Abstract
A sensitive spectrophotometric assay for ionized calcium has provided direct evidence for calcium complexes in aqueous bicarbonate solutions under physiological conditions as regards calcium and bicarbonate concentration, ionic strength, pH, and temperature. The results were consistent with a CaHCO+3 complex with an apparent stability constant of 2.6 litres/mole and a less well-determined CaCO°3 complex with an apparent stability constant around 53 litres2/mole2.