A Study of Nitrite Oxidation at Platinum Microelectrodes

No pre sen te tra ba lho são apre sen ta dos es tu dos en vol ven do a oxi da ção ele tro quí mi ca de ni tri to na su per fí cie de mi cro e le tro dos de pla ti na de ra i os com pre en den do a fa i xa de 5 a 25 μm. Obser vou-se que o me ca nis mo da re a ção ele tró di ca não se res trin ge a pro ces so pu ra men te di fu si o nal e que a res pos ta do ele tro do é lar ga men te in flu en ci a da pela for ma ção de óxi dos de pla ti na. Uti li zan do pro ce di men to en vol ven do pré-condicionamento do ele tro do em meio de KCl foi pos sí vel de sen vol ver um mé to do am pe ro mé tri co bas tan te sim ples para a de ter mi na ção de ni tri to com li mi te de de tec ção na fa i xa de 2 μM.


Intro duc ti on
Ni tri te and ni tra te are im por tant in the che mistry of the en vi ron ment sin ce both spe ci es play an im por tant role in the analy sis of na tu ral wa ters and ot her en vi ron men tal mate ri als.This has led to many re cent stu di es of analy ti cal pro ce du res for the ir de ter mi na ti on.The se arch for sim ple and ra pid met hods of analy sis of both ions has in clu ded the de ve lop ment of flow in jec ti on pro ce du res, many of them cou pled to ion chro ma to grap hic ins tru ments.The de tec ti on of the analy tes has been car ri ed out both spec trop ho to metri cally and elec tro che mi cally; the ad van ta ges of elec troche mi cal de tec ti on are the ra pid res pon se and the avo i dan ce of po ten ti ally uns ta ble com pounds pro du ced in some of the co lo ri me tric met hods.
The cat ho dic be ha vi our of ni tra te and ni tri te is al ways si mi lar.Both ions re qui re highly aci dic con di ti ons for the re duc ti on to take pla ce, ni tro gen, ni trous oxi de and am monia be ing the pro ducts of the elec troly sis un der some ex peri men tal con di ti ons 1 .Analy ti cal met hods ba sed on the re duc ti on pro ces ses have al ways to con si der sour ces of inter fe ren ce such as hydro gen evo lu ti on, jus tif ying the use of mer cury elec tro des in ca taly tic po la ro grap hic or ad sorp ti ve cat ho dic strip ping vol tam me tric met hods [2][3][4] .Ho we ver, in con trast to ni tra te, ni tri te may also be oxi di sed.Hen ce, seve ral pa pers have des cri bed the ano dic oxi da ti on of ni tri te at pla ti num [5][6][7] and vi tre ous car bon 8,9 and the re has been a re cog ni ti on that mo di fi ca ti on of the Pt sur fa ce im pro ves the per for man ce of the ano de as a de tec tor.On the ot her hand, the re ap pe ars to have been no at tempts to un ders tand the me cha nism of ni tri te oxi da ti on or the dif fi cul ti es found with un mo di fi ed Pt sur fa ces.The re fo re, in this pa per we have in ves ti ga ted furt her the oxi da ti on of ni tri te on Pt micro e lec tro des [10][11][12][13] .
The use of mi cro e lec tro des as an analy ti cal tool for the de ter mi na ti on of ni tri te has al re ady been re por ted 14 .On car bon and gold mi cro fi bres it was shown that good ste ady sta te vol tam mo grams could be ob ta i ned by using the li ne ar swe ep vol tam metry tech ni que.The li mi ting cur rents of the se vol tam mo grams were pro por ti o nal to the analy te con cen tra ti on and the de tec ti on li mits es ti ma ted sug ges ted the use of the met ho do logy for mo ni to ring ni tri te in na tu ral wa ters.
The ano dic oxi da ti on of ni tri te is known to oc cur at mode ra te po ten ti als ( ca. 1.0 V vs. SCE) at pla ti num elec tro des, ho we ver dif fi cul ti es as so ci a ted with the sur fa ce pas si va tion by pla ti num oxi des res tricts this ap pro ach as a met hod for the analy te de ter mi na ti on.Accor dingly, the met hod Ar ti cle pre sen ted in this pa per de als with the am pe ro me tric de tec tion of ni tri te on pla ti num sur fa ces.In this pa per a ca re ful treat ment of the elec tro de sur fa ce is des cri bed and it will be shown that af ter such, pla ti num mi cro e lec tro des cons ti tu te a re li a ble sen sor for the elec tro che mi cal de tec ti on of ni tri te.

Expe ri men tal
Instru men ta ti on Pla ti num mi cro e lec tro des were fa bri ca ted by se a ling mi cro wi res into glass, cut ting per pen di cu lar to the wire and po lis hing with emery pa per and then with alu mi na pow der (1, 0.3 and 0.05 µm) on a po lis hing cloth.The ir ra dii were chec ked by me a su ring the ste ady sta te cur rent for fer ric yani de so lu ti ons of known con cen tra ti on and com pa ring to the equa ti on that des cri bes the ste ady sta te cur rent for a micro e lec tro de (Eq.1).A small be a ker was used as a cell, the re fe ren ce elec tro de was a com mer ci al SCE (KCl).All electro che mi cal ex pe ri ments were per for med with a Hi-Tek wa ve form ge ne ra tor and the cur rents were am pli fi ed with a la bo ra tory-built am pli fi er.No de oxy ge na ti on was ne cessary and all ex pe ri ments were car ri ed out at room tem pe ratu re, 293 ± 2 K.

Che mi cals
So lu ti ons were pre pa red by dis sol ving the analy ti cal gra de re a gents in Mil li po re wa ter.Stan dard ni tri te so lu tions (pre pa red with the NaNO 2 salt) were stan dar di sed using a re com men ded pro ce du re from the li te ra tu re 15 .

Re sults
Fi gu re 1 shows cur rent (I) vs. po ten ti al (E) plots re corded at 10 mV s -1 for three freshly po lis hed Pt mi cro disc elec tro des in a so lu ti on of 1 mM NO 2 -in 0.1 M KCl (pH = 7).At the lar gest mi cro disc ra di us, 25 µm, the oxida ti on of ni tri te le ads to a well for med oxi da ti on wave at E 1/2 = +730 mV vs. SCE and with E 3/4 -E 1/4 ≈ 50 mV.The li mi ting cur rent pla te au, ho we ver, does not ex tend over a wide po ten ti al ran ge and po si ti ve to +950 mV, the cur rent de cre a ses aga in.Mo re o ver, in con trast to the ex pec ted beha vi our at a mi cro e lec tro de at slow scan ra tes [10][11][12][13] , it can be seen that the re ver se scan shows con si de ra ble hyste risis; at all po ten ti als the ano dic cur rent is sig ni fi cantly lower.Almost no hyste ri sis is ob ser ved if the scan is re ver sed at +850 mV.The fi gu re also shows that at the smal ler mi cro discs, the se trends are exa cer ba ted and, with a 5 µm mi cro disc, it is more dif fi cult to iden tify a sig ni ficant pla te au in the I -E res pon se.On the ot her hand, when the "pla te au cur rents" are plot ted ver sus the disc ra dii, a line ar plot is ob ser ved, see the in set to Fig. 1.This im pli es that un der the se con di ti ons and at po ten ti als in the ran ge 850-900 mV, the oxi da ti on of ni tri te is mass trans port con trol led.The mass trans port li mi ted cur rent to a mi crodisc elec tro de is gi ven by [10][11][12][13] I L = 4nFDcr (1) whe re n is the num ber of elec trons trans fer red in the electro che mi cal pro cess (here n = 2 as su ming that the fi nal pro duct of the ano dic oxi da ti on is ni tra te), F is the Fa ra day cons tant, D is the dif fu si on co ef fi ci ent of the elec tro ac ti ve spe ci es, c is the con cen tra ti on and r is the ra di us of the micro e lec tro de.Mo re o ver, the slo pe of the li ne ar plot le ads to a va lue for the dif fu si on co ef fi ci ent of ni tri te which is 1.2 x 10 -5 cm 2 s -1 , clo se to that ex pec ted for a small ani on in aque ous so lu ti on 16 and in agre e ment with the va lue obta i ned by Sil va et al. 14 using data from a chro no am pe rome tric ex pe ri ment with a car bon fi ber mi cro e lec tro de in 0.05 M Na 2 SO 4 .The in set to Fig. 1 also shows the re produ ci bi lity of the "li mi ting cur rents" (for each ra di us, three ex pe ri men tal po ints are shown); with ca re ful po lis hing of the mi cro discs, the cur rents were re pro du ci ble to ±5%.Cyclic vol tam mo grams at Pt elec tro des, area 0.10 cm 2 , con fir med that the oxi da ti on of ni tri te is com ple tely ir rever si ble.
The se re sults would sug gest that the oxi da ti on of ni trite at a Pt ano de is mass trans port con trol led over a nar row ran ge of po ten ti als and on the ti mes ca le of the se ex pe riments.It is, ho we ver, cle ar from fi gu re 1 that the cur rent de cre a ses as the po ten ti al is ta ken po si ti ve to +900 mV and it was also found that the cur rent de ca yed if the po tenti al was held at +850 mV for an ex ten ded pe ri od.Both these ob ser va ti ons are com pa ti ble with the con clu si on that pla ti num oxi de for med on the ano de sur fa ce inhi bits ni trite oxi da ti on.Fi gu re 2 il lus tra tes the in flu en ce of pH on the I -E plots for ni tri te oxi da ti on at a Pt mi cro disc elec tro de, ra dius 12.5 µm in 0.1 M KCl.Com pa ri son of Figs.2(a) and (b) de mons tra tes that even a small in cre a se of the pH to 9 led to al most com ple te loss of the oxi da ti on wave at E 1/2 = +730 mV.Sin ce it se e med un li kely that the ni tri te ion is uns ta ble in this mildly al ka li ne so lu ti on, vol tam mo grams were re cor ded over the same po ten ti al ran ge for the electroly tes in the ab sen ce of ni tri te.The se vol tam mo grams, re cor ded at a much hig her cur rent sen si ti vity than Figs.2(a)-(c) are shown as Figs.2(d)-(f).It can be seen that plati num oxi de for ma ti on is al ways ir re ver si ble, but the extent of its for ma ti on and the po ten ti al whe re it is for med va ri es strongly with pH.At pH 9, a well for med oxi da ti on peak is seen at E p = +590 mV vs. SCE and the char ge as soci a ted with the sur fa ce oxi da ti on is ≈1.5 mC cm -2 sug gesting a fi nal thick ness of 1-5 mo no la yers de pen ding on the real mi cro rough ness of the po lis hed mi cro discs.In contrast, at pH 7 the ano dic cur rents are much lo wer at all poten ti als con si de red, ie, much less pla ti num oxi de is for med, and the vol tam mo gram re cor ded bet we en +400 mV and +1000 mV shows no peak but an in cre a sing anodic cur rent over most of the po ten ti al ran ge.At pH 2, the sur fa ce oxi da ti on cur rents are de cre a sed furt her.It can be seen that the vol tam mo grams for the oxi da ti on mir ror the sta te of oxi da ti on of the pla ti num sur fa ce and it ap pe ars that the pre sen ce of pla ti num oxi de inhi bits ni tri te oxi da tion.At pH 9, ni tri te oxi da ti on would not oc cur un til a poten ti al whe re the Pt is al most fully co ve red with the oxi de and hen ce no oxi da ti on is ob ser ved.At pH 7, ni tri te oxi dati on oc curs on a sur fa ce with only a low oxi de co ve ra ge but by +1000 mV the oxi de co ve ra ge is be co ming sig ni ficant and star ting to inhi bit the ni tri te oxi da ti on.At pH 2, even less oxi de is for med and the vol tam mo gram for ni trite oxi da ti on shows a bet ter li mi ting cur rent pla te au and less hyste ri sis on the re ver se scan.It was, ho we ver, no ted that, in the aci dic so lu ti on, the li mi ting cur rent is ≈20% lower than that ob ser ved at pH 7.This is not due to the decom po si ti on of ni tri te in aci dic so lu ti on sin ce the li mi ting cur rent did not chan ge with time and, if the pH of the acidic so lu ti on was in cre a sed back to 7, the li mi ting cur rent in cre a sed to the ex pec ted va lue.Pre su mably, the lo wer limi ting cur rent re sults from n < 2 but the che mistry in this so lu ti on has not been in ves ti ga ted furt her.
In view of the abo ve re sults, it might be con clu ded that the elec tro che mi cal analy sis is best car ri ed out with an oxi de free Pt elec tro de.Ho we ver, at tempts to de ter mi ne ni tri te using chro no am pe ro me tric res pon se of a po lis hed Pt elec tro de were not very suc cess ful.Although it was pos si ble to ob ta in a tran si ent whe re the cur rents had about the ex pec ted va lu es, wha te ver va lu es were cho sen for the ini ti al and me a su re ment po ten ti als, the I -t tran si ent was ne ver re pro du ci ble.More im por tantly, re pe ti ti ons without re po lis hing the elec tro de led to qui te dif fe rent currents.Hen ce, a dif fe rent ap pro ach was em plo yed.
The po ten ti al of the Pt mi cro disc was held at +1000 mV in 0.1 M KCl un til the back ground cur rent had re ached a ste ady sta te, the elec tro che mi cal pro cess li kely being the for ma ti on of pla ti num oxi des; this re qui red about 400 s and the fi nal cur rent at a mi cro disc (ra di us 12.5 µm) was typi cally ≈10 pA.Ali quots of a con cen tra ted ni tri te so lu ti on were then ad ded and the res pon se of the mi crodisc no ted.Fi gu re 3 shows the re sults from such an ex pe riment.Du ring the pe ri od be fo re the ad di ti on of ni tri te, the cur rent drops with time, ini ti ally ste eply and then slowly be fo re it asympto tes to a ste ady sta te va lue.After the ad diti on of ni tri te was com men ced, it can be seen that the current in cre a ses li ne arly with ni tri te con cen tra ti on, see in set, and that 2 µM ni tri te is ea sily de tec ted.In fact, the ste ady sta te cur rent va ri es with ni tri te con cen tra ti on over the ran ge 0-20 µM ac cor ding to the equa ti on with a re gres si on co ef fi ci ent, r = 0.99995.Si mi lar li ne ar cali bra ti on plots could be ob ta i ned for ni tri te con cen tra ti ons up to 1 mM.Mo re o ver, the se ex pe ri ments are very re produ ci ble alt hough the slo pe of this I vs. c plot is only ≈35% of that ex pec ted for a dif fu si on con trol led 2e -oxi da ti on.Fi gu re 4 shows the re sults from a si mi lar ex pe ri ment but one whe re the oxi da ti on of the Pt sur fa ce was car ri ed out in 0.1 M KCl so lu ti on al re ady con ta i ning 14 µM ni tri te (as would be the case for an analy sis using a stan dard ad diti on pro ce du re) and furt her ali quots of ni tri te were then added to the so lu ti on.In this case the re gres si on line is with r = 0.9986.The re sults re ma in re pro du ci ble but the res pon se to ni tri te is hig her; the slo pe of the I vs. \c NO ccol \ \ \ \ plot is lar ger than when the Pt sur fa ce oxida ti on is car ri ed out in a ni tri te free so lu ti on.Uncer ta in ti es in the back ground cur rent would be the li mi ting fac tor in de ter mi ning the lo west de tec ti on li mit.Ove rall, the elec trode oxi di sed in the pre sen ce of ni tri te se ems to be more sensi ti ve and this pro bably in di ca tes the pre sen ce of less oxi de on the sur fa ce.One ex pla na ti on would be that ni tri te ion, as for exam ple chlo ri de, ad sorbs on the Pt sur fa ce and the re fore inhi bits oxi de for ma ti on.

Dis cus si on
Many ano de re ac ti ons in aque ous so lu ti on oc cur via me cha nisms whe re an oxi di sed pla ti num spe ci es on the sur fa ce is an es sen ti al in ter me di a te in the pro cess.The oxida ti on of car bon mo no xi de is one exam ple which oc curs via a che mi cal re ac ti on betwe en ad sor bed CO and sur fa ce spe ci es such as Pt -OH 17,18 .
The ano dic oxi da ti on of ni tri te is cle arly not this type of reac ti on sin ce the for ma ti on of oxi di sed Pt spe ci es on the surfa ce inhi bits the re ac ti on.This im pli es that ni tri te oxi da ti on oc curs via a di rect elec tron trans fer me cha nism, whe re nitra te is the fi nal pro duct in a 2e -re ac ti on.This is in ac cordan ce with cou lo me tric stu di es car ri ed out at pla ti num elec tro des by Gu i del li et al. 19 .Ba sed on vol tam me tric and po ten ti os ta tic data, the aut hors pro po sed a de ta i led me chanism for the ano dic oxi da ti on of ni tri te which is li kely to invol ve the for ma ti on of NO 2 as a first step: fol lo wed by the dis pro pror ti o na ti on re ac ti on of NO 2 This in ter pre ta ti on would cer ta inly be com pa ti ble with the ob ser va ti on that the half wave po ten ti al for ni tri te oxida ti on va ri es lit tle with pH.Accor ding to Gu i del li et al. 19 , in the ran ge of pH 3.3-7.0 the main spe ci es in so lu ti on is NO 2 -and the half wave po ten ti al is in de pen dent of pH.Howe ver, at more aci dic con di ti ons the pro to na ted spe ci es pre va ils in so lu ti on and the elec tro che mi cal re ac ti on in volves the re duc ti on of HNO 2 , with a shift of 60 mV per de cade of H + con cen tra ti on.
The re are se ve ral known exam ples of sim ple elec tron trans fer re ac ti ons inhi bi ted by pla ti num oxi de la yers 20,21 .It is pro ba ble that the ap pa rent ca taly sis of ni tri te oxi da ti on re por ted at some mo di fi ed elec tro des 6 re sults only from a chan ge in pla ti num oxi de co ve ra ge; the pre sen ce of the porous poly mer co a ting is li kely to lead to a de cre a se in sur face oxi da ti on.
The need to pre-oxidise the Pt sur fa ce in or der to ob ta in re pro du ci ble re sults lo wers the sen si ti vity of the met hod but it is still pos si ble to see a cle ar chan ge in sig nal on ad diti on of 2 µM ni tri te.Cer ta inly, the di rect oxi da ti on at a pre-oxidised Pt mi cro disc pro vi des a sim ple and ra pid appro ach to ni tri te analy sis when it is pre sent in con cen tra tions in the ran ge 5-1000 µM.In prac ti ce, ho we ver, be ca u se of the re la ti vely po si ti ve po ten ti al used for the oxi da ti on of ni tri te, it is li kely that many spe ci es ca pa ble of di rect anodic oxi da ti on will be sour ces of po ten ti al in ter fe ren ce.In Fi gu re 4. Cur rent time res pon se du ring the ad di ti on of ni tri te each equi va lent to a so lu ti on con cen tra ti on of 2 µM (b -j).In this case the pre-treatment of the freshly po lis hed Pt mi cro disc elec tro de was car ried out in a 0.1 M KCl + 14 µM ni tri te so lu ti on (a).Elec tro de po ten ti al = +1000 mV.Elec tro de ra di us 12.5 µm.The in set shows the plot of I vs. C ni tri te .such a case, the di rect ano dic oxi da ti on may be pre fer red as a de tec tor for flow in jec ti on met hods af ter pre vi ous se pa rati on of the che mi cal spe ci es using ion chro ma to graphy.In the se ap pli ca ti ons, the Pt sur fa ce is re a dily pre-oxidised in aque ous 0.1 M KCl in the ab sen ce of ni tri te and in ter fe rences are un li kely af ter the chro ma to grap hic se pa ra ti on.
The use of a mi cro disc elec tro de adds fle xi bi lity to the elec tro a naly sis.It al lows the stra igh tfor ward mi ni a tu ri sa tion of de tec tor cells whi le the sig nal to no i se is sig ni fi cantly im pro ved, the reby lo we ring the lo west de tec ti on li mit.In ad di ti on, the ap pli ca ti on of mi cro e lec tro des would al low the de ter mi na ti on to be made in me dia of low io nic strength and also re du ces the de pen den ce of the sig nal on the flow of the analy te past the mi cro disc.

Fi gu re 1 .
Cyclic vol tam mo grams for freshly po lis hed Pt mi cro disc elec tro des in a so lu ti on con ta i ning 1 mM ni tri te and 0.1 M KCl (pH = 7).Elec tro de ra dii are (a) 25 µm (b) 12.5 µm and (c) 5 µm.Po ten ti al scan rate 10 mV s -1 .The in set shows the cor res pon ding plot of I L vs r and the mul ti ple po ints for each ra di us in di ca tes the re pro du ci bi lity of the ex pe ri ments.

Fi gu re 2 .
Cyclic vol tam mo grams for freshly po lis hed Pt mi cro disc elec tro des in a so lu ti on con ta i ning 1 mM ni tri te and 0.1 M KCl (a) at pH 7 (b) ad jus ted to pH 9 by the ad di ti on of KOH (c) ad jus ted to pH 2 by the ad di ti on of H 2SO 4. The fi gu res (d)-(f) show the res pon ses for the Pt mi cro disc elec tro de in 0.1 M KCl at the same pH wit hout the addi ti on of ni tri te.Elec tro de ra di us 12.5 µm.Po ten ti al scan rate 20 mV Fi gu re 3. Cur rent time res pon se du ring (a) the pre-treatment of the freshly po lis hed Pt mi cro disc elec tro de in 0.1 M KCl and (b) du ring the ad di ti on of ali quots of ni tri te each equi va lent to a so lu ti on con centra ti on of 2 µM.Elec tro de po ten ti al = +1000 mV.Elec tro de ra di us 12.5 µm.The in set shows the plot of I vs. c NO 2 -.