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I n t h e d e t e r m i n a t i o n of f l u o r i d e i o n w i t h a f l u o r i d e i o ns e l e c t i v e e l e c t r o d e , t i r o n i s t h e most e f f e c t i v e o f masking a g e n t s t e s t e d f o r aluminum. The h i g h e r t h e pH, t h e more e f f e c t i v e t h e masking, b u t t h e h y d r o x i d e i o n r a i s e s t h e l o w e r l i m i t o f d e t e r m i n a b l e f l u o r i d e i o n c o n c e n t r a t i o n a t h i g h e r pH.T h e r e f o r e , measurements a t pH 6 .O a r e p r e f e r a b l e .

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This research is part of a generalinvestigation of a new fire assay procedure for the platinum metals. Iridium is satisfactorily collected in the copper-nickel-iron buttons, in amounts from 9 pg. to 4.59 mg. Losses to slag and pot walls are negligible.