Eduardo Almeida Neves

Determinação e distribuição de ácido ascórbico em três frutos tropicais

An iodimetric titration was applied for determination of ascorbic acid (AA) in three tropical fruits: araca (Eugenia sp.), acerola (Malphigia sp.), and orange (Citrus sinenses). According to superficial color, the fruits were classified as immature (green), medium maturity (green-yellowish), and mature (yellow or red). The fruits were frozen to minimize AA oxidation. The AA contents of araca showed a gradient of concentration between the upper part and the down part of the fruit. The concentration in the upper part (C1) was always lower than the concentration in the down part of the fruit (C2). Similar behavior was observed for acerolas and oranges, which presented a mean concentration in the upper part of 17.7 mg g-1 and 0.612 mg L-1 and in the down part of 19.6 mg g-1 and 0.665 mg L-1, respectively. It can be concluded that the three fruits presented a heterogeneous distribution of AA and the variation was more pronounced for immature fruits, i.e. fruits less suitable for ingesting.

Flow Injection Spectrophotometric Determination of Free and Total Sulfite In Wines Based on the Induced Oxidation of Manganese(II)

Abstract The sulfur(IV) content in wines was determined with a flow injection spectrophotometric system by treating each sample in the donor channel with sulfuric acid in order to release free SO2, which diffuses through a PTFE membrane to the acceptor solution containing Mn(II) in acetate buffer at pH 5.5. The induced oxidation of Mn(II) takes place proportionally to the free S(IV) content and the oxidized species formed reacts with iodide to form iodine which can be spectrophotometrically measured at 352 nm. A linear plot of absorbance vs S(IV) concentration is observed up to 26 mg SO2 L−1 with a detection limit of 1.0 mg L−1. Matrix effects required the standard additions of sulfite to the samples for obtaining accurate results. A previous treatment of the sample with tris-(hydroxymethylaminomethane) and EDTA decomposed the hydrogen-sulfite/aldehyde adducts and allowed the determination of the total S(IV) content. As expected, total S(IV) concentration in wines has been found to be much higher than fre...

Differences in thermal decomposition of Ag(I), Mn(II), Fe(II) and Fe(III) complexes of cyclic dithiocarbamates

Abstract The thermal decomposition of pyrrolidinedithiocarbamate (Pyr) and piperidinedithiocarbamate (Pip) complexes of Ag(I), Mn(II), Fe(II) and Fe(III) have been investigated by thermogravimetry and differential scanning calorimetry. The decomposition intermediates and final products were identified from their X-ray diffraction patterns. Changes in their IR spectra were correlated with their thermogravimetric profiles. The hydrated compounds decomposed without loss of water and oxides were detected as the final decomposition products even in nitrogen atmosphere.

Determination of sulfur dioxide in air on the basis of the catalyzed autoxidation of Co(II) in an azide medium

A method used for the determination of S(IV) in precipitation was adapted for measurements of SO2 in air. A solution containing tris(hydroxymethyl)-aminomethane (TRIS) and sodium azide is used to trap SO2 during 30 min at an air flow of 15–40 L/h−1. The addition of a solution containing perchloric acid, CO2+ and Mn2+ leads to a formation of a mixture of azido-cobaltate(III) complexes (pH 5.2–5.4 for the final HN3/N3−-buffer), proportional to the total [S(IV)] in the absorption solution. Spectrophotometric measurement is performed at 365 nm with a slope of (3.07±0.03)×104 L·mol−1·cm−1 of the calibration curve. Detection limit is 8×10−8 M in the working solution with 5 cm light path cuvettes. This results in a relative detection limit of 2.8 μg SO2/m3 under the sampling conditions chosen. Interference of oxidants and H2S is avoided by inserting in the sampling train a cartridge containing FeSO4·7H2O and silver wool upstream of the absorber. Oxides of nitrogen do not interfere. The method provides results comparable with those obtained with a coulometric monitor based on the I2/I− system.

A study of the stability of Mn(III) and its redox reaction with Co(II) in aqueous azide medium

Abstract Mn(III) was generated in a coulometric cell by the oxidation of Mn(II) in azide medium at 2 M ionic strength and 22 °C. The system exhibited an excellent Nernst behaviour. The interaction of Mn(II/ III) with Co(II/III) according to the overall reaction Co(III)+ Mn(II)⇌Co(II)+ Mn(III) was studied as a function of azide concentration. The equilibrium constant for this reaction increases from 0.0239 at 2 M azide to 0.0289 at 0.1 M azide. Kinetic measurements were also performed to measure the forward and reverse rate constants for the equilibrium which resulted in an equilibrium constant of 0.0175 at 1 M azide. The results are discussed in reference to the catalytic role of Mn(II/III) in the sulfite catalyzed autoxidation of Co(II).

Thermal stability and bonding in the silver complexes of ethylenediaminetetraacetic acid

Abstract The solid silver-EDTA complexes AgH 3 Y·(3/4)H 2 O; Ag 2 H 2 Y; Ag 3 HY and Ag 4 Y·(1/2)H 2 O (Y −4 is ethylenediaminetetraacetate anion) obtained by reaction of the Ag + ions with sodium salts of EDTA in different degrees of neutralization in aqueous media have been characterized by means of potentiometric titrations, IR spectrometry, elemental analysis and powder X-ray diffraction (XRD). Thermogravimetry (TG) and Differential Scanning Calorimetry (DSC) were used to investigated the thermal stability of the complexes as well as the decomposition pathways. The thermoanalytical results associated with infrared data lead to the conclusion that the silver-carboxylate group bonding is primarily ionic.

Spectrophotometric Determination of Iodate in Table Salt

Potassium iodate in table salt can be spectrophotometrically determined at two well defined UV absorption maxima (288 and 352 nm), after being converted to I3- by reaction with iodide in the presence of phosphoric acid Calibration curves with slopes of 7.320xa0xxa0104 and 1.103xa0xxa0105 L mol-1 cm-1 were obtained at 352 and 288 nm, respectively, at 22 °C. Typical results of 37.39 (±0.15) and 63.67 (±0.16) mg KIO3 per kg of salt were obtained with samples of 0.15-0.21 g, comparable with results from a standard but less precise iodometric titration of large samples (20-50 g). The method has been found adequate for checking homogeneity of iodine distribution in table salt, as small weighed samples are normally required.

The Sulfite Accelerated Autoxidation of Cobalt(II) in the Presence of Tris(Hidroxymethyl)Aminomethane

Abstract The oxidation of cobalt(II) complexes in tris(hidroxymethyl)-aminomethane (TRIS) buffers in aqueous medium, by dissolved oxygen, is strongly accelerated by sulfur(IV). The process is still faster in the presence of Mn2+. The reaction is adequate for S(IV) determination as it is stabilized by the amine and provides an analytical curve, with a slope as high as 7.4 × 103 L.mol−1.cm−1 at 230 nm referred to the concentration of cobalt(III) formed, proportional to the sulfite concentration. The method is being used for S(IV) determination in sulfur hexafluoride aged under electrical stress. It presents the advantage of avoiding the use of toxic sodium azide from the former procedure.

New aspects of the reaction of silver(I) cations with the ethylenediaminetetraacetate ion

The highly neutralized ethylenediaminetetraacetate (EDTA) titrant (95-99% as Y(4-) anion) precipitates with Ag(+) cations to form the Ag(4)Y species, in aqueous medium, which is well characterized from conductometric titration, thermal analysis and potentiometric titration of the silver content of the solid. The precipitate dissolves in excess Y(4-) to form a complex, AgY(3-). Equilibrium studies at 25 degrees C and ionic strength 0.50 M (NaNO(3)) have shown from solubility and potentiometric measurements that the formation constant (95% confidence level) beta(1) = (1.93 +/- 0.07) x 10(5) M(-1) and the solubility products are K(S0) = [Ag +](4)[Y(4-)] = (9.0 +/- 0.4) x 10(-18) M(5) and K(S1) = [Ag +](3)[AgY(3-)] = (1.74 +/- 0.08) x 10(-12)M(4). The presence of Na(+), rather than ionic strength, markedly affects the equilibrium; the data at ionic strength 0.10 M are: beta(1) = (1.19 +/- 0.03) x 10(6) M(-1), K(S0) = (1.6 +/- 0.4) x 10(-19) M(5) and K(S1) = (1.9 +/- 0.5) x 10(-13) M(4); at ionic strength tending to zero; beta(1) = (1.82 +/- 0.05) x 10(7) M(-1), K(S0) = (2.6 +/- 0.8) x 10(-22) M(5) and K(S1) = (5 +/- 1) x 10(-15) M(4). The intrinsic solubility is 2.03 mM silver (I) in 0.50 M NaNO(3). Well-defined potentiometric titration curves can be taken in the range 1-2 mM with the Ag indicator electrode. Thermal analysis revealed from differential scanning calorimetry a sharp exothermic peak at 142 degrees C; thermal gravimetry/differential thermal gravimetry has shown mass loss due to silver formation and a brown residue, a water-soluble polymeric acid (decomposition range 135-157 degrees C), tending to pure silver at 600 degrees C, consistent with the original Ag(4)Y salt.

Complexing power of alkanesulfonate ions: the lead-methanesulfonate system

The interaction between lead(II) and methanesulfonate ions was studied by conventional d.c. polarography at 2.0 M ionic strength and pH 4.0. Two stepwise lead complexes are formed in solution: [Pb(CH3SO3)]+, ß1 = 1.76 ± 0.03 M−1 and [Pb(CH3SO3)2], β2 = 0.25 ± 0.05 M−2 which are reversibly reduced at the DME. At high ligand concentrations, above 1.0 M, a third lead complex, [Pb(CH3SO3)3] 433= 0.30±0.07M −3, is formed.