TY - JOUR
T1 - Determination of the protonation enthalpy of humic acid by calorimetric titration technique
AU - Kimuro, Shingo
AU - Kirishima, Akira
AU - Sato, Nobuaki
N1 - Funding Information:
This study has been supported by the Grant-in-Aid No. 24760713 for Young Scientists (B) of The Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT).
Publisher Copyright:
© 2014 Elsevier Ltd. All rights reserved.
PY - 2015/3
Y1 - 2015/3
N2 - In this study, the calorimetric titration technique was used to determine the protonation enthalpy of two reference humic acids and polyacrylic acid. First, we obtained the apparent protonation constant of two kinds of humic acid purchased from IHSS (International Humic Substances Society) and polyacrylic acid by potentiometric titration. Second, we obtained the protonation enthalpy of them by calorimetric titration. The protonation enthalpy of humic acid was affected by pH and the ionic strength of bulk solution. From the comparison of ΔH between humic acid and polyacrylic acid, it was concluded that the pH dependence of ΔH is attributed to the heterogeneity of humic acid. And ΔH of phenolic hydroxyl group in humic acid is strongly influenced by the electric double layer of humic acid's surface. This is considered to be a reason of the ionic strength dependence of ΔH. On the other hand, Gibbs free energy of the protonation of humic acid is affected by the electrostatic attraction with the progress of dissociation of functional groups such as carboxyl group and phenolic hydroxyl group. Consequently, the thermodynamics of the protonation of humic acid is affected by the polyelectrolyte effect and the heterogeneity.
AB - In this study, the calorimetric titration technique was used to determine the protonation enthalpy of two reference humic acids and polyacrylic acid. First, we obtained the apparent protonation constant of two kinds of humic acid purchased from IHSS (International Humic Substances Society) and polyacrylic acid by potentiometric titration. Second, we obtained the protonation enthalpy of them by calorimetric titration. The protonation enthalpy of humic acid was affected by pH and the ionic strength of bulk solution. From the comparison of ΔH between humic acid and polyacrylic acid, it was concluded that the pH dependence of ΔH is attributed to the heterogeneity of humic acid. And ΔH of phenolic hydroxyl group in humic acid is strongly influenced by the electric double layer of humic acid's surface. This is considered to be a reason of the ionic strength dependence of ΔH. On the other hand, Gibbs free energy of the protonation of humic acid is affected by the electrostatic attraction with the progress of dissociation of functional groups such as carboxyl group and phenolic hydroxyl group. Consequently, the thermodynamics of the protonation of humic acid is affected by the polyelectrolyte effect and the heterogeneity.
KW - Calorimetric titration
KW - Humic acid
KW - Protonation modeling
KW - Thermodynamic quantities
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U2 - 10.1016/j.jct.2014.10.010
DO - 10.1016/j.jct.2014.10.010
M3 - Article
AN - SCOPUS:84910630596
VL - 82
SP - 1
EP - 8
JO - Journal of Chemical Thermodynamics
JF - Journal of Chemical Thermodynamics
SN - 0021-9614
ER -