Buffer Standards for the Physiological pH of the Zwitterionic Compound, HEPPSO from 5 to 55 °C

The values of the thermodynamic second dissociation constant, pK2, and related thermodynamic quantities of N-(2-hydroxyethyl)piperazine-N′-2-hydroxypropanesulfonic acid (HEPPSO) have already been reported from 5 to 55 °C, including 37 °C, by the emf method. This paper reports the results for the pH of one chloride-free buffer solution containing the composition: (a) HEPPSO (0.08 mol⋅kg−1)+NaHEPPSO (0.04 mol⋅kg−1). The remaining seventeen buffer solutions contain a saline medium of ionic strength I=0.16 mol⋅kg−1, matching closely that of physiological fluids. Conventional pH values, denoted as paH, for all eighteen buffer solutions from 5 to 55 °C have been calculated. The operational pH values, designated as pH, with residual liquid-junction corrections for five buffer solutions, one without NaCl, and four with buffer solutions in saline media of I=0.16 mol⋅kg−1 are recommended as pH standards in the range of physiological application. These are based on the NBS/NIST standard scale for pH measurements.