Buffers for the Physiological pH Range: Acidic Dissociation Constants of Zwitterionic Compounds (ACES and CHES) in Water from 5 to 55 °C

The precise electromotive force (emf) measurements of the cells without liquid junction, Pt|H 2 |ACES ( m 1 ) + NaACESate ( m 2 ) + NaCl ( m 3 )|AgCl, Ag and Pt|H 2 |CHES ( m 1 ) + NaCHESate ( m 2 ) + NaCl ( m 3 )|AgCl, Ag, have been made to collect emf data from (5 to 55) °C, with molalities m 1 = m 2 = m 3 in the range 0.01 to 0.1 mol·kg - 1 . The hydrogen and silver + silver chloride electrodes were used. The second thermodynamic dissociation constants (p K 2 ) of N -(2-acetamido)-2-aminoethanesulfonic acid (ACES) and 2-( N -cyclohexylamino)ethanesulfonic acid (CHES), along with the thermodynamic quantities associated with the ionization of ACES and CHES, have been calculated. The values of p K 2 for ACES are expressed by the equation p K 2 = 2548.60/( T /K) − 20.03159 + 3.2172 ln( T /K), and for CHES, by p K 2 = 1917.66/( T /K) + 5.7940 − 0.4971 ln( T /K) with standard deviations of ±0.000 58 and ±0.000 97, respectively.