POTENTIOMETRIC DETERMINATION OF ANIMAL STABILITY-CONSTANTS

Potentiometric titration was used to determine the logarithms of the s tepwise equilibrium constants for the species formed between morpholin e and formaldehyde in aqueous solution, ionic strength 0.5 and 2.5M (K Cl) at 25 degrees C. The instrumental and computational techniques dev eloped for metal-ligand stability constant determination were applied. Formaldehyde is equivalent to the metal-ion and is represented by M w hile neutral morpholine is equivalent to the ligand and is represented by L. The stability constants of the following equilibria were determ ined by non-linear regression (figures in parentheses are at ionic str ength 2.5M KCl): M + L reversible arrow ML (hemi-aminal) log K-1 = 2.9 0+/-0.02 (2.980+/-0.004); ML + L reversible arrow ML(2) (bis-animal); log K-2 = 1.3+/-0.2 (1.41+/-0.07); MLH reversible arrow ML + H+ (proto nated hemi-aminal) pK(a) = 5.87+/-0.01 (6.411+/-0.005); ML(2)H reversi ble arrow ML(2) + H+ (protonated bis-aminal) pK(a) = (7.6+/-0.2). The pK(a) of the protonated bis-aminal could only be determined at the hig her ionic strength. The results are in good agreement with reported va lues determined using the classic formol titration. The automated titr ation system acquired the full time course of the pH change upon each titrant addition allowing a kinetic analysis to be performed as well a s an equilibrium analysis. The forward and reverse rate constants for M + L reversible arrow ML were 0.77M(-1) sec(-1) and 8.1 x 10(-4) sec( -1), respectively.