The partial molal volume and compressibility of Tris and Tris–HCl in water and 0.725 m NaCl as a function of temperature

The apparent molal volumes and compressibilities of Tris and Tris–HCl have been determined in water (5–45 °C) and 0.725 m NaCl (5–25 °C). The changes in the volume (Δ V ) and compressibility (Δ κ ) for the dissociation of Tris–H + Tris - H + = Tris + H + as functions of temperature in water and 0.725 m NaCl have been determined from these measurements. The values of Δ V and Δ κ have been used to determine the effect of pressure on the dissociation constant for Tris–H + ( K P / K 0 ). ln ( K P / K 0 ) = - ∆ V / ( R T ) P + 0 .5∆ k / ( R T ) P 2 These results will be useful in the calibration of pH systems making in-situ measurements at high pressure in seawater. In 0.725 m NaCl at 5 °C and a pressure of 2000 bar, the dissociation constant is reduced by 29%.