Application of protocols devised to study Bi(III) complex formation by voltammetry : the Bi(III)-picolinic acid system

Abstract

Bi(III) coordination chemistry has been largely neglected due to the difficulties faced when studying these systems even though Bi(III) is used in various medicinal applications. This study of the Bi(III)– picolinic acid system by voltammetry applies the rigorous methodologies already developed to enable the study of Bi(III) systems starting in very acidic solutions to prevent precipitation. This includes calibrating the glass electrode accurately at these low pHs, compensating for the diffusion junction potential below pH 2 and determining the reduction potential of uncomplexed Bi(III) which cannot be directly measured. The importance of including nitrate from the background electrolyte as a competing species is highlighted, especially for data acquired below pH ∼ 2. From analysis of the voltammetric data, it was not clear whether a ML3OH species formed in solution or whether it was a combination of ML4 and ML4OH. Information from crystal structures and electrospray ionizationmass spectrometry measurements was thus used to propose the most probable species model. The log β values determined were 7.77 ± 0.07 for ML, 13.89 ± 0.07 for ML2, 18.61 ± 0.01 for ML3, 22.7 ± 0.2 for ML4, and 31.4 ± 0.2 for ML4OH. Application of these methodologies thus opens the door to broaden our understanding of Bi(III) complexation.