The dc arc is considerably more sensitive than the plasma are in determining rhenium, using the rhenium emission line at 3460.5 Å and cobalt at 3453.5 Å as the internal standard. The plasma arc is twice as precise as the dc arc. The internal standard technique does not increase precision of the plasma arc for rhenium but is useful to indicate proper functioning of the arc aspirator assembly. Use of a 1:1 alcohol—ethanol solvent for rhenium increases sensitivity of plasma arc determinations by a factor or two. The extracting solvent used to remove rhenium from molybdenite, 4-methyl-2-pentanone, is not spectrographically useful in the plasma arc at the wavelengths used for rhenium determination.
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