The protonation equilibria for 20 standard alpha-amino acids in solutions have been studied pH-potentiometrically. The dissociation constants (pK(a)) of 20 amino acids and the thermodynamic functions (Delta G degrees, Delta H degrees, Delta S degrees, and delta) for the successive and overall protonation processes of amino acids have been derived at different temperatures in water and in three different mixtures of water and dioxane (mole fractions of dioxane were 0.083, 0.174, and 0.33). Titrations were also carried out in water ionic strengths of (0.15, 0.20, and 0.25) mol.dm(-3) NaNO3, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent (dioxane), temperature and ionic strength influencing the protonation processes of amino acids is presented and discussed to determine the factors which control these processes. (c) 2006 Elsevier Ltd. All rights reserved.