Samples Containing Rhenium

Overview

While preparing his periodic table, Mendeleev found no element to occupy position 75. He therefore proposed the existence of this element and named it dvi-manganese (meaning that it is two [dvi] positions down from a known element in the same group), and described some of its properties. Element 75, rhenium (named after the river Rhine in Europe), was not discovered until about 55 years later, in 1925, by Noddack, Tacke, and Berg, through its X-ray spectrum, and is the last "stable" element to be discovered.

Rhenium is an extremely rare metal that is found in Cu and Mo ores at low ppm levels. Re is rarely found in minerals at concentrations in excess of 20 ppm. Re is a silvery, white, dense metal that is similar in appearance to Pt and Ir. Chemically, Re is unlike Mn, and is not found in minerals associated with Mn. Re has a high melting point (3180 °C), second only to tungsten (W).

The chemistry of Re does not resemble that of any other element, except technetium, which is primarily man-made and will not be discussed in this series.

Sampling and Handling

The measurement of Re is uncommon, and there is essentially no risk of contamination when any type of sample handling apparatus/device is used.

For general information on sampling and subsampling, see Sampling and Subsampling.

The Metal

The metal dissolves readily in nitric acid, and is the most popular acid to use for metal dissolution:

3Re + 7 HNO₃ = 3HReO₄ + 7NO + 2 H₂O

The metal is slowly soluble in sulfuric acid and insoluble in HF and HCl. All modes of acid attack carry the risk of loss of some volatile perrhenic acid (HReO₄). Although the literature is somewhat vague on this point, it is suggested that acid digestions be performed under reflux conditions or in closed vessels.

Metallic Re is stable in air below 1000 °C. When fused with NaOH, it forms a yellow melt from which Na₂ReO₄ may be obtained. Alkali perrhenates can be melted without decomposition. Due to the potential volatility of HReO₄ and Re₂O₇, fusion with NaOH, NaOH + NaNO₃, or Na₂CO₃ (preferred, because Pt crucibles can be used, and NaNO₃ can be added if a stronger oxidizing environment is needed) is common. Finally, an aqueous mixture of NaOH and H₂O₂ readily dissolves the metal.

Oxides, Minerals and Ores

Any rhenium material may be converted to a soluble perrhenate by fusion with Na₂O₂. Fusion with Na₂CO₃, or with Na₂CO₃ plus NaNO₃, is about equally general as a mode of attack, and has the advantage that Pt crucibles can be used.

Organic Matrices

Dry ashing of organic materials, foodstuffs, plant, and blood and sewage sludge, as a preliminary decomposition step, is not suggested for samples containing Re, unless an alkaline reagent such as Na₂CO₃ is added. If ashing is used, it is suggested to keep the temperature low (400 to 450 °C max) and to use an ashing aid, such as high-purity sodium carbonate. If the sample is high in silica, subsequent fusion of the ash with sodium carbonate is suggested. Heating of the ash to fumes with sulfuric and hydrofluoric acids to rid the sample of silica is not suggested.

For more on ashing, please see the following paper: Ashing

Detailed Elemental Profile

Chemical compatibility, stability, preparation, and atomic spectroscopic information is available by clicking the element below. For additional elements, visit our Interactive Periodic Table.