ISSN:
1572-8927
Keywords:
Zwitterions
;
ampholyte
;
dissociation constant
;
glycine
;
solute-solvent interactions
;
ionization
;
activity coefficient
;
methanol-water
;
aqueous mixture
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
Notes:
Abstract The two thermodynamic dissociation constants of glycine at 11 temperatures from 5 to 55°C in 50 mass % methanol-water mixed solvent have been determined from precise emf measurements with hydrogen-silver bromide electrodes in cells without liquid junction. The first acidic dissociation constant (K 1)for the process HG+⇌H++G± is expressed as a function ofT(oK) by the equation pK 1 = 2043.5/T − 9.6504 + 0.019308T. At 25°C, pK 1is 2.961 in the mixed solvent, as compared with 2.350 in water, with ΔH°=1497 cal-mole−1, ΔG°=4038 cal-mole−1, ΔS°=−8.52 cal-°K−1-mole−1, and ΔC p o =−53 cal-°K−1-mole−1. The second acidic dissociation constant (K 2)for the process G±⇌H++G− over the temperature range studied is given by the equation pK 2 = 3627.1/T − 7.2371 + 0.015587T. At 25°C, pK 2is 9.578 in MeOH−H2O as compared with 9.780 in water, whereas ΔH° is 10,257 cal-mole−1, ΔG° is 13,063 cal-mole−1, ΔS° is −9.41 cal-°K−1-mole−1, and ΔC p o is −43 cal-°K−1-mole−1. The protonated glycine becomes weaker in 50 mass % methanol-water, whereas the second dissociation process becomes stronger despite the lower dielectric constant of the mixed solvent (ω=56.3 at 25°C).
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00647198
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