[en] The authors evaluated several calorimetric assays for ReO₄^-, and discovered that all were flawed. They evaluated atomic absorption spectroscopy as a technique to determine sub-millimolar concentrations of ReO₄^-, and discovered that it is not sensitive enough for their use. However, they discovered that ICP-AES can be used to determine concentrations of ReO₄^- down to 0.25 ppm. They next determined that ReO₄^- can be quickly extracted (10 minutes or less) from aqueous HNO₃ using the commercial extractant Aliquat-336 nitrate diluted with 1,3-diisopropylbenzene. Higher concentrations of extractant led to higher values of K_d (the distribution ratio). K_d was lower as the nitrate concentration of the medium increased, and was also lowered by increasing the acidity at constant nitrate ion concentration. The authors performed parallel studies with TcO₄^-, determining that K_d(ReO₄^-) and K_d(TcO₄^-) track similarly as the conditions are changed. An effort was made to prepare substituted pyridium nitrate salts that are soluble in organic solvents to be used as alternate extractants. However, in all cases but one, the salts were also soluble to some extent in the aqueous phase, significantly limiting their usefulness as extractants for these purposes. Many of the new extractant salts would partition between the organic solvent and water so that 10% of the extractant salt was in the aqueous phase. Only 1-methyl-3,5-didodecylpyridium nitrate did not show any measurable solubility in water. However, this compound was not as good an extractant as Aliquat-336. A considerable effort was also made to find suitable alternative solvents to 1,3-diisopropylbenzene. Several ketone solvents with flash points above 60 C were tested, and two of these, 2-nonanone and 3-nonanone, were superior to 1,3-diisopropylbenzene as a diluent