Pulse polarography: Part X. Formaldehyde hydration in aqueous acetate and phosphate buffer solutions

Formaldehyde hydration has been studied with pulse polarography in acetate and phosphate buffers, at 25°C. It could be shown that in both buffer systems extrapolation to zero buffer concentration produces the equilibrium constant K_d=CH₂O]/CH₂(OH)₂]=(6.31±0.05) 10^?4 in pure water, a value of as yet unmatched precision. In the buffers mentioned two novel features appear in the hydration mechanism: (a) buffer acids form addition compounds with CH₂O where the, corresponding equilibria are rapidly established. The equilibrium constants have been derived. (b) H₂PO₄^?/HPO₄^2? buffers are exceptional in that the polarographic specific rate of hydration as a function of the concentration of this buffer is less than expected on the basis of a normal linear dependence. This is explained, as in the case of free-aldehyde glucose 23], on the basis of a solvent (water)-shared associate of (presumably) H₂PO₄^? with CH₂O which decreases the rate of diffusion of the latter, the more so as the buffer concentration increases. It is pointed out, that the occurrence of such associates can explain the excess activity of H₂PO₄^? as an acid catalyst in aldehyde hydration 16].