Microcalorimetric studies. Part 2.—Enthalpies of reaction of Cu(II), Ni(II) and Zn(II) with glycine, serine, threonine and histidine

Abstract

Measurements are reported of the enthalpies of reaction of Cu(II), Ni(II) and Zn(II) with neutralized glycine, serine, threonine and histidine solutions over a broad range of pH. There is a sharp change in enthalpy of reaction above pH∼10 in Cu-serine and Cu-threonine solutions, possibly due to the formation of hydroxypolynuclear complexes in more alkaline solutions. Ni-serine and Zn-serine systems do not show a similar change in enthalpy of reaction at high pH, but precipitation may occur. The enthalpies of formation of the bis-histidino complexes with Cu(II), Ni(II) and Zn(II) are reported; using measured enthalpies of ionization of the histidine acids, reaction enthalpies of Ni(II) with histidine over a range of experimental conditions were calculated, and compare satisfactorily with the observed results.