Beryllium-chrome-azurol-S complexes

Abstract

Complex formation between the aquated beryllium cation and the tetrabasic dyestuff chrome-azurol-S (H₄CAS) has been studied by spectrophotometric, potentiometric, electrophoretic, and ion-exchange techniques. At least two complexes are shown to exist within the pH range 3-6. The major species is the anionic complex BeHCAS- for which log β₁₁, = 4.66 ± 0.08 at 25º and at ionic strength 0.lM (NaClO₄) Coordination of beryllium is considered to occur at the quinonoid-,). carboxylato site in the HCAS8- ligand. The stability constant for the dinuclear species Be₂CAS^O is shown to be log β₂₁ = 15.8 ± 0.1 under the same medium conditions. The dinuclear complex interferes with spectrophotometric measurements at beryllium concentrations exceeding 2.5 x 10^-4. The acidity constants of H₄CAS were determined at 25º and at an ionic strength 0.IM (NaClO,) as pK_2a = 2.25 ± 0.10,pK_3a, = 4.88 ± 0.05, pK₄, = 11.75 ± 0.05, whilst pK_{1a =} -1.2 ± 0.4.