Indian Journal of ChemistryVol. 29A, October 1990, pp. 967-970

Kinetics and mechanism of gamma ray induced reaction betweenhexaaquochromium (Ill) and EDTA

K S R Anjaneyulu, N R Anipindi & M N Sastri *School of Chemistry, Andhsa University, Visakhapatnam 530 003

Received 12 October 1989; revised and accepted 1 January 1990

The rate of reaction between Cr(HzO)r and EDTA has been found to be enhanced when irradiat-ed with gamma radiation forming Cr(Y)(H20t. The kinetics of this reaction has been studied and amechanism proposed. Prolonged irradiation of the reaction niixture leads to the formation of Cr(VI)and polymeric Crtllf) species.

The reaction between hexaaquochromium (III) andEDTA is slow and very much dependent on pH.This reaction has been the subject of study by sev-ral workers. Rao et al) observed the catalytic ef-fect of carbonate/bicarbonate ion on the forma-tion of Cr{III)-EDTA complex, and developed arapid chelatometric method for the determinationof Cr{III) with EDTA. Bhat and Pathak/ andOhashi? independently studied the catalytic effectof nitrite and sulphite on this reaction. While theformer explained the catalysis on the basis ofCr{III)-Cr{II) couple formation, the latter groupsuggested that the catalysis is due to the labilisingeffect of these oxyanions by rapid coordination tothe Cr{III) centre. Yamamoto and Ohashi" ob-served that HZ02 catalyses the reaction and sug-gested a mechanism involving oxidation of Cr{III)by HzOz. We have presently observed enhancedrate of reaction between Cr{III) and EDTA whenthe reaction is subjected to gamma irradiation.The results of the title investigation are reportedin this paper.

Materials and Methods

SolutionsHexaaquochromium (III) perchlorate was pre-

pared by reducing chromium trioxide (E. Merck,pro analysis) with a slight excess of formic acid inthe presence of perchloric acid and it was standar-dised by oxidising to Cr{VI) with alkaline HzOzand titrating against standard hypo solution. A 0.2mol dm - 3 solution of disodium salt of EDTA wasprepared by dissolving requisite quantity in dis-tilled water and it was standardised against stand-ard calcium solution. Sodium perchlorate (to

maintain ionic strength) was prepared by neutralis-ing weighed amount of anhydrous sodium carbon-ate with perchloric acid, and its concentrationchecked by passing an aliquot through a Dowex50W-X8 cation exchange (H+ -form) resin columnand estimating the resultant H+ ions. All otherreagents used were of reagent grade.

EquipmentAbsorbances in the visible region were mea-

sured with a Unicam SP 600 spectrophotometerfitted with 1 em matched glass cells. A Shimadzuatomic absorption! flame emission spectropho-tometer model 640-12 was used for the estimationof chromium at low concentrations. pH measure-ments were made using a Toshniwal CL 46 digitalpH meter. A 5000 curie gamma chamber (BhabhaAtomic Research Centre, Bombay) was used forgamma irradiations. The dose rate was determinedusing a Fricke dosimeter and was found to be3.2 x 105 rads/hour.

ProcedureRequisite quantities of EDTA and NaCI04 (to

maintain ionic strength) were taken together anddiluted to about 90 ml. The pH was adjusted tothe required value and the contents were trans-ferred to 100 ml volumetric flask. The reactionflask was kept in a beaker to which ice cubes wereadded to maintain the temperature of the reactionmixture at 4°C. This was done to avoid tempera-ture effects arising out of heating because of gam-ma radiation. An aliquot of hexaaquochromium(III) kept at 4°C was added to the reaction flaskand the contents were made upto 100 ml with wa-ter kept at the same temperature. The pH of the

967

INDIAN J CHEM, SEC A, OCTOBER 1990

reaction mixture was again measured immediatelyat 4°C and this was taken as the initial pH of thesolution. The beaker containing the reaction flaskwas kept in the gamma chamber. Aliquots of thereaction mixture were withdrawn periodically andtheir absorbances were measured immediatelyagainst water blank at 545 nrn where the productexhibited an absorption maximum. The time takenfor the operation was about 2 min. Under theseconditions no reaction occurred between thereactants at 4°C in the absence of gamma radia-tion and withdrawal of an aliquot from the gammachamber amounted to freezing the reaction. De-pending upon the experimental conditions, upto40-60% change in absorbance was followed andthe pseudo-first order rate constants evaluated byplotting 10g(A..,- At) against time. The value ofA.., was calculated taking the E value of Cr(III)-EDTA complex as 204 at 545 nm",

Results and DiscussionHamm? observed that the rate of formation of

Cr(III)-EDTA complex does not depend on [ED-TA] and it is very sensitive to pH of the reactionmixture. However, Beck and Judie reported thedependence of rate on [EDTA] and they interpret-ed this dependence by assuming the rate-deter-mining step to be the formation of an outer-spherecomplex. They explained the linear dependence ofrate on pH to stepwise deprotonation of EDTAand also to formation of hydroxochromium (III)species. In the present studies it has been noticedthat the rate of formation of Cr(III)-EDTA com-plex is dependent on [EDTA] and a plot of kobSversus [EDTA] is linear with an intercept on therate axis (Fig. 1). It has been noticed that the rateof the reaction increases with pH and also linearlydepends on dose rate (Table 1). The order in [sub-strate] is unity.

Mechanism and rate lawUnder the conditions employed in these studies,

Table 1- Variation of rate constant with dose rate[Cr(Hp~+l = 0.005 mol dm-3; pH = 4.50 ± 0.5; [EDTA]

= 0.08 mol om - 3; I! = 1.0; temp. = 4.0 ± O.1·C

Dose rate x 10- 5 kobs X 103 Dose rate x 10- 5 k"bsX lO'(rads/hr) (s - I) (rads/hr) (s - I)

3.20 7.38 1.52 3.4R

2.48 6.26 U4 3.04

2.:'3 581 1.20 2.70

968

14r-------------------------------,12

10

o[cr (HZO)!+] ,0·003mol dm-3

TEMP. , 4·0± O·I·C}J ,1·0DOSE·, 3·2 Xl05radSlhr

4

°0~~---74--~6~~8--~10--~1~2--~14~-+.16~[EDTA]Xl0Z mol dm-3

Fig. 1-Variation of rate constant with [EDTA]

Cr(III) undergoes hydrolysis to give hydroxy spe-cies (Eq. 1)

K

Cr(H20)~+ ;=!Cr(H20)50H2+ +H+ ... (1)

[K=first hydrolysis constant of Cr(H20)~+1. Thepredominant species of EDTA in the pH rangeemployed in these studies is H2y2 - and this reactswith Cr(H20)~+ and Cr(H20)50W+ to form ionpairs (Eqs 2 and 3).

K,

Cr(H20)~+ + H2y2- ;=!lCr(H20)~+· H2y2-} ... (2)

I~

Cr(H20)50H2+ + H2y2- ;=!ICr(H20)50H2+ 'H2y2-}II

... (3)

The ion pairs (1) and (II) react with OH radicalsformed during radiolysis of water (see Eqs 4 and5)

. slowI+OH ----+Cr(IV)-EDTA+OH-k

... (4)

. slowII +OH -----Cr(IV)-EDTA+OH-

kOH... (5)

Thus the hydroxyl radicals assist in the forma-tion of the products by oxidising Cr(III) to Cr(IV)which is a very labile species. This leads to therate law:

Rate = k[I][OH"]+ kotl[II][OH"] ... (6)

ANJANEYUW et aL: y-RAY INDUCED REACTION BElWEEN Cr(HPM + & EDTA

From Eqs (1-3) we get

[1]= KI[Cr(H20)~+l[H+][H2y2-]. [H+]+K

[11]=K2(Cr(H20)~+lK[H2y2-][H+]+K

... (7)

... (8)

Substituting these values in Eq. (6), the rate lawtakes the form (9)

Rate = [Cr(H20)~+MH2 y2-][OH']

x !kK1[H+]+ kOHKK2j[H+]+ K ... (9)

The contribution due to I can be neglected be-cause under the experimental conditions. Cr(III)exists mostly as Cr(H20 )sOH2 +. Hence,kOHKK2~ kK1[H+]

Rate=[Cr(H20M+1[H2Y2-IOH·]![~~~K~j

· .. (10)

· .. (11)

The reciprocal of kobs can be expressed as

· .. (12)

where C = [H2y2 -IOH']

The EDTA independent path may be due to oxi-dation of Crill{H20 )sOH2 + to labileCrlV{H20)sOH3+ (in a stow step) which in a seriesof fast steps reacts with EDTA to give the pro-ducts. A plot of 1/ kobs versus [H +] is found to belinear (Fig. 2). From the slope and intercept of thisplot the first hydrolysis constant K of hexaaquoch-romium (III) is evaluated. The K-value obtained inthese studies is 1.6 x 10 - s M- 1 at 4°C and I-i = 1.0as against the reported value of 8.3 x 10- sM- 1 at25°C and I-i = 0.35. The small value of K obtainedpresently may be due to the temperature effect.

Yamamoto and Ohashi" studied the reaction inthe presence of H202 and observed that the rateof the reaction is independent of [EDTA] and firstorder dependent on [H202] and [Cr(III)]. Theysuggested that H202 oxidises Cr(III) to give Cr(IV)and OH' radicals which also oxidise Cr(III) to

4·0r-------------------:::<n

35

30

2·5

Crt H2Olt0 0003mol dm-3

[EDTA} 008 mol dm-3

}J 0'0-DOSE 032 X105rods/hr

1-0

~~~l ~2~3~4·~5-.6~~7~8~9~10"1~1-1~2~13~14~[H+]Xl0"mol dm-3

Fig. 2- Variation of II kob• with [H+]

Cr(IV). The resultant Cr{IV) is very labile andreacts with EDTA to give Cr{IV)-EDTA specieswhich in a series of fast steps gives Cr{III)-EDTAcomplex. They attributed increase in rate with pHto the formation of Cr(H20hOH2+ which is a la-bile species. The spectra of Cr(III) solutions at dif-ferent pH values are shown in Fig. 3. A bathoch-romic shift has been noticed in the spectra at boththe peaks. At pH 4.9 the peak at 410 nm is shift-ed to 430 nm and that at 575 nm to 585 nm. Em-erson and. Graven" assigned these maxima toCr(H20)sOH2+. It was also observed that colourof solution turned grassy green. A solution ofCr(H20)~ + at pH 4.9 was passed through a Do-wex 50W-X8 cation exchange resin (H+ -form) andthe resin bed was washed with doubly distilled wa-ter, adjusted to pH 4.9 and eluted with 0.5M sodi-um perchlorate adjusted to a pH of 4.9. A greenband separated which slowly moved down the co-lumn indicating its lower charge than Cr(H20)~+.It can therefore be concluded that this green spe-cies is the dipositive Cr(H20)sOH2+ species.

Draganic et aL9 determined the G-values forvarious radiolysis products in oxygenated aqueousformic acid solutions on irradiation with a 60COsource over the pH range of 1.3 to 13.0, and didnot observe any change in the yields. They report-ed the following G-values: G -H,o:4.09; GH:0.55;GoH:2.72; Ge~:2.63; GH,o,:0.68; GH,:0.45Another interesting conclusion drawn by theseworkers is- that G.- == GOH== 2.67 in the pH range••3-13.

The reactive species that contribute to the en-hanced rate offormation of Cr(III)-EDTA are H202,OU', e ~ and H. The Molecular ~~"jrogen perox-ide also contributes to the formation of the pro-duct species", However, this reaction is very slowwhen compared with that by OR radicals and canbe neglected. If aquated electrons were to play arole in the reaction, this could occur through the

969

INDIAN J CHEM, SEC A, OCTOBER 1990

:21

~2•••\Jc ao•.wou

2~16~\J

!~: I~

IE-e..J

o:I •

4

pHA.------A 4· 90~4·60_____ 3·••

_~.'4

~~0---3~70~~3~90~-4~1~0--~43~0~~4~5~O--4~7~0--4~9~O~~5~10~~5~3~0--~55~0~~5~70~~5~9~0--~6I~0--~6~'0~-6~50WAVEI.ENGTH, nm

Fig. 3-Spectra of chromium (III) at different pH values (pH adjusted with NaOH)

reduction of Cr(ll} to labile Cr{II} in which casethe kObs for EDTA independent path should begreater than the observed value. But the values in-dicate that this reaction path does not contributesignificantly. The reduction of Cr{IV}-EDTA toCr{III}-EDTA by aquated electrons cannot beruled out. A similar role can be expected fromatomic hydrogen also. The formation of Cr(V)-EDT A intermediate cannot be expected underthese conditions because aquated electrons arepresent.

Nature of productsGamma ray irradiation of a mixture of

Cr{HzO}g+ and EDTA in acid medium results inthe formation of a purple coloured solution. Thespectrum of the reaction mixture corresponds tothat of Cr(YXH20t on irradiation for 8 hr. Thisproduct is stable towards irradiation for about 28hr. The product solutions irradiated for 8-36 hrwere passed through an Amberlite IR-45 anionexchange resin column. The adsorbed anionicCI\III) species was separated by eluting with 0.5mol dm - 3 NaCl indicating that the product ismononegative (cis-Cr{ oxMHzO); gets eluted with0.5M NaCl). Further irradiation results in changein colour gradually to reddish purple, red, lightblue and finally to greenish yellow. The pH of thesolution gradually increases and attains a maxi-mum value of 8.30 (after 136 hr of irradiation). Inview of these observations, irradiation of a pre-pared sample Cr(Y}{HzOt was carried out in the

970

presence of large excess of EDT A and similar re-sults were obtained.

Ion exchange studies of solutions irradiated forabout 60 hr showed the presence of positivelycharged Cr{III) species. These cationic speciescould be eluted from Dowex 50WX8 resin bedwith 6.0M HN03. It shows that this cationic spe-cies must be dimeric or polymeric. The reactionsolutions also contained Cr{Y XHzO}- andCr(YXOHj2- and this was confirmed by ion ex-change studies and spectrophotometry. The green-ish yellow products obtained on prolonged irradia-tion {for 136 hr and beyond} contained Cr{VI) tothe extent of 60-65% of total chromium. It is like-ly that the remaining chromium exists as cationicCr(III) species probably in dimeric and polymericforms.References1 Rao V K, Sundar D S & Sastri M N, Z anal Chern, 218

(1966) 93.2 Bhat T R & Phatak G M, J inorg nucl Chem, 28 (1966)

3058.3 Ohashi K, Suzuki T, Kubo K & Yamamoto K, Bunseki Kag-

aku,25(1976)693.4 Yamamoto K & Ohashi K, Bull Chern Sac Japan, 49 (1976)

2433.5 Furlani C. Morpurgo G & Sartori G, Z anorg Chern, 303

(1960) I.6 Hamm R E, JAm chem Sac, 75 (1953) 5670.7 Beck M T & Judit G G, Ac/a chim Hung, 70 (1971) 23H.H Emerson K & Graven W M, J inorg nucl Chern, II (1959)

309.9 Dragonic I G. Nenadovic M T & Dragonic Z D. J phys

Chern. 73 (1969) 2564.l1