Q. Explain about Thiosulphuric Acid?

Thiosulphuric acid, H₂S₂0₃, has never been isolated but its salts are well known which contain the thiosulphate ion, S₂O^2-₃. The best' known thiosulphate is the sodium salt, Na₂S₂O₃.5H₂O. It is obtained by boiling a Solution of sodium sulphite with sulphur followed by filtration and crystallisation.

Na₂SO₃ + S ---------> Na₂S₂O₃

Sodium thiosulphate is a colourless, crystalline solid highly soluble in water and commercially known as hypo. The thiosulphate ion is unstable in the presence of acids breaking to give H₂O, SO₂ and S.

Na₂S₂O₃+ 2HCl --------------> 2NaCl + SO₂+ S + H₂O

It functions as a reducing agent, reducing chlorine to chloride. This reaction is utilised for removing excess chlorine from bleached fabrics.

Na₂ S₂O₃ + 4Cl₂ + 5H₂O----------------> 2NaHSO₄ + 8HCl

The milder oxidising agent iodine behaves rather differently giving sodium tenathionate instead of Na₂S0₄, though it is also converted into iodide.

Na₂S₂O₃+ I₂------------> 2NaI + NA₂S₄ O₆                sodium tetrathionate

This reaction is used in iodometric titrations for determination of copper. Sodium thiosulphate is used in photography for 'fixing' the negative. It removes the unreacted light sensitive silver bromide from photographic plates to avoid further reaction of light.

AgBr+ 2Na₂S₂O₃--------------> Na₃ [Ag (S₂O₃] ₂+ NaBr