DC-polarography and cyclic voltammetric studies of some mono and bis azo compounds derived from aromatic primary amines and 2,3-dihydroxynaphthalene in aqueous solutions

The DC and CV behavior of some mono and bis azo compounds based on aromatic primary amines and 2,3-dihydroxynaphthalein was investigated in Britton-Robinson buffer series. The obtained results indicated that these compounds undergo an irreversible reduction leading to cleavage of the N=N center with the formation of amine compounds. However, for the derivative m-CH3 on the aniline ring in alkaline solutions, the reduction stops at the stage for saturation of the N=N center. The E1/2 and Ep shifted to more negative potentials with rise of pH and the values of il are not much influenced except for the m-CH3 derivative. The total number of electrons involved in the reduction process was determined by controlled potential coulomety and calculated from Ilkovic equation. The effect of substituents on the electrode pathway was discussed. Based on the data obtained the electroreduction mechanism was suggested and discussed.

azo compounds based on aromatic primary amines and 2,3-dihydroxynaphthalene in which the azo linkage is in the o-position to the OH group of the dihydroxynaphthalene linkage. It is aimed to through light on the electrode reaction pathway of these azo compounds and effect of substituents in the phenyl ring on the reduction mechanism.

EXPERIMENTAL
All azo compounds used in the present investigation were pure chemicals from BDH or Aldrich. The azo compounds were prepared by diazotizing the aromatic amines then coupling with 2,3-dihydroxynaphthalene. For mono azo compounds, one equivalent of the diazonium salt was added slowly to the solution of 2,3dihydroxynaphthalene. For bis-azo compounds, two equivalent of the diazonium salt were added to the solution of dihydroxynaphthalene. The purity of the compounds was tested by elemental analysis, IR, 1 HNMR and UV/V is spectroscopy. The compounds obtained have the following structural formula: ethanol to the buffer solutions was necessary to affect good solubility of the azo compounds. The reduction of the mono-and bis-azo compounds (I & II) at the dropping mercury electrode gave a single reduction wave (Fig.1). The limiting current (i l ) for the bis azo compounds is almost twice as that of the mono azo ones. The value of the current in strong acid media (pH 2) corresponds to the reduction of the N=N centers by the uptake of 4 electrons giving two amine molecules: The reduction current was slightly influenced by rise of pH for both series I&II ( 5-10% decrease from pH 2 to pH 11) which can be attributed to decreased diffusion coefficient of the reducible species with rise of pH. This results from the ionization of the OH groups leading to anionic form which exert repulsive action with the electrode surface. Also, the decreased participation of adsorption to the electrode reaction can contributed to this decrease in i l at higher pH. The effect of mercury pressure on the limiting current revealed that the reduction process is mainly controlled by diffusion with some adsorption or kinetic contribution. The value of the of the exponent x in the relation 20 : i l = kh x amounts to (0.65-0.43), the value in general decreases with rise of pH.
Analysis of the reduction waves using the fundamental equation for the polarographic waves 18 showed that the electrode reaction is irreversible. The transfer coefficient () and the number of electrons participating in the rate-determining step (n a ) were determined from the reciprocal slope of logarithmic analysis (S 1 ) and given in Table 1. The most probable  values were obtained for n a =1, which denoted that the rate-determining step should involve one electron. The half-wave potential (E 1/2 ) of the reduction waves shifted to more negative values with rise of pH indicating the contribution of H + to the electrode reaction and that the proton uptake should precede the electron transfer 16 . The E 1/2 -pH curves comprised two segments with inflection point at pH 6-7. The number of protons 5×10 -3 M stock solutions of the azo dyes in consideration were prepared by dissolving the accurate weighed required amount in ethanol then completed to the volume in a calibrated measuring flask. Buffer solutions used for control of pH were the components of the Britton-Robinson 18 universal series containing 50% by volume ethanol to affect complete dissolution of the organic compounds.
DC-polarograms were recorded on an ink recording Sargent Welch polarograph using a dropping mercury electrode (DME) with m = 1.7 mgs -1 at mercury height (h) = 50 cm. The mercury used was purified according to the recommended method [19]. Cyclic voltammetry measurements were done by the aid of polarographic analyzer model 264A-PAR from (EG&G) provided with the electrode assembly 303A with a hanging mercury drop as working electrode (HMDE), a Pt wire as counter electrode and Ag/AgCl reference electrode. X-Y recorder model RE 0091 (Houston Instrument Division, EG&G) was used for recording the voltammograms.    involved in the rate-determining (p) was determined from the slopes of E 1/2 -pH plots (S 2 ) and the plots of logarithmic analysis (S 1 ) using the relation 21 :

RESULTS AND DISCUSSION
The value of (p) as determined for all azo compounds under study was found to be unity Table 1.

Cyclic voltammetry
The mono-and bis-azo compounds (I & II) were studied by cyclic voltammetry at the HMDE in buffer solutions containing 50% by volume ethanol. Three pH media (pH 3, 7,9) were chosen for this study. The voltammograms were recorded at different sweep rates from 20-500 mV/s. As shown in Fig.2, the voltammograms consist of a single cathodic peak but not anodic peaks were appeared in the reverse scan. This denoted the irreversible nature of the electrode reaction. On increasing the sweep rate () the peak potential (E p ) shifted to more negative values confirming the irreversibility of the reduction process. The values of the transfer coefficient () were determined by making use of the Galus equation 22 which relates the E p and the n a . E p = -1.14 (RT / n a F) + (RT / n a F) lnk o f , h /D 1/2 ) -RT / n a F ln(n a ) The plots of E p versus logarithm of sweep rate (ln) yielded straight lines of slopes (S 3 ), from these slope values n a were calculated and given in Table 2. The most probable values of á were obtained at n a =1. This indicated that the ratedetermining step of the electrode reaction should involve one electron as confirmed before from DC polarography.
On plotting the peak current (i p ) as a function of the square root of sweep rate ( 1/2 ) linear relations passing through the origin were obtained which indicated that the electrode reaction is mainly governed by the diffusion of the reducible species 23 .

Effect of substituents
From dc-polarography, the half-wave potential (E 1/2 ) values were found to be influenced by the nature of the substituent in the phenyl ring. Generally, it was found that E 1/2 shifted to more negative potential with decreased electron acceptor character of the substituent or when its electron donor character increased. Thus, the plots of E 1/2 as a function of Hammett constants ( x ) gave linear relation (Fig. 3) indicating the validity of the Hammett free energy relationship in the form 24 : From the value of the slope, , amounting to 0.10 at pH 3. The positive slope of E 1/2 - x plots indicates that the reduction process is influenced by nucleophilic attack in the potential determining  ( Table 3).
ii -Using controlled potential electrolysis (cpe): In this method, the background current was measured for the supporting electrolyte (buffer), then the desired potential was applied to the working electrode and the depolarizer solution (1x10 -3 M) was introduced into the electrolysis cell. The electrolysis process allowed to proceed till completion of the reduction process and the accumulated charge (Q) was read directly from the digital coulometer. The average number of electrons n involved in the overall electrode reaction was then calculated using the well known relation: in which F is the Faraday constant (96487 coulombs), w is the weight of depolarizer in gram, M its molecular weight. The data obtained of cpe measurements were collected in Table (3 The products of electrolysis of azo compounds I a , II a , I d and II d were concentrated by extraction with diethyl ether then subjected to TLC test. The TLC extract of the four azo compounds showed two spots in acid solutions (pH 3), while in alkaline solutions (pH 9), azo compounds I d and II d gave only one spot. These results denoted that for azo compounds I d and II d , the reduction process in alkaline solutions led only to saturation of the N=N centers, and the hydrazo compound is obtained, while in acid solutions the cleavage of the N=N bond occurs giving the aniline derivative and the aminodihydroxynaphthalein.

Sequence of the electrode reaction
The data obtained from dc-polarography, cyclic voltammetry and controlled potential electrolysis of mono-and bis-azo compounds under study revealed that the rate-determining step of the electrode reaction involved one electron and one proton and the protonation of the depolarizer should precede the electron transfer. In the light of the explanation of Zuman 21 and the data gained in the present study, the sequence of electrode reaction would be H + , e, e, H + according to Scheme 1. ... (2) ... (3) ... (4) ... (5) ... (6) ... (7) ... (8)