Thermodynamic Quantities for the Ionization Reactions of Buffers

262 GOLDBERG, KISHORE, AND LENNEN
Values adjusted to TÄ298.15 K and IÄ0
pK rH°/(kJ mol 1 ) rC p /(J K 1 mol 1 ) Reference
Reaction „1…: H2L ¿ ÄH ¿ ¿HL
1.78 3.51 76LEW/HAN
Reaction „2…: HLÄH ¿ ¿LÀ 6.3 04JOH
6.3 10HOL
6.31 28MOR
6.18 29BRO/WYN
6.273 49KIL
6.14 3.3 76LEW/HAN
6.288 76WAU
6.45 82TAK/YAG
6.25 83CAP
6.27 87KIT/ITO
6.4 97KOB/SUG
6.35 2.6 86 98FUK/TAK
Comments: The selected pK value for reaction 2 encompasses the results of what we believe to be the most reliable of the studies 49KIL, 76WAU. There
is a reasonable agreement of this selected pK value with the other results. We adopt rH°/(kJmol1 )3.0, the rounded average of the calorimetric results
for reaction 2.
TABLE 7.18. CAPS
Other names 3-cyclohexylamino-1-propanesulfonic acid; CAS No. 1135-40-6
Empirical formula C 9H 19NO 3S
Molecular weight 221.32
Ionization reaction
Evaluation: AAC
Structure:
HL H L , where HLC 9H 19NO 3S
Selected values at TÄ298.15 K and IÄ0:
pK10.499, rG°/(kJ mol 1 )59.929, rH°/(kJ mol 1 )48.1, and rC p /(J K 1 mol 1 )57
Values from literature
pK rH°/(kJ mol 1 ) T/K I Methods and comments Reference
10.35 48.5 298.15 0.01 M Potentiometric titration—glass electrode; and calorimetry. Dilute
solutions 0.005–0.01 M were titrated with 0.1 M NaOH or 0.1 M
HCl.
76MCG/JOR
10.60 298.15 ? Potentiometric titration of sample 0.01–0.014 M with 0.1 M HCl
or 0.1 M NaOH.
87KIT/ITO
10.39 298.15 0.10 M Potentiometric titration—glass electrode. 97ORA/AZA
11.094 278.15 0 Electrochemical cell—no liquid junction. The pK value at T 97ROY/MOO
10.940 283.15 0
10.790 288.15 0
10.643 293.15 0
10.499 48.22 298.15 0
10.359 303.15 0
10.223 308.15 0
10.168 310.15 0
10.087 313.15 0
9.958 318.15 0
9.831 323.15 0
J. Phys. Chem. Ref. Data, Vol. 31, No. 2, 2002
278.15 K in the published paper 97ROY/MOO contains a
typographical error. The value given here is correct 2001ROY. The
value of rH° given here was calculated from pKs measured at
several temperatures. The value rC p /(J K 1 mol 1 )93 was also
calculated from the temperature dependency of Roy et al.’s
97ROY/MOO reported pKs.