The second acidic dissociation constants of protonated piperazine-N,N′-bis-2-hydroxypropane-sulfonic acid (POPSO sesquisodium salt) have been determined at 12 different temperatures from (278.15 to 328.15) K including 310.15 K. Electromotive-force measurement technique was used employing hydrogen-silver chloride cells without liquid junction. The results of pK
2 are given by the equation: pK
2 = -1041.77/T + 51.0459 - 6.97646lnT. The uncertainty of the fit is ±0.0008. At 289.15 K, pK
2 = 7.8029; whereas, at 310.15 K (body temperature), pK
2 = 7.6862. Thus, the buffer solutions of POPSO and its sodium salt are useful for pH control in the physiological pH region of (7.0 to 8.5). The changes of Gibbs free energy (
G°), enthalpy (
H°), entropy (
S°) and heat capacity
Cp° were computed from the temperature derivative of the pK
2 for the dissociation of the zwitterionic acid POPSO
±-3 = POPSO
-4 + H
+ in the standard state. At 298.15 K, these results are compared with those of similar components, which are the derivatives of the parent compounds TAURINE, PIPERAZINE and MORPHOLINE.