Luiz M. Aleixo

Development of a chemically modified electrode based on carbon paste and functionalized silica gel for preconcentration and voltammetric determination of mercury(II)

Abstract A mercury-sensitive chemically modified electrode (CME) based on modified silica gel-containing carbon paste was developed. The functional group attached to the silica gel surface was 3-(2-thiobenzimidazolyl)propyl, which is albe to complex mercury ions. This electrode was applied to the determination of mercury(II) ions in aqueous solution. The mercury was chemically preconcentrated on the CME prior to voltammetric determination by anodic stripping in the differential-pulse mode. A calibration graph covering the concentration range from 0.08 to 2 mg 1 −1 was constructed. The precision for six determination of 0.122 and 0.312 mg 1 −1 Hg(II) was 3.2 and 2.9% (relative standard deviation), respectively. The detection limit for a 5-min preconcentration period was 0.013 mg 1 −1 . A study for foreign ions was also made.

Potentiometric study of acid—base properties of humic acid using linear functions for treatment of titration data

Abstract A pontentiometric study of the acid-base properties of humic acid was made with the objective of determining the p K a value and concentration of each titratable group. The use of linear equations in the treatment of titration curves led to the characterization of six titratable groups, which permits the acid-base properties to be explained and will be useful in studies of soil and environmental chemistry.

Direct voltammetric determination of Mo(VI) in plants: the need for a multivariate study of interferences

A multivariate modelling procedure using a second order composite design showed that the adsorptive stripping voltammetry (AdSV) technique used for molybdenum determination in a N,N-dimethylformamide (DMF)–ethanol–water homogeneous ternary solvent system (HTSS) using-benzoinoxime (BO) as the complexing agent and a sodium acetate–acetic acid buffer as the supporting electrolyte is much more tolerant to the presence of phosphorous (as phosphate) and iron than it could be presumed. Instead of the concentration ratios of P/Mo = 100 and Fe/Mo = 500, determined by univariate experiments, these values were respectively raised to 97,500 and 4200 when the phosphate and Fe(III) levels are varied simultaneously from 0.625 to 2.500 and from 0.006 to 0.150 mg l −1 , respectively, in the voltammetric cell, keeping the molybdenum concentration constant at 3.00 gl −1 . This allowed us to propose a straightforward AdSV–HTSS procedure for the determination of Mo(VI) in plants. The AdSV results compared favourably with those obtained by using graphite furnace atomic absorption spectrometry (GFAAS) and with the results of samples from the International Plant-Analytical Exchange (IPE) programme of Wageningen University (The Netherlands).

Determination of molybdenum in steel by adsorptive stripping voltammetry in a homogeneous ternary solvent system.

A new alternative approach for the determination of molybdenum in steel is proposed, using adsorptive stripping voltammetry (AdSV). The determinations are performed in a homogeneous ternary solvent system (HTSS) composed of N,N-dimethylformamide, ethanol and water, with alpha-benzoinoxime (alpha BO) as the complexing agent and a sodium acetate-acetic acid buffer as the support electrolyte. The HTSS composition was optimized by mixture design modelling. The AdSV measurements were performed in the differential pulse mode using an accumulation potential of -1050 mV. Under these optimized experimental conditions, the Mo(VI)-alpha BO reduction current peak potential is observed at potentials near -1250 mV, much lower than those usually reported, and the calibration plot follows the polynomial equation I = 0.359 + 0.265 [CMo(VI)] - 0.015 [CMo(IV)]2 (r2 = 0.997), for Mo concentrations up to 10.0 micrograms L-1. There is a linear range in this calibration plot for Mo(VI) concentrations up to 0.20 microgram L-1, defined by the equation I = 0.353 + 0.385 [CMo(VI)] (r2 = 0.980). In both cases, I is the absolute value for the current in microA and CMo(VI) is the concentration of Mo in microgram L-1. The detection limit for this linear concentration range was estimated as 20 pg L-1. A RSD of 0.43% is associated with the signals at a Mo(VI) level of 0.72 microgram L-1. From the common method-interfering species tested, only iron at Fe/Mo(VI) ratios above 500 and vanadium and tungsten at M/Mo(VI) ratios above 100 appear to affect the analytical response significantly. Phosphorous may also reduce the analytical signal at P/Mo(VI) ratios above 100, due to the formation of the competitive P-Mo complex. The suggested routine procedure was tested by analyzing four stainless steel samples and the results compared well with the ICP-AES measurements. The higher sensitivity of this method permits direct determination of Mo(VI) in steels, eliminating the need of analyte concentration or separation steps in the sample processing procedure.

Determination of total sulphur in ethanol by reduction using Raney nickel and detection by cathodic stripping voltammetry

A procedure for the determination of total sulphur in ethanol by reduction of the sulphur compounds to sulphide using Raney nickel, conversion to H2S and detection by differential-pulse cathodic stripping voltammetry (DPCSV) is presented. The supporting electrolyte that gave the best results was a sulphide antioxidant buffer (SAOB II, 10%). The calibration graph covered the concentration range 1–40 p.p.b., and the method could be used for ethanol samples with sulphur contents ranging from 15 ng g–1 to 20 µg g–1. The precision and accuracy of the method compare well with values in the literature.

Procedure for the simultaneous determination of tartaric and citric acids and total carbonate by potentiometric titrimetry and its application to antacid analysis.

The application of a linear algebraic method to the potentiometric titration of a mixture of tartaric and citric acids and sodium carbonate and sodium hydrogen carbonate (previously treated with strong acid), permits the determination of the concentration of each individual acid and the total equivalent of carbonate in antacids. It is also possible to determine individual concentrations of sodium carbonate and sodium hydrogen carbonate by combining the results of the described titration with the acidimetric titration of a sample previously treated with an excess of strong base.

Voltammetric determination of Zn(II) in Zn-Fe alloy electroplating baths using square-wave voltammetry

A routine analytical method for zinc (II) determination in Zn-Fe alloy galvanic baths was developed employing square-wave voltammetry with the static mercury drop electrode (SMDE) as working electrode. Real alloy bath samples were analyzed by the standard addition method and recovery tests were undertaken. The supporting electrolytes used in the analyses were 1.0 mol L-1 NH3 / 0.2 mol L-1 NH4Cl or 0.1 mol L-1 citric acid (pH=3), presenting peak potentials for zinc (II), respectively, at -1.30 V and -0.99 V vs. Ag|AgCl (saturated KCl). The proposed voltammetric method showed a linear response range at 25 °C between 1.0 x 10-5 and 2.2 x 10-4 mol L-1 for zinc (II), in both electrolytes studied. The interference levels for some metals, such as Cu (II), Pb (II), Cr (III) and Mn (II), which could prejudice Zn-Fe alloy deposition, were evaluated. These ions did not present significant degrees of interference in the zinc (II) determination. The zinc (II) recovery tests for the proposed method exhibited a good agreement with the reference method, showing relative errors lower than 3.0%.

Immobilized enzyme flow-injection conductimetric system for the determination of acetylcholine

A flow injection system for the determination of acetylcholine, based on the use of immobilized cholinesterase in glass beads, a diffusion membrane and a conductimetric cell is described. In this approach acetic acid formed by the action of acetylcholinesterase on acetyl choline permeates through a PTFE membrane and is received by a stream of deionized water, which carries the solution to the conductivity cell. Linearity for acetylcholine concentration and conductimetric response with a useful range of 10–5–10–3 mol l–1 was observed.

Determination of glucose in instant coffee with an enzyme electrode

Abstract An amperometric method utilizing an enzyme electrode was developed for the determination of the glucose content of instant coffee. The electrode was constructed by combining an oxygen electrode with a β- d -glucose oxidase-immobilized collagen membrane. Samples of instant coffee were analysed and the results obtained by the proposed method and by the established AOAC method agreed well (r=0.9964). The sensor was stable for more than 2 weeks and for 320 analyses.

Formalin and paraformaldehyde as end-point indicators in catalytic thermometric titrimetry

The use of formalin solution as an end-point indicator in the titration of both acids and bases in aqueous solutions was investigated. In both instances, a lowering in temperature at the end-point, due to depolymerization of poly(oxymethylene glycols), was observed. Endothermic end-points were also obtained by using paraformaldehyde as an end-point indicator in titrations of acids in aqueous solutions. The use of paraformaldehyde in the titration of very weak acids and of some sparingly soluble acids solubilized in micellar suspensions of cationic detergents was also investigated.