Comparative study of methods for the determination of airborne fluorides
Some current methods for the determination of both particulate and gaseous compounds of fluorine, present in air as pollutants, are described and compared. The operation included sampling of the air, the separation of fiuoride from interfering substances and its final determination. Results obtained for the separation of fiuoride by distillation and by other techniques of microdiffusion are presented, as weil as those obtained by the application of different methods to the subsequent determination of fiuoride. The analytical method used by the authors is described, giving some results obtained in its application to the control of air pollution caused by an aluminium smelting plant. In July 1966 we were asked to determine the extent of air pollution by fluorides caused by an aluminium plant in an area situated in the North of Italy. The whole study covered a period of two years, and followed the improvements introduced by the industry on the abatement system. Consequently, many hundreds of samples of air, stack effluents and other emissions have been analyzed, both for gaseous and partic~late fluorides. To perform this task, the complete method of analysis, including sampling, separation and determination of airborne fluorides, was checked in each of these steps; we shall now try to give a bri"ef report on the analytical conclusions we have been able to draw from this experience. Considering first the operation of sampling, it isaweil known fact that the collection of a representative sample is the hardest task of the entire analytical procedure when discrimination of gaseous and particulate fiuorides is required, as was the case with our controls. The difficulties arise from the high reactivity of the hydrogen fluoride which tends to interact with the particulates always present in this type of emission. At the time when we faced this problem, several techniques had been applied to the separation of the gaseous compounds prior to the collection of particulates in an attempt to prevent this interaction (Pack and co-workersl, 1960, Adams and co-workers2, 1963), but the procedures were applicable to ambient air, while we had to sample also stack gases, which are characterized by a higher concentration of hydrogen fluoride, particulates and water as compared with samples of air. It was only in 1968 that a stack sampling technique for particulate and gaseous fluorides was reported by Dorsey and Kemnitz3• For practical reasons we decided to collect first the particulates and afterwards the gaseous fluorides, and from the proposed methods we decided to use membrane filters followed