BUFFERS FOR THE PHYSIOLOGICAL PH RANGE - ACIDIC DISSOCIATION-CONSTANTS OF ZWITTERIONIC COMPOUNDS (ACES AND CHES) IN WATER FROM 5 TO 55 DEGREES-C

The precise electromotive force (emf) measurements of the cells withou t 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) degrees C, wit h 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 s econd thermodynamic dissociation constants (pK(2)) of N-(2-acetamido)- 2-aminoethanesulfonic acid (ACES) and 2-(N-cyclohexylamino)ethanesulfo nic acid (CHES), along with the thermodynamic quantities associated wi th the ionization of ACES and CHES, have been calculated. The values o f pK(2) for AGES are expressed by the equation pK(2) = 2548.60/(T/K) - 20.03159 + 3.2172 ln(T/K), and for CHES, by pK(2) = 1917.66/(T/K) + 5 .7940 - 0.4971 ln(T/K) with standard deviations of +/-0.000 58 and +/- 0.000 97, respectively.