Temperature of maximum density behaviour of ammonia in water

Abstract

The temperature of maximum density (TMD) of aqueous ammonia solutions at low concentrations (0.019–0.16 mol kg^–1) has been measured. The data have been analysed by applying the hydrolysis correction to evaluate the structural part (Δθ^mol_str) of the change in TMD due to unhydrolysed ammonia molecules. From the calculation of the characteristic constant, a, ammonia is found to be water structure stabilising. This water structure strengthening ability of ammonia is corroborated by density measurements of aqueous ammonia solutions at different low concentrations over a limited range of temperatures around 4 °C. The negative entropy of hydration, positive ΔC^∞_p and negative dB^str_η/dT found for aqueous ammonia also support this finding. The negative limiting excess expansibility, α^∞E₂, of ammonia undergoes a change in sign at 5 °C.