Combustão Iniciada por Micro-ondas: um novo método para a volatilização de Cloro e Flúor em cimento.

Abstract

A new method for simultaneous determination of chlorine and fluorine by ion chromatography (IC) in Cement Portland was developed by volatilization of these analytes through the microwave-induced combustion (MIC). Several parameters were optimized, such as the use of an auxiliary volatilization (microcrystalline cellulose or graphite), the form of manipulation of the samples (pellets or wrappedd in polyethylene film), the relation between the sample mass and the auxiliary volatilization, the absorption solution (H2O or NH4OH 10, 25, 50, 100, 150 or 200 mmol l-1 ) and also a reflux step of 5 or 10 min.. Additionally, microwave assisted extraction (MW-AE), with extracting solutions as H2O, NH4OH 50 or 100 mmol l-1 or HNO3 6 mol l-1 was also evaluated and the analytes were also determined by IC. The potentiometry with ion selective electrode (ISE) was evaluated as an alternative method for determination of the analytes. Furthermore, Cl was also determined by the reference method C114-13 of American Society for Testing and Materials (ASTM). The results obtained after MW-AE were in agreement (around 90%) with the reference method for Cl, while F could not be determined in samples from the extractions, due to low concentrations extracted (lower than limit of detection, LOD = 35.9 mg kg-1 ). However, through the volatilization by MIC using the relation of 300 mg cellulose and 100 mg of sample, wrappedd in polyethylene film, were obtained agreement higher than 90% for Cl with the reference method (ASTM), and between 98 and 103% with the reference values for Cl in certified reference materials (CRMs) FLX-CRM 101 (Cement) and PACS-2 (Sediment Marine). For F, due to absence of a CRM with similar matrix and with reported values for this element, were performed spike recoveries, as a mixture of CRM NIST (Oyster Tissue) with cement samples, and the recoveries were around of 96%. These results were obtained using 6 ml of ultrapure water as absorbing solution with 5 min of reflux step. The LOD of Cl and F in theproposed method was 99.1 mg kg-1 and 17.7 mg kg-1 , respectively. The relative standard deviation in the developed method, for both analytes, was always less than 7%.