Spectro-electrochemical characterization and quantification of Rutin in aqueous media.

From UV-Vis spectrophotometric measurements the acidity constants of Rutin in aqueous media, at 25 °C and 0.1 M ionic strength, were determined as: pK1 = 4.392 ±â€¯0.167, pK2 = 7.130 ±â€¯0.050, pK3 = 8.661 ±â€¯0.042 and pK4 = 12.354 ±â€¯0.020 and the molar absorptivity coefficients of all the Rutin pH-dependent species were reported as a function of wavelength. Furthermore, the electrochemical behavior of Rutin at neutral pH was investigated using a bare carbon paste electrode, CPE. It was found that both: Rutin electrochemical oxidation and reduction are reversible, adsorption-controlled processes, involving 2 electron transfers. Moreover, the bare CPE was used for the electrochemical quantification of Rutin in neutral aqueous media, displaying the following features: (1.078 ±â€¯0.440) µM, (3.594 ±â€¯0.400) µM and (0.308 ±â€¯0.014) µA µM-1 for the detection and quantification limits and sensitivity, respectively, within the 1-11 µM linear range. Meanwhile the spectrophotometric method displayed the following analytical features: (3.385 ±â€¯1.318) µM, (11.283 ±â€¯3.114) µM and (0.0120 ±â€¯0.0001) AU µM-1 for the detection and quantification limits and sensitivity, respectively within the 11-110 µM linear range. In like manner, the bare CPE is also shown as a robust electrochemical sensor that allows Rutin quantification even in the presence of ascorbic acid, commonly found in Rutin samples.

New insights on the spectrophotometric determination of melatonin pKa values and melatonin-ßCD inclusion complex formation constant.

Using UV-Vis spectrophotometry a stability study of melatonin at different pH values was done in aqueous media, finding that at acidic pH melatonin is unstable when interacting with the environment, however it becomes stable protecting it from light and oxygen. From the UV-Vis spectra and SQUAD software, melatonin pKa values, in a completely protected aqueous medium, were estimated as 5.777±0.011 and 10.201±0.024. Using the same techniques, the melatonin and ß-cyclodextrin inclusion complex formation constants were assessed at pH3, 7 and 11.5, giving the values of log ß=(3.07±0.06), (2.94±0.01) and (3.07±0.06) M-1, respectively. From the global acidity formation constants and the complexes' formation constants, the molar fractions were determined for each species of MT and MT-ßCD, to build the molar fraction-[ßCD]-pH 3D diagram and the molar fraction-pH 2D diagrams, where it was possible to observe the predominance of the MT species with and without ßCD. A voltammetric study at pH3, allowed obtaining a value of log ß=(3.15±0.01) M-1, which corroborates that obtained through UV-Vis spectrophotometry, supporting strongly the rationale behind using simple, straightforward techniques.

Spectrophotometric quantification of the thermodynamic constants of the complexes formed by dopamine and Cu(II) in aqueous media.

The thermodynamic constants of the complex Cu(II)-dopamine in aqueous solution were evaluated from spectrophotometric data using the software SQUAD. It was found that there exist Cu(II):DA complexes with 1:1 and 1:2 stoichiometries and that their predominance depends on both the solution pH and the [Cu(II)]/[DA] ratio. Moreover, it is shown that the solubility of Cu(OH)2(s) increases drastically when these complexes are thermodynamically stable.

Study on the stability of noradrenaline and on the determination of its acidity constants.

Noradrenaline is a catecholamine which has been largely recognised to play a very important role in biological systems. In view of the neurotransmitter's alleged importance, this work aimed at showing the influence of time on its spectral behaviour using different analytical methods and determining its acidity constants through spectrophotometric titration and by the so-called point-by-point analysis, where the samples are freshly prepared for each pH value investigated at the instant required. Because the catecholamines are light-sensitive and likely to react with the oxygen in the surrounding air, both methods used preclude its incidence onto the samples being analysed under the presence of a nitrogen atmosphere maintained over the solutions. The constants obtained through point-by-point analysis were log beta1 = 30.71+/-0.16, log beta2 = 22.00+/-0.15 and log beta3 = 11.69+/-0.16.

Study on the stability of the serotonin and on the determination of its acidity constants.

The present work aimed at describing the spectral behaviour of the serotonin and to evaluate its acidity constants using three different methods, using two spectrophotometry titrations and a third method that involved point-by-point analysis, which permitted to monitor closely and determine the evolution of the serotonin species in solution as a function of time. The three methods allowed estimation of three acidity constants associated to the same number of functional groups that form part of the molecule. The results given by the point-by-point analysis were: log(beta1) = 24.95 +/- 0.12; log(beta2) = 20.20 +/- 0.10; log(beta3) = 10.89 +/- 0.018.

Study on the stability of adrenaline and on the determination of its acidity constants.

In this work, the results are presented concerning the influence of time on the spectral behaviour of adrenaline (C(9)H(13)NO(3)) (AD) and of the determination of its acidity constants by means of spectrophotometry titrations and point-by-point analysis, using for the latter freshly prepared samples for each analysis at every single pH. As the catecholamines are sensitive to light, all samples were protected against it during the course of the experiments. Each method rendered four acidity constants corresponding each to the four acid protons belonging to the functional groups present in the molecule; for the point-by-point analysis the values found were: log beta(1) = 38.25 +/- 0.21, log beta(2) = 29.65 +/- 0.17, log beta (3) = 21.01 +/- 0.14, log beta(4) = 11.34 +/- 0.071.

Evaluation of the acidity constants of the 4-hidroxy-5-[salicylideneamino]-2-7-naphthalenedisulfonic acid (Azomethine-H) using UV-vis spectrophotometry.

The time stability of the azomethine-H species was determined not to be better than 10 min in the absence of oxygen and light, however under phosphate buffered conditions the azomethine-H species remained stable for longer periods, as indicated by the spectrophotometric behaviour. Nevertheless, the analysis time still exceeded the stability allowance. Therefore, the determination of the acidity constants of the Azomethine-H species was studied by means of UV-vis spectrophotometry in buffered media by means of the point-by-point analysis and data processing with SQUAD to refine the resulting constants, which were: pK(a1) = 3.39, pK(a2) 7.36 and pK(a3) 8.73. The latter were associated to the corresponding acid-base equilibria of the amine and hydroxy groups constituting the molecule.

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.