Indian Journal of ChemistryVol. 30A. February 1991. pp 15R-1() I

Osmium(VIII) oxidation of chromium(III)'in aqueous alkaline medium

S M Tuwar, S T Nandibewoor & J R Raju*DepartmentofChemistry,KarnatakUniversity,Dharwad580 003

Received 4 June 1990; revised 27 August 1990; accepted 22 October 1990

Osmium(VIII}oxidation of Cr(III}in aq. alkaline medium ([OH-];;;.0.20 mol dm -.1)at 24°C is foundto follow the rate law,- d[Os(VIII}]Tldt= kK~[Os(VIII}h [Cr(III}lT/(l+ K~[OH-1T}The constants K 2 and k respectively represent the equilibrium formation constant of the speciesOsOS(OH}3- in the step Os04(OH}~-+OH-~OsOs (OH}3-+H20 and rate constant ofOsO,(OH}3- + Cr(OHL- interaction (Cr(OH); as Cr(III) species is known to occur predominantly in al-kaline medium}.The experimental values of K z and k are 7.5 ± 0.4 dm' mol- 1 and 83.3 ± 2.0 drn' mol- 1

S-I, the main active species being OsOs(OH}.1-and Cr(OH};.

The amphoteric nature of Cr(lII) in basic mediumhas recently been shown I to be due to the predom-inance of Cr( OH); and not Cr( OH)i -. The exist-ence of Cr(OH); was also supported by the kineticevidence gathered during oxidation of Cr(lII) byhexacyanoferrate{III) in alkaline medium", The re-dox potentials of Os(VI)/Os(VllI) (- 0.30Y) andCr(llI)/Cr(VI) ( + O.l3Y) couples in alkaline medi-um show that Os{VIll) should be able to oxidiseCr(lII) in such a medium. Preliminary studiesshowed that such a reaction is amenable to kineticinvestigation and hence the title study.

Materials and MethodsReagent grade chemicals and doubly distilled wa-

ter were used throughout. Osmium tetroxide (John-son Mathey) was dissolved in 0.50 mol dm -.1 sodi-um hydroxide and the solution standardised byadding excess standard hexacyanoferrate{II) in 1.0mol dm - 3 hydrochloric acid to it and, by titratingthe unreacted hexacyanoferrate{II) after 1 hr, withCe(IY) using ferroin as an indicator. The Cr(IlI) so-lution was obtained by dissolving the double salt,Cr2(S04k K]S04' 24H20 (BDH) in water. The ac-tual [Cr(lIl)) was estimated by oxidising it to Cr(VI)with persulphate in the presence of Ag(I), the Cr(VI)being titrated with iron(II). Chromium{VI) solutionwas made by dissolving potassium dichromate(BDH, AR) in water. Osmium(VI) solution was ob-tained by mixing equivalent concentrations ofCr(lII) and Os(VIlI) in 0.50 mol dm - 3 sodium hy-droxide. Sodium perchlorate and sodium hydroxidewere used to adjust the ionic strength and alkalinityin reaction solutions respectively. All solutions werealways prepared afresh for each kinetic run.

ISH

Kinetic run- The reaction was followed spectro-photometrically (Hitachi 150 spectrophotometer)under second order conditions at 24° ± O.OYC bymixing the reactant solutions which also contained

. the required amounts of sodium hydroxide and so-dium perchlorate. The absorbances of the aliquotsof the reaction mixture were measured in matchedquartz cells of 1 em path length at regular time inter-vals at 280, 321 and 372 nm, as three of the fourspecies, Os(VllI), Os(VI) and Cr(VI) (to the exclu-sion of Cr(lII) which absorbs in the visible region)absorb practically in the entire UV/visible region;further no single A.max is available for anyone ofthese species. The absorption of Cr(lII) at thesethree wavelengths was negligible. In view of theoverlapping absorption regions, the reaction couldbe followed only by recording the absorbances atthe three wavelengths mentioned above and solv-ing' the sets of triple simultaneous equations with aprior knowledge of the relevant molar absorbancyindices (E) at the respective wavelengths. The adher-ance of Os(VIll), Os(VI) and Cr(Yl) to Beer's lawhad been individually tested at the above threewavelengths in the concentration range of5.0 x 10-" to 5.0 X 10-4 mol dm ": in 0.20 moldm -.1 sodium hydroxide (Table 1). Initial rates weremeasured by the plane mirror method and were re-producible within 10%. In the case of Os(VIII) spe-cies, the absorbance depended on [OH-). Conse-quently, the molar absorbancy index (E) had to heevaluated for each [OH -) in the reaction by a test ofthe application of Beer's law at the three wave-lengths (Table 2). No such dependency on [OH-)was observed in the case of Os(VI) and Cr(VI).

Kinetic runs at pH > 12 gave satisfactory results.

TUWAR et al.: OSMIUM(VIlI) OXIDATION OF CHROMIUM(IlI)

Table l=-Molar absorbancy indices" for Os(VIIJ), Cr(VI) andOs(VI) at different wavelengths in 0.20 mol dm -3 NaOH

Species 280 nm 321 nm 372 nm

Os(VIll)

Cr(V\)

Os(VI)

2190

300

720

1270

4670

320

1550

3300

960

"Molar absorbancy indices (E) agreed within ± 1.5%

Table 2-Molar absorbancy indices" for Os(VIII) at differentwavelengths at different [OH -]

[OW]moldm-.1

372 nm

321 nm280 nm

0~2 0~4 0~8 0.10 0.20 0.30

800 920 1050 1090 1270 13701640 1930 2045 2060 2190 21901550 1550 1550 1550 1550 1550

"Molar absorbancy indices (E) agreed within ± 1.5%

However, when [OH-] < 0.020 mol drn ":', the reac-tion mixture tended to become turbid presumablydue to precipitation of Cr( OH)3; hence the studywas restricted to runs under conditions of pH > 12.It was found that this reaction can be carried out inall glass vessels, quartz vessels or polyacrylate ves-sels with no significant variation in the results. Theatmospheric oxygen also had no influence on the re-sults.

ResultsStoichiometry-Reaction mixtures containing dif-

feren t [Os(VIll)] and [Cr( ill)] and fixed [0 H -] of0.20 mol dm - 3 were kept aside for 8 hr at 24°C andthen analysed as follows. Osmium(VIII) was extract-ed with chloroform and its concentration measuredin the organic layer spectrophotometrically at 282nm (E282 = 1870). ChrorniumrVl ) left behind afterextraction of Os(VIII), was titrated with iron(ll) so-lution after acidifying the reaction mixture. Chrorn-iumrlll) could be measured at 590 nm (E = 25 ± 1).The results support a 3:2 stoichiometry for the reac-tion: 30s(VIII) + 2Cr(ill) = 30s(VI) + 2Cr(VI).

Reaction order-Orders in each reactant werefound from log-log plots of initial rates against con-centrations. Order in Os(VIIl) was - 0.93 between2.50 x 10-5 and 2.25 x 10-4 mol dm-3 of [Os(VIII)]at fixed [Cr(III)] and [OH-] of 1.5 x 10-4 mol dm-3

and 0.20 mol dm - 3 respectively (Table 3). Order inCr(III) was around unity as studied between0.50 x 10-4 and 5.0 x 10-4 mol dm-3 [Cr(IlI)] atfixed [Os(VIII)] of 2.25 x 10-4 mol dm - 3 and [OH-]of 0.20 mol dm - 3. The effect of [OH -] on the reac-tion was studied from 0.020 to 0.30 mol dm - 3 at

Table 3-Effect of [Os(VIII)], [Cr(III)] and [OW] on Os(VIII)-Cr(IlI) reaction at 24°C and 1=0.30 mol drn ":'

[Cr(IlI)] x 10' [Os(VIlI)]x 104 [OW]mol dm-.1 mol dm-.1 mol

dm-.1

0.50

0.801.00

1.50

3.005.001.501.501.50

1.501.501.50

1.501.501.501.501.501.501.50

2.25

2.25

2.25

2.25

2.252.250.250.500.65

0.700.901.60

2.252.252.252.252.252.252.25

0.20

0.20

0.20

0.20

0.200.200.200.200.20

0.200.200.200.200.020.040.080.100.200.30

(Initial rate) x l O"mol dm-3 S-l

Expt. Calc .•

0.55 0.56

0.86 0.891.20 1.15

1.66 1.68

3. IO 3.37

5.35 5.620.24 0.190.44 0.380.53 0.49

0.59 0.530.74 0.671.33 1.201.66 1.68

0.36 0.370.60 0.641.00 1.051.26 1.211.66 1.68

2.10 1.96

"Initial rates were calculated from Eq. (3) using Kz and k as7.5 ± 0.4 drn ' mol- I and 83.3 ± 2.0 drn' mol- I s " I respectively.

fixed [Os(VIII)] and [Cr(III)] and I= 0.30 mol dm-3

(Table 3) and the order in [OH -] was found to be -0.60.

Effect of added products and ionic strength-In-itial addition of products, Cr(VI) and Os(VI) be-tween 3.0 x 10 - 5 and 3.0 x 10 - 4 mol dm - 3, keeping[Os(VIII)] and [Cr(IlI)] and ionic strength fixed, didnot have any significant effect on the reaction. Therate increased from 1.66 x 10 - 6 to 2.50 x 10 - 6 moldm - 3 S - 1 with increase in ionic strength from 0.20to 0.90 mol dm - 3, other conditions being constant.

As observed in an earlier study' increase in tem-perature favours the formation of dimeric, trimeric,etc. forms of Cr(lII). Presumably, a part of Cr(III)which undergoes polymerisation is more difficult tooxidise and our attempts at finding the rates at high-er temperatures actually resulted in marginal dec-reases in rates. Hence, temperature effect on thereaction could not be studied.

DiscussionThe reaction is approximately first order in

Cr(III) and fractional order each in [Os(VIII)] and[OH -]. Osmium(VIII) is known to form differentcomplexes with OH - in basic media as shown in

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INDIAN J CHEM, SEe. A, FEBRUARY 1991

Eqs (1) and (2) with equilibrium constants of KJ andK2 having the values of (24 and 6.8 drn ' mol- J re-spectively".Os03(OH)3 - +OH- ~Os04(OH)~- + H20 KJ ••• (1)Os04(OH)~- + OH- ~OsOS(OH)3- + H20 K2 ...(2)

It is seen that a progressive increase in[Os04(OH)~-1 and [OsOs(OH)3-1 occurs with in-crease in (OH-] and, at (OH-] of 0.20 mol drn ":',used in this study, virtually all Os(VIII) is present asOs04(OHn- and OsOs(OH)J-.- Indeed,[OsOS(OH)3-] may be calculated on the basis of KJ

and K2 to be about 20% at an [OH-l of 0.020 moldm-·1 when total [Os(VIII)] = 2.25 X1O-.J mol dm-3,and over 90% of the total [Os(VIII)] when [OH -] is0.20 mol dm ":'. It is also found that the absorbanceof Os(VIll) does become almost constant beyond an[OH-] of around 0.15 mol dm ":' indicating the pre-dominance of one species, presumably OsOs(OH)3-. Because of this reason and the fact that theinitial rate is a function of [OH -] with fractional or-der in [OH-], the main oxidant species is likely tobe OsOs( OH)3 - and its formation in the equilibri-um (2) is of importance in the reaction. As for the re-ductant, Cr(IIl), the forms CrOH2+, Cr(OH);,Cr(OHh and Cr(OH)i besides polymeric speciesare known in basic solutions J. In acid medium,CrOH2+ and Cr(OH); exist and, between pH 7 and10, Cr(OHh in colloidal form is known. The am-photerisrn of Cr(III) is due to formation of Cr(OH);and this is the form in which almost the entire dis-solved Cr(III) exists' above pH 12 under the condi-tions employed in this reaction. The absorption

9.D

"'E~~

N~

"fx .•0

~~

~'"2-

10

- 3 -I1 I(OH J dm mol

Fig. I-Verification of rate law (3) ([Cr(III)] = 1.50 x 10-4•

[Os(VIII)] = 2.25 x 10 -~, /= 0.30 (mol dm - 3), temp. 24°C)

160

spectra of Cr(III) in the aqueous medium in the visi-ble region in the presence and absence of OH - wereall similar except that some hyperchromicity isfound in the spectrum in the presence of alkali. Amechanism in terms ofOsOs(OHj3- and Cr(OH)ican account for the experimental results as inScheme 1 where Os(VIll) species, formed in apreequilibrium step (i), interacts with the Cr(III) spe-cies in a slow step (ii) (see Scheme 1). Scheme 1leads to rate law (3) where K2 and k represent theformation constant of OsOs(OHP- (step i) and therate constant of

K,

Os04(OH)2; + OH- ~ OsOs(OHj'- + H20 ... (i)

- kOs05(OH)3- + Cr(OH)~- OsOs(OH).J- + Cr(OH)4

(or Os(VIl) + Cr(IV)) ...(ii)

fastOs(VlII)+Cr(IV)-Os(VII)+Cr(V) ... (iii)

fastOs(VIll) + Cr(V)-+Os(VII) + Cr(Vl) ... (iv)

_ fastOs(VII)+Cr(OH)4 -Os(VI)+ Cr(IV) etc ... (v)

Scheme 1

the slow step (ii). In Eq. (3), the denominator mustalso contain the factor (1 + K2 [Os(VlII)hl which isleft out since

- u1:Os(VIII)h kK2[ Os(VIII)l.J Cr(llI)].J OH -]Tdt (1 + K2[OH-h)

... (3)it approximates to unity. It may be noted thatOs(VIII) oxidation of Cr(III) (Scheme 1) takes placein single equivalent steps giving rise to species suchas Os(VII) and Cr(IV). The one equivalent stepswritten in Scheme 1 are in agreement with the factthat single electron transfer steps are more probablethan others. Intervention of oxidation states of Oslower than Os(Vl) in the mechanism is unlikely inview of the reasonable stability of the OstVl) speciesin an alkaline medium.

Intervention by the free radicals in the reaction isexpected, if Scheme 1 is found valid but radical sca-venging tests using the monomers acrylamide andacrylonitrile failed as Os(VIII) was found to oxidisethe monomers themselves. The mechanism shownin Scheme 1 and rate law (3) may be verified by aplot of [Os(VIII)l.JCr(lII)h/ratc versus lI[OH-hwhich should be linear (from Eq. 3) and the slopeand intercept should lead to the values of K2 and k.

/1 TUWAR et al.: OSMIUM(VIII) OXIDATION OF CHROMIUM(II1)

Such a plot is shown in Fig. 1 with K2 = 7.5 ± 0.4dm ' mol-1 (K2 = 6.8 dm ' mol-1 at 50°C from earli-er work") and k= 83.3 ± 2.0 dm ' mol-1 s -1. The va-lues of K2 and kwere further used to calculate ratesfor several experimental situations. Rates calculatedin this way have been found to be comparable withthe experimentally measured rates. The marginal in-crease in rate with increase in ionic strength is as ex-pected qualitatively from the mechanism ofScheme 1 where ions of like charge interact in therate-determining steps.

It was also found that added ions like Li + andCI- had practically no effect on the reaction. Theseresults and the fact that the species of Os(VIII) andCr(Ill) in alkaline medium are HOOsOs and

Cr(OH)i (coupled with the high value of k) indicatethat the oxidation presumably occurs by an innersphere mechanism. This conclusion is supported bythe results of earlier work-".

References1 Dhanpat Rai, Saas B M & Moore D A, Inorg Chern, 26

(1987) 345.2 Tuwar S M, Nandibewoor S T & Raju J R, Trans met Chern,

1990 (In press).3 Vogel A I, Textbook of quantitative inorganic analysis (ELBS,

Longman, New York) (1978) p. 763.4 Bhatt L, Sharma P D & Gupta Y K, Indian J Chern, 23A

(1984) 560.5 Swinehart J H, J inorgnucl Chern, 27 (1967) 2313.6 LancasterJM & Murray RS, J chernSoc(A)(1971) 2755.

161