Electrochemical oxidation of oxalic acid and hydrazinium nitrate on platinum in nitric acid media

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_{Rockombeny, L.C. and Féraud, Jean-Pierre and}

Queffelec, Benoit and Ode, Denis and Tzedakis, Theodore (2011)

Electrochemical oxidation of oxalic acid and hydrazinium nitrate on

platinum in nitric acid media. Electrochimica Acta, vol. 66. pp. 230-238.

ISSN 0013-4686



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Electrochemical

oxidation

of

oxalic

acid

and

hydrazinium

nitrate

on

platinum

in

nitric

acid

media

L.C.

Rockombeny

a

_,

_J.P.

_Féraud

a

_,

_B.

_Queffelec

b

_,

_D.

_Ode

a

_,

_T.

_Tzedakis

c,∗ a_{Commissariatàl’ÉnergieAtomique/Marcoule,DTEC/SGCS/LGCI,BP17171,30207BagnolssurCèze,France} b_{SIER,16AvenuedupetitLac,95210St.Gratien,France}

c_{LaboratoiredeGénieChimique,UMR5503CNRS,UniversitéPaulSabatier,118,routedeNarbonne,31062Toulouse,France}

a

b

s

t

r

a

c

t

Severalstudiesintheliteraturehaveinvestigatedtheelectrochemicaleffectsofoxalicacidandhydrazine

onvariousmaterialsinneutral(pHbufferedto7),basicorweaklyacidicmedia(pH6).Thepresentwork

proposeselectrochemicaltechniquesthatallowforthestudyoftheelectrochemicalbehavior,onaPt

electrode,ofoxalicacidandhydraziniumnitratetobetterunderstandtheiroxidationmechanismsin

anitricacidmediumatapHbelow1;inaddition,someexperimentswerecarriedouttodefinean

electrochemicalmethodthatwouldallowforthesimultaneousdetectionofthesespecieswhenpresent

withinprocesseffluentinveryacidicsolutions.Somephysicaldataregardingoxalicacidandhydrazinium

nitratewerealsodetermined:anodicoxidationofhydraziniumnitrateandoxalicacidwereobservedat

0.2Vand0.7V(vs.Ag/AgCl),respectively.Thediffusioncoefficientsofhydraziniumnitrateandoxalic

acidwerefoundtobe5.2×10−6_and2.9×10−7_cm2_s−1_{,respectively.Anexperimentaldesignapproach}

demonstratedtheinfluenceofnitricacidconcentrationsonthediffusioncoefficientsofthesespecies.

1. Introduction

Numerouspapershaveaddressedtheelectrochemicalbehavior ofoxalicacidandhydrazine[1–20],oftenwithrespecttobiology. Oxalicacidandhydrazinearedetectedbyelectrochemicalmethods inaqueoussolutionforpHlevelsrangingfrom6to7.

Worksontheoxidationofoxalicacidhavefocusedon design-inganoxalate-specificsensor;theyhavestudiedthemechanism ofdegradationduringanodicoxidation.Cyclicorlinear voltamme-try[1,2,4–7,9–11]andchronoamperometry[3–5,8]arethemain electrochemicaltechniquesthathavebeenused.Variousmassive materials(Pt [1],glassy carbon[1,4],etc.)and modified metals (dimensionallystableanodes,DSA,TicoatedbyPtorIrO2orRuO2

[1],boron-dopeddiamond[7],etc.)areusedasanodes.Graphite modifiedbypalladiumnanoparticles[4,6]andrhodium[8,11]have alsobeenexaminedascatalystsfortheelectrochemicaloxidation ofoxalicacid.Theresultsshowthattheoxidationpotentialofoxalic acidisinfluencedbythetypeofelectrodeusedaswellasits sur-faceand composition.Adsorption(especially onPt),passivation andinteractionphenomenacaninfluencetherateofthisreaction. Cyclic voltammetry[12–18], amperometricdetection and/or differential pulse voltammetry [12] have been used to study

∗Correspondingauthor.Tel.:+33561558302;fax:+33561556139. E-mailaddress:tzedakis@chimie.ups-tlse.fr(T.Tzedakis).

theoxidationofhydrazineonmodifiedglassycarbonelectrodes [12–16].Zincoxide[17]andcarbonnanotubes[18]havealsobeen usedforthedevelopmentofahydrazineelectrochemicalsensor.As foroxalicacid,theoxidationofhydrazinemaybeinfluencedbythe typeofelectrodeusedaswellasitssurfacestateandcomposition andsolutionpH.

Abibliographicalreviewhasrevealednostudiesinvestigating thedevelopmentofamethodfordetectingbothoxalicacidand hydrazinepresentinthesamemedium.

Thepurposeofthepresentworkwastocarryoutvoltammetric measurementsonoxalicacidandhydraziniumnitrateona plat-inumelectrodeinaconcentratednitricacidmediumtoobtaina betterunderstandingoftheiroxidationmechanismsandtodevelop anelectrochemicalmethodfortheirsimultaneousdetectionwhen presentwithinprocesseffluentinveryacidicsolutions.Some phys-icaldataregardingoxalicacidandhydraziniumnitratewerealso obtained.

2. Experimentalprocedures

Thechemicalsusedwereofanalytical grade(purity>98.5%). SolidoxalicacidwasprovidedbySigmaAldrich(CAS:61353566). Nitric acid possessed a density =1.42gcm−3 _{(purity 65%).}

Hydraziniumnitratewasalaboratory-preparedsolution. Concen-tratednitricacidwasaddedtohydrazinehydrate(CAS:78035378) until complete neutralization at pH 4.5, forming hydrazinium

Fig.1.[Ox.ac.]oroxalicacid’sconcentrationdependenceonthecurrent–potentialcurves,obtainedonPtrotatingdisk.ω=1000RPM:supportingelectrolyte2MHNO3;(1)

nooxalicacid;(2)1mM;(3)5mM;(4)10mM;(5)25mM;(6)50mM;(7)75mM;(8)100mM;potentialscanrate:0.005Vs−1.Inset:thevariationofanodiccurrentat1.1V vs.theoxalicacidconcentration.

nitrate;thisreactionishighlyexothermicandcanbeexplosive. Par-ticularcautionisrequiredwhenusinghydrazinehydratebecause itisaCMR(carcinogenic,mutagenicandreprotoxic)substance.

Allelectrochemicalmeasurementswerecarriedoutusingan AutolabPGSTAT30potentiostat/galvanostatcontrolledwithGPES 4.9softwareandathree-electrodesetupwitha saturatedsilver referenceelectrode(Ag/AgCl,3MKCl),aplatinumwire counter-electrodeandaplatinum-diskworkingelectrode.

3. Resultsanddiscussion

3.1. ElectrochemicalkineticsonPtrotatingdiskelectrode 3.1.1. Oxalicacid

Linearvoltammograms,obtainedatthesteadystate(5mVs−1)

withaPtrotating-diskelectrode(ω=1000RPM)andvarious con-centrationsof oxalic acidin 2MHNO3, are shown in Fig. 1. A

signalindicating the oxidation ofoxalic acidtocarbon dioxide onPtaccordingtoequation(3.1.1.1)wasobservedforpotentials higherthan0.7V;simultaneously,bubblesappearedatthe elec-trodesurface.Forlowconcentrations(curves(1)–(3)),theobtained signalshowsapracticallyconstant‘limitingcurrent’,withaslight decreaseforpotentialshigherthan1.4V.Forconcentrationsabove 10mM,apeak-shapedcurvewasobtainedandthemagnitudeofthe currentdecreasednearlytozeroforpotentialshigherthan_∼1.2V; partialandreversiblepassivationoftheplatinumelectrodecaused bythegaseouscarbondioxideproducedontheelectrodesurface couldexplainthisdecrease.Indeed,allofthecurvesobtainedcanbe reproducedwithoutmechanicaltreatment.Theoxidationreaction canbewrittenasfollows:

HCOO COOH_→ 2CO2+2H++2e− (3.1.1.1) Alinear dependence oftheanodic current(Imaxrecorded at

1.1V) vs. the oxalic acid concentration observed in the range 1–100mM(inset,Fig.1)reflectsamass-transferlimitation,even ifthemagnitudeofthecurrentdecreasesforhigherpotentials:

Imax at 1.1V (A)=1.77×10−5[Ox.Ac.]_(mmolL−1₎,

where R2=0.9975.

Tafelplots (lni=lni0+˛nF/RT)forcurves obtainedin 1mM

oxalicacidwereusedtodeterminetheexchange-currentdensityi0,

theelectron-transfercoefficient˛andtheintrinsicheterogeneous electron-transfer coefficient k0 _(cms−1_{). The exchange-current}

density (i0=nFk0C0) wasfound tobe 2×10−7_Acm2_{. Assuming}

n=2,˛andk0_{werefoundtobe0.3and10}−6cms−1,respectively.

Thevalueof k0 _{indicates thatoxalic acidcan beconsideredan}

irreversiblesystem. 3.1.2. Hydraziniumnitrate

LinearvoltammogramsobtainedatthesteadystatewithaPt rotating-diskelectrodeandvariousconcentrationsofhydrazinium nitratein2MHNO₃areshowninFig.2(a).

ConsideringthatthefinaloxidationproductisN2,theoverall

oxidationreactionofhydraziniumnitratecanbewrittenasfollows: N2H+5 →N2+5H++4e− (3.1.2.1)

Thefollowingresultswereobtained:

• Forhydraziniumnitrateconcentrationslowerthan10mM, classi-callyshapecurveswereobtained,containingonewavebeginning at∼0.2–0.4V.Thewave“plateau”withalimitingcurrentclearly indicates amass-transfer limitation(diffusionlimitation). The magnitude of its limiting current increases linearlywith the hydraziniumnitrateconcentrationsaccordingtotherelationship (3.1.2.2).

I/A=7.37×10−5·[H Nitrate] R2=0.9991 (3.1.2.2) • Forhigherconcentrationsofhydraziniumnitrate(50–75mM),

thecurvesindicatethreeimportantmodifications:

-ThebeginningoftheI/Ecurveforhydraziniumnitrateoxidation shiftstolowerpotentials whentheconcentrationincreases, meaningthatthecorrespondingelectroactivespeciesis oxi-dizedmoreeasily.

-Increasingthehydraziniumnitrateconcentrationcausestwo signalstoappear,at0.2–0.6Vand0.7–1.2V.

-Afterthepotentialofthesecondsignalwasreached,the mag-nitudeofthecurrentdecreaseindicatesthepassivationofthe platinumelectrode.Nevertheless,thecurvesobtainedcanbe reproducedwithoutmechanicaltreatment,whichsuggeststhat this passivationis reversible.Thefinal oxidation product of hydraziniumnitrateisgaseous nitrogenandthepresenceof agaseouslayerattheinterfacecouldexplainthedecreasein themagnitudeofthecurrent.

Twopossibleexplanationscouldjustifythisbehavior:

Assumingthatoneelectronwasexchangedthroughan elemen-tarystep,forhydrazinium nitrateoxidation,theglobalreaction

Fig.2.(a)Hydraziniumnitrate’sconcentrationdependenceonthecurrent–potentialcurvesobtainedonPtrotatingdisk;ω=1000RPM:supportingelectrolyte2MHNO3;

(1)nohydraziniumnitrate;(2)1mM;(3)5mM;(4)10mM;(5)25mM;(6)50mM;(7)75mM;potentialscanrate:0.005Vs−1.(b)Hydraziniumnitrateconcentration dependenceofthemaximumanodiccurrent(1.1V).

(3.1.2.1)canbedecomposedaccordingtothefollowingsimplified scheme: H2N NH3+→H2N NH2++e– +H+ (3.1.2.1.a) H2N NH2+→HN NH2++e– +H+ (3.1.2.1.b) HN NH2+↔ HN NH+H+ (3.1.2.1.c) HN NH _→ HN NH+_{+e– (3.1.2.1.d)} HN NH+_{→N2+e– +2H}+ _(3.1.2.1.e)

(1)Forhydraziniumnitrateconcentrationslowerthan10mM,the curvesshowonlyonewave;thus,wecanassumethatthefirst electronicexchange((3.1.2.1.a)and/or(3.1.2.1.b))wasthe lim-itingstep;thismaycorrespondto1or2electronsexchanged andundertheseconditions,theexchangeoftheremaining elec-tronstoobtainN2((3.1.2.1.b)or(3.1.2.1.c)to(3.1.2.1.d))occurs,

during‘faster’steps.

Whenthehydraziniumnitrateconcentrationincreases,high current values ledto higher intermediate (HN NH2+)

con-centrations. In addition, high H+ _{concentrations, could be}

disadvantageousfortheequilibrium(3.1.2.1.c),whichmaythen causetheintermediate(HN NH2+)toaccumulate.Underthese

conditions,theoxidationoftheseintermediates(HN NH₂+_)to

nitrogenmayoccurathigherpotentials(0.7–1.2V)andleadto thesecondsignal.

(2)Moisy et al. [21] presented the following chemical equilib-riumbetweenHNO3andhydraziniumnitrateforstronglyacidic

media:

N2H+₅+HNO3↔N2H2+₆ +NO−3 (3.1.2.3)

Forhydraziniumnitrateconcentrationslowerthan10mM, thesolutioncontainsmainlyN₂H5+;thus,thecurvesshowonly

onewaveindicatingtheoxidationtonitrogenaccordingtothe previousstatements.Whenthehydraziniumnitrate concen-trationincreases,theequilibrium(3.1.2.3)shiftstotheright and N2H62+appears;itsoxidationatmoreanodicpotentials

(0.7–1.2V)canbeobservedseparatelyfromthewaveofN2H5+

oxidation.

Nevertheless,considering eitherexplanation((1) or(2)),the dependenceofthemaximumcurrent(at1.1V)onthehydrazinium nitrate concentration for concentrations higher than 10mM (Fig.2b)seemstobequasi-linearinspiteofacertaindispersion ofthepoints.

Icurrentat1.1VA=−6.13×10−5[HNitrate]mmolL−1, R2=0.979

(3.1.2.4)

Thislinearevolutionappearstobe‘normal’becausethefinal product(N2)and,consequently,theoverallelectronnumber(4e−)

are the same. In addition, according to either the first or sec-ondexplanation,theoverallconcentrationofhydraziniumnitrate remainsthesame;therefore,theoverallcurrent(at1.1V)changes linearlywiththeconcentration,inspiteofthepartialpassivation oftheelectrode.

Tafelplots(lni=lni0+˛nF/RT)forthecurvesobtainedin1mM

hydraziniumnitratewerealsousedtodeterminei0,˛andk0.i0was

foundtobe4.4_×10−5Acm2,assuming_n=2;_˛and_k0werefoundto

be0.2and10−4_cms−1_{,respectively.Thisvalueofk}0_indicatesthe

slightlyirreversibleelectrochemicalbehaviorofthehydrazinium nitrate/nitrogensystem.

3.2. Electrochemicalcharacterizationofthesystemsby transient-statecyclicvoltammetryandsteady-statelinear voltammetry

3.2.1. Oxalicacid

Toobtainabetterunderstandingoftheoxalicacidoxidation mechanism,variouscyclicvoltammogramswererecorded with-outstirringatdifferentpotentialscanrates.ThePt-diskanodewas immersedina5mMsolutionofoxalicacidcontaining2Mnitric acid.Theanalysisofthevoltammograms(Fig.3(a))showsthatthe magnitudeofthenetcurrentoftheanodicpeakobservedat∼1V increaseslinearlywiththesquarerootofthepotentialscanrate (Fig.3(b)).ThisfindingindicatesthatoxidationonPtisnot lim-itedbyadsorptionphenomena[1]butratherisdiffusionlimited. Ontheotherhand,theanodiccurvecontainsonlyonesignal, indi-catingthatthetwoelectronsareexchangedsimultaneously.The cathodicpeaksatE=0.4Vwerealsoobtainedduringthestudyof thesupportingelectrolyteandarenotduetotheoxalicacid/carbon dioxidesystem.

Therepresentationofthepotentialoftheanodicpeak(E_peak) vs. thelogarithm of thepotential scan rateis indicative ofthe reversibilityofthesystem[22].Forasimpleelectrochemical sys-tem,E_peakvarieslinearlywithlnrinaccordancewith

Epeak=E0−_˛ R×T ×n˛×F



0.78+lnD 0.5 k0 +ln ˛×n˛×F×r0.5 R×T



(3.2.1.1) whereE0_{isthestandardpotentialoftheelectroactivesystem(V),}

n˛isthenumberofelectronsexchangedinthecharge-transferstep,

k0_{istheintrinsicheterogeneouselectronictransferrateconstant}

(cms−1_{),Risthegasconstant(8.31Jmol}−1_K−1_{),Tistheabsolute}

-0.0001 -0.00008 -0.00006 -0.00004 -0.00002 0 0.00002 0.00004 0.00006 0.00008 0.0001 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

E (Volt vs SCE)

I / A

I = 0.0002.r

1/2

R

2

= 0.9992

0 0.00001 0.00002 0.00003 0.00004 0.00005 0.00006 0.00007 0.4 0.3 0.2 0.1 0

r

1/2

/ (V/s)

1/2

Ip / A

Ep = 0.018.ln(r) + 0.88 R2 = 0.992 0.93 0.935 0.94 0.945 0.95 0.955 0.96 0.965 0.97 0.975 5.5 4.5 3.5 2.5 ln (r) Ep / V

(a)

(b)

(c)

(5) (4) (3) (2) (1)

Fig.3.(a)PotentialscanratedependenceontheshapeofcyclicvoltammogrammsobtainedonaPtrotatingdiskelectrode(S=0.125cm2_{),immersedin5mMoxalicacid:}

supportingelectrolyte2MHNO3withoutstirring;(1)20mVs−1;(2)50mVs−1;(3)75mVs−1;(4)100mVs−1;(5)150mVs−1;(b)theanodicpeakcurrentIpeakvs.r1/2and(c)

dependenceofthepeakpotentialEpeakvs.ln(r).

Fig. 3(c) shows the linear evolution of Epeak vs. lnr

(E_peak(V)=0.881+0.018lnr_(Vs) with R2_{=0.992), indicating that}

theoxidation ofoxalicacidtocarbon dioxideisanirreversible process.Theslopeofthisline,(RT)/(2˛n˛F)=0.018,indicatesthat

˛×n˛≈0.7.Becausetwoelectronsareexchanged,˛≈0.4.Onthe otherhand,thediffusivityofoxalicacidandthestandardpotential E0_{mustbeknowntodeterminek}0_{fromtheY-interceptofthisline:}

0.881=E0+_˛ R×T ×n˛×F



0.78+lnD 0.5 k0 +0.5ln ˛×n˛×F R×T



. Inthisstudy,thevalue0.7Vvs.Ag/AgClnearEI=0,inagreement

withFig.4(a),waschosenasE0_toestimatek0_.Underthese

con-ditions,thediffusioncoefficient(Doxalicacid)canbedeterminedby

theLevichequation(3.2.1.2):

Ilim,i=0.62×n×S×F×D2/3_i ×ω1/2×−1/6_×Csol

i [22](3.2.1.2)

whereIlim,iistheanodiclimitingcurrent,nistheoverallnumberof

electronsinvolved,C_isolistheconcentrationofspeciesiinsolution (molm−3_{),ωistherotationalspeedoftheelectrode(rads}−1_),is

thekinematicviscosityofthesolution(m2_s−1_)andSisthe

geo-metricsurfaceareaoftheworkingelectrode(m2_)immersedina

5mMsolutionofoxalicacid.Severallinearvoltammogramswere recordedatvariousangularvelocitiesωandthepotentialscanrate was0.005Vs−1_(Fig.4(a)).

Theincreaseintheanodiclimitingcurrentwithωreflectsan increaseinthefluxoftheelectroactivespeciesattheworking elec-trodeinterface.Theplotoftheanodiclimitingcurrents(Ilim)vs.the

squarerootoftherotationalspeed(√ω)inFig.4(b)showsalinear evolutionandallowsD_oxalicacidtobedetermined.UsingtheLevich equation(3.2.1.2)and consideringthat n=2e−_,S=0.03cm2 _and

=10−2_cm2_s−1_{,Doxalicacidwasfoundtobe2.9×10}−7_cm2_s−1_.For

comparison,chronoamperometry(Cottrellequation[22])and/or voltammetry(Randles–Sevcikequation[22])studiesconductedat

neutralpH[6,7,19]revealedvaluesofD_oxalicacidintherange10−8

to10−5_cm2_s−1_{.The wide dispersionofthe Doxalicacid values is}

surprisingandsuggestspossibleinteractionsbetweenoxalicacid moleculesand/orbetweenoxalicacidandnitricacidmolecules.

Consideringthediffusivity ofoxalic acid(2.9_×10−7_cm2_s−1₎

andthereportedvalueofE0 _{foroxalicacid,theinterceptofthe}

straightline(Fig.3(c))showsthatk0

≈1.6×10−6cms−1.Thisvalue

isvery sensitivebecauseofthehighuncertaintyin the estima-tionoftheintercept(Fig.3(c)),butitverifiesthevalue10−6_cms−1

obtainedwithTafel’sLawinSection3.1.1.Astudyofthesensitivity oftheoxalicaciddiffusioncoefficientdependingontheoperating conditionsisdetailedbelow.

3.2.2. Hydraziniumnitrate

Severalcyclicvoltammogramswererecordedoveraverywide range ofpotentialscan rates (20mVs−1 to27.5Vs−1)tobetter

understandthemechanismoftheanodicoxidationofhydrazinium nitrate.The potentiostat used wasnot compatible with poten-tialscanrateshigherthan27.5Vs−1.Theworkingelectrodewas

immersedina5mMsolutionofhydraziniumnitratein2Mnitric acid. The resulting cyclic voltammograms (Fig. 5) indicate an increase inthemagnitude ofthecurrent of theoxidation peak observedatE=0.4–0.5V.Thecathodicpeakwasalsoobtained dur-ingthestudyofthesupportingelectrolyteandthuswasnotdueto thehydraziniumnitrate/nitrogensystem.Forpotentialscanrates higherthan 3Vs−1_{,a second signalappearsat 0.8V, indicating}

thattheoxidationofhydraziniumnitrateonPtinvolvesatleast twosteps,witharelativelystableintermediate.Athirdoxidation shoulderappearsat_∼1.2Vwhenrexceeds20Vs−1_.Hydrazinium

nitrateisoxidizedonPtintwoorthreesuccessivestepsandtends toconfirmthemechanism(3.1.2.1a)–(3.1.2.1e).

Themagnitudeof theoxidation peakcurrent I_peak is plotted vs. √r in Fig.6(a).For potentialscan rates ranging from20 to 200mVs−1_{,Ipeak increases linearlywith √r, indicating that the}

Fig.4.(a)InfluenceoftheangularvelocityofthePtrotatingdisk(S=0.125cm2_{)anodeonthelinearvoltammogramms,obtainedatthesteadystate(potentialscanrate:}

0.005Vs−1).Oxalicacid5mMin2MHNO3;(1)250rpm,(2)500rpm,(3)750rpm,(4)1000rpm,(5)1250rpm,(6)2000rpm;(b)thevariationofanodiclimitingcurrentsvs.

ω1/2_.

oxidationofhydraziniumnitrateonPtislimitedbydiffusion.For higherpotentialscanrates(intherange0.2–27.5Vs−1_),theslope

of thestraight linedecreasestothehalfof thepreviousvalue, indicatingthatmasstransferremainsthelimitingphenomenon. Inaddition,thenumberofexchangedelectronswasdividedby2. Apossibleexplanationcouldbethatafirstbi-electronicoxidation occurs for high values of r, followed by a second bi-electronic oxidationoftheintermediateelectrogeneratedattheelectrode.

Fig.6(b)presentstheevolutionofthepeakpotentialvs.the log-arithmofthepotentialscanrate.Twodifferentevolutionscanbe observed:

Fig.5. Influenceofthepotentialscanrateontheshapeofcyclicvoltammogramms obtainedonPtrotatingdisk,immersedin5mMhydraziniumnitratein2MHNO3;

(a)(1)20mVs−1;(2)50mVs−1;(3)75mVs−1;(4)100mVs−1;(5)150mVs−1;(6) 175mVs−1;(7)200mVs−1;(b)(7)200mVs−1;(8)800mVs−1;(9)3.2Vs−1;(10) 12.8Vs−1;(11)27.5Vs−1.

(i) Forpotentialscanrateslowerthan1Vs−1_{,theincreaseofthe}

peak potentialvs. lnrwasvery slow (Epeak(V)=0.345+0.011

ln r_(Vs−1₎; R2=0.81), meaning that the corresponding

elec-troactivesystemis practicallyreversible undertheselected operating conditions. A similar treatment of oxalic acid (the diffusivity is discussed in next section) shows that k0

≈8×10−3cms−1 and˛×n˛≈1.Thevalueofk0 confirms theslightlyirreversibleelectrochemicalnatureofthissystem. (ii)For potential scan rates higher than 1Vs−1, Fig. 6(b)

shows a linear evolution of the peak potential vs. lnr (Epeak(V)=0.22+0.0321 lnr_(Vs−1₎; R2=0.94). The ratio ofthe

slopesforevolutions(i)and(ii)isequalto3,whichisrelated toalowervalueof˛×n˛forthecorrespondingelectroactive

system.Consideringthatonlythefirstbi-electronicoxidationof hydraziniumnitratewasobservedforhighrvalues,thevalueof n˛in˛× n˛couldbelowerandmayprovideapossible

expla-nationfortheobservedresults.Ontheotherhand,operating athighpotentialscanratesleadstoI/Ecurvesbeingstrongly

y = 1.34.10-3.r1/2 R2 = 0.994 y = 5.69.10-4.r1/2 + 8.86.10-4 R2 = 0.995 0.0E+00 1.0E-03 2.0E-03 3.0E-03 4.0E-03 5 4 3 2 1 0

r

1/2

/ (V/s)

1/2

Ip

/

A

y = 1.09.10-2.ln(r) + 0.345 R2 = 0.813 y = 3.21.10-2_{.ln(r) + 0.222} R2 = 0.939 0.3 0.4 0.5 0.6 10 8 6 4 2

ln (r)

E

p

(

V

)

v

s

S

C

E

(a)

(b)

Fig.6. (a)EvolutionoftheanodicpeakcurrentIpeakvs.r1/2;

Fig.7.(a)InfluenceoftheangularvelocityofthePtrotatingdiskontheshapeofcurrent–potentialcurves,obtainedwith5mMhydraziniumnitratein2MHNO3.(1)250rpm;

(2)500rpm;(3)750rpm;(4)1500rpm;(b)thevariationofanodicmaximumcurrent(1–1.1V)vs.ω0.5_.

influencedbytheohmicdropcausedbythesolutionbetween theworkingandreferenceelectrodes.Toobtainthetruecurves, theohmicdrophastobecompensateddynamically; unfortu-nately,theinstrumentsthatareavailableareunabletoperform thisoperation.

Toevaluatethediffusivityofhydraziniumnitrate, voltammo-gramswereplottedatthesteadystateforvariousangularvelocities ofthePt-diskanode(Fig.7a);theworkingelectrodewasimmersed ina5mMsolutionofhydraziniumnitrateandthepotentialscan ratewas0.005Vs−1_{.Strongoscillations(noise)inthelimitingpart}

ofthecurrentpotentialcurveswerecausedbynitrogenbubbles. Forlowangularvelocities,thecurvesshowonesignalcontaininga diffusionwaveatpotentialsrangingfrom0.5to1.5V.Forangular velocitieshigherthan750rpm,thecurvesclearlyindicateafirst sig-nal(0.2–0.6V)withaplateauat0.55V,followedbyasecondsignal forpotentialshigherthan0.7V.Evenifthemagnitudeofthissecond signaldecreasesforpotentialsabove1.2V,thisdropdoesnot irre-versiblyaffecttheelectrodesurfaceandthesecurvescanbe repro-ducedwithoutanyelectrodetreatment.Apossibleexplanationfor thecurrentdecreasecouldbethe“masking”oftheelectrodebythe largeamountofnitrogenelectrochemicallygeneratedattheanode. Thepreviously discussed acid–baseequilibrium (cf (3.1.2.1)) allowsvariousformsofhydraziniumnitratetoappearandcould explainthepresenceoftwoverydistinctsignals.Inaddition,at highstirringrates,thehighcurrentamplituderesultedinlarge nitrogenbubbles,maskingthedisksurface.Intermediateformsof hydraziniumnitratecouldthenaccumulateattheinterfaceand theiroxidationcouldbeseparatelyobserved.

Alinearevolutionofthelimitingcurrent(at0.7V)vs.thesquare rootoftheangularvelocity(Fig.7(b))wasobserved,meaningthat thecurrentwaslimitedbymass-transferphenomena.According totheLevichequation(3.2.1.2)andtakingintoaccountthatn=4, S=0.03cm2 _{and =10}−2cm2s−1, the value of _{Dhydraziniumnitrate}

wasfound to be5.2_×10−6_cm2_s−1_{.For comparison, studies of}

theoxidationofhydrazine inseveralmedia[12,15,18]reported valuesofDhydrazinerangingfrom2.5×10−6to8.2_×10−6cm2s−1,

similartothevalueofD_{hydraziniumnitrate}foundinthiswork. How-ever,hydraziniumnitrateexhibitscomplexchemistryinnitricacid media[21];astudyofthesensitivityofthisdiffusioncoefficient dependingontheoperatingconditionsisdetailedbelow.

3.3. Effectsoftheoperatingconditionsonthediffusion coefficients

The previous experiments demonstrate the high sensitivity of the measurement of the diffusion coefficients due to the

existence of molecular interactions and/or chemical equilibria, whichmaypossiblyberelatedtotheactivityofnitricacid.The approachdetailedbelowwasimplementedtostudythisinfluence inanexperimentaldesigncontext.Themethodsusedtoanalyzethe resultscanbeassociatedwithaphysical-responsemodelbasedon experimentalparameterswithaminimumnumberoftests. Var-iousexperimentaldesignswereimplementedandanalyzedwith thesoftwareprogramLumiere4.45©_.

3.3.1. Influenceofoxalicacidandnitricacidconcentrationson Doxalicacid

In this study, given the number of factors (2) to be stud-iedandassumingalinearexperimentalmodel,a2p_{full-factorial}

experimental design was used. The factors studied were the concentrations of oxalicacidand nitricacid; theresponse was D_oxalicacid determinedbychronoamperometryusingtheCottrell equationwithanappliedpotentialof1.1V.Theresultsare summa-rizedinTable1(a),whereX1istheoxalicacidcodedvariable,X2is

thenitricacidcodedvariableandK,theD(X1,X2)ratioofD(−1,−1),

isadimensionlessresponse.

Theanalysisofthisexperimentaldesignincodedvariablesand responseKprovidedthecorrelationmatrix(Table1(b)).The diago-nalof“1”showsthatthemaineffectsandsecond-orderinteractions donotoverlap.

Table1

(a)Effectofnitricacidonthediffusioncoefficientofoxalicacid;(b)correlation matrix;(c)histogramofeffects.

(a)

X1 X2 [Oxalic

acid](mM)

[Nitricacid](M) Doxalicacid(cm2s−1) K

−1 −1 1 0.2 1.7E−08 1 1 −1 10 0.2 1.2E−08 0.71 −1 1 1 2 5.3E−07 31.1 1 1 10 2 4.3E−07 25.6 (b) X1 X2 X1×X2 X1 1 X2 0 1 X1×X2 0 0 1 (c) Effects % X2 97.99 X1 1.11 X1×X2 0.9

Thecoefficientsofthepolynomialresponsevs.themaineffects andinteractionswerecalculatedbymultiplelinearregressionin theLumiere program.Table1(c)lists thecoefficientsaccording totheirmagnitudeinthepolynomial.Thistableshowsthe sig-nificantinfluenceofthenitricacidconcentrationinthemedium onDoxalicacid.Theplotofthispolynomialresponseasafunction

ofX1 andX2 indicatesthattheeffectoftheoxalicacid

concen-trationonD_oxalicacid islimited atlow nitricacidconcentrations (<1M)butincreasesforhighernitricacidconcentrations_(≥1M). Thisbehaviormayreflecttheexistenceofinteractions between oxalicacidmolecules,forexamplethehydrogenbond[20], depend-ingonthenitricacidconditionsinthemedium.Adeviationofless than15%betweenthemathematicalmodelandtheexperimental resultreflectsthesatisfactoryagreementbetweenthemodeland theexperiment.

3.3.2. Influenceofthehydraziniumnitrateandnitricacid concentrationsonD_{hydraziniumnitrate}

Although Dhydraziniumnitrate was consistent with the data

reportedintheliterature,itseemed interestingwithrespectto thestudyofoxalicacidtodeterminetheinfluenceoftwofactors, theconcentrationsofhydraziniumnitrateandnitricacid,onthe D_{hydraziniumnitrate}response.

According to Moisy et al. and given the existence of two N2H5+/N2H62+chemicalequilibriumconstants(Ka=0.09[23]and

Ka=0.4[24])from1Minnitricacid,threefullexperimentaldesigns

wereimplementedinthreelevels.Thus,thepeaks,themidpoint ofeachsideand thecenterofasquare definetheexperimental matrix.

TheD_{hydraziniumnitrate}responsewasdeterminedby chronoam-perometrywiththeCottrellequationforanappliedpotentialof 0.75V.TheresultsaresummarizedinTable2(a)–(c),whereX1is

thehydraziniumnitratecodedvariable,X2isthenitricacidcoded

variableandK,theD(X1;X2)ratioofD(−1,−1),isadimensionless

response.ForTable2(b)and(c),thehydraziniumnitrate concen-trationswereeffective;therefore,theconcentrationofhydrazine introducedintothemediumwasdeterminedfromthedifferent equilibriumconstantssuchthatthemeanhydraziniumnitrate con-centrationwasbetween0.1mMand1M.

Theanalysisoftheseexperimentaldesignsincodedvariables andresponseKprovidedthecorrelationmatrix(Table2(d)).The diagonalof“1”showsthatthemaineffectsandsecond-orderand quadraticinteractionsdonotoverlap.

Thecoefficientsofeachpolynomialresponseasafunctionof themaineffectsandinteractionswerecalculatedbymultiplelinear regressionintheLumiereprogram.Foreachpolynomialresponse, thevaluesofthesecoefficientsdemonstratethesignificant influ-enceofthehydraziniumnitrateandnitricacidconcentrationson D_{hydraziniumnitrate}.Theplotsofthesepolynomialresponsesasa func-tionofX1andX2indicatethefollowing:

• For low nitric acid concentrations (< 1M), Dhydraziniumnitrate

decreasesasthenitricacidconcentrationincreases.Inthiscase, itispossibletoconcludethatthereisagoodcorrelationbetween themathematicalmodelandtheexperiment;bothrevealedthe existenceofamaximumvalueforahydraziniumnitrate concen-trationof0.55mM.

• Forhighnitricacidconcentrations(≥1M),whenKa=0.09[23],

Dhydraziniumnitrate again decreased as the HNO3 concentration

increased.However,aboveapproximately0.7mMhydrazinium nitrate,D_{hydraziniumnitrate}increasedwiththenitricacid concentra-tion.Inthiscase,themathematicalmodelwaspoorlycorrelated withtheexperimentalvalues.

• For highnitricacidconcentrations(≥1M),whenKa=0.4[24],

D_{hydraziniumnitrate} again decreased as the HNO3

concentra-tion increased;however, above a nitricacid concentrationof

Table2

Effectofnitricacidonthediffusioncoefficientofhydraziniumnitrate:(a)with 0.2≤[nitric acid]<1M; (b) with 1≤[nitric acid]<2M and Ka=0.09; (c) with

1≤[nitricacid]<2MandKa=0.4;(d)correlationmatrix.

(a)

X1 X2 [Hydrazinium

nitrate](mM)

[Nitricacid](M) Dhydraziniumnitrate

(cm2_s−1) K −1 −1 0.1 0.2 6.2E−08 1 0 −1 0.55 0.2 5.7E−06 92 1 −1 1 0.2 2.3E−06 37 −1 0 0.1 0.6 5.2E−08 0.83 0 0 0.55 0.6 4.3E−06 70 1 0 1 0.6 1.9E−06 31 −1 1 0.1 1 5.6E−08 0.9 0 1 0.55 1 3.1E−06 50 1 1 1 1 2.3E−06 37 0 0 0.55 0.6 4.0E−06 65 0 0 0.55 0.6 4.4E−06 71 0 0 0.55 0.6 4.0E−06 65 0 0 0.55 0.6 4.5E−06 73 (b) X1 X2 [Hydrazinium nitrate](mM)

[Nitricacid](M) Dhydraziniumnitrate

(cm2_s−1) K −1 −1 0.1 1 6.6E−05 1 0 −1 0.55 1 4.4E−06 0.06 1 −1 1 1 2.5E−06 0.04 −1 0 0.1 1.5 5.7E−05 0.9 0 0 0.55 1.5 3.3E−06 0.05 1 0 1 1.5 1.7E−06 0.02 −1 1 0.1 2 5.0E−05 0.8 0 1 0.55 2 3.8E−06 0.06 1 1 1 2 1.4E−06 0.02 0 0 0.55 1.5 3.0E−06 0.05 0 0 0.55 1.5 3.7E−06 0.06 0 0 0.55 1.5 3.4E−06 0.05 0 0 0.55 1.5 2.9E−06 0.04 (c) X1 X2 [Hydrazinium nitrate](mM)

[Nitricacid](M) Dhydraziniumnitrate

(cm2_s−1) K −1 −1 0.1 1 1.7E−07 1 0 −1 0.55 1 2.9E−06 17 1 −1 1 1 8.0E−07 4.7 −1 0 0.1 1.5 8.7E−08 0.5 0 0 0.55 1.5 1.9E−06 11.2 1 0 1 1.5 1.0E−06 5.9 −1 1 0.1 2 1.1E−07 0.6 0 1 0.55 2 2.4E−06 14.1 1 1 1 2 8.0E−07 4.7 0 0 0.55 1.5 2.0E−06 11.8 0 0 0.55 1.5 1.8E−06 10.6 0 0 0.55 1.5 2.2E−06 12.9 0 0 0.55 1.5 1.9E−06 11.2 (d) X1 X2 X1×X2 X12 X22 X1 1 X2 0 1 X1× X2 0 0 1 X2 1 0 0 0 1 X2 2 0 0 0 0 1

approximately1.5M,D_{hydraziniumnitrate}increasedwiththenitric acidconcentration.In this case, there wasa goodcorrelation betweenthemathematicalmodelandtheexperimentalvalues; bothrevealedtheexistenceofamaximumvaluewhenthe con-centrationof hydrazinium nitratewas0.55mM.These results confirmedthatthevalueofKa=0.4[24]closelyapproximatesthe

Fig.8.CurrentpotentialcurvesobtainedonthePtrotatingdiskanodeimmersed withinprocesseffluentsolutioncontainingbothhydraziniumnitrateandoxalicacid at0.1Min2MHNO3.1000rpm;25◦C;potentialscanrate:0.005Vs−1.

3.4. Electrochemicalbehaviorofbothcomponentsinprocess effluent

Hydraziniumnitrateandoxalicacidwerestudiedin process effluentin0.1Mand2Mnitricacid,respectively.AsshowninFig.8, thetwosignalsobservedareattributedtohydraziniumnitrateand oxalicacid.Thefirstsignal,whichisthehydraziniumnitratesignal, showsamaximumfollowedbyaslightdecreaseincurrentcaused bynitrogenbubbles.Thesecondsignal,whichisattributedtothe oxalicacidaffectedby nitrogenbubbles,isverynoisy.Despites thesenoiseproblems,theresultsshowthat thesespeciescould bedetectedseparately and inthe case oftheircontinuous and simultaneousdetection,anoptimized electrochemicalcellmust bedesignedtoensuretherapidand continuousremovalofthe electrogeneratedbubblesandtoimprovethesensitivityofthe elec-trochemicalresponseofthesystem.

4. Conclusion

Theoxidationofoxalicacidandhydraziniumnitratewas stud-iedona Pt anodein nitricacid media.Theoxidation of oxalic acidwasobservedat0.7V(vs.Ag/AgCl)andcarbondioxide bub-bles wereobserved on thesurface of the electrode. Within an analyticalframework,theinfluenceofthisphenomenonmustbe reducedbecauseoftheeffectontheelectroactivesurfaceofthe electrode.Theelectrokineticparameters determinedcorrespond toanirreversiblesysteminwhich twoelectronsareexchanged simultaneously.Intheoxalicacidconcentrationrangefrom1mM to100mM,alinearevolutionofthelimitingcurrentvs.the con-centrationwasproposed.

The oxidation of hydrazinium nitrate begins at a potential of 0.3V (vs.Ag/AgCl) and yields a separatesignalfrom that of oxalicacid. Theformationofnitrogenbubbleswasobservedfor concentrationsabove5mM.Cyclicvoltammetrystudieswere per-formedat highscanrates totrytounderstand themechanism ofhydraziniumnitrateelectrooxidation.Forhighvaluesofr,the resultsdemonstratethattheoxidationofhydraziniumoccursin morethanonestep.Alinearevolutionofthelimitingcurrentvs. theconcentrationofhydraziniumnitratewasobservedinthe con-centrationrangebetween1mMand75mM.

The diffusion coefficients of oxalic acid and hydrazinium nitratein nitricacid mediawere determined; the values were 2.9×10−7_cm2_s−1_and5.2×10−6_cm2_s−1_{,respectively.Aspartof}

thisdetermination,theexperimentaldesignapproachesthatwere implementedrevealedtheinfluenceofthenitrateconcentration onthesediffusioncoefficients.Foroxalicacid,thisinfluencecanbe

explainedbytheexistenceofhydrogeninteractionsbetweenthe molecules.Fornitricacidconcentrationshigherthan1Minnitric acid, hydrazinium nitrate forms divalent hydrazinium cations, whosepresencestronglyinfluencesthevalueofthehydrazinium nitratediffusioncoefficient.

Inacidicmedia,thetwospeciescanbedetectedatthesame time,althoughbubblingeffectsinterferewiththewave measure-ments.Thisworkprovidesinterestingperspectiveswithrespectto theelectrochemicalanalysisofthesespecies,includingthe under-standingoftheiroxidationprocesses.

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