Sulfuric acid dissolving elemental selenium

The colored solutions produced on dissolving elemental selenium in sulfuric acid were first observed by Magnus in 1827 80). Since then a number of workers have investigated the nature of selenium solutions in sulfuric acid, oleum, and sulfur trioxide, providing 81) a substantial amount of data but little understanding of the system. Recently, it has been shown that these solutions contain the yellow Se and green Seg " polyatomic cations 82). 

Elemental red selenium, like sulfur, when dissolved in organic liquids, reacts with thallous sulfide paper (see page 433). An addition compound is formed which is resistant to dilute acids and hydrogen peroxide. Since it is black, it cannot be seen on the black thallous sulfide paper, in contrast to the analogous red-brown compound formed by considerable quantities of sulfur. However, the unaltered thallous sulfide can be dissolved away by bathing the paper in dilute acid or hydrogen peroxide solution, and the spot at which the drop of selenium solution was applied then shows up as a dark brown to black fleck. The latter consists of the selenide-sulfur complex, or of thallous sulfide, protected from the action of the solvents by a coating of the complex. 

Selenium has a valence of 2- when combined with hydrogen or metals. In oxy-compounds it has 4-f and 6+ valence, similar to sulfur. Like sulfur, selenium forms dioxides, trioxides and other oxygen compounds, especially if heated. The most common oxidation reactions are conversion of elemental selenium to selenium dioxide, which dissolves in water to form selenious acid (H2Se02). The... 

Elemental sulfur, selenium, and tellurium give colored solutions when dissolved in a number of strongly acidic media. It has been shown that SjJ, Sg+, S4+, Seg+, Tc4+, and Tcg+ are present in such solutions. These cations are formed by the oxidation of elements by H2S2O7 or S2O6F2 for example,... 

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