A simple spectrophotometric assay for micromolar amounts of lanthanum in the presence of calcium and phosphate

Abstract A sensitive spectrophotometric assay for micromolar amounts of lanthanum in the presence of calcium and phosphate (as hydroxyapatite) was developed utilizing the change in absorption (at 652 nm) when the dye arsenazo III was complexed with lanthanum. Arsenazo III was used at a level of 25 μM and the solution pH was maintained at 3.1 with 0.2 M sodium acetate. Lanthanum concentrations down to 0.5 μM could be reliably assayed. Calcium ion did not complex well with arsenazo III at pH 3.1. With calcium present at 100 μM and lanthanum at 10 μM, the assay was 115 times more sensitive for lanthanum. The assay is simple, rapid, reproducible and, unlike the assay using radioactive lanthanum, can be performed at any time.