Abstract Five geminal bis (phosphonates) containing an amino group in the α, γ or δ position of the carbon chain have been synthesised and their acid–base and complexation properties with Cu 2+, Zn 2+, Ca 2+, Mg 2+ and Na+ ions studied by potentiometry. Determination of the stability constants of the complexes with divalent metal ions is complicated by the formation of precipitates. The results of these studies show a negligible ...