Practical acid dissociation constants, temperature coefficients, and buffer capacities for some biological buffers in solutions containing dimethyl sulfoxide between 25 and −12 °C

Abstract

Acid dissociation constants ( K a ), usually expressed as p K a (- log K a ) can be considered as indices of acid-base equilibria in solution and their evaluation under the solution conditions that exist during the exposure of biological systems to low temperatures are as important as the measurement of pH per se. The assignment of pH∗ standards to define pH∗ scales in the binary mixed solvent, dimethyl sulfoxide-water (27), has provided the basis for measuring the p K a ∗ values of some biological buffers in mixtures of Me 2SO and H 2O which have particular relevance to studies which demonstrate the “pH-dependent” recovery of smooth muscle after low-temperature storage (9, 31). “Practical” ionization conslants in water (p K′ a ) and in 20% ( w w ) and 30% ( w w ) dimethylsulfoxide-water (p K a ∗) have been measured by potentiometric titration of a range of zwitterionic buffer compounds at 25, 0, −5.5, and −12 °C together with the respective buffer capacities and temperature coefficients. Measurements have been made with reference to the relevant standard states for each solvent system, thereby endowing the values with as much thermodynamic significance as possible.