Murty, K. S.

Murty, R. V. S.

Rao, N. Venkateswara

Rao, P. V. Subba

English

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iIndisjoJournal of Chemlatl'1Vol. f5A. January1977,pp. 16·18I, Kinetics of Oxidationof Methyl Orange& Methyl Red by V (V)*P. V. SUBBA RAO, N. VENKATESWARA RAO, R. V. S. MURTY & K. S. MURTYDepartmentof Chemistry,AndhraUniversity,WaltairReceived9 February 1976; accepted26 APril 1976Thekineticsof oxidationofmethylorangeandmethylredbyV(V)hasbeeninvestigatedinaqueousperchloricandsulphuricacidmedia. In perchloricacidmedium,reactionobeystheratelaWl~~ [V(V)]=k'[VOt] [Dye]+k'K1[V01] [Dye][H+)2HzPO.,HSO,andCI ionsarefoundtoacceleratehereactionin theorder,H2P04- >HS04- >CI-.This has beenascribedto additionalvanadium(V)speciesformedwith theseanions. Theactivationparametershavebeendeterminedandplausiblemechanismproposed.I""'JiHOUGH kineticsof oxidationof a numberof.1i organic compoundsby va adium(V)haveI beenreported,kinetic study of oxidationofaroniatic azo compoundsby V(V) has not beenmad~so far. A detailedkineticstudyof oxidationof methylorangeand methylred in aqueousper-chlo#cand sulphuricacid media has now beenarr*d out. The influenceof the ddedanionsH2P?i, HSO, and CI- on the reaction has alsobeen~investigated.The resultsenableus to under-stan the nature of the speciesinvolved in thereac'onandtheirrelativereactivities.MatfrialsandMethodsAIl thechemicalsusedwereof AR grade. Sodiumvana~atesolut onwas pr paredand standardizedacco~dingto the methodof GopalaRao and co-work rsl. Met ylorangeandmethylred solutionswere!standardizedaccordingto the method ofKne*t andHibbert2.Ra e measurements- The courseof the reactionwasf llowedby m asuringtheopticaldensityof theunre<4cteddyewith a Klett-Summersoncolorimeterusing!54-0nmfilter. UndertheseconditionsBeer'slaw asobeyedbyboththedyeswhichhaveabsorp-tion aximain thisrange.Stochiometry- Thestoichiometryof thereactionwas eterminedby mixingmethylorangeor methylredi 3M H2S04 with a knownexcessof V(V) andthee cessV(V) back-titratedwith standardMohr'ssalt, after the completionof the reactionwhichisindic*ed by thedischargeof thepink colourof thedyeiIj acidsolution. Onemoleof eitherof thedyeswas fPund to consumefour molesof V(V). Theauthorsalsofoundthattheproductsarethecorres~pondi~gnitroso compoundswhich were detectedusing~pottestsprescribedby Feigl3. The testfornitro k:ompounds3gavea negativeresult. It wasfurtherfoundthatundertheexperimentalconditionsemployedin thekineticstudy,therewasno detect-ablei*teractionbetweenV(V) and dimethylanilineduringthetimetakenfor followingthe kineticrun,showi1gthat -N(CH3)2 group presentin these*Part1of this workwaspresentedat the Indian ScienceCongres~heldat Chandigarh.16dyesis notinvolvedin theoxidation. It is believedthatthesiteofattackbyV(V) isthe-N =N - bondwhich by the uptakeof two oxygenatomsyieldsnitroso compounds.This is in conformitywith thestoichiometryof the reaction.The authorsemployedthe vinyl polymerizationtestdescribedby WatersandLittler4for thedetec-tion of anyfreeradicalintermediatein t,hereaction.Thetestgavea negativeresult.ResultsThe reactionis foundto be first orderin methylorangeor methylredasevidencedby theplot of log(OD) versus time, which was linear under theconditions[V(V)]~[Dye]. The pseudo-firstorderconstant(k1)wascalculatedfromthe slopeof thelinearplot.The valuesof k1 were determinedat varying[V(V)] keepingthe concentrationsof eitherof thedyesandH+ion constant. Plot of k1versus[V(V)]waslinearpassingthroughoriginshowingthat thereactionis firstorderwithrespecto V(V) (Fig. 1).The ratedatain acidsolutionsof constantionicstrengthyield a linear plot of k1 against[H+]2inboth perchloricand sulphuricacidmedia(Fig. 2).However, a positive interceptwas observedinboththeacidsindicatingthat thereactioninvolvestwopaths,onewitharateindependentofH+ionandtheotherwith a ratedirectlyproportionalto [H+]2.In the calculationof [HT]in H2S04 mediumwhichhas a low seconddissociationcomtantof 0·0097any contribution by HSO, has been neglectedbecauseit does not introduceappreciableerror.Thussulphuricacidwasalsoregardedasamonobasicacid like perchloricacid and the ionic strengthintherunswasmaintainedconstantbyaddingNaHS04-The resultsindicatethat the reactionis muchslower in perchloricacid than in sulphuricacidmedium. Kinetic runs werealso madeby addingsodiumbisulphateor sodiumchloridemaintainingtheionicstrengthconstantwith sodiumperchlorate.A linear increaseof rate with [HSO,] or [Cl-] isobservedin the caseof oxidationof both the dyes.leavingan intercepton k1 axis (Fig. 3). In thecaseof dihydrogenphosphateion plot of k1versus.11111 II;!'SUBBA RAO er al.: V(V) OXIDATION OF METHYL ORANGE & METHYL RED17,·6VO'++ HaO+VO++ -N=N-6- H+O~o,.,YO<S+-N=N- -H+VO;+ 2H+32'22824I~20'c;:'e;:;- 16Q)(.; 12FastVO; + VO++ 2H+_ 2VO++ +HaOScheme1- Mechanismof V(V) oxidationof dyesThis mechanisminvolvestwo-electrontransfer(O-transfer),VeIn) formedbeingrapidlyoxidizedbyV(V) to V(IV). The suggestedmechanismappearsmoreprobablebecauseamechanisminvolvingone-electrontransfermust lead to free radical inter-mediatesforwhich,however,thetestgaveanegativeresult. The mechanismleadsto the rate-law.:-d(V(V)] =k'[VO"'J[Dye]+k'Kl[VO}J[H+JIl[Dye]dt,.. +VOl + -N =N- __ vo·+ -N=N-H+ HIO 6- H++ FastVO; + -N = N- -_VO+ + -NO + -NO6- H+0·2 0·4 0·6 0.8 100 1.2[HSO;] OR [CI1,MFig. 3- Plotsof kl versus[HSO&](0) and[CI-](8) {[vMl= 1·0X to-1M;[methylorange]= 2·4X 10'IM; [HCIOJ =2'OM) and [CIO&]=4·0M (curve1); and [HCIOJ =3-SMand[CIO~= 4'SM (curve2); temp.=30°±o-l°}asVOt, V(OH)i+or VOI+relatedby theequilibria(1 and 2).VO;+HT+HaO~V(OH)i+ 0)VO;+2H+~VOI++HzO (2)The hydrogenion-independentpath presumablyinvolvesVOt, andthe lineardependenceof rateon[H+].rulesoutequilibrium(1). Thusthehydrogenion-dependentpathinvolvesVOI-f. Methylorangeand methylred havingpK valuesof 3·46and 5·0canbeconsideredto existin protonatedformsunderthe acid conditionsemployed.On the basis ofresultsgathereda mechanismshownin Scheme1maybe proposed.4403·02·0,·0-8.017.0!7'0 6·06'0 5,~5·EE'-'.•••4·01Q)(-43·01I2·01!,·IH2PO&]waslinearpassingthroughoriginshowingdirectdependenceof rate on [HzPO,J.RlJO.ctivespecies ofV(V)- In mineralacidsolutionsbelow pH 2·2 V(V) is known to be presentas VO; andat higheracid concentrationsV(OH)i+andV03+(r~f.6,7)werealsosupposedtobeformed5.In aqueousperchloricacidmediumcomplexationofthesespecieswith perchlorateion maybeneglectedthoughin sulphuricacidmedium,,8,VOt is believedto existasVO.HSO". Therefore,in perchloricacidmediumtheactivespeciesof V(V) mayberegardedFig. 2- Plotsof kl versus[H+]·{[V(V)]= t·O X to 1M'[methylorange]=2·4X to-1M; [CIO~=3·0M (curvet)and [HSO&]= 3·0M (curve 3). [V(V)]= 1·0X to-I M(methyl red]= 1·4x to-1M; [CIO~=2·0M (curve 2)[HSOt]= 2·0M (curve4); temp.= 30°±0'1°}.10 20 30(VANADIUM(V>}X 104,MFig. t - Plotsof kl versus[V(V)] {For curvet. [methyl<>range]= 2·4X to-1M; [H.SOJ =O'SM; [HS04]= O·SM.For curve2 [methylred]= 2·4X to-1M;[H.SOJ = O'SM;[HSO,]=O'SM; temp.=30°±0'1°}INDIAN J. CHEM., VOL. 15A, JANUARY 1977ITABL~ 1- RELATIVE ACCELERAtINGEFFECTS OF ANIONS{[V(V)]=1·0 x 10-3M; [HClOJ =4'OM; t-l =4·0;[Anion] =0'2M; temp.=30o±0'1°}A~on Cl- HSO,& H2PO~ No addedanionTh~rate-lawexplainsthe linear dependenceofrateqn [H+J2with a p.Jsitiveinte~ceptonrateaxis.The ~aluesk' andkWKl for theoxidaticmof methyloran~ andmethylredwerefoundto be 0,05,0,06(!lo, 3.p and temp.,30°)and 0'5, 0,85 (IL, 2·0 andtemp" 30°)respectively.TM lineardecreaseofrateonaddedHSO" H"PO,and Ct!- suggeststhat the speciesVO"HSO",VO"H2PO"ajndV02Cl aTebetteroxidantsthanVO~orV03+.The resultsin Table1 indicatethe followingorderof acceleratingeffect:~P04>HSO,>Cl- perhapsdue]:J theincreasein thestabilityof thecomplexesV02HkPO,,>V02RSO,,>VO"el. It is interestingtonote that the stabilitiesof chloride,sulphateandphosPlhatecomplexesof iron(III) are also in thesamerrders. ThecomplexformationbetweenV03+andthes anionswasnot consideredbecauseV03+isnot s,p.pp)sedto be presentin appreciableconcen-tratiopin comparisonwithVO;.In ~ulphuricacid mediumor in the presenceofHSO&lH2PO, andCl- ions,therefore,the followingstepsJrnustalsobe consideredin additionto thosepostulatedin perchloricacidmedium(Scheme1):! K,VOt+A- ~ V02A whereA-=HSO" H2PO, or el-k'" +VO~A+-N =N~VO++-N=N -+A-i IH+ O· H+Th~ratewill nowbedt~}V)J=k'[VOtl[Dye)+k"Kl[VqJ[H+J2[DyeJ+K2k'[VOt][Dye}[HSO,J4·61 4-84 12·67 4·22Lineardependenceof therate on [(H+)J2with aninterceptshowsthatinadditiontoYO; andV02HSO",V03+alsoastheoxidizingspeciesinsulphuric acidmedium.The valuesof k"K2 in the presenceof HSO, fortheoxidationof methylorangeandmethylredwerefoundto be 3'0 (fL=4'0and temp.30·C)and 2·0(1l=3'0andtemp.30°C)respectively.As hasalreadybeenstatedthe plot of kl versus[H2PO,,]is linearwithoutanyinterceptwhereastheplotsof k1versus[HSO,J or [CI-J gave intercepts.This showsthatin therate-lawk"K" in the caseofH2PO, may be so high that the other terms inrate-lawcan be neglected.This showsthe highcatalytic activity of H2PO. in comparisonwithCl- or HSO,.The energyof activation and entrepyof acti-vation for the oxidation of methyl orangeandmethyl red calculatedfrom Arrheniusplots are:12·0±1'0 kcaljmole,-39 ±1 cal deg-lmole-land14,0±1·0 kcaljmole, -32 ±1 cal deg-l mole-lrespectively.AcknowledgementTwo of theauthors(R.V.S.M. andK.S.M.) thankthe CSIR, New Delhi, and the AndhraUniversityfor theawardof researchfellowships.References1. GOPALARAo, G., RAO, V. P. & MURTY, B. V. S. R .• Z.analyt. Chem.,147 (1955),161.2. KNECHT, E. & HIBBERT, E., New yeductionmethodsin volumetricanalysis (Longmans, Green, London)"1925.3. FEIGL, F., Spot tests(Elsevier,New York), 1954,120.4. WATERS,W. A. & LITTLER, J. 5., J. chem.Soc., (1959),1299.5. ROSSOTTI,F. J. C. & ROSSOTTI,R., Acta chem.scand.,10(1956),957.6. LITTLER, J. S. & WATERS,W. A., J. chem.Soc., (1959),4046.7. WEST,D. M. & SKOOG,D. A., J. Am. chem.Soc.,82 (1960),280.8. MEHROTRA,R. N., Indian J. Chem.,12 (1974),365.9. YATSIMIRSKII, K. R, Instability constantsof comPlexcom-pounds (PergamonPress, Oxford), 1960,102.181111'1

Kinetics of Oxidation of Methyl Orange & Methyl Red by V(V)*

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Kinetics of Oxidation of Methyl Orange & Methyl Red by V(V)*

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