Complexation of M–(buffer)x–(OH)y Systems Involving Divalent Ions (Cobalt or Nickel) and Zwitterionic Biological Buffers (AMPSO, DIPSO, TAPS and TAPSO) in Aqueous Solution

The complexation behavior of eight M–(buffer) x –(OH) y systems involving two divalent ions (cobalt and nickel) and four zwitterionic biological buffers (AMPSO, DIPSO, TAPS and TAPSO) were characterized. Complex formation was detected for all eight M–(buffer) x –(OH) y systems studied, but fully defined final models were obtained for only four of these systems. For systems involving cobalt or nickel with AMPSO or TAPS, a complete characterization of the systems was not possible in the studied buffer pH-range. Metal complexation was studied by glass-electrode potentiometry (GEP) and UV-Vis spectroscopy at 25.0 °C and I =0.1 mol⋅dm −3 KNO 3 ionic strength. For the Ni–(L) x –(OH) y and Co–(L) x –(OH) y systems, with L = TAPSO or DIPSO, the proposed final models and overall stability constants were obtained by combining results from both techniques. For the Ni–(L) x –(OH) y systems, the measured values of the stability constants are log 10 β NiL =3.0±0.1 and log 10 β NiL2 =4.8±0.1 for L = TAPSO, and log 10 β NiL =2.7±0.1 and log 10 β NiL2 =4.6±0.1 for L = DIPSO. For the Co–(L) x –(OH) y systems, the overall stability constants are log 10 β CoL =2.2±0.1, log 10 β CoL2 =3.6±0.2 and log 10 β CoL(OH) =7.6±0.1 for L = TAPSO, and log 10 β CoL =2.0±0.1 and log 10 β CoL(OH) =7.8±0.1 for L = DIPSO. For both buffers, the CoL(OH) species is characterized by a major structural rearrangement.