Second dissociation constants of 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS) and 3-(cyclohexylamino)-2-hydroxy-1-propanesulfonic acid (CAPSO) from the temperatures 278.15 K to 328.15 K

The second dissociation constant pK2values for 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS) and 3-cyclohexylamino)-2-hydroxy-1-propanesulfonic acid (CAPSO), useful secondary standard buffers within the pH range of 9.7 to 10.5 for CAPS, and 8.9 to 10.0 for CAPSO, have been determined fromT=278.15 K toT=328.15 K. Electromotive force measurements of cells of the type: Pt·H2(g,p°=101.325 kPa)·buffer (m1) Na salt of buffer (m2), NaCl (m3)·AgCl·Ag, wheremdenotes molality, were used. The pK2for the ionization of CAPS in the experimental temperature range is expressed by the equation: pK2=1046.64·(K/T)+35.10191−4.9342·ln(T/K), whereas that for CAPSO is given by: pK2=2106.21·(K/T)+9.08772−1.11049·ln(T/K). The thermodynamic quantities have been calculated and the results obtained are compared with those for zwitterionic compounds which are the derivatives of taurine.