Solute-solvent effects in the acidic dissociation of the ampholyte N-tris(hydroxymethyl)methylglycine (“tricine”) in 50 mass % methanol-water solvent
Tóm tắt
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 cells of the type Pt|H2(g, 1 atm), tricine buffer, Br−, AgBr|Ag over the range 5 to 50°C (pK
1)and 5 to 60°C (pK
2).The standard thermodynamic quantities ΔHo, ΔSo, and ΔC
p
o
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 tricine 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.
Tài liệu tham khảo
R. G. Bates and H. B. Hetzer,J. Phys. Chem. 65, 667 (1961).
S. P. Datta, A. K. Grzybowski, and B. A. Weston,J. Chem. Soc., 792 (1963).
M. Woodhead, M. Paabo, R. A. Robinson, and R. G. Bates,J. Res. Nat. Bur. Std. 69A, 263 (1965).
D. H. Everett and W. F. K. Wynne-Jones,Proc. Roy. Soc. (London) 169A, 190 (1938).
R. G. Bates and G. D. Pinching,J. Res. Nat. Bur. Std. 42, 419 (1949).
S. Goldman, P. Ságner, and R. G. Bates,J. Phys. Chem. 75, 826 (1971).
H. S. Harned and R. W. Ehlers,J. Am. Chem. Soc. 54, 1350 (1932).
R. A. Robinson and R. H. Stokes,Electrolyte Solutions (Butterworths, London, 1970), 2nd rev. ed., Appendix 12.1.
R. N. Roy, R. A. Robinson, and R. G. Bates,J. Am. Chem. Soc. 95, 8231 (1973).
E. J. King,J. Am. Chem. Soc. 73, 155 (1951).
R. G. Bates and R. A. Robinson, inChemical Physics of Ionic Solutions, ed. by B. E. Conway and R. G. Barradas (Wiley, New York, 1966), Chap. 12.
R. N. Roy, R. A. Robinson, and R. G. Bates,J. Chem. Thermodyn. 5, 559 (1973).
R. G. Bates,J. Res. Nat. Bur. Std. 47, 127 (1951).
H. S. Harned and R. A. Robinson,Trans. Faraday Soc. 36, 973 (1940).
N. W. Please,Biochem. J. 56, 196 (1954).
M. Paabo, R. G. Bates, and R. A. Robinson,J. Phys. Chem. 70, 247 (1966).
D. H. Everett and W. F. K. Wynne-Jones,Trans. Faraday Soc. 48, 531 (1952).
M. Paabo, R. A. Robinson, and R. G. Bates,J. Am. Chem. Soc. 87, 415 (1965).
M. Paabo, R. A. Robinson, and R. G. Bates,Anal. Chem. 38, 1573 (1966).
E. E. Sager, R. A. Robinson, and R. G. Bates,J. Res. Nat. Bur. Std. 68A, 305 (1964).
R. G. Bates,J. Electroanal. Chem. 29, 1 (1971).
R. A. Robinson and R. G. Bates,J. Res. Nat. Bur. Std. 70A, 553 (1966).
J. O'M. Bockris and A. K. N. Reddy,Modern Electrochemistry (Plenum, New York, 1970), Vol. I, Chap. 2.
K. S. Pitzer,J. Am. Chem. Soc. 59, 2365 (1937).