M. Romero-Romo

Spectrophotometric and electrochemical determination of the formation constants of the complexes Curcumin–Fe(III)–water and Curcumin–Fe(II)–water

The formation of complexes among the Curcumin, Fe(III) and Fe(II) was studied in aqueous media within the 5-11 pH range by means of UV-Vis spectrophotometry and cyclic voltammetry. When the reaction between the Curcumin and the ions present in basic media took place, the resulting spectra of the systems Curcumin-Fe(III) and Curcumin-Fe(II) presented a similar behaviour. The cyclic voltammograms in basic media indicated that a chemical reaction has taken place between the Curcumin and Fe(III) before that of the formation of complexes. Data processing with SQUAD permitted to calculate the formation constants of the complexes Curcumin-Fe(III), corresponding to the species FeCur (lob beta110 = 22.25 +/- 0.03) and FeCur(OH)- (log beta111 = 12.14 +/- 0.03), while for the complexes Curcumin-Fe(II) the corresponding formation constants of the species FeCur- (log beta110 = 9.20 +/- 0.04), FeHCur (log beta111 = 19.76 +/- 0.03), FeH2Cur+ (log beta112 = 28.11 +/- 0.02).

Chemical and electrochemical considerations on the removal process of hexavalent chromium from aqueous media

Two methods were used to remove Cr(VI) from industrial wastewater. Although both are based in the same general reaction: 3Fe(II)(aq) + Cr(VI)(aq) ⇌; 3Fe(III)(aq) + Cr(III)(aq) the way in which the required amount of Fe(II) is added to the wastewater is different for each method. In the chemical method, Fe(II)(aq) is supplied by dissolving FeSO4 · 7(H2O)(s) into the wastewater, while in the electrochemical process Fe(II)(aq) ions are formed directly in solution by anodic dissolution of an steel electrode. After this reduction process, the resulting Cr(III)(aq) and Fe(III)(aq) ions are precipitated as insoluble hydroxide species, in both cases, changing the pH (i.e., adding Ca(OH)2(s)). Based on the chemical and thermodynamic characteristics of the systems Cr(VI)–Cr(III)–H2O–e− and Fe(III)–Fe(II)–H2O–e− both processes were optimized. However we show that the electrochemical option, apart from providing a better form of control, generates significantly less sludge as compared with the chemical process. Furthermore, it is also shown that sludge ageing promotes the formation of soluble polynuclear species of Cr(III). Therefore, it is recommended to separate the chromium and iron-bearing phases once they are formed. We propose the optimum hydraulic conditions for the continuous reduction of Cr(VI) present in the aqueous media treated in a plug-flow reactor.

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.

Multicomponent Click Synthesis of New 1,2,3-Triazole Derivatives of Pyrimidine Nucleobases: Promising Acidic Corrosion Inhibitors for Steel

A series of new mono-1,2,3-triazole derivatives of pyrimidine nucleobases were synthesized by one-pot copper(I)-catalyzed 1,3-dipolar cycloaddition reactions between N-1-propargyluracil and thymine, sodium azide and several benzyl halides. The desired heterocyclic compounds were obtained in good yields and characterized by NMR, IR, and high resolution mass spectrometry. These compounds were investigated as corrosion inhibitors for steel in 1 M HCl solution, using electrochemical impedance spectroscopy (EIS) technique. The results indicate that these heterocyclic compounds are promising acidic corrosion inhibitors for steel.

Synthesis of New 1,2,3-Triazole Derivatives of Uracil and Thymine with Potential Inhibitory Activity against Acidic Corrosion of Steels

Ten 1,4-disubstituted 1,2,3-triazoles were synthesized from one of 1-(azido-methyl)benzene, 1-(azidomethyl)-4-fluorobenzene, 1-(azidomethyl)-4-chlorobenzene, 1-(azidomethyl)-4-bromobenzene or 1-(azidomethyl)-4-iodobenzene, generated in situ from sodium azide and the corresponding benzyl halide, and dipropargyl uracil or dipropargyl thymine. Optimal experimental conditions were established for the conventional click chemistry. The corrosion inhibiting properties of some of these compounds, which were determined by means of an electrochemical technique, are also presented.

Electrochemical quantification of the antioxidant capacity of medicinal plants using biosensors.

The working area of a screen-printed electrode, SPE, was modified with the enzyme tyrosinase (Tyr) using different immobilization methods, namely entrapment with water-soluble polyvinyl alcohol (PVA), cross-linking using glutaraldehyde (GA), and cross-linking using GA and human serum albumin (HSA); the resulting electrodes were termed SPE/Tyr/PVA, SPE/Tyr/GA and SPE/Tyr/HSA/GA, respectively. These biosensors were characterized by means of amperometry and EIS techniques. From amperometric evaluations, the apparent Michaelis-Menten constant, Km′, of each biosensor was evaluated while the respective charge transfer resistance, Rct, was assessed from impedance measurements. It was found that the SPE/Tyr/GA had the smallest Km′ (57 ± 7) μM and Rct values. This electrode also displayed both the lowest detection and quantification limits for catechol quantification. Using the SPE/Tyr/GA, the Trolox Equivalent Antioxidant Capacity (TEAC) was determined from infusions prepared with “mirto” (Salvia microphylla), “hHierba dulce” (Lippia dulcis) and “salve real” (Lippia alba), medicinal plants commonly used in Mexico.

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.

Cd(II) and Pb(II) Separation from Aqueous Solution using Clinoptilolite and Opuntia Ectodermis

This work presents the conditions for Pb(II) and Cd(II) removal from aqueous solution using two different sorbent materials, namely, clinoptilolite and Opuntia ectodermis, which were characterized before and after entering into contact with the metal-containing aqueous media, using scanning electron microscopy (SEM), thermal analysis (TGA) and surface area (BET). Metal removal was found to depend on the initial metal concentration in aqueous solution, pH and the mass/volume ratio. The Pb(II) and Cd(II) uptake process was maximum at pH 4 for both sorbents, which showed an adsorption capacity that was adequately described by the Freundlich adsorption isotherm. The Thomas model was used to describe the adsorption data from column studies, the sorption capacity was 12.21 Pb(II) mg per gram of Opuntia ectodermis, while 7.71 Pb(II) mg per gram of clinoptilolite were adsorbed; lower values were obtained for Cd(II) removal. It was demonstrated that the use of low cost materials for the treatment of Pb(II), and Cd(II), containing wastewater is an effective and economical alternative method.

Cr(VI) Removal From Wastewater Using Low Cost Sorbent Materials: Roots of Typha Latifolia and Ashes

This work presents conditions for hexavalent chromium (Cr(VI)) removal from aqueous solution using different sorbent materials, namely: pyrolytic ashes of an industrial sludge from wastewater treatment and roots of Typha latifolia. The sorbent materials were characterized using scanning electron microscopy (SEM) and surface area using theBrunauer-Emmett-Teller (BET) technique, before and after the contact with the chromium-containing aqueous media. An overall Cr(VI) concentration reduction of 45 % was achieved using the roots of Typha latifolia whereas in the case of pyrolytic ashes a 60 % removal was observed. The percentage removal was found to depend on the initial Cr(VI) concentration in aqueous solution, pH and temperature. The Cr(VI) uptake process was maximum at pH 2 and a temperature of 40 °C for both sorbents. These materials showed a Cr(VI) adsorption capacity that was adequately described by the Langmuir adsorption isotherm. It was demonstrated that the use of waste materials for the treatment of Cr(VI)-containing wastewater is an effective and economical alternative method.

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.