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 MLOH species formed in solution or whether it was a combination of ML and MLOH. Information from crystal structures and electrospray ionization-mass 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 ML, 18.61 ± 0.01 for ML, 22.7 ± 0.2 for ML, and 31.4 ± 0.2 for MLOH. Application of these methodologies thus opens the door to broaden our understanding of Bi(III) complexation.

A Novel Approach to Monitoring of the Diffusion Junction Potential in Speciation Studies by Polarography under very Acidic Conditions. Part II: The Quasi‐Reversible Cu(II)‐Picolinic Acid System

“…These included (i) the determination of reversible potential values from quasi‐reversible reduction waves, (ii) correcting for the large junction potentials ( E j was about 30 mV at pH 0.3), (iii) the fact that E j varies with pH, (iv) the slightly larger shift in E 1/2 for Tl(I) compared to that for Cu(II) due to the changing ionic strength and (v) the indirect determination of E (Cu) using E (Tl) ave and Δ E (M) ave data. Additionally, it is critical that pH be determined as accurately as possible, thus methodologies developed to calibrate the glass electrode under acidic conditions were also employed 2. Taking all these into consideration, the fact that the stability constants compared very well to the literature data shows the rigor of these protocols as well as robustness of polarography as a technique for determining these values.…”

“…no. 6.0234.100) 2, was calibrated by the titration of standardised solutions of 0.5 M HNO 3 with 0.5 M NaOH and the calibration plot was calculated. Because the accurate measurement of pH is critical in this work, a deviation from linearity of the GE response in the very acidic region had to be accounted for.…”

“…Because the accurate measurement of pH is critical in this work, a deviation from linearity of the GE response in the very acidic region had to be accounted for. To this effect, a quadratic function was fitted to experimentally recorded GE potential readings in the highly acidic region to minimise E j errors 2. In this work pH signifies −log[H + ].…”

“…This is due to the large and pH‐dependent diffusion junction potential ( E j ) that is formed between the RE filling solution and the sample solution. Glass electrode potentiometry (GEP) has even further limitations under very acidic conditions because mass‐balance equations, which are solved for the free hydrogen ion concentration from the deprotonation of the ligand caused by the complexation reaction, cannot be accurately solved when the background solution contains more than about 10 −2 M strong acid 1, 2. Additionally, irrespective of the technique that is used, the pH of the solution has to be measured and the E j constrains the accuracy with which this can be done.…”