Temperature of maximum density behaviour of SO₂, H₂S and CO₂ in water

Abstract

Temperatures of maximum density (TMD) of aqueous SO₂<em>, </em> H₂S and CO₂ solutions at low concentrations (0.003-0.03 mol kg^-1) have been measured to study the effects of these gases on the structure of water. Due to large degree of dissociation of SO₂ in water yielding H⁺ and HSO<img src='/image/spc_char/3_bar.gif' border=0> ions, dissociation corrections are applied to evaluate the structural part (Δ<em>θ</em><img src='http://www.niscair.res.in/jinfo/equation132.gif' border=0>) of the change in TMD exclusively due to undissociated SO₂ molecules. This correction is not found necessary for H₂S and CO₂ which on account of their small degree of dissociation produce very small number of ions. The structural part of the change in TMD (Δ<em>θ</em><img src='http://www.niscair.res.in/jinfo/equation132.gif' border=0>) at various concentrations for these gases has been used to evaluate the characteristic constant 'a' from the sign and magnitude of which the gases SO₂, H₂S and CO₂ are shown to be water-structure stabilisers. This is corroborated by the positive ΔC<img src='http://www.niscair.res.in/jinfo/equation133.gif' border=0> for these gases dissolved in water. These findings are also supported by the formation of stable hydrates of SO₂ and H₂S from phase diagram and X-ray study respectively.