Acid-Base Behavior In 50-Percent Aqueous Methanol: Thermodynamics of the Dissociation of Protonated Tris(hydroxymethyl)aminomethane and Nature of the Solvent Effect *

Elec tro mutive-force me thods a nd ce ll s without liquid junc tion have been uti lized to de te rmine the ac id ic d issociati on co ns tant of protona ted tri s(hydrox ymethyl)a minome th a ne 12-amino-2-(hydrox yme th yl)1, 3p ropa nedi ulJ in 50 wt pe rcent methanol so lve nt a t seve n te mperatures fro m ]0 to 40 °C. Th e c hange of the di ssoc iat ion cons ta nt with te mpera ture has been used to calcula te the c ha nges of en tha lpy, entrop y, and heat capac ity when the di ssocia ti on takes place in the s ta ndard s tate. Compari so ns with earli e r measureme nts in the aqueous medium revea l no grea t d ifl'e re nces in the entha llJ Y and entropy, s ugges tin g tha t wate r partic ipates in prefe re nce to me tha nol in the pro tolyti c react io n even in 50-pe rce nt mc thanol. It is shown th a t e lectros tati c co ns ide rati ons a lone a re una ble to explain the so lve nt efl'ec t on the di ssociation ene rgy, a nd a subs ta nt ia l " bas ic ity e fl'ec t" is indicated . The ac tivit y-coe ffi c ie nt te rm for the a mine hydroc hloride in eq uimola l a mine-sa lt bufl'e rs has been eva lua ted and compa red with s imila r da ta in the wa te r so lve nt.