RESUMO
The behavior of a modified carbon paste electrode (CPE) for nickel determination by anodic adsorptive stripping voltammetry (AAdsSV) was studied. The electrode was built incorporating the Dowex 50W x 12 (H(+) form) ion exchanger to a Nujol-graphite base paste. Ni(2+) was preconcentrated on the electrode surface in open circuit conditions, with the reduction (-1300 mV)/reoxidation step carried out in HCl solution (pH 3). During deposition time (5 min), the hydrogen evolution did not present obstacle in the quantification of nickel. For 12 min of accumulation and 5 min of deposition, nickel can be quantified up to 600 mug l(-1). The detection limit was 0.005 mug l(-1) at a linear potential scan rate of 200 mV s(-1). Interferences from Hg(2+) and Ag(2+) up to a concentration of 1 and 0.6 mg l(-1), respectively, were eliminated with the aid of the anion exchanger Dowex-2 (mesh 200-400) which was added to the sample in the preconcentration step. The tolerance for some metal ions such as Cu(2+), Cd(2+), Fe(3+), Zn(2+), Co(2+) and Pb(2+) was improved in the same way. The method was applied for the determination of nickel in samples of tap and mineral water. At the concentration level of 50 mug l(-1) of Ni (2+), the results were in good agreement with those obtained using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). For a Ni(2+) concentration of 5 mug l(-1), the results obtained showed better accuracy than those obtained by ICP-AES.
RESUMO
A chemically modified electrode (CME) was constructed and evaluated for use as an end-point indicator in the automatic titration of Ni(II) with EDTA. The CME consisted of a graphite paste prepared by mixing a strong acid ion exchanger containing 4-(3,5-dichloro-2-pyridylazo)-1,3-diaminobenzene and graphite powder-Nujol paste. This mixture showed high mechanical resistance in strongly acidic and alkaline solutions (6 M HCl-pH 12). The CME did not require any special conditioning prior to use. It could be used over long periods (5-6 months) of continuous work without renewing either the electroactive surface or paste. In buffered solutions (pH 3-4.5), automatic direct potentiometric titrations could be carried out over a wide interval of Ni(II) concentrations, ranging from 3 to 6000 ppm, with satisfactory accuracy and precision. For practical analysis, the electrode was applied to the determination of nickel in two certified composition alloys. Interferent ions were previously separated by applying an ion-exchange procedure.
RESUMO
A simple and sensitive potentiometric sensor for end-point detection in the automatic titration of Zn(II) with EDTA was prepared and studied. The sensor was based on a conventional carbon paste which was mixed during preparation with 4-(3,5-dichloro-2-pyridil-azo)-1,3-diaminobenzene (3,5-Cl2PADAB). pH effects, buffer concentration, reagent content into carbon paste and the presence of foreign salts on the electrode response were studied. Titration curves with sharp end-point breaks were obtained in Zn(II) concentration range from 0.5 to 6550 ppm. The electrode was easily made and with very inexpensive materials. A titrimetric method for the determination of zinc in insulin by automatic potentiometric end-point detection is described. It was applied to the determination of the metal in two commercial pharmaceutical preparations. The results were in good agreement with those obtained by the U.S. Pharmacopeia standard method.
RESUMO
In slightly acid solutions Cu(II) and 4-(3,5-dichloro-2-pyridylazo)-1,3-diaminobenzene (3,5-C12PADAB) form a red-purple complex( = 5.0 x 10(4)1.mole(-). cm(-1) at 547 nm). The complex is readily broken by EDTA, with a sharp change of colour to the yellow of the free reagent (Deltalambda(max) = 110 nm at pH 3.5). This colour contrast and the reversibility of the formation reaction, together with the solubility of the complex in water, make the pyridylazo reagent useful as a metallochromic indicator for copper titration, at pH 2.7-5.5. The Cu(II)-3,5-C12PADAB indicator system has been satisfactorily applied to the determination of sulphur in cast iron, "useful oxides" (Al(2)O(3) + Fe(2)O(3)) in samples of commercial aluminium sulphate, and total phosphorus in a synthetic mixed fertilizer.