THERMODYNAMIC CONSTANTS OF N-(2-HYDROXYETHYL)PIPERAZINE-N'-3-PROPANESULFONIC ACID (HEPPS) AND (3-[N-MORPHOLINOL])-2-HYDROXYPROPANESULFONIC ACID (MOPSO) FROM THE TEMPERATURES 278.15 K TO 328.15 K

Electromotive-force measurements were taken for cells without a liquid junction of the type: Pt \ H-2(g, p(0) = 101.325 kPa) \ buffer (m(1)) , Na salt of buffer (m(2)), NaCl (m(3)) \ AgCL \ Ag, where m denotes m olality. Values of the second thermodynamic dissociation constant pK(2 ) for N-(2-hydroxyethyl)piperazine-N'-3-propanesulfonic acid (HEPPS) a nd (3-[N-morpholinol])-2-hydroxypropanesulfonic acid (MOPSO) have been determined from T = 278.15 K to T = 328.15 K. These zwitterionic comp ounds are useful as secondary standard buffers in the pH range 7.6 to 8.2 for HEPPS, and 7.2 to 6.4 for MOPSO. The pK(2) values for the ioni zation of HEPPS in the experimental temperature range are expressed by the equation: pK(2) = 359.23.(K/T) + 21.0324 - 2.50647.ln(T/K); while , for MOPSO, pK(2) = 432.25.(K/T) + 22.99923 - 3.08415.ln(T/K). The va lues of the thermodynamic quantities are compared with those for struc turally related compounds. (C) 1997 Academic Press Limited.