Um sistema de análises por injeção em fluxo é proposto para a determinação espectrofotométrica de sulfito em vinhos brancos. O método envolve conversão do analito a SO2, difusão gasosa através de uma membrana semi-permeável de Teflon®, coleta em um fluxo alcalino (pH 8), reação com verde de Malaquita (VM) e monitorização a 620 nm. O projeto do sistema foi simplificado empregando-se uma membrana tubular concêntrica. Foram investigados: concentrações dos reagentes, pH dos fluxos doador e receptor, intensidade iônica, temperatura, temporização, adição de agentes tensoativos e presença de interferentes em potencial presentes na matriz vinho. A sensibilidade analítica melhorou (ca. 100%) adicionando-se cloreto de cetil piridinio (CPC). O sistema proposto é robusto e deriva em linha base não é observada durante períodos de até 4 horas de operação. São requeridos apenas 400 miL de amostra e 0.32 mg VM por determinação. O sistema processa 30 amostras por hora, fornecendo resultados precisos (d.p.r. < 0.015 para 1.0 - 20.0 mg L-1 SO2) e concordantes com os obtidos por um procedimento alternativo.

A flow-injection system is proposed for the spectrophotometric determination of sulphite in white wines. The method involves analyte conversion to SO2, gas diffusion through a Teflon® semi-permeable membrane, collection into an alkaline stream (pH 8), reaction with Malachite green (MG) and monitoring at 620 nm. With a concentric tubular membrane, the system design was simplified. Influence of reagent concentrations, pH of donor and acceptor streams, temperature, timing, surfactant addition and presence of potential interfering species of the wine matrix were investigated. A pronounced (ca. 100%) enhancement in sensitivity was noted by adding cetylpyridinium chloride (CPC). The proposed system is robust and baseline drift is not observed during 4 h operating periods. Only 400 muL of sample and 0.32 mg MG are required per determination. The system handles 30 samples per hour, yielding precise results (r.s.d. < 0.015 for 1.0 - 20.0 mg L-1 SO2) in agreement with those obtained by an alternative procedure.

The aim of this work was to propose two different didactic experiments, which can be used in practical classes of analytical chemistry courses. More flexible experiments related to the theme, giving some options to the instructor are proposed. In this way, the Experiment 1 was divided in two parts. In the first one, the visualization of two distinct phases separation is emphasized: the rich and the poor phases in surfactant. In the second part, the metal pre-concentration (Co as example) is emphasized. The Experiment 2 has three different parts. In the first one, the complex formation is pointed out, in the second one, the pH influence is shown and in the last one, the influence of the complexation time is demonstrated.

This review deals with the general use of the surfactants in Analytical Chemistry. Principal characteristic of the micelle is the improvement in selectivity and/or sensitivity of the analytical determination with emphasis on the catalytic reaction and "cloud point" extraction.