Georgina Alarcón-Ángeles

Spectrophotometric study on the stability of dopamine and the determination of its acidity constants

The interest in determining the acidity constants of the catecholamines stems from the fact that they play rather an important biological role. The present work reveals the effect of different parameters such as oxygen, light, analysis time and pH on the dopamine oxidation process, where oxygen has an effect on the dopamine oxidation of 40% and up to 20% is attributed to exposure to light as a function of the pH. The application of adequate control on the said parameters (which ensured stability of the dopamine) facilitated the determination of the corresponding three acidity constants, 9.046+/-0.147, 10.579+/-0.148 and 12.071+/-0.069.

Simultaneous Electrochemical Determination of Adrenaline and Ascorbic Acid: Influence of [CTAB]

A carbon paste electrode modified with the cationic surfactant cetyltrimethyl ammonium bromide (CTAB) was used to carry out an electrochemical study at pH 3 in 0.1 M NaCl by differential pulse voltammetry to discern the effect of the surfactant on the electrochemical signals of adrenaline (AD) and ascorbic acid (AA). It was shown that in the absence of CTAB, AA and AD voltammetric signals overlap in the 500-600 mV potential interval. However, in the presence of CTAB, the AD oxidation process was not favored, whereas the opposite is true for the AA, which leads to an effective peak separation. This method permitted to obtain reliable analytical parameters to undertake the determination of AD in the presence of AA as the detection limit was 7.20 ± 0.04 μM,the quantification limit was 24.32 ± 0.08 μM,the sensitivity was 2.478 ± 0.067 μA mM -1 , and the linearity range was 0.01-0.27 mM. This approach was used effectively to determine AD in real pharmaceuticals obtaining satisfactory results. Moreover, it is shown that the simultaneous determination of AD and AA may be performed and that AD can be adequately determined even in the presence of both AA and uric acid.

Guest-host complex formed between ascorbic acid and β-cyclodextrin immobilized on the surface of an electrode.

This work deals with the formation of supramolecular complexes between ascorbic acid (AA), the guest, and β-cyclodextrin (β-CD), the host, that was first potentiodynamically immobilized on the surface of a carbon paste electrode (CPE) throughout the formation of a β-CD-based conducting polymer (poly-β-CD). With the bare CPE and the β-CD-modified CPE, an electrochemical study was performed to understand the effect of such surface modification on the electrochemical response of the AA. From this study it was shown that on the modified-CPE, the AA was surface-immobilized through formation of an inclusion complex with β-CD, which provoked the adsorption of AA in such a way that this stage became the limiting step for the electrochemical oxidation of AA. Moreover, from the analysis of the experimental voltammetric plots recorded during AA oxidation on the CPE/poly-β-CD electrode surfaces, the Gibbs’ standard free energy of the inclusion complex formed by the oxidation product of AA and β-CD has been determined for the first time, ∆G0inclus = −36.4 kJ/mol.

Development a Boron Potentiometric Determination Methodology Using a Carbon Paste Electrode Modified with a β-Cyclodextrine- Azomethine-H Inclusion Complex

In this work it is shown that a carbon paste electrode modified with a β-Cyclodextrine-Azomethine-H inclusion complex is useful for the potentiometric determination of H3BO3 in aqueous solution where Azomethine-H plays the role of recognition agent for H3BO3. A Nernstian slope was obtained for the E vs. log [H3BO3] however after the first determination, the amount of Azomethine-H on the electrode surface diminished to about 80 % of the initial value. In order to improve the electrode time-life two polymers: glutaraldehide and nafion were used to avoid the loss of the recognition agent.