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ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
ADDENDUM T r a c e Determination of F l u o r i n e
Sir: Two instruments of the type described in paper ( I ) have been in almost constant use for four years in our laboratory. Some experiences should be reported. (1)Caution: The method is rather insensitive to incomplete combustion. Even if small amounts of soot pass through the tube, correct results can still be obtained. This has caused some carelessness of the analysts. Combustions were carried out so quickly that the exit capillary of the hydrogenation chamber was clogged. In one case, hydrogen passed backward into the combustion part of the tube and caused a small explosion. In order to avoid this risk, a T-tube was introduced between stopcock H1 and the reactor tube, ( 1 ) Figure 1. The open end of the T-tube leads to the bottom of a 50-cm high water-filled open tube, which functions as a safety valve. If the exit capillary is clogged, the hydrogen will pass out through it and not force its way backward. Also capillary U1 was widened to 3 mm. (2) No tungsten(V1) oxide should ever get into the combustion chamber between U1 and U2. If this happens, the color reagent is bleached and the analyses are lost. In order to avoid any risk, we put a wad of quartz wool into the combustion tube close to orifice U2. This quartz wool must be removed when traces of sulfur are determined with the instrument, to avoid blanks. (3) Some minerals like calcium phosphate do not release all their fluorine with tungsten(V1) oxide, even when they are very finely ground and thoroughly mixed with it. They do, however, liberate it with the phosphate flux described earlier (2). (4) Volatile organic liquids and oils are conveniently analyzed in the following manner. Fill spoon S with tungsten(V1) oxide powder and place it a t opening Q in the combustion tube. Close Q with a serum cap. Fill the combustion tube with nitrogen and inject the sample through the serum cap onto the tungsten(V1) oxide in the spoon. Move the spoon toward the furnace and pyrolize the sample slowly under nitrogen. Finally apply oxygen and burn all residues. Stopcock H3 is no longer needed with this procedure. LITERATURE C I T E D (1) W. J. Kirsten, Anal. Chem., 48, 84 (1976). (2) W. J. Kirsten and 2. H. Shah, Anal. Chem., 47, 184 (1975).
Wolfgang J. Kirsten Department of Chemistry Swedish University of Agricultural Sciences S-750 07 Uppsala, Sweden.
