Solute-Solvent Effects in the Acidic Dissociation of the Ampholyte N-Tris(hydroxymethyl)methylglycine ("Tricine") in 50 Mass % Methanol-Water Solvent

The pK values for the two acidic dissociation steps of the ampholyte N-tris(hydroxymethyl)methylglycine ("tricine") in 50 mass % methanol-water solvent have been determined by emf measurements of eells of the type Pt/H(2)(g, 1 atm), trieine buffer, Br(-), AgBr\Ag over the range 5 to 50 degrees C (pK(1)) and 5 to 60 degrees C (pK(2)). The standard thermodynamic quantities Delta H degrees, Delta S degrees, and Delta C degrees(p) for the two dissociation processes have been derived and are compared with the corresponding values for tricine and the parent glycine in water and with those for other acids in 50 mass % methanol-water solvent. Both trieine and protonated tricine become weaker acids when methanol is added to the aqueous solvent. It appears that a strong stabilization of the zwitterion in water is responsible for this behavior. This conclusion is supported by comparing the changes in entropy and heat capacity for the dissociation of tricine with the values of these quantities for the dissociation of "model" acids of simple structure, such as ammonium ion and acetic acid.