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

Abstract

The use of polarography to accurately determine stability constants of complexes formed under very acidic conditions (below pH 2) is demonstrated. The diffusion junction potentials, which must be accounted for below pH 2, were evaluated by applying protocols developed where Tl(I) is used as an internal reference. The Cu(II)-picolinic acid (2-pyridinecarboxylic acid) system studied was chosen since the CuL + species only exists in solution below pH 2 under the conditions used and literature data exists to confirm the accuracy of procedure. Additionally, the reduction of Cu(II) was quasi-reversible and procedures to determine the reversible half-wave potentials were investigated. Average log  values of 7.75  0.09 for CuL + and 14.8  0.1 for CuL 2 were obtained, which compared well to literature data.

Measurements and Modeling To Determine the Reduction Potential of Uncomplexed Bi(III) in Nitrate Solutions for Application in Bi(III)-Ligand Equilibria Studies by Voltammetry

“…37,38 Since we wanted to work at moderate ionic strengths so that formation constants generated could have some application to the biological field, a nitrate background was chosen as the compromise between solubility of hydrolysis products and reversibility of the reduction process. The successful study of quasi-reversible systems had been previously been demonstrated, 39,40 however, as will be discussed, in this work it was found that the DC reduction waves could actually be considered as reversible when working in 0.25 -0.5 M nitrate solutions.…”

“…The reduction of Tl(I) was fully reversible across the pH range, as noted before. 39,41 Although it was expected that Bi(III) reduction would be quasi-reversible in a nitrate medium, 30,45 the  values obtained were very close to one (typically 0.951.00) provided data points at the onset of mercury oxidation were omitted when fitting the wave. Therefore, since I d does not change with the extent of reversibility and E 1/2 is dependent on the steepness of the wave, which in this case is Nernstian (since   1 in Eq 2), the reduction of Bi(III) was considered as reversible for our purpose (see Section 2 of the Supporting Information for further discussion).…”

Application of Protocols Devised to Study Bi(III) Complex Formation by Voltammetry: The Bi(III)–Picolinic Acid System

“…In previous studies, no complexation was observed for [PA T ]:[Tl T ] up to 100 for measurements made up to pH 8. 23,24 Figure 1 shows the plot of experimental E 1/2 (Bi) (○) vs pH where [PA T ]:[Bi T ] = 197, as well as the E 1/2 values after correction for E j (×). Both the E(Bi) and E c (Bi) values as determined for this experiment are also displayed.…”

“…It is thus important to include nitrate as a competing ligand especially in the pH range where these species are significant. Unfortunately, the 3D-CFC software used to determine stability constants from polarographic data (as applied in previous work (23,24)) can only accommodate two ligands simultaneously, one of which is always hydroxide and the other is the ligand being investigated. As a guide, the species distribution diagram (SDD) of 1 × 10 -5 M Bi(III) in 0.5 M nitrate (in the absence of PA) is shown on the secondary axis in Figure 1.…”

“…(30) Their work, however, uses less rigorous methodologies and the role of the Bi(III) nitrates was not yet realized. (30) The work presented here is the culmination of various studies done up to this point, (22)(23)(24)(25) as it illustrates how the protocols that were designed to ultimately study Bi(III)-ligand systems are applied.…”