É proposto um método para a determinação de fluoreto em carvão mineral. O mesmo é baseado no emprego da pirohidrólise para a separação do fluoreto da amostra e posterior determinação do analito por potenciometria direta, com eletrodo íon seletivo. O sistema utilizado para a realização da pirohidrólise foi desenvolvido e a influência do V 2 O 5 como catalisador na reação de pirohidrólise foi investigada, sendo verificado que o mesmo não influencia sobre a liberação do fluoreto presente na amostra. Amostras certificadas e não certificadas de carvão mineral foram analisadas. Resultados precisos (desvio padrão relativo inferior a 7%) e exatos (recuperação do fluoreto de acordo com os valores certificados) foram obtidos. O limite de quantificação (LOQ) do método foi de 5,0 µgF/g de amostra, usando 20 mg de amostra e volume final de 10 mL. A freqüência de análise foi de 5 amostras h -1 .A method for the determination of fluoride in coal using pyrohydrolysis for sample decomposition is proposed. An apparatus was constructed, and the influence of vanadium pentoxide (V 2 O 5 ) as a catalyst for the pyrohydrolysis reaction was investigated. It was verified that V 2 O 5 does not influence the release of fluoride from coal. Certified and non-certified samples were analysed using an ion selective electrode (ISE) for analyte detection. Precise (RSD < 7%) and accurate (recovery in accordance with certified values) results were obtained. The limit of quantification (LOQ) of the method was 5.0 µgF/g of sample, using 20 mg of sample and a final dilution to 10 mL. The sample frequency was 5 samples h -1 .Keywords: coal, fluoride, pyrohydrolysis, ion electrode, vanadium pentoxide, catalyst
IntroductionFluorine is one of several trace elements receiving much attention owing to its possible harmful ecological effects. This element is considered essential to both plant and animal life at low concentrations but is toxic at high concentrations. The potential toxicity of fluorine has been classified as "high" for terrestrial life and "low" for aquatic life. 1,2 Due to the high volatility of the element, the widespread world consumption of coal may result in significant fluorine emissions. 2 From this point of view, it is important to establish accurate, precise and rapid methods for determination of this element.With the exception of a few instrumental methods, 2,3 the determination of fluoride in coal is carried out after sample dissolution. Most published methods related to the determination of fluoride in solid samples thus involve distillation, pyrohydrolysis, alkaline fusion, combustion bomb or acid decomposition in a bomb, typically followed by spectrophotometric or potentiometric measurements. 3,4 In some instances, these methods are complex and time consuming (mainly methods based on distillation and fusion), may give incomplete breakdown of fluorinecontaining compounds, analyte losses during the depressurisation of the bomb (decomposition in a combustion bomb), or by volatilisation (alkaline fusion), if the fusion temperature is...