Buffer Standards for pH Measurement of N-(2-Hydroxyethyl)piperazine-N'-2-ethanesulfonic Acid (HEPES) for I=0.16 mola⟨...kg-1 from 5 to 55 A°C

The values of the second dissociation constant, pK (2), of N-(2-hydroxyethyl) piperazine-N'-2-ethanesulfonic acid (HEPES) have been reported at twelve temperatures over the temperature range 5 to 55 A degrees C, including 37 A degrees C. This paper reports the results for the pa (H) of eight isotonic saline buffer solutions with an I=0.16 mola <...kg(-1) including compositions: (a) HEPES (0.01 mola <...kg(-1)) + NaHEPES (0.01 mola <...kg(-1)) + NaCl (0.15 mola <...kg(-1)); (b) HEPES (0.02 mola <...kg(-1)) + NaHEPES (0.02 mola <...kg(-1)) + NaCl (0.14 mola <...kg(-1)); (c) HEPES (0.03 mola <...kg(-1)) + NaHEPES (0.03 mola <...kg(-1)) + NaCl (0.13 mola <...kg(-1)); (d) HEPES (0.04 mola <...kg(-1)) + NaHEPES (0.04 mola <...kg(-1)) + NaCl (0.12 mola <...kg(-1)); (e) HEPES (0.05 mola <...kg(-1)) + NaHEPES (0.05 mola <...kg(-1)) + NaCl (0.11 mola <...kg(-1)); (f) HEPES (0.06 mola <...kg(-1)) + NaHEPES (0.06 mola <...kg(-1)) + NaCl (0.10 mola <...kg(-1)); (g) HEPES (0.07 mola <...kg(-1)) + NaHEPES (0.07 mola <...kg(-1)) + NaCl (0.09 mola <...kg(-1)); and (h) HEPES (0.08 mola <...kg(-1)) + NaHEPES (0.08 mola <...kg(-1)) + NaCl (0.08 mola <...kg(-1)). Conventional pa (H) values, for all eight buffer solutions from 5 to 55 A degrees C, have been calculated. The operational pH values with liquid junction corrections, at 25 and 37 A degrees 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 mola <...kg(-1).