Polythionic acid

Skeletal formula of polythionic acid

Polythionic acid is an oxoacid which has a straight chain of sulfur atoms and has the chemical formula H₂S_nO₆ (n > 2). Trithionic acid (H₂S₃O₆), tetrathionic acid (H₂S₄O₆) are simple examples. The compounds of n < 80 are expected to exist, and those of n < 20 have already been synthesized. Dithionic acid (H₂S₂O₆) does not belong to polythionic acid due to the difference in the property.

Stability

Under the condition of strong acidity about pH 1, most of polythionic acids are stable.

Synthesis

Various polythionic acids are produced in Wackenroder solution in which sulfur dioxide and hydrogen sulfide dissolve. The reactions may be as follows although the mechanisms are not clear.

H₂S + H₂SO₃ → H₂S₂O₂ + H₂O

H₂S₂O₂ + 2H₂SO₃ → H₂S₄O₆ + 2H₂O

H₂S₄O₆ + H₂SO₃ → H₂S₃O₆ + H₂S₂O₃

Occurrence

Polythionic acids are often found in crater lakes. There are various kind of ions containing sulfur atoms derived by hydrogen sulfide and they make the strong acidity condition. It it observed that polythionates in crater lakes are drastically decreased before an eruption occurs.^[1] The phenomenon may be useful to predict volcanic activity.

References