Almost all of the applications of trihydric alcohols are included in the more than 150 uses of glycerol e.g., humectant, solvent, plasticizer, component of pharmaceuticals, and as a derivative employed in plastics, coatings, explosives and foods.1 Moreover, glycerol is one of the most important compounds to be monitored in a sugar-fermentation plant in order to improve the quality control of the process. 2 In the soap industry, monitoring of the glycerol concentration in lixivia and such is relevant in view of the related economical aspects.Advanced analytical characteristics are required for industrial control, and an increasing number on the applications of chemically modified electrodes as amperometric detectors in flow systems have been reported. [3][4][5][6][7][8][9] These include electrodes based on copper, cobalt and nickel for the electrocatalytic oxidation of carbohydrates in an alkaline solution at low potential.Nickel electrodes exhibit catalytic activity in the oxidation of a wide variety of organic compounds in alkaline solutions. The Ni(III) species is generated on the electrode surface at potentials in the region of 0.4 to 0.5 V vs. Ag/AgCl. The reactions are:Glycerol can be determinated by monitoring the Ni(II) and re-oxidizing this species by applying a suitable potential. Despite the versatility, simplicity and suitability of the nickel-chromium alloy as an amperometric detector, and the relevance of glycerol monitoring for the industrial production of soaps, detergents and such, the use of Ni-Cr alloy electrodes for this determination had not yet been reported.The main purpose of the present work was thus to develop a simple, fast and low-cost flow-injection procedure for glycerol determination in lixivia. Glycerol electrocatalytic oxidation in an alkaline medium at low potential was exploited. For this task, a Ni-Cr wire was used as an electrochemical detector, and its chemical composition and electrochemical characteristics were investigated. Experimental ApparatusThe electrochemical system comprised a MQPG-01 Microquimica computerized potenciostat (Florianópolis-SC, Brazil) equipped with Ag/AgCl (3 mol l −1 KCl) reference and platinum auxiliary electrodes (spiral, 0.2×100 mm) sealed in alkaline glass. The working electrode was made of Ni-Cr (ca. 80:20 w/w) by sealing the metal wire (o.d. 1.0 mm) in a glass tube so as to leave about 1-cm length exposed (microcylinder). Unless otherwise indicated, the potential scan rate was 100 mV s −1 . Flow-injection experiments were carried out in a system with an IPC-8 Ismatec peristaltic pump (Zürich, Switzerland) furnished with Tygon pumping tubes, a home-made injector 10 and 0.8 A flow-injection amperometric method for glycerol determination in samples relevant to the industrial production of soaps, detergents and such is presented. A nickel-chromium alloy micro-cylindric electrode was used as the electrochemical detector, which after a pretreatment exhibited good repeatability and stability. The method involved glycerol oxidation by an inactive Ni(OH...
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 SO 2 , 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 empregandose 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 µL 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 SO 2 ) 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 SO 2 , gas diffusion through a Teflon ® semipermeable 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 µL 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 SO 2 ) in agreement with those obtained by an alternative procedure.Keywords: concentric tubular membrane, sulfite, spectrophotometric flow analysis, micellar medium, gas diffusion IntroductionGas diffusion is a very effective means to enhance selectivity in analytical chemistry, and was implemented in a flow-injection system in the pioneer work dealing with the development of an improved procedure for determination of total CO 2 in plasma. 1 Since then, the strategy has been often exploited, as it is efficiently accomplished in flow systems. 2,3 Generally, a gaspermeable membrane is placed between two liquid streams, one of them -the donor stream -containing the sample under processing. The analyte is converted to a volatile chemical species that is removed from the sample matrix and collected by other stream -the acceptor stream.Sulphite is an important preservative in the food and beverage industry, especially with regard to wine production 4-8 and its content should be strictly controlle...
Um sistema de análises químicas por injeção em fluxo empregando uma mini-coluna de AgCl(s) é proposto para a determinação de nitrogênio total em plantas por espectrometria de absorção atômica com chama. O método se baseia na remoção de íons Ag+ por amônia com formação do complexo diamino argentato(I) e posterior direcionamento da amostra a um espectrômetro de absorção atômica onde prata é monitorada. Nitrogênio foi determinado em sete amostras de referência (três replicatas) com precisão e exatidão comparáveis às do procedimento condutimétrico. O sistema é estável, apresentando apenas pequenas variações em sensibilidade (< 2 %) durante períodos de operação de 4 horas. A velocidade analítica é de aproximadamente 100 h-1 e a repetibilidade das medidas é satisfatória (desvio padrão relativo em geral < 0,02).
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