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Abstract
The present thesis includes three chapters, introduction, experimental and results and discussion.
In the first chapter, a literature survey of the previous studies on the polarographic behaviour of both azo dyes and Schiff bases. It includes also the potentiometric studies on azo-compounds and Schiff bases and their complexes with different metal ions. Also the studies on corrosion inhibition of carbon steel are included.
The experimental part (chapter II) includes preparation of the azo-dye and Schiff bases compounds derived from 2-amino-3-hydroxy pyridine, instruments and solutions which were used for the measurements using different techniques.
Chapter (III) includes the results and discussion and it consists of three parts:
Part (A):
It includes the results and data obtained from polarographic studies of the azo dye and Schiff base compounds in aqueous buffer solutions of different pH values (2-12) containing 10% (v/v) ethanol for azo-dyes (first series) and Schiff bases (second series).
The polarographic behaviour of azo compounds of the first series (Ia-e) and Schiff bases of second series (IIa-e) exhibited a single polarographic wave within the entire pH range except for compounds Ib and Ie. Generally, the limiting current (il) of the reduction waves of azo compounds is considered to be almost pH-independent, revealing that the total number of electrons consumed in the reduction process is the same in both acidic and alkaline solutions. But for Schiff base, the wave height in acidic solutions is almost twice that in alkaline ones, i.e., the reduction process of Schiff bases involve 4 electrons in acid solutions and 2 electrons in alkaline medium.
On the other hand, the half-wave potential of the waves get shifted to more negative potentials on increasing pH of the electrolysis medium denoting that hydrogen ions are consumed in the reduction process. The plots of the half-wave potential (E1/2) vs. pH for all the compounds give satisfactory linear correlations consisting of one segment except Ie consist of two segments, the breaks occurred at pH = 5.31.
The effect of mercury height on the limiting current denoted that the reduction process of all compounds is mainly diffusion-controlled with some adsorption contribution, since the values of the exponent x in the relation (il = KhX) equal to 0.4-0.8 for first series (Ia-e) and 0.35-0.79 for the second series (IIa-e).
Logarithmic analysis of the polarographic waves using the fundamental equation of polarography confirmed the irreversible nature of the waves. The plots of Ed.e. vs. log [i/(id-i)] give linear correlations. from the slopes of these plots, values of the transfer coefficient () were calculated and found to be less than unity at na = 2. The number of protons (ZH+) participating in the rate determining step was determined from slopes of logarithmic analysis and those of E1/2-pH curves and found to be one for all depolarizers in both acidic and alkaline solutions, i.e. the rate-determining step involved two electrons (na = 2.0) and one proton (ZH+ = 1.0).
The kinetic parameters of the electrode reaction; the rate constant (Kof,h) and the activation energy (G*) were evaluated from DC-polarographic measurements. The obtained results revealed that the values of Kof,h are found to be decreased and subsequently the activation energy (G*) increased on increasing pH of the electrolysis medium, which denoted the more irreversible nature of reduction process on going from acidic to alkaline part of the pH range.
Effect of substituents on E1/2 was considered, and the plots of E1/2 as a function of Hammett substituent constant (x) at pH values 5.0 and 9.0 showed linear correlations with positive slopes. Generally, it was found that the electron withdrawing group (m-NO2) shifts the E1/2 to less negative value i.e., accelerates the reduction process, whereas electron-donating groups (p-CH3, p-OCH3, p-N(CH3)3) shift the E1/2 to more negative values, i.e. retards the reduction process.
The diffusion coefficient values of the depolarizers of series I & II and the total number of electrons consumed in the overall reduction process were determined using Ilkovic equation. The results obtained revealed that the reduction process of the azo compounds (Ia-e) consumed 4 electrons / depolarizer molecule, whereas 8 electrons are consumed for compound 3-NO2 (Ie) in both acidic and alkaline solutions. For Schiff bases (IIa-e) the reduction process consumed 4 and 2 electrons in acidic and alkaline solutions, respectively. The results were confirmed by controlled potential coulometry (cpc) measurements. The suggested mechanistic pathway of the electrode reaction was established and discussed.
Part (B):
It includes potentiometric studies of compounds under investigation and their complexes with Mn2+, Fe2+, Co2+, Ni2+ and Zn2+ ions. from potentiometric titrations of azo dyes and Schiff bases the proton-ligand stability constants log KH are determined as well as the formation constants of the complexes log KM.
An inspection of the results reveals the pKH values of series Ia-e and series IIa-e are influenced by the inductive or mesomeric effect of substituents. The general trend of these compounds revealed that, compounds containing electrodonating groups have higher pKH values (a lower acidic character which enhanced the electron density by their high positive inductive or mesomeric effect, so a stronger O–H bond is formed. On the other hand, compounds containing electron withdrawing groups have lower pKH values since it have the opposite effect. For Ia, Ic and IIa compounds the order of stability constant for metal ions under consideration is Zn2+ > Ni2+ > Co2+ > Fe2+ > Mn2+ which indicate that the stability increases with decrease of ionic size i.e. from Zn2+ to Mn2+. The other orders of increasing stability of other ligands with metal ions not follow the ionic model and formed through covalent bonding between the central metal ion and ligand metals.
Part (C):
It contains the results of galvanostatic polarization. The effect of azo dyes and Schiff bases under investigations as inhibitors on the cathodic and anodic polarization of carbon steel in 0.1 M HCl solution was studied. Corrosion rate (Icorr.) was found to decrease with increase of the concentration of the additives. The polarization curves indicate that these compounds influence both cathodic and anodic processes (i.e. mixed inhibitors).
The inhibition efficiency azo compound decreases according to the following sequence:
Id > Ib > Ic > Ia > Ie
The order for Schiff bases is:
IIe > IId > IIb > IIc > IIa
The results showed that the adsorption of azo dye and Schiff base compounds on the carbon steel surface follows Langmuir adsorption isotherm. The values of the equilibrium constant of adsorption K and Goads of the inhibitors adsorbed on the surface of carbon steel were calculated. The adsorption process of azo dyes and Schiff bases on the carbon steel surface was found to be spontaneous from the negative value of Goads. Also, the mechanism of interaction between azo dyes and Schiff bases with steel surface to form stable complex was established and discussed. The inhibition efficiency increases with increasing the concentration of azo and Schiff base compounds and the presence of electrodonating group such as –OCH3, -N(CH3)2, -(CH3), -OH, -Cl is more inhibiting effect than electro withdrawing group as –NO2. The strong electrodonating group facilitates the formation of stable complex because it increase the electron density at the ring and thus, decreases the withdrawing nature of it.