A Comparative Study of the Polarographic Behavior of Cobalt ( II ) , Nickel ( II ) , Manganese ( II ) , and Zinc ( II ) in Azide Solutions

O comportamento eletroquímico dos metais cobalto(II), níquel(II), manganês(II) e zinco(II) foi estudado por polarografia em diferentes concentrações do ligante azoteto. As ondas de redução do cobalto(II) e níquel(II) apresentam deslocamento para potenciais mais positivos com o aumento da concentração de azoteto na solução. A antecipação máxima é de 300 mV para o cobalto(II) e de 370 mV para o níquel(II). No caso do zinco(II) registrou-se apenas uma pequena antecipação de 27 mV em baixas concentrações do ligante, ocorrendo em seguida o deslocamento da onda para potenciais mais negativos. O manganês(II), de comportamento diferente dos demais metais estudados, mostra apenas deslocamento no sentido de aumento da sobretensão.

The pseudohalide 1 azide, N 3 -, acts as a bridg ing ligand in re dox re ac tions in ho mo ge neous me dia, mak ing the electron trans fer easy 2 .In non-homogeneous me dia, the cat alytic ef fect is also ob served due to the ad sorp tion of the ligand on the mer cury sur face, with the long axis re main ing per pen dic u lar to the metal sur face 3 .A shift in the re duc tion wave to wards more pos i tive po ten tials is ob served in the pres ence of the ligand 4 , as com pared with the wave of the metal aquo ion.The azide can there fore be clas si fied as a cat a lytic ligand 5 .
Studies of the elec tro chem i cal be hav ior of some met als in azide me dia show that the ex ten sion of the cat a lytic action changes from one metal to an other 6 .The ac tion of azide as a cat a lytic ligand de pends not only on ligand ad -sorp tion on the elec trode sur face, but also on the na ture of the metal pres ent.
The re sults ob tained by means of sys tem atic polarographic stud ies of the elec tro chem i cal be hav ior of some met als be long to the Irving-Williams or der 7 at the pres ence of azide are shown in this pa per.

Ex per i men tal
All re agents were chem i cally pure or of an a lyt i cal grade.The azide and per chlor ate so lu tions were stan dardized by the gravimetric method.The per chlor ate so lu tions of co balt(II), nickel(II), man ga nese(II), and zinc(II) were pre pared by re act ing perchloric acid in ex cess with the metal car bon ate.These so lu tions were stan dard ized by Atomic Emis sion Spec tros copy with In duc tive Ar gon Plasma 8 .The free-acid pres ent in the stock so lu tions was de ter mined by the method of stan dard ad di tion 9 .The ionic Ar ti cle strength was main tained con stant in 2.0 mol/L with the addi tion of so dium per chlor ate.
The nor mal and dif fer en tial pulse polarograms were car ried out with an EG&G-PARC 273A potentiostat attached to a Static Mer cury Drop Elec trode 303A cell stand.The work ing elec trode was a me chan i cally con trolled drop ping mer cury elec trode.The ref er ence elec trode was Ag/AgCl, and the aux il iary elec trode was a plat i num wire.All so lu tions were deoxygenated with ar gon, and the temper a ture was al ways 25 °C.The con cen tra tions of cobalt(II), man ga nese(II), and zinc(II) were 1 x 10 -3 mol/L, and for nickel(II) the con cen tra tion was fixed at 0.5 x 10 -3 mol/L to avoid polynuclear com plex for ma tion.The azide con cen tra tion was var ied from 0 to 2.0 mol/L.
The ex per i ments were car ried out with suc ces sive ad ditions of a metal so lu tion in 2.0 mol/L NaN 3 to a so lu tion in 2.0 mol/L NaClO 4 of the same metal.Since the metal concen tra tion in both so lu tions was the same, its con cen tra tion and the ionic strength re mained con stant, chang ing only the azide con cen tra tion.
Perchloric acid was added to so lu tions in the ex per iments with co balt(II) and zinc(II) in or der to pre vent the hy dro ly sis of the metal.The ad di tion of strong acid to the sys tem re sults in a buffer so lu tion formed by the weak acid, HN 3 , and its salt NaN 3 .The cor rect acid con cen tra tion was de ter mined us ing the Henderson-Hasselbach equa tion 10 .The ad di tion of perchloric acid was avoided as pos si ble due to the great ox i dant power of the HN 3 formed, which can cause cat a lytic waves, as ob served in the co balt(II)/azide sys tem 11 .

Re sults and Dis cus sion
The nor mal pulse re duc tion wave of co balt(II) in azide me dia shows a pro gres sive an tic i pa tion, with a split into two waves with E1/2 equal to -1.070 and -1.370 V vs. Ag/AgCl, re spec tively (Fig. 1).The cur rent as so ci ated with the first wave (E1/2 = -1.070V) in creases with the grad ual ad di tion of azide.With the sec ond wave (E 1/2 = -1.370V) the op po site oc curs: the cur rent de creases un til it dis appears for an azide con cen tra tion equal to 0.70 mol/L.In pre vi ous re search it was shown that the first wave cor responds to the re duc tion of the metal com plex spe cies, and the sec ond, to the free uncomplexed metal 12 .The split of the waves orig i nates in the fact that the polarographic exper i ments in volve time in ter vals too short to per mit the trans for ma tion of the metal aquo ion into the complexed spe cies af ter the later are re duced on the mer cury sur face.At high azide con cen tra tions, there is al most no metal aquo ion in so lu tion, and the re duc tion wave of this spe cies disap pears.
The nor mal pulse polarograms re corded for co balt(II) show a signficant de crease in the value of the lim it ing current at high azide con cen tra tions, prob a bly due to the kinetic prob lems as so ci ated with the elec trode pro cess un der these con di tions.
The max i mum an tic i pa tion of the to tal re duc tion wave in azide me dia as com pared with the metal re duc tion wave in perchorate me dia is ob tained for the ad di tion of 0.70 mol/L azide.This an tic i pa tion is equal to 300 mV.
In the in ter val of azide con cen tra tion from 0.7 to 1.0 mol/L, the E 1/2 of the re duc tion wave re mains prac tically con stant, and at con cen tra tions higher than 1.0 mol/L a shift in the re duc tion wave oc curs to wards more neg a tive po ten tials.The max i mum value, 30 mV, is ob tained for 2.0 mol/L azide.
The polarographic re duc tion wave of nickel(II) in perchlor ate me dia shows E 1/2 equal to -1.020 V vs. Ag/AgCl.With the ad di tion of azide, a split ting into two waves occurs with E 1/2 equal to -0.650 and -1.020 V vs. Ag/AgCl, respec tively (Fig. 3).This split ting is ob served for ligand con cen tra tions from 0.05 to 0.10 mol/L, and the first wave (E 1/2 = -0.650V) in creases, while the sec ond de creases with the pro gres sive ad di tion of azide.For ligand con cen -  tra tions equal to 0.15 mol/L or higher, the curves show only one wave with E 1/2 equal to -0.650 V.
The vari a tion of the re duc tion wave height of nickel(II) with the ad di tion of azide is not lin ear with azide con cen tration.A cur rent limit is reached with the ad di tion of 0.50 mol/L azide.For the aquo ion re duc tion wave, the cur rent de creases rap idly, and it be comes zero near 0.20 mol/L azide.
When the azide con cen tra tion is higher than 1.0 mol/L, the re duc tion wave shifts to wards more neg a tive po ten tials, and the most neg a tive value of E1/2 (cor re spond ing to a shift of 50 mV) was ob tained for an azide con cen tra tion of 2.0 mol/L.
The cur rent max i mum re corded for the re duc tion wave of nickel(II) was re ported in pre vi ous pa pers for the lig ands thiosulfate 13 and thiocyanate 14 , how ever the ex pla na tion of the pres ence of this max i mum is not given.Pre vi ous pa pers on max ima in pulse polarography re ported that the in crease in the pulse width can elim i nate the prob lem 15,16 .This ar tifice was used in the ex per i ments with nickel(II), and the results ob tained with pulse widths lon ger than 50 ms showed that the cur rent max ima de crease, but are not com pletely elim i nated.This in di cates that in the nickel(II)/ azide system there are other prob lems in the elec trode pro cess, result ing in the max ima and the in crease in the lim it ing cur rent ob served in both tech niques used.
The cur rents re corded by means of both of the techniques used (Figs. 3 and 4) show a sig nif i cant in crease with in creas ing azide con cen tra tion.This fact can be ex plained for ex per i ments with dif fer en tial pulse polarography, assum ing that this tech nique is sen si tive to the ki netic ef fects of the elec trode pro cess 17 .Thus, the in crease in the cur rent de notes a mi nor ir re vers ibil ity of the elec trode pro cess.
The ef fect of azide on the re duc tion wave from man ganese(II) dif fers com pletely of the re sults ob tained for cobalt(II) and nickel(II).The ad di tion of azide to a so lu tion of man ga nese(II), at an ionic strength of 2.0 mol/L per chlorate, causes a grad ual shift in the re duc tion wave to wards more neg a tive po ten tials (Fig. 5).The max i mum shift, 140 mV, was re corded for the ad di tion of 2.0 mol/L azide.
The dif fer en tial pulse polarograms for man ga nese(II) show a sig nif i cant de crease in cur rent for azide con cen trations higher than 0.50 mol/L (Fig. 6), in di cat ing a de crease in the re vers ibil ity of the elec trode pro cess.
For zinc(II), the ad di tion of azide at con cen tra tions lower than 0.030 mol/L causes a shift in the re duc tion wave to wards more pos i tive po ten tials in com par i son with the wave of the metal aquo ion (Fig. 7).The max i mum an tic ipa tion ob served was 27 mV.
For azide con cen tra tions higher than 0.030 mol/L, the re duc tion wave starts to shift to wards more neg a tive po tentials.The max i mum shift to the neg a tive po ten tial, 96 mV, was ob tained with the ad di tion of 2.0 mol/L azide.
An im por tant fact ob served for zinc(II) in the polarograms ob tained in both tech niques (Figs. 7 and 8) is the clear mod i fi ca tion of the re vers ibil ity in the pres ence of  azide.Small con cen tra tions of azide are suf fi cient to cause a great change in the re vers ibil ity of the elec trode pro cess, as ob served in the nor mal pulse polarograms -the curves in the pres ence of azide are more ver ti cal than the curve obtained in 2.0 mol/L per chlor ate -and the dif fer en tial pulse polarograms are more pointed in azide me dia.

Con clu sions
The re sults ob tained for co balt(II), nickel(II), man ganese(II), and zinc(II) in azide me dium con firmed the cat alytic ac tion of the ligand on the re duc tion pro cess, and the fact that this ac tion is de pend ent on the metal pres ent.
The cat a lytic ef fect was higher for nickel(II), fol lowed by co balt(II) and zinc(II).For man ga nese(II) this ef fect was not ob served.
The re sults ob tained show that the over all ac tion of the azide on the metal re duc tion pro cess is a re sult of two factors: 1) the ad sorp tion of the ligand on the mer cury sur face, form ing a bridge that makes the elec tron trans fer eas ier (ki -netic fac tor), con se quently shift ing the wave to wards more pos i tive po ten tials, and 2) the for ma tion of more sta ble complexed spe cies with a higher num ber of more sta ble coor di nated lig ands which need a higher overpotential to be re duced (ther mo dy namic fac tor), and there fore shift ing the wave to wards more neg a tive po ten tials.An other pos si ble im por tant fac tor in the over all re sults ob tained is the chem ical ki net ics of sub sti tu tion of the wa ter mol e cules by azide, which may be an ex pla na tion for the de crease in the lim iting cur rent (at high azide con cen tra tions) ob served for cobalt(II).
For co balt(II) and nickel(II), the elec tro chem i cal kinetic fac tor is pre pon der ant up to azide con cen tra tions of 0.70 and 0.50 mol/L, re spec tively, and then the re duc tion waves shift to wards more pos i tive po ten tials.Af ter an azide con cen tra tion of 1.0 mol/L for both met als, the thermo dy namic fac tor be comes re spon si ble for the shift of the re duc tion waves to wards more neg a tive po ten tials, due to the for ma tion of more sta ble com plexes.
For zinc(II), low azide con cen tra tions (lower than 0.030 mol/L) are suf fi cient to make the elec tron trans fer eas ier, and makes the elec trode pro cess more re vers ible.For higher azide con cen tra tions, how ever, a shift to wards more neg a tive po ten tials is ob served, in di cat ing the prepon der ance of the ther mo dy namic fac tor.
The low value ob served for man ga nese(II) ex plains the elec tro chem i cal be hav ior of this metal in azide me dia.The in ter ac tion be tween the metal and the ligand ad sorbed on the mer cury sur face be ing weak, the ef fect of the ligand as a bridge is not ver i fied.The pres ence of the ad sorbed ligand   makes the elec tron trans fer from the elec trode to the metal aquo ion pres ent in so lu tion more dif fi cult, shift ing the reduc tion wave to wards more neg a tive po ten tials.

Ac knowl edg ments
The au thors thank FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) for fi nan cial sup port.