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Thermodynamics of the dissociations of glycine in 50 mass % aqueous monoglyme from 5 to 55°C
Authors:Rabindra N. Roy  James J. Gibbons  Ronald Snelling
Affiliation:(1) Department of Chemistry, Drury College, 65802 Springfield, Missouri;(2) Present address: Department of Chemistry, Louisiana State University, 70803 Baton Rouge, Louisiana
Abstract:Electromotive-force measurements on cells without liquid junction have been used to determine the pK1 and pK2 values of glycine in 50 mass % aqueous monoglyme at 11 temperatures from 5 to 55°C. The change in the first dissociation constant is given as a function of the thermodynamic temperatureT by the equation pK1=–2058.6/T+15.421–0.019169T, whereas that for the second dissociation constant is given by the equation pK2=1200.5/T+6.7211–0.0042897T. At 25°C, the pK1 is 2.806 in the mixed solvent, as compared with 2.350 in water; hence, protonated glycine becomes a weaker acid in the mixed solvent. The pK2 is 9.453 in the mixed solvent, whereas that in water is 9.780, suggesting that the second dissociation process becomes stronger in terms of acidity. The thermodynamic quantities DeltaGo, DeltaHo, DeltaSo, and DeltaCpo have been calculated, and the results have been discussed with respect to preferential solvation and also compared with similar data for the same two processes in 50 mass % methanol.
Keywords:Zwitterions  ampholyte  dissociation constants  glycine  solute-solvent interactions  ionization  activity coefficient  monoglyme-water  aqueous mixtures
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