Buffer Standards for pH Measurement of N-(2-Hydroxyethyl)piperazine-N′-2-ethanesulfonic Acid (HEPES) for I=0.16 mol⋅kg−1 from 5 to 55 °C

The values of the second dissociation constant, p K 2 , of N -(2-hydroxyethyl) piperazine- N ′-2-ethanesulfonic acid (HEPES) have been reported at twelve temperatures over the temperature range 5 to 55 °C, including 37 °C. This paper reports the results for the p a H of eight isotonic saline buffer solutions with an I =0.16 mol⋅kg −1 including compositions: (a) HEPES (0.01 mol⋅kg −1 ) + NaHEPES (0.01 mol⋅kg −1 ) + NaCl (0.15 mol⋅kg −1 ); (b) HEPES (0.02 mol⋅kg −1 ) + NaHEPES (0.02 mol⋅kg −1 ) + NaCl (0.14 mol⋅kg −1 ); (c) HEPES (0.03 mol⋅kg −1 ) + NaHEPES (0.03 mol⋅kg −1 ) + NaCl (0.13 mol⋅kg −1 ); (d) HEPES (0.04 mol⋅kg −1 ) + NaHEPES (0.04 mol⋅kg −1 ) + NaCl (0.12 mol⋅kg −1 ); (e) HEPES (0.05 mol⋅kg −1 ) + NaHEPES (0.05 mol⋅kg −1 ) + NaCl (0.11 mol⋅kg −1 ); (f) HEPES (0.06 mol⋅kg −1 ) + NaHEPES (0.06 mol⋅kg −1 ) + NaCl (0.10 mol⋅kg −1 ); (g) HEPES (0.07 mol⋅kg −1 ) + NaHEPES (0.07 mol⋅kg −1 ) + NaCl (0.09 mol⋅kg −1 ); and (h) HEPES (0.08 mol⋅kg −1 ) + NaHEPES (0.08 mol⋅kg −1 ) + NaCl (0.08 mol⋅kg −1 ). Conventional p a H values, for all eight buffer solutions from 5 to 55 °C, have been calculated. The operational pH values with liquid junction corrections, at 25 and 37 °C have been determined based on the NBS/NIST standard between the physiological phosphate standard and four buffer solutions. These are recommended as pH standards for physiological fluids in the range of pH = 7.3 to 7.5 at I =0.16 mol⋅kg −1 .