Oswaldo E. S. Godinho

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.

Oxalate determination in urine using an immobilized enzyme on sorghum vulgare seeds in a flow injection conductimetric system

A flow-injection (FI) method was developed for the determination of oxalate in urine. It was based on the use of oxalate oxidase (E.C. 1.2.3.4) immobilized on ground seeds of the BR-303 Sorghum vulgare variety. A reactor was filled with this activated material, and the samples (200 µL) containing oxalate were passed through it, carried by a deionized water flow. The carbon dioxide produced by the enzyme reaction permeated through a microporous PTFE membrane, and was received in a water acceptor stream, promoting conductivity changes proportional to the oxalate concentration in the sample. The results obtained showed a useful linear range from 0.05 to 0.50 mmol dm-3. The proposed method, when compared with the Sigma enzymatic procedure, showed good correlation (Y = 0.006( + 0.016) + 0.98( + 0.019)X; r = 0.9995, Y = conductivity in µS, and X = concentration in mmol dm-3), selectivity, and sensitivity. The new immobilization approach promotes greater stability, allowing oxalate determination for 6 months. About 13 determinations can be performed per hour. The precision of the proposed method is about + 3.2 % (r.s.d).

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.

Titration of proteins in dimethyl sulphoxide–water mixtures

The acid–base behaviour of arginine, lysozyme and ovalbumin has been studied by potentiometric and catalytic thermometric titrimetry in a mixture of dimethyl sulphoxide–water, with acrylonitrile as the end-point indicator in the latter technique. It was observed that, with the exception of the SH groups, all the protonated groups, including the guanidine groups of lysozyme and ovalbumin, were titrated by catalytic thermometric titrimetry. By using potentiometric titrimetry, all the titratable groups of ovalbumin were determined, whereas the guanidine groups of lysozyme were not determined by this technique.

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.

A new spot test for fluoride

SummarySolution of silver ion, which is stable in the presence of appropriate concentrations of ferrous and ferric salts, is reduced by ferrous salts if fluoride is added. This fact has been utilized to develop a spot test for fluoride. 2μg of fluoride ion can be detected. A method for the removal of fluoride from interfering ions is also described.ZusammenfassungSilberlösungen, die in bestimmten Konzentrationen von Eisen(II)- und Eisen(III)-salzen beständig sind, werden von Eisen(II)-salzen reduziert, wenn man Fluorid zusetzt. Auf dieser Grundlage wurde eine Tüpfelreaktion für Fluorid ausgearbeitet. 2 μg F− können damit nachgewiesen werden. Eine Methode zur Abtrennung von Fluorid von störenden Ionen wurde ebenfalls angegeben.

Paraformaldehyde as an end-point indicator in hydrolytic thermometric titration of metal ions and iodine

The use of paraformaldehyde (PF) as an end-point indicator in hydrolytic titrations involving precipitation, complexation and redox reactions was investigated. The decrease in temperature caused by depolymerization of PF, catalysed by hydroxyl ions produced by hydrolysis of excess of titrant, was employed to locate the end-point of the titration. The results of titrations of alkaline earth metals with carbonate, several metal ions with EDTA and iodine with arsenite are presented.