Incorporation and Voltammetric Determination of Silver ( I ) at an Electrode Electrochemically Modified with a Poly-Benzidine Film

Eletrodos modificados com filmes de poli-bendizina são quimicamente estáveis no intervalo de pH en tre 0 e 7 e podem ser ciclados de -0.200 a 0.850 V sem evidência de decomposição. Conseqüentemente, neste intervalo de potencial, eles podem ser usados como ma te rial eletródico para a oxidação ou redução de diferentes espécies. Uma propriedade interessante deste eletrodo modificado é sua capacidade de capturar e quantificar o íon Ag(I) presente na solução. Para obter resultados reproduzíveis a etapa de acumulação foi realizada numa solução de AgNO3 com um tampão de acetato de pH = 7. Após redução para Ag(0) na mesma solução, a etapa de determinação foi executado por voltametria cíclica. A resposta analítica corresponde à oxidação para Ag(I) e a quantia de prata incorporada ao filme foi determinada a partir da carga do pico anódico. O uso potencial deste método para a determinação analítica da prata é discutido.


In tro duc tion
In the con text of metal ion anal y ses, chem i cally mod ified elec trodes (CMEs) pos sess ing prop er ties have emerged as one of the most at trac tive ar eas for re search [1][2][3][4] .Among their ad van tages are their in her ent high se lec tiv ity and sen si tiv ity.Methods that have been used for elec trode mod i fi ca tion in clude phys i cal coat ing, co va lent at tach ment and electropolymerization.
Par tic u larly, electropolymerization of fers an ef fec tive and ver sa tile way to de posit poly mer film on a sub strate elec trode [5][6][7][8] .
The electrooxidation of ar o matic amines is well known, and dif fer ent as pects of their mech a nism have been elu cidated 9,10 .Pre vi ous stud ies show that when ar o matic amines such as naphtidine 5 , 1-naphtylamine 6 , and benzidine (Bz) 7 are electrooxidized, they fol low a com plex re ac tion to give a poly mer that ad heres to the elec trode sur face.These polymers which ad here to a base metal are con duc tors, and their dop ing grade can be con trolled 9,11,12 .
Re cently, a num ber of in ves ti ga tors have dem on strated a new vari a tion in elec tro chem i cal trace-metal anal y sis in which analyte preconcentration is car ried out at CMEs.The preconcentration is achieved by a purely non-electrochemical de po si tion step, and the subsequant quan ti fi ca tion of the ac cu mu lated analyte can be car ried out by nor mal voltammetric mea sure ments 4,13 .

Ar ti cle
CMEs have been used as work ing elec trodes in the deter mi na tion of sil ver and mer cury 14,15 .In both cases, the metal ions were chem i cally preconcentrated on CMEs.
In this ar ti cle we re port the in cor po ra tion of Ag(I) ion into poly-benzidine (poly-Bz) films.The dis tri bu tion of the Ag(I) ion be tween the so lu tion and the film is es tab lished, and a prob a ble an a lyt i cal ap pli ca tion is given.

Ex per i men tal
Chem i cals and so lu tions Benzidine (Bz) from Fluka was used as re ceived.All of the other re agents were an a lyt i cal grade un less oth er wise stated.Wa ter was ob tained from LABCONCO equip ment model 90901-01.The Bz so lu tions used to ob tain the film were pre pared by weigh ing Bz and dis solv ing it in the solvent-supporting elec tro lyte so lu tion (0.3 M NaClO 4-0.1 M HClO 4).The con cen tra tion of Bz was de ter mined by spectro pho to met ric meth ods ( λ máx = 250 nm, log ε = 4.2) as previ ously de scribed 7,8 .
Metal stock so lu tions were pre pared by dis solv ing the ap pro pri ate salts in a sol vent-supporting elec tro lyte so lution or in an ac e tate buffer so lu tion.The con cen tra tion of metal was de ter mined by potentiometric ti tra tion us ing a stan dard KCl so lu tion.Working so lu tions were freshly prepared by di lut ing from the stock so lu tion.Ac e tate buffer so lu tions were ob tained by ad just ing 0.2 M so dium ac e tate so lu tions to pH = 7 with gla cial ace tic acid.All so lu tions were deaerated by bub bling pure ni tro gen.

Ap pa ra tus
Cy clic voltammetric and electropolymerization ex per iments were car ried out with an EG&G PAR 273 potentiostat/galvanostat.The out put sig nal was re corded on a Hewlett-Packard Model 7015B X-Y plot ting sys tem.The val ues of the charges at the voltammetric peaks were de ter mined by the in te gra tion of the cur rent-time curves.A com pen sat ing po lar planimeter (In ter na tional Trade As soci ates, Ja pan) was used for this pur pose.In each case, the ap pro pri ate back ground cur rent val ues were sub tracted 6 .Sim i lar re sults are ob tained if the sig nal is pro cessed dig itally, us ing the cor re spond ing EG&G PAR soft ware, however in this case the sig nal is much noiser than in the an a log i cal one.
The UV-visible spec tra were ob tained with a Hewlett-Packard 8452A spectrophotometer.The work ing tem per a ture was con trolled us ing a LAUDA K4R ther mostat-cryostat.A con ven tional three-electrode elec tro chemi cal cell was used 16 .A Pt wire with a sur face area of 0.361 cm 2 was used as the work ing elec trode, and a Pt wire with a sur face area of 2 cm 2 in spi ral form was used as the coun ter elec trode.The ref er ence elec trode was an aque ous sat urated cal o mel elec trode (SCE).

Gen er a tion of the films
The poly-Bz films were ob tained from a 5 x 10 -4 M Bz so lu tion in a sup port ing elec tro lyte at pH = 2.5, by cy cling the po ten tial be tween 0 and 0.800 V at a scan rate of v = 0.100 V s -1 .The tem per a ture of the cell was main tained at 35± 0.1 °C.These ex per i men tal con di tions led to the forma tion of an ad her ent film, which was me chan i cally sta ble and had a very re pro duc ible elec tro chem i cal re sponse 8 .In ad di tion, the film thick ness could be in creased by in creasing the num ber of po ten tial cy cles.
Once the poly mer film was gen er ated, the work ing elec trode was rinsed with sup port ing elec tro lyte so lu tion.Then, it was trans ferred to an other cell con tain ing the same elec tro lyte so lu tion in the ab sence of the mono mer, but at pH = 1 and 20 °C.The po ten tial was cy cled be tween -0.100 and 0.500 V for about 10 min.The film thus ob tained is very sta ble for at least 24 h.

Elec tro chem i cal be hav ior of poly-Bz
A typ i cal voltammogram of the poly-Bz elec trode, prepared as de scribed above, is shown in Fig. 1.Its re dox response shows only one peak sys tem, the an odic and cath odic peaks ap pear ing at 0.185 and 0.160 V re spectively.Al though the voltammetric re sponse was dis cussed in de tail in a pre vi ous pa per 8 , some char ac ter is tics should be noted.The film can be re peat edly cy cled from -0.200 V to 0.850 V with no ev i dence of de com po si tion.It can be used in this po ten tial range as elec trode ma te rial for se lective ox i da tion or re duc tion of dif fer ent electroactive species whose re dox po ten tials also oc cur in the same range.The peak cur rent de pends lin early on the scan rate in the range of 0.005-0.100V s -1 , and the peak po ten tial is in depend ent of v be tween 0.005 and 0.150 V s -1 .
The value of the an odic charge at the voltammetric peak (Q film ) was de ter mined by in te grat ing the cur rent-time curve.As shown in Fig. 1, the base line was taken as a straight line join ing points a and b.
As pre vi ously re ported 8 , the re dox be hav ior of the film is strongly de pend ent on the pH of the elec tro lyte so lu tion.It has re pro duc ible be hav ior in the pH in ter val of 1-3.5, but at pH > 4 the elec tro chem i cal re sponse is prac ti cally lost.Fig ure 2 shows a typ i cal voltammogram of the film when it is trans ferred to the ac e tate buffer so lu tion at pH = 7.The ox i da tion cy cle shows only an in creas ing cur rent with no de fined peak, while in the re duc tion a peak at 0.100 V is observed.
Al though the film re sponse is not ob served at pH > 4, the film is not de stroyed and re cov ers its orig i nal re sponse when the pH is re stored to 1.

In cor po ra tion of Ag(I) ion into poly-Bz film
This analyte was cho sen be cause its re dox po ten tial is within the range in which the poly-Bz elec trode is elec trochem i cally sta ble.The gen eral pro ce dure in cluded the follow ing steps: i) Sta bi li za tion and char ac ter iza tion of the poly-Bz elec trode, as de scribed above: the an odic charge was de termined af ter the elec trode po ten tial was kept at -0.100 V for 10 min to al low the com plete re duc tion of the film and to ob tain ma te rial with re pro duc ible be hav ior.
ii) Preconcentration step: the poly-Bz elec trode was placed in an open cir cuit in a well-stirred so lu tion of AgNO 3 in a sup port ing elec tro lyte so lu tion.Ac cu mu la tion was car ried out for a known amount of time, af ter which it was re moved and rinsed with the elec tro lyte so lu tion.
iii) Re duc tion to Ag(0): the elec trode was trans ferred to the anal y sis cell con tain ing the sup port ing elec tro lyte so lution.The ac cu mu lated Ag(I) was re duced at a -0.100V for 5 min.
There is a pos si bil ity of the spon ta ne ous re duc tion of Ag(I) by the re duced film dur ing the pre-concentration step.To test this pos si bil ity, step iii was omit ted.It was observed that the charge (Q Ag ) does not give re pro duc ible values, and is al ways smaller than the charge ob tained af ter step iv.iv) De ter mi na tion step: per formed by cy clic voltammetry.The an a lyt i cal re sponse cor re sponds to sil ver ox i da tion to Ag(I) and the quan tity of sil ver in cor po rated into the film was de ter mined from the charge in volved in the an odic peak (Q Ag ).
Ex per i ments were per formed with a mod i fied elec trode sta bi lized at pH = 1, as de scribed in the Ex per i men tal sec -Fig ure 2. Cy clic voltammogram of the poly-Bz mod i fied elec trode in ac e tate buffer so lu tion at pH 7 and 20 °C.v = 0.025 V s -1 .3. Cy clic voltammogram of the poly-Bz mod i fied elec trode at 0.025 V s -1 in sup port ing elec tro lyte so lu tion at pH 1 and 20 °C, af ter be ing in con tact with a so lu tion con tain ing sil ver at a con cen tra tion of 1 x 10 -3 M for 5 min.tion.Typ i cal re sults af ter the in cor po ra tion of sil ver into the poly-Bz elec trode are shown in Fig. 3.The peak at +0.325 V is as so ci ated with the ox i da tion of the de pos ited sil ver, and shows a cor re spond ing cath odic peak at +0.110 V.

Fig ure
The an odic peak at +0.510 V cor re sponds to the ox i dation of poly-Bz film.The shift of the po ten tial peak with respect to the one in Fig. 1 may be due to the fact that the re dox sites of the film complexed by Ag(I) make the re dox pro cess of the ligand more dif fi cult.
How ever, ex per i ments per formed at sev eral Ag(I) concen tra tions show lack of reproducibility, par tic u larly at low ion con cen tra tions.This prob lem may be due to vari a tions in the H + con cen tra tion in the film, since these ions seem to com pete with Ag(I) ions for the same "site" in the film.
In light of these re sults the stud ies were per formed at pH = 7 in tro duc ing some vari a tions in the sta bi li za tion and ac cu mu la tion steps.The, the mod i fied elec trode sta bi lized at pH = 1 was trans ferred to an ac e tate buffer so lu tion at pH = 7 for about 15 min to al low the protonic equi lib rium in the poly mer ma trix.Then ac cu mu la tion step was car ried out in a AgNO 3 so lu tion in the same ac e tate buffer.Ac cord ingly, steps iii and iv were per formed in the ac e tate buffer so lution.
The cy clic voltammetric re sponse ob tained un der these new con di tions is shown in Fig. 4. The sharp peak at +0.530 V is due to the reoxidation of Ag(0).The sharp ness of the wave and the rel a tively flat back ground cur rent in this po -ten tial re gion makes it eas ily quan ti fi able.How ever, as can be ob served in Fig. 4, the re duc tion pro cess is ill-defined and seems to oc cur in a po ten tial zone near the film re duction.In prin ci ple, it can be in ferred that the re duc tion of the sil ver ion to me tal lic sil ver is a very slow pro cess.The observed ki netic lim i ta tion may be due to (1) the high re sis tivity of poly-Bz at -0.100 V in pH = 7 so lu tion or (2) the slow Ag(0) phase for ma tion pro cess un der these con di tions.Nev er the less, if the po ten tial is main tained at -0.100 V for some time, a sec ond pos i tive sweep yields a voltammetric re sponse vir tu ally iden ti cal to the first.
From these re sults, we can de duce that the poly-Bz film en abled the ac cu mu la tion and hence the preconcentration of Ag(I), be ing re pro duc ible at pH = 7.

Gen eral an a lyt i cal pro ce dure
The an a lyt i cal ap pli ca tion of this pro ce dure is based on a work ing curve which cor re lates the amount of Ag(I) accu mu lated in the mod i fied elec trode and its con cen tra tion in so lu tion, main tain ing cer tain pa ram e ters con stant, such as the ac cu mu la tion time and the charge of the film (Q film ).Re gard ing the Q film , it should be noted that it is quite dif ficult to ob tain iden ti cal films.Thus, the work ing curves are con structed by nor mal iz ing Q Ag us ing the ra tio Q Ag /Q film .The ex per i ments were per formed at pH 7 as de scribed, for dif fer ent con cen tra tions of AgNO 3 so lu tion in ac e tate buffer dur ing the ac cu mu la tion step.The re sults are shown in Fig. 5.As can be ob served, the Q Ag /Q film ra tio in creases with the Ag(I) con cen tra tion un til it reaches a con stant value of 25.Thus, the sat u ra tion charge of Ag(I) is much Fig ure 4. Cy clic voltammogram of the poly-Bz mod i fied elec trode at 0.025 V s -1 in ac e tate buffer so lu tion at pH = 7 and 20 °C, af ter be ing in con tact with a so lu tion con tain ing sil ver at a con cen tra tion of 1 x 10 -3 M for 5 min.higher than the poly mer it self.This may in di cate that a redox site has sev eral groups in volved in the co or di na tion of Ag(I).
The sat u ra tion value is the same for films of dif fer ent thick nesses and is in de pend ent of the AgNO 3 con cen tration higher than 1 x 10 -3 M.This in di cates that the in cor pora tion of Ag(I) ions fol lows a def i nite stoichiometry and there is a spe cific in ter ac tion be tween these ions and the poly mer ma trix.Hence, the plot in Fig. 5 re sem bles a dis tribu tion or par ti tion of an ion be tween a so lu tion and an ion ex change resin 17 .This can be re garded as an "ex change iso therm" and ther mo dy nam i cally in ter preted as sum ing an equi lib rium be tween the Ag(I) ion in side and out side the film.
Re gard ing the pos si bil ity of us ing this method for the an a lyt i cal de ter mi na tion of Ag(I), the in set of Fig. 5 shows that Q Ag /Q film is lin ear with C Ag in the in ter val 0 < C Ag < 1.5 x 10 -4 M. The min i mum charge that can be de tected by cyclic voltammetry un der the con di tions used in these ex per iments for the re duc tion peak of Ag(I) is 2.5 µC.Thus, con sid er ing a film with a charge of 10 µC, the min i mum con cen tra tion of Ag(I) that can be de tected is about 2 x 10 -6 M. It should be noted that this is an ap prox i mate cal cu lation, but one which shows the po ten tial util ity of this method, and which de serves de tailed stud ies us ing more sen si tive de tec tion tech niques, such as dif fer en tial pulse voltammetry.

Con clu sions
Elec trodes mod i fied with poly-Bz films show the ability to cap ture Ag(I) in so lu tion.The pro cess is based on the preconcentration of Ag(I) ions in the poly mer ma trix.The ex is tence of a con stant re la tion for the sat u ra tion quan ti ties Q Ag /Q film and its high value ( i.e. , 25) in di cates the strong spe cific in ter ac tions in volved be tween Ag(I) and the polymer.
The an a lyt i cal sig nal is the strip ping re sponse due to silver ox i da tion ob tained af ter the re duc tion of the preconcentrated Ag(I) ions.The an a lyt i cal de tec tion method seems to be quite prom is ing, es pe cially if tech -niques more sen si tive than cy clic voltammetry are used for de tec tion.

Ac knowl edg ments
Fi nan cial sup port from the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), the Consejo de Investigaciones Científicas y Tecnológicas de la Provincia de Córdoba (CONICOR), and the Secretaría de Ciencia y Técnica from the Universidad Nacional de Río Cuarto is grate fully ac knowl edged.

Fig ure 1 .
Fig ure1.Voltammetric re sponse of the poly-Bz mod i fied elec trode in sup port ing elec tro lyte so lu tion at pH 1 and 20 °C.v = 0.025 V s -1 .

Fig ure 5 .
Fig ure 5. a) De pend ence of Q Ag /Q film on the con cen tra tion of Ag(I).b) In set: the lin ear re gion of the same plot.