscillation period and increased the amplitude of the potential oscillations.

For the rnalonic acid system a decrease in pH increases the oscillation period

principaUy as a result of increased liberation of iodide-iodine from previously

iodinated organic material. Oxidation of the organic material to carbon dioxide

Hazards involving acetone-hydrogen peroxide mixtures should be noted/7/ .

330

COOKE: OSCILLATING SYSTEM

I" B

- - \~- - -

1 minute

WfW

Fig. 1. Effect of sulfuric acid on the oscillations. Pt electrode vs S..~. E. A-E malonic acid: CH2(COOH) 2 5.4x10-2M, IG 3 3.1 x ~0" M, Mn(II) 0.68 xl02M, HoOo 65.0xi0 2M; H2SG4:A)4.6 x I0" M, B) ll.0 x 10"2M, C) I2.3x~0"2M, D) 14.6 xl0"2M, E) 19.2 x I0"2M. T = 27.2 + 0.2 ~ F -H acetone: (CH3)pCO 4.2 M, IO~ 1.54 x 10"2M, Mn(II) 0.68 x 10"2M, H2SG4: F) I . 5 x 10=ZM. G) 3.1 x 10"2M, H) 7.7 x 10"2M. T = 2 6 . 2 + 0.1~

is very much greater for the malonic acid reaction than for the acetone system.

Typical osciUations vs. acidity for the sulfuric acid-malonic acid and acetone

systems are shown in Fig. 1. For the manganese (II)-malomic acid system the +

initial induction period is proportional to [H ] over a wide concentration range.

For small variations about the typical oscillatory conditions (Fig. 1) an increase

in hydrogen peroxide, iodate and manganese(II) concentration decreases the

induction period. At high manganese(H) concentration the induction period

331

COOKE: OSCILLATING SYSTEM

H202 HIO3HI02? Mn (111) HIO ~__ 12 . ~ / ~

H21203 Mn(,) §

Mn (111) +OH-+ OH H2102 0 H+H 2 0 2 -" H 2 0 + H 0 2 H 2 02 HO2+H202 -'.,-H20 § 02.+OH

phase I : production of iodine phase 2: consumption of iodine to sink

tar the acetone system the sink is CH 2 ICOCH 3

Fig. 2.

again increases. The use of partially iodinated malonic acid can remove the

induction period. An appreciable induction period is not observed in the acetone

system.

An outline of the expected steps within the system excluding organic peroxo

species is shown in Fig. 2. From the iodide, iodine and redox potential traces it is

observed that the reaction may be considered as switched rapidly between two

steady states dependent on the iodide ion concentration and is thus composed of

two gross phases.

A) The production of iodine via the manganese(II) catalyzed HIO -H O reaction 8 2 2

This isolated reaction exhibits exceedingly complex behavior as expected from

the HIO3-H 2 02 reaction/3, 4 /which is itself capable of osciUatory behavior. The

manganese (II) reaction is noticeably photosensitive in the visible region. In total

darkness, except for the light required for occasional spectrophotometric observation

and for small variations about typical concentrations for the production of good

oscillations in the malonic acid or acetone systems (Fig. 1) the rate of production

iodine is increased by an increase in the iodate, hydroxortium ion, manganese(II)

and hydrogen peroxide concentrations. At low manganese(II) concentration the

reaction exhibits an induction period and is autocatalytic. The amount of iodine

produced is limited and is subsequently reoxidized to iodate over a period of hours.

332

COOKE: OSCILLATING SYSTEM

Oscillations have not been observed but the iodine may be returned by replenishment

of the hydrogen peroxide. It is noted that the I2-H202 reaction is inhibited by the

H202 both in the presence and absence of manganeme(II). At high manganese(II)

concentration the limiting concentration of iodine decreases. Over the accessible

range cerium (III) behaves similarly to manganese(IlL

It is significant that the present reaction analogous to the Belousov-Zhabotinskii

oscillatory system/8/is catalyzed by Ce(III) and Mn(II) but not by Fe(II) and

V(IV). Iodate cannot replace bromate in the Beleousov-Zhabotimkii reaction but

the iodate-iodide and bromate-bromide reactions do not obey the same rate laws

/6/ . The important species HIO 2 is not as readily available as HBr% in the

bromate reaction. HIO 2 is, however, formed in the rate-determining step in the

HIO3-H O reaction. It would seem possible that a mechanism analogous to the 2 2

HBrO 2 oxidation of Mn(II) and CeXlII)/4. 5/is operative in the present case with

comequent autocatalytic production of HIO 2 and subsequent rapid reduction of

M (n+l) species by hydrogen peroxide ~/15/ .

B) The comtmaption of iodine-iodide

The state responsible for production of iodine ( ~ high HIO2/Mn(III ) availability)

is dependent on the iodide concentration being low. Thus as the iodide concentration

rises there is roached a point when production of this state is inhibited and the

iodine concentration no longer increases. Iodineo iodide and tri-iodide ion are

now present and subject to at least four possible fates.

+

I2 + CH2(COOH) 2 -+ CHI(COOH)2 +I- +H + I 2 + CHI(COOH),2 ~ CI2(COOH) 2 +I" +H (1)

I 2 + 5H202 -* 2103 + 2H* + 4H20

21" + H202 + 2H + -+ 12 + 21"120

51" + IO3 + 6H + -+ 312 + 3H20

Mn(II) ahd Ce(III) may also be oxidized by OH and O2H/15, 16/.

(2)

(3)

(4)

333

COOKE: OSCILLATING SYSTEM

The principal consumption of iodine is via iodinat ion of the organic species which

is also the principal soume of iodide ion. This react ion is zero order in iodine for

both malon ic a c i d / 9 , 10 / and ace tone /11 / and proceeds via an enol iza t ion

mechanism. It is thus expected to be a c i d - c a t a l y z e d . The rate of removal of

iodine is observed to increase with increasing acidi ty in the acetone system but

the effect is compl ica ted by l iberat ion of iodine from previously iodinared

ma te r i a l in the case of malonic acid. In the oscil latory system the iodinat ion

react ion together with the reoxidat ion of iodide produced mainta ins a steady state

iodide concentrat ion until the iodine concentrat ion decreases appreciably .

Eventually the iodide concentrat ion decreases to a point where the production of

the state responsible for production of iodine via the manganese(II) ca t a lyzed

react ion is no longer inhibi ted, the remaining iodide is rapidly removed and the

cyc l e commenced again.

It is noted that the H202-I 2 r e a c t i o n / 2 / i s inhibi ted by i od ide /12 , 13/ . This

together with observations on the manganese ca ta lyzed react ion suggests that this

react ion is not responsible for removal of apprec iab le iodine in the oscil latory

system.

For the oxidation of iodide/3, 4 / the iodate reaction is by far the more rapid.

Several factors suggest that the pr incipal fate of iodide ion involves iodate.

A decrease in iodate concentrat ion decreases the iodine produced per cyc le . The

iodide ion concentrat ion can rise more rapidly at low iodate concentrations. At

high iodate concentrations not a l l the iodine is removed in the cyc l ing but the

oscil lat ions are from tow iodine concentrat ion to high iodine concentrat ion. The

removal of iodide via HIO 2 formed in the HIO -H O react ion remains a 3 2 2

possibil i ty.

C) The effect of the one electron reductant

The use of acetone enables the observation that the effect of manganese(II)

-H 0 reaction. At low on the oscil latory system is analogous to that on the HIC 3 2 9.

334

COOKE: OSCILLATING SYSTEM

1 m inu te

Fig. 3. Effect of manganese(I I) on the acetone system. Pt electrode vs S. C. E. (CH3)2CO 3.14 M, 10 3 2.8 x 10"2M, H2S_C~t 7.7 x 10-2M, H202 68.0 x 10-2M; Mn(II): A) 5 x 10-4M, B) 6 x 10"*M, C) 81 x 10"4M. T = 26.2 + 0.2~

manganese(II) concentration an induction period is observed in each cycle before

iodine is produced (Fig. 3). An increase in the manganese(II) concentration removes

this period and thus decreases the duration of the phase during which the iodine is

produced. A further increase in the manganese(II) concentration decreases the

iodine produced per cycle but also continues to shorten the period during which

the iodine is produced. Over the accessible range of concentrations the behavior

of the cerium(m) system appears analogous. The malonic acid reaction in both

instances is considerably affected by regeneration of iodide-iodine from the

iodinated organic material. This eventually results in precipitation of iodine and

consequently oscillations in the malonic acid system are not easily reinitiated after

the initial wave train is complete. Oscillations in the acetone system may be

reinitiated with iodate.

Acknowledgement

I thank Laporte Industries Ltd. (Warrington) for donation of the inhibitor-free

hydrogen peroxide and The Chemical Society for the provision of a research grant.

335

COOKE: OSCILLATING SYSTEM

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