الفهرس 
VoltammetryOnly 14 pages are availabe for public view
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Abstract
The work in this Thesis aimed to elucidate the electrode reaction pathways of some biologically active compounds {levobunolol HCl (LV.HCl), Mesterolone (MST) and Pipazethate HCl (PZ.HCl)} in buffered solutions of various pH values, and identification of their adsorptive behavior at the mercury or unmodified carbon past electrode (Bare CPE) or that modified with sodium montomorillonite clay (Na-MMT modified CPE). This work was aimed also to optimize electroanalytical methods for trace determination of the investigated compounds in their bulk forms, formulations and in spiked human serum at the mercury electrode (using cathodic stripping voltammetry method), unmodified carbon past electrode (Bare CPE) or that modified with sodium montomorillonite clay (Na-MMT modified CPE (using anodic stripping voltammetry method). The methods are based on the combination of a preconcentration step with the advanced voltammetric measurement which generates extremely favorable signal-to-background ratio that characterizes stripping voltammetric analysis. Compared to solid electrode materials, mercury is a very attractive choice of material because it has a high hydrogen overvoltage that greatly extends the cathodic potential window, and possesses a highly reproducible, readily renewable, and smooth surface. Disadvantages of the use of mercury are its limited anodic range (due to the oxidation of the mercury) and its toxicity. Solid electrodes with extended anodic potential windows have attracted considerable interest. Carbon paste electrode (CPE) is a mixture of an electrically conducting graphite powder and hydrophobic adhesive (organic liquid as binder). It has been extensively used in electro-analytical chemistry because of its excellent properties: wide potential range (from –1.40 to 1.30 V), easy preparation, convenient surface renewal, low residual current, porous surface, and low cost. However, most of the recent studies on CPE showed that its sensitivity is relatively poor. This may be attributed to its high surface hydrophobicity which is mainly responsible for high overpotential (irreversibility) and sluggishness of the kinetics of the electrode process, resulting in weak electrochemical responses. Often undesirable overpotential at CPEs can be suppressed by means of surface hydrophilization by intensive electrolysis or chemical treatment, erosion by surfactants in situ, or using special modifiers. In spite of the wide range of electrode modifiers, clays have attracted the interest of electrochemists, in particular for their analytical applications. Clay minerals are naturally occurring materials, cheap and widely available. Their well-defined layered structure and flexible adsorptive properties make them interesting materials that can be used as catalysts or catalytic supports, or sorbents for inorganic and organic compounds. Montmorillonite (MMT) clay is of a 2. 1 layered structure, a single layer of aluminum octahedral sheet sandwiched between two layers of silicon tetrahedral sheets. Montmorillonite particles carry two kinds of electric charges: a surface charge (a variable (pH dependent) charge resulting from proton adsorption/desorption reactions on the surface of hydroxyl groups) and a structural (permanent) negative charge resulting from isomorphous substitutions within the clay structure. As a consequence of this negative potential, montmorillonite has negative electrophoretic mobility and very important cation adsorption and cation exchange properties. The negative charges of the layers are compensated by cations Ca2+ or alkali-metal ions bounded between the layers. These cations can be exchanged by other inorganic or organic cations (Cation exchange capacity of montmorillonite is typically between 0.80 and 1.50 mmol g.1 and anion exchange about four times lower). Also, these cations are strongly hydrated in the presence of water to render natural clay hydrophilic. Owing to the hydrophilic environment at surface and low organic carbon content, natural clay is an effective adsorbent for ionic organic species. On other side, the OH and O atoms on the broken edges of montmorillonite hydrolyze and form Lewis acid or Lewis base functional groups that are the sources of the pH-dependent charge. Based on the molecular structure of montmorillonite, cations can also be adsorbed by either electrostatic attraction on the basal plane or formation of covalent bonds with the functional groups on the broken edges. So, MMT has been widely used to modifying electrodes to improve their determination sensitivity towards anions, cations, and organic compounds. The thesis comprises of three main chapters: Chapter I: This chapter contains a general introduction concerning the principles of different voltammetric techniques. Chapter II: This chapter contains descriptions of nature of the investigated biological active compounds (Pharmaceutical compounds) and their formulations. Description of the apparatus used in the measurements, methods of preparation of solutions, methods of preparation of unmodified carbon past to use it as bare CPE and modified carbon past with sodium montomorillonite clay (Na-MMT clay) to use it as modified CPE. Procedures used for assay of the investigated compounds in their bulk forms, formulations and in spiked human serum or urine were also described. Chapter III: This chapter contains literature survey concerning the published analytical methods for quantification of each of the investigated compounds. It contains also the results and discussion of the electrochemical behavior and quantification studies of the examined compounds at the hanging mercury DROP electrode (HMDE) and unmodified carbon past electrode (Bare CPE) or that modified with sodium montomorillonite clay (Na-MMT modified CPE). The different studies carried out in the present thesis and the obtained results can be summarized in the following points: 1- The electrochemical behavior of LV.HCl drug (which is commonly used in the topical treatment of increased intraocular pressure in patients with chronic open-angle glaucoma or ocular hypertension) was studied in the B-R universal buffer of various pH values at the HMDE using cyclic voltammetry. The compound has been reduced in a single 2-electron irreversible cathodic wave over the entire pH range which attributed to reduction of the >C=O double bond of the reactant molecule to –CHOH. The peak potential (Ep) of this wave shifted to more negative potentials with the increase of pH of the medium, indicating the participation of H+ ions in the electrode reaction of LV.HCl. However, at pH . 11.0, an ill-defined second small reversible reduction peak appeared at more negative potentials beside the main one. The voltammogrames were also recorded at different scan rates (v). from the slopes of (Ep – pH) and (Ep – ln v) plots, the numbers of protons (ZH+) and electrons (na) involved in the rate-determining step of reduction process were calculated. Accordingly, the electrode reaction pathway of LV.HCl at the mercury electrode was suggested and discussed. 2- The electrochemical behavior of MST drug (is a sex hormone. It can be used to treat disorders in which the body does not produce enough male hormones naturally, or it can be used to enhance the already-normal levels of male hormones for muscle build-up. It can also be prescribed for some types of sexual dysfunction, such as disinterest, impotency, and low sperm count) was studied in the B-R universal and borate buffer of various pH values at HMDE using cyclic voltammetry. At scan rate (v . 50 mVs–1); the reduction of MST took place in two splitting peaks of unequal height. The 2nd splitting cathodic peak is ill-defined and less developed than the first one. No any corresponding anodic peaks were observed which may be attributed to a chemically coupled process. As the scan rate increased, the 2nd splitting peak became more developed and a small broadic anodic peak was also observed. At v . 300 mVs–1, the 1st and 2nd splitting cathodic peaks also overlapped and the intensity of the anodic peak increased gradually with increasing scan rates but the ratio of ipa/ipc is still less than unity even at scan rate 500 mVs–1. These two splitting cathodic peaks can be attributed to the reduction of the carbonyl group of MST via two-one electron per molecule, wherever, the single peak at v . 300 mVs–1, can be attributed to the reduction of the carbonyl group of MST via two electrons to the dianion species. Analysis of the effect of scan rates and concentration of the reactant on the voltammograms indicate the formation of a dimmer by an ErevC2irr mechaniem (reversible electron transfer followed by irreversible second order chemical reaction) at the monoanion level of the studied compound. Electron transfer coefficient (.) indicates that the system is quasi-reversible. Number of electrons (na) involved in the rate- determining step of reduction process was calculated and the electrode reaction pathway of MST at the mercury electrode was suggested and discussed. 3- The electrochemical behavior of PZ.HCl drug (which is a non narcotic antitussive drug that acts by suppressing irritable and spasmodic cough) was studied at the bare CPE. The voltammograms exhibited no obvious oxidation peak indicating that the electrochemical activity of PZ.HCl at the bare CPE is very poor and/or bare CPE possesses poor adsorption ability towards PZ.HCl. The probable reason may be attributed to that PZ.HCl is in the cationic/protonated form (in this work pKa was found to be 9.0; due to deprotonation of the basic aliphatic-nitrogen atom of piperidine ring). However, carbon pastes have typically hydrophobic surface which, in aqueous solutions, repel hydrophilic species involved in the electrode transformations of numerous redox systems of both inorganic and organic origin. The hydrophobicity of the pasting liquid at the CPE surface may hinder the access of the PZ.HCl species to the electrode surface and adsorption of its hydrated species in the lipophilic layer formed by the organic liquid. It may also decrease the electron transfer rate (slower kinetic), causing a high oxidation over- potential (irreversibility) and shift the response of interest towards the corresponding potential limit and therefore obscured or completely overlapped by higher background currents. Whereas, a well-defined anodic peak of enhanced peak current magnitude was observed using the developed Na-MMT modified CPE reflecting the electrocatalytic efficiency of the Na- MMT clay. This may be due to its high adsorption ability to PZ.HCl attributed to its well-layered structure, a very high surface area (150 to 200 m2 g–1) and the negative charge held by its platelets that promote electrostatic interactions with cationic or protonated PZ.HCl species. This negatively charged of hydrophilic layered silicate can facilitate reaching of protonated PZ.HCl species to the electrode surface. As a consequence it facilitates its adsorption at Na-MMT modified CPE surface and hence it makes its oxidation easier than that at the bare CPE Cyclic voltammograms recorded in solutions of pH 2 to 11 exhibited a single main well-defined anodic peak over the pH range of 2 to 6, which is due to oxidation of the aliphatic nitrogen atom of piperidine ring of the analyte molecule (protonated form). While at pH . 7.0, the main oxidation peak splits into two peaks. The new splitting peak appeared at less positive potential with respect to the main oxidation one is due to uncharged basic form (a conjugate base is oxidized at less positive potentials than the corresponding acid form). from the Ep versus ln v plots, values of . (symmetry transfer coefficient) and number of electrons transferred in the rate-determining step were estimated. Electrode reaction mechanism of Pipazethate HCl at the developed Na- MMT modified CPE surface was suggested. 4- The interfacial adsorptive behavior of LV.HCl and MST at the mercury electrode surface and PZ.HCl at the CPE modified with Na-montmorillonite natural clay was identified. This was carried out by recording cyclic voltammogram of each of the investigated analyte following its preconcentration onto the working electrodes by adsorptive accumulation under open circuit conditions and then at a given applied preconcentration potential (Eacc) for a certain preconcentration time (tacc).. Cyclic voltammogrames of 1st and 2nd cycles of the three compounds under investigation indicated their strong adsorptive behavior at the working electrode surface. This behavior was very promising application in the trace quantification of the investigated (LV. HCl and MST) compounds by adsorptive cathodic and (PZ.HCl) by anodic stripping voltammetry techniques using different potential- waveforms. Beside, charge transfer resistance value change (from 200.59 . for bare CPE to 219.21 . for that modified with 7% (w/w) Na-MMT) was characterized by electrochemical impedance spectroscopy (EIS). However, its value was increased to 259.69 . at 9% (w/w) Na-MMT modified CPE. This leads to hinder the electron transfer process, distort appreciably the symmetry of the peak and increase the background current (decrease signal response to background noise ratio). This status reflected the competitive relationship between the adsorption and conductive abilities of the electrode. Furthermore, SEM was used to analyze the morphological of surface of the CPE before and after modification with the layer lattice Na-MMT clay. SEM image of CPE modified with 7%(w/w) Na-MMT clay reflects the highest surface area compared with the others due to its small particle size of discrete spherical particles morphology and distinguishable internal holes. BET adsorption isotherm indicated that the surface areas for bare CPE and that modified with 5 and 7% (w/w) Na-MMT were 1.31, 1.43 and 1.78 m2/g, respectively. To account for absorption behavior of the PZ.HCl at the developed modified CPE, Langmiur isotherm equation related to surface coverage is used. A plot of C/isat vs. C at 200 s yields good linearity, with a correlation coefficient (r) of 0.999. from the intercept of the line, . (monolayer binding constant) of 8.49×105 mol–1 L was obtained, indicating that Langmuir isotherm is valid for such system. The obtained values of the separation factor or equilibrium parameter RL (0.19 - 0.55) lie between 0 to 1 at all initial PZ.HCl concentrations confirm the adsorption process of PZ.HCl at modified Na-MMT CPE is favorable. The calculated value of .Go (– 33.83 KJmol–1), indicates that the adsorption mechanism of PZ.HCl onto the developed 7% (w/w) Na-MMT modified CPE can be mainly described in terms of physical interaction (for instance by electrostatic or Van-der- Vaals forces) with slightly chemisorption contribution. 5- Based on the strong adsorptive character of LV.HCl and MST at the HMDE surface and PZ.HCl at the CPE modified with Na-montmorillonite natural clay, rapid, simple, selective and sensitive adsorptive cathodic or anodic stripping voltammetric methods were developed for their trace quantifications in their bulk forms using different potential-waveforms {linear-sweep (LS) and square-wave (SW)}. Validation of the described stripping voltammetric methods for assay of the bulk forms of the investigated compounds was examined via evaluation of the linear dynamic range, limit of detection (LOD), limit of quantitation (LOQ), repeatability, reproducibility, precision, selectivity, robustness and inter-laboratory precision. The described stripping voltammetric methods were sensitive and reliable for determination of the examined compounds at trace levels. 6- The developed stripping voltammetric methods were successfully applied to the direct determination of each of the examined compounds (LV. HCl, MST and PZ.HCl) in their commercial pharmaceutical formulations {(Betagan® drops), (cidoviron tablets) and (Selgon ® tablets)}, respectively, without the necessity for samples pretreatment and / or time-consuming extraction steps prior to the analysis. The satisfactory results obtained by means of the described stripping voltammetric methods were statistically compared with those obtained by a reported method. 7- The developed stripping voltammetric methods were also successfully applied to the direct determination of the three compounds under investigation in spiked human serum samples at trace levels without the necessity for samples pretreatment and /or time-consuming extraction steps prior to the analysis. The methods are sensitive, simple, precise, and applicable to a wide range of concentration of the compounds under investigation, besides being less time-consuming and offering economic assay of these compounds comparable, at least, to the reported methods. All the described methods could be recommended for use in quality control and clinical laboratories. Three publications were extracted from the obtained results of the present Thesis. Two of these publications were published in international scientific Journals. The third one is still under reviewing.