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Chloroselenic Acid, (SeO3.HCl)2

Chloroselenic Acid, (SeO3.HCl)2, has been described as a pale yellow liquid obtained when dry hydrogen chloride and selenium trioxide are allowed to combine in a cooled vessel. It has a density of 2.26, and solidifies at -46° C. It fumes in air owing to the escape of hydrogen chloride, which gas is also evolved on heating, selenium and selenium dioxide remaining.

Chloroselenic acid dissolves readily in water, much heat being evolved and selenic and hydrochloric acids being formed. It is decomposed by alcohol with precipitation of selenium. It dissolves without decomposition in selenium oxychloride, but is insoluble in benzene, carbon tetrachloride, chloroform and ether.

Molecular weight determinations give values in agreement with the double formula.

When selenium dioxide is similarly treated with dry hydrogen chloride, two additive compounds, SeO2.4HCl and SeO2.2HCl, are obtained. The former is a yellow acicular crystalline solid, stable at 0° C., whilst the latter is a yellow liquid stable up to 170° C. and dis- tillable at this temperature with some decomposition into its constituents. The liquid can absorb hydrogen chloride in amounts varying with the temperature. By the action of dehydrating agents selenium oxychloride may be obtained.

With hydrogen bromide, selenium dioxide produces a rather more stable crystalline compound, SeO2.4HBr.
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