Indian Journal of ChemistryVol. 35A, December 1996, pp. 1084-1090

Osmium{VIII) catalysed oxidation of antimonyilll) by alkalinehexacyanoferratetlll) and analysis of osmium{VIII) in microamount by

a kinetic method

P L Timmanagoudar, G A Hiremath & S T Nandibewoor*

Department of Studies in Chemistry, Karnatak University, Dharwad 580 003, India

Received 15 April 1996; revised 19 June 1996

The osmiumt Vill) catalysed hexacyanoferratetlll) oxidation of antimony(ill) in aqueous alkalinemedium at 25°C obeys the rate law

d[Fe( CN)~-]-r-

dtkK2K3[Sb(ill)][Fe(CN)~-][OH-][Os(VIll)]

1 + K2[OH ] + K2K3[Fe(CN)~ ][OH ]

where K2 is the equilibrium constant for the reaction

K,

[Os04(OHhF- +OH- ~ [Os05(OH)P- +H20

K3 is the formation constant for the complex formation between alkaline species of osmium(VIll)and hexacyanoferrate(ill) and k is the rate constant of the slow step. The data suggest that the reac-tion goes through catalyst-oxidant complex formation followed by the reaction of substrate withcomplex in the rate determining step. The reaction constants involved in the mechanism are derived.The title reaction has been utilised for the analysis of osmium(VIll) in the range 0.019 p.g em -3 to0.66 p.gcm-3•

Although arsenic(III) and antimony(IIIl belongto the same group in the Periodic Table and havestriking similarities, arsenic(III) compounds areused as insectisides and weed killers while thoseof antimony(III) are employed as medicaments 1.

Antimony(III) oxide is slightly soluble in waterand dilute acid, but rather more soluble in-alkali".However, the species of antimony(III) in alkalinesolution are not so well developed as is the casein acid medium.

Osmium(VIII) is known to be an efficient cata-lyst in several redox reactions, particularly in al-kaline solution.'. The reduction potentials" of thec.ouples Fe(CN)~- /Fe(CN)2-, Os(VIII)/Os(VI) andSb(V)/Sb(III) of + 0.45 V, + 0.30 V and - 0.58 Vrespectively, in alkaline medium makes the osmi-um(VIII) catalysed oxidation of antimony(III) byhexacyanoferrate(III) more feasible. Due to thevarious intermediate species of osmium(VIII) andantimony(III) involved, this reaction has been un-dertaken to learn more about the alkaline redoxchemistry of these species.

Furthermore, there is also possibility of com-plex formation in the reaction. The uncatalysed 'and chromium(III) catalysed" reaction betweenantimony(III) and hexacyanofertate(III) in alkalinemedium- have been studied previously. A microa-mount of osmium(VIII) is sufficient to catalyse thereaction in alkaline medium in the reaction and avariety of mechanisms are possible. Herein. wereport the results of the study of the osmium(VIII)catalysed oxidation of antimony(III) by hexa-cyanoferratedll) in alkaline medium.

Materials and MethodsReagent grade chemicals and doubly distilled

H20 were used throughout. A stock solution ofFe(CN)~- was prepared by dissolving a knownweight of K3Fe(CN)6 (BDH) in H20 and the con-centration was ascertained by iodometric titra-tion 7. The [Fe(CN)2 -] was prepared by dissolvingK4Fe(CN)6 (BDH) in H20. Antimony(III) solutionwas made by dissolution of Sb203 (E. Merck, AR)

TIMMANAGOUDAR et aL: OSMIUM(Vill) CATALYSED OXIDATION OF ANTIMONY(II1) 1085

Table I-Effect of [Fe(CN~-1 [Sb(II1)1 (Os(vm)) and [OH-] on the Os(vm) catalysed oxidation of Sb(II1) by Fe(CN)~- in al-kaline medium at 25 ±O.I°C and 1-1.0 mol dm ?

10· [Fe(CN)~-] W[Sb(II1)) 106[Os(vm)) 10 [OH-] 106 (Inl. rate)(mol dm+") (mol dm-3) (mol dm=t) (mol dm=) (mol dm ? S-I)

Expt. Calc.·

0.7 1.5 1.5 5.0 0.45 0.461.0 1.5 1.5 5.0 0.61 0.632.0 1.5 1.5 5.0 1.13 1.14

3.0 1.5 1.5 5.0 1.56 1.55

5.0 1.5 1.5 5.0 2.20 2.187.0 1.5 1.5 5.0 2.70 2.65

3.0 0.3 1.5 5.0 0.25 0.303.0 0.5 1.5 5.0 0.45 0.513.0 1.0 1.5 5.0 1.03 1.00

3.0 1.5 1.5 5.0 1.56 1.55

3.0 2.0 1.5 5.0 1.90 2.013.0 3.0 1.5 5.0 3.00 3.09

3.0 1.5 0.3 5.0 0.32 0.313.0 1.5 0.5 5.0 0.55 0.523.0 1.5 1.0 5.0 1.05 1.03

3.0 1.5 1.5 5.0 1.60 1.55

3.0 1.5 2.0 5.0 2.10 2.073.0 1.5 3.0 5.0 3.18 3.10

3.0 1.5 1.5 2.0 1.22 1.23

3.0 1.5 1.5 3.0 1.41 1.39

3.0 1.5 1.5. 5.0 1.57 1.55

3.0 1.5 U 7.0 1.63 1.63

3.0 1.5 1.5 10.0 1.76 1.72

*Initial rates were calculated using values of k, K2 and Kj 2.50x103±1O dm! mol-I S-I, 6.35±0.2 dm" mol-I and1666.6 ± 50 dm ' mol " ':Error ±4%.

in 3.0 mol dm - 3 of HCI and was standardised"against KBr03 using methyl red indicator in thepresence of 4.0 mol dm-3 of HCl. Antimony(V)was prepared from KSb(OH)6.1-H20 (BDH). Os-mium(V1II) solutions were prepared. by dissolvinga weighed amount of OS04 (Johnson Matthey) in0.5 mol dm-3 of NaOH and its concentration wasascertained? by addition of excess of standardFe(CN)~- in 1.0 mol dm-3 HCI, followed by titra-tion of unreacted Fe(CN)~ - with standard ceri-um(IV) using ferroin indicator.

The appropriate amount of alkali just to neutra-lise the acid present in antimony(III) solution wasadded in the reaction mixture. The concentration

of liberated NaCI is also taken into account incalculating the ionic strength. NaOH and NaCI04

were used to provide the required alkalinity andto maintain the ionic strength respectively in reac-tion solutions.

Kinetic studiesKinetic runs were followed under the condi-

tions where [Sb(III)] was present nearly in ten foldexcess over [Fe(CN)~-] at a constant temperatureof 25 ± 0.1°C unless otherwise stated. The reac-tion was initiated by mixing the thermally equili-brated' solutions of Fe(CN)~- and Sb(III) whichalso contained the required amounts of Os(V1II),

1086 INDIAN J CHEM. SEe. A, DECEMBER 1996

NaOH and NaCI04• The reaction was followedby measuring the absorbance of Fe(CN)~ - in thereaction solution in a 1 em cell in a thermostattedcompartment of a Hitachi lS0-20 spectropho-tometer at 420 nm where the other constituentsof the reaction mixture do not absorb significant-ly. The application of Beer's law for Fe(CN)~- at420 nm under the reaction conditions had earlierbeen verified giving E = 1060 ± 20 dm-' mol- I

em - I. The choice of initial rates was made owingto the non-linearity of the plots of log [Fe(CN)~-]versus time after about SO to 60% of the reactionand also due to the fractional order in [Fe(CN)~- ].Initial rates were obtained by plane mirror meth-od '? and were reproducible within ± 4%.

In view of the moderate concentration of alkaliused in the reaction medium, attention was alsogiven to the effect of surface of the reaction ves-sels on the kinetics. Use of polythene/acrylic wereand quartz or polyacrylate cells gave the same re-sults as with glass vessels and cells indicating thatthe surfaces play no important role in the reactionrate.

The effect of dissolved oxygen on the rate ofthe reaction was checked by preparing the reac-tion mixture and following the reaction in an at-mosphere of nitrogen. No significant differencebetween the results obtained under nitrogen andin presence of air was observed. In view of ubiq-uitous contamination of carbonate in basic solu-tions, the effect of carbonate on the reaction wasalso studied. added carbonate upto1.a x 10 - 3 mol dm - 3 showed no effect on reac-tion rate. However, fresh solutions were usedwhile performing the experiments.

ResultsStoichiometry

Different sets of concentrations of reactants inO.S mol dm - 3 alkali at a constant ionic strengthof 1.0 mol dm - 3 and with a constant amount(1.S x 10 - 6 mol dm - 3) of catalyst were kept for24 h at 2SoC under a N2 atmosphere. The un-reacted Fe(CN)~ - was estimated by acidifying thereaction mixture with sulphuric acid in the pres-ence of zinc sulphate which prevents the backreaction of Fe(CN)~- with Sb(V). Sb(IIl) was de-termined by the method mentioned in experimen-tal section. The total oxidant was then found byiodometry and after accounting for the Sb(V)formed, the remaining oxidant concentrationcould be derived. The results indicate that theconsumption ratio of [Fe(CN)i-]-[Sb(IIl)] is2.0 ± 0.1 as shown in Eq. (1).

Os(VIlI)2Fe(CN)i - + Sb(III) •

2Fe(CN)2- +Sb(V)

Reaction orderThe orders were determined from log-log plots

of initial rate versus concentration at a constantalkalinity of O.SO mol dm - 3 and ionic strength of1.0 mol dm "". At a fixed concentration of osmi-um(VIlI) of 1.S x 10 - 6 mol dm - 3 the apparent or-der in [Fe(CN)~-] in the range of 7.0 x 10-5-

7.0 x 10 - 4 mol dm - 3 was significantly less thanunity and the order in [Sb(III)] in the range .of3.0 x 10-4-3.0 X 10-3 mol dm-3 was unity(Table 1).

The order in [Osmium(VIlI)] was unity in therange 3.0 x 10-7-3.0 X 10-6 mol dm-3 with allreactant concentrations and conditions being con-stant (Table 1). With increasing concentration ofalkali, the rate increased (Table 1).

Effect of added products and Cl:Initially added products hexacyanoferrate(II)

and antimony(V) in the concentration range of1.0 x 10-4 to 1.0 x 10-3 mol dm " by keepingother conditions and reactant concentrations con-stant did not affect the reaction rate significantly.

Chloride ions might also affect the results[Complex formation with antimony (III)I. How-ever, chloride ions added up to 2.S X 10-2 moldm - 3 did not change the results significantly.

Effect of ionic strength and solvent pol~rity .The ionic strength of reaction solution was van-

ed between O.S to 2.S mol dm-3 with NaCI04 atconstant concentrations of oxidant, reductant,catalyst and OH- at 3.0 x 10-4, 1.S X 10-3,

1.S X 10-6 and 0.50 mol dm "? respectively. The in-itial rate increased with increase in ionic strength andthe plot of log (rate) versus j1!2 was linear with a posi-tive slope.

The relative permitivity (ET) effect was studiedby varying the tert-butyl alcohol content in thereaction mixture with all other conditions beingconstant. Attempts to measure the relative permi-tivity of the media failed. However, they werecomputed from the values of the pure liquids asin earlier work!'. There was no reaction of sol-vent with oxidant under experimental. conditionsused. Initial rates increased with decrease in rela-tive permitivity of the medium. The plot of log(rate) versus II ET was linear.

Effect of added ionsThe effect of added cations and anions on the

TIMMANAGOUDAR et 01.: OSMIUM(VIII) CATALYSED OXIDATION OF ANTIMONY(IIl) 1087

k 4- r; J3-}'~duct(A) + Sb(OH)~ ~ Sb(OH)4 + FdCN)6 +LOSOS(OH)

[Sb(lVl]

[Sb (IV)]

Schorn. 1

reaction rate was studied at constant ionicstrength (1.0 mol dm -,) maintained by sodiumperchlorate. When sodium ions were replaced bylithium ions (lithium perchlorate) the rate in-creased significantly but in the case of potassiumions (potassium perchlorate) rate increased onlymarginally. Similar replacement of perchlorateions by sulphate ions (Na2S04) and nitrate ions(NaN03) did not have any effect on the reactionrate.

lSJTectoJter.nperatureThe rate constants k of the slow step of

Scheme 1 were obtained from the intercepts of[Sb(III)] [Os(VIII)]/rate versus 1/[Fe(CN)~-] plotsat three different temperatures and were used forthe calculation of activation parameters. The va-lues of k (dm! mol-I S-I) are 2.5 x 103, 4.0 X 103,7.0 X 103 at 259, 34° and 40°C respectively. Fromthese data I:!.H'" and I:!.S'" were calculated to be43.2±2 kJ mol-1 and -33.0±4 J K-1 mol-1 respectively.

DiscussionThe uncatalysed oxidation of antimony(III) by

hexacyanoferrate(lli) in concentrated acid medi-um has been studied 12and the reaction proceedswith slower rate. And also osmium(VIII) failed tocatalyse the reaction in acid medium. The solubi-lity of antirnony(III) in acid medium is also one ofthe major difficulties in carrying out the reactionHowever, in alkaline medium antimony(III) exhi-bits greater solubility hence kinetics of the titlereaction can be studied conveniently. Anti-mony(III) at pH> 11 exists 13 as Sb(OH)i and isbelieved to be the active species in the reactionsystem.

Osmium(VIII) is an efficient catalyst in alkalinemedium and is widely believed 14to give hydroxy-lated species in such media. The different com-plexes of osmium(VIII) with OH - in alkaline me-

Table 2-;-Effect of [OH'-] on osmium(VIII) species" and ini-tial rates of osmium VIII) catalysed hexacyanoferrate(III) oxida-

tion of antimony(III) in aqueous alkaline media at 25°C

[Fe(CN)~-] = 3.0 x 10-4,[Sb(III)] = 1.5 x 10-';[Os(VIII)]= 1.5 x 10-6 and l= 1.0 (mol dm~')

[OH-] lO~a" al a. 106 (InI.rate)(moldm-') (mol dmv s :"]

0.2 R.13 0.40 0.53 1.22(1.3 4.37 0.31 0.64 1.410.5 I.R6 0.22 0.76 1.570.7 1.02 0.17 0.R2 1.631.0 0.51 0.13 0.87 1.75

* a", al and a2 are the fractions of tOtal Os(VIII) of the spe-cies [OsO,(OH);], [Os04(OHg-] and [Os05(OH)'-] respec-tively.Error ±4%.

10 2'() 10

4 , ,., 1 8

I J I6 ~ "f• 6..•~

~2 34 1·4 '0 4N.!

2E.-

N$! $!

!1 U .; 2

J ..~0 ,.0 0

0 ()'1 ().4 1-2

[OH-) • mol dm-l

Fig. I-Variation of concentration of osmium(VIII) specieswith [OW] (I) [Os03(OH);] (II) [Os04(OHg-] (III)[OsO,(OH)'-] and (IV) rate with [OH-] (Conditions as in

Table 2).

dia are shown in equilibria (2) and (3), where theequilibrium constants K 1 and K2 have values of24 and 6.8 dm ' mol-I respectively 14.

K,[OsO,(OHM- + OH- ;=' lOsO.j(OHhF- + H20

... (2)

[Os04(OHlzF- + OH- .= [OsOs(OHlP- +H20

... (3)

Interestingly, the absorbance of osmiuffi(VIII)becomes almost constant for [OH -] above -0.15 mol dm-3, indicating the predominance ofone species" presumably [OsOs(OH)P-. The in-crease in rate with increase in [OH-] can be attri-buted to the formation of [OsOs(OH)P- species.Infact, above [OH-]=0.15 mol dm-3, the pre-

1088 INDIAN J CHEM. SEC. A, DECEMBER 1996

Table 3-0smium(VIII) analysis by kinetic method

[Sb(III)]= 1.5 x 10-3, [Fe(CNM-]= 3 x 10-4, [OH-]=0.5,1= 1.0 mol dm - 3; temp. = 25°C

[Os(VIIIW /-lg em - 3Standard

Taken Found* Deviation

0.047 0.050 ±O.o3

0.105 0.100 ±0.040.162 0.158 ±0.Q2

0.215 0.210 ±0.040.272 0.270 ± 0.03

0.329 0.330 ±O.020.386 0.390 ±0.030.443 0.445 ±O.o3

0.500 0.503 ±0.030.552 0.550 ±·0.D1

0.608 0.610 ±0.020.666 0.665 ±0.D1

*Average of five sets.

dominant species of osmium (VIII) reported 14 is[OsOs(OH)P- only. In the present study in orderto confirm the active species of catalyst we havecalculated the concentrations of different hydrox-ylated species of osmium(VIII) from the reportedvalues of K 1 and K2 (see infra) and are shown inTable 2. The concentrations of [Os03(OH)3)- and[Os04(OH}z]2- show a decreasing trend with in-creasing [OH-) while the species [OsOs(OH)P-shows an increasing trend with increasing [OH-)as does the rate of the reaction (Table 2). Also inthe plot of [OH -) versus concentration of differ-ent species of osmium(VIII) and that of with rate,the species [OsOs(OH)P- vary parallely with therate (Fig. 1) indicating its predominance in suchsolutions in the reaction system. Such type ofcharacterisation of osmium(VIII) species has beenreported in earlier study".

The order of significantly less than unity in [ox-idant) reveals the possibility of involvement ofcomplex formation between .the oxidant and cata-lyst in the reaction system in a pre-equilibriumstep. The complex formed by the mere associa-tion of oxidant and catalyst is short lived andreactive, therefore may be considered as an ad-dtk'l. Thus formed adduct reacts with Sb(OH),\ inthe 1,,\;;' determining step to give Fe(eN)~- as theproduct of oxidant and Sb(OH)4 {Sb(IV)1 as thereactive Intermediate of Sbilll) by regeneratingthe catalyst in its original form. Further, in the faststep thus formed Sb(IV) utilising another mole-

'"

..s ;:; t: ; &

loJ[Sb(mloslvm)V('Ala),mOI d,,;3.

Fig. 2-A. Plot of [Sb(III)] [Os(VIII)]/rate versus1/[Fe(CN)~-]. B. Plot of [Sb(III)] [Os(VIII)]Irate versus

II[OH-]; (Conditions as in Table 1).

cule of oxidant gives the final produet of Sb(III)as Sb(V) along with Fe(eN)~-, which satisfies thestoichimetric observations. Hence, in accordancewith the generally well-accepted principle ofnon-complimentary oxidations taking place insequences of one electron steps, a mechanism asin Scheme 1may be envisaged.

The order of unity in [Sb(III)) and fractional or-der in [Fe(eN)~ -) is well accommodated inScheme 1, which incorporates all the experimentalresults. Since the order in [Sb(III)) is unity, thepresence of the monomeric species of anti-mony(III) in the present reaction condition ismost likely which contrasts with the opinion ofMeites and Schlossel". Although the experimentalevidence for the formation of a Sb(IV) complex asan intermediate was not observed, such a specieswas observed in earlier studies'". Attempts to ob-tain UV-vis spectral evidence for adduct forma-tion were in vain; no appreciable change resultingin the spectra of the oxidant and catalyst in thepresence of one another. A feeble interaction isprobably involved. However, evidence for thecomplex formation is obtained kinetically. Suchcomplex formation between the catalyst and oxi-dant has been observed in other studies''-". Theadduct might be formed through - eN - bridg-ing. Indeed, complex formation of iron and osmi-um(VIII) by cyanide linkages and other similarcomplexes have been observed'",

TIMMANAGOUDAR et al.: OSMIUM(VIll} CATALYSED OXIDATION OF ANTIMONY(m} 1089

0·2 00

00·24

0

CHO

••<I0·1&

0·12

3

106 [oscvnnl, mol dm-J

Fig. 3-Plot of 6.X versus [Os(VIII)] for Osmium(VIIl} analy-sis by a kinetic method; (Conditions as in Table 3). (6.X isthe difference in absorbance of catalysed and uncatalysed

reaction at 3rd minute).

Scheme 1 leads to rate Eq. (4)

d[Fe( CN)~-]dt

kK2K3[Sb(III)][Fe(CN)~-][Os(VIII)][OH-]1 + K2[OH-] + K2K3[Fe(CN)~-][OH ]

... (4)

Since, K2[Os(VIII)] ~ 1 and K3[Os(VIII)] ~ 1,they approximate to unity (in view of low concen-tration of osmium(VIII) used).

The mechanism in Scheme 1 and rate law (4)may be verified by rearranging it in the form of(5) and (6)

[Sb(III)][Os(VIII)] 1rate kKJ[Fe(CN)~

... (5)

[Sb(III)][Os(VIII)] 1rate

1[ 1 I+- +1k K3[Fe(CN)t]

... (6)

From E .,(5), plot of [Sb(ID)][Os(VIII)]/rate versusl/iFe(CN)~-] and from (6) plot of [Sb(ID)][Os(VI-IIlVrate versus I/[OH-] are expected to be linear.This is found to be so as shown in Fig. 2. Fromthe slope and intercept, the values of K2, K3 andk at 25 ± 0.1 °C were derived as 6.35 ± 0.2 dm 'mol", 1666.6 ± 50 dm? mol-I and 2.5 x 103 ± 10dm ' mol-I s -I respectively. Using these values,rates over the range of different conditions werecalculated and compared with experimental data(Table 1). There is a good agreement betweenthem. The value of K2 obtained is near the valuereported earlier!".

The effect of ionic strength and dielectric con-stant of the medium on the rate of the reactionare also in the expected direction on the basis ofthe reaction between the negatively charged spe-cies as in Scheme 1. The adduct formation in themechanism of Scheme 1 is expected to lead to anegative entropy of activation and this is found tobe the case. A relatively low value of entropy ofactivation and high value of rate constant (k ofslow step of Scheme 1) coupled with effect ofadded ions indicate that the oxidation presumablyoccurs by inner sphere mechanism. This conclu-sion is supported by earlier work 18.

Analysis of osmium( VII!) in microamounts by akinetic method

The kinetic method of analysis of inorganic andorganic species in micro amounts are receivingconsiderable interest, since they are highly sensi-tive, selective, reproducible and less expensive.The methods play important role in environmen-tal analysis. In continuations:" on kinetic methodof analysis, we have utilised the title reaction forthe analysis of osmium(VIII) in micro amounts.

The osmium(VIII) catalysed oxidation of anti-mony(ID) by alkaline hexacyanoferrate(III) underthe conditions studied leads to antimony(V) andhexacyanoferrate(II) via formation of a catalyst-oxidant complex as shown in Scheme 1.

Method recommended for analysis-A knownset of concentration of hexacyanoferrate(III), anti-mony(III), alkali and osmium(VIII) (e.g.3.0 x 10-4, 1.5 X 10-3, 0.5 and 1.5 x 10-6 moldm - 3 respectively) was thermally equilibrated andmixed. The absorbance of hexacyanoferrate(III) atthird rnin was taken at 420 nm. With same concen-.

1090 INDIAN J CHEM. SEe. A, DECEMBER 1996

tration of reactants and alkali, osmium(VIII) wasvaried, the reading at third min being taken in eachcase. With difference in absorbance of hexacyano-ferrate(llI) between catalysed and uncatalysedreactions as ~X, a calibration curve of ~X versus[Os(VIII)] (Fig. 3) was used to determine un-known concentrations of osmium(VIII). The re-sults are shown in Table 3. The results of analysiswere satisfactory in the concentration range of os-mium(VIII) from 1.0 x 10-7 to 3.5 X 10-6 moldm-3• Above these concentrations the experimen-tal curve ~X versus [Os(VIII)] deviated from line-arity.

Interference for few metals upto a concentra-tion range of 2.0 x 10 - 4 mol dm - 3 was tested.Apart from reductants which reduce hexacyanofer-rate(llI) and the oxidants which oxidise antimony(llI), it was found that Zn2+, Te4+, Se4+,Mg2+, Pb2+ have no effect, whereas, Cu2+,Mn2+, NF+, C02+ and Li " interfere. The methodfinds its application in analysis of osmium(VIII) inmicro amounts in its complex forms and suchother similar forms. Analysis in this range becomeincreasingly important in recent years, since verydilute aqueous solution is used as a biologicalstain and as a catalyst for many reactions. It hasbeen reported that osmium(VIII) is harmful toeyes, nose and throat even in dilute concentr-ations.

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