‘Non-coordinating’ buffers for studies involving metal ions

Abstract

Buffer systems are of outstanding importance for solution chemistry. When it comes to kinetic and thermodynamic studies on metal ions in buffered aqueous solution the extent of complex formation between the buffer applied and the metal ions studied should be negligibly small. In addition, there should be no or at least only a minor catalytic effect of the buffer on systems which are subject to general acid catalysis. 2,6-Lutidine (=2,6-dimethylpyridine) and 2,4,6-collidine (=2,6-trimethylpyridine) have often been applied as buffer compounds for the pH range 6.5–8.0 because of their restricted coordination properties due to steric hindrance through the two methyl groups neighbouring the donor nitrogen. A series of lutidines L carrying substituents in the 3- and/or 4-position has been synthesized and characterized with respect to yield upon synthesis, solubility in water, and UV absorption. ▪ L = 3-X 3-4-X 4-2,6-dimethylpyridine pK a values of the free bases L and complex formation constants for the aquo ions Ag +, Mg 2+, Ca 2+, Ba 2+, Zn 2+, Cu 2+, Ni 2+, and Ce 3+ as determined by potentiometric titration in aqueous solution are presented. A sequence of 2,6-lutidine type buffers is suggested covering the pH range 3–8 in small steps. The formation constants for the 1:1 complexes of divalent and trivalent aquo metal cations are small (mean value: 1.7 M −1) and nearly independent of both the nature of the metal and the pK a of the substituted 2,6-lutidines studied. These results are interpreted as being indicative of weak complex formation sterically restricted to ‘outer sphere’ interaction. It is shown that the acids LH + do not act as catalysts for the dissociation of nickel(II) triglycine complex which is known to be subject to general acid catalysis.