Thallous hydroxide, TlOH

Thallous hydroxide, TlOH, crystallises from aqueous solution in yellow needles. A solution of the hydroxide is readily prepared by shaking an excess of thallium turnings with water in a vessel through which a stream of oxygen is passing, or by double decomposition between thallous sulphate and baryta.

Thallous hydroxide is readily dissociated into thallous oxide and water: -

2TlOH ⇔ Tl₂O + H₂O,

the equilibrium pressure reaching the value 760 mm. at 139°, according to the measurements of Bahr. The hydroxide also darkens in colour in vacuo when exposed to light, but the nature of the chemical change is unknown.

Thallous hydroxide is readily soluble in water and alcohol. One litre of the saturated aqueous solution at t° contains x grams of the hydroxide, where x and t are as follows: -

t°	0°	20°	30°	40°	50°	60°	70°	80°	90°	100°
x	254.4	352.7	402.8	495.0	594.5	733.1	888.8	1062.6	1260.8	1505.0

Thallous hydroxide is a comparatively strong base. The aqueous solution is colourless and has a strongly alkaline reaction. It precipitates metallic hydroxide by double decomposition, readily absorbs carbon dioxide, and attacks glass and porcelain. Accordingly, unless special care is taken in its preparation, thallous hydroxide is always contaminated with a little carbonate.

Thallous hydroxide is oxidised to dark brown thallic hydroxide by ozone. The reaction has been proposed as a test for ozone, but is given by other oxidising agents, e.g. hydrogen peroxide.

The hydrate, TlOH.H₂O, has been obtained, but the conditions under which it is produced are not known.