Re-evaluation of Sn(II)-Br− Speciation in Concentrated Bromide Media by Cyclic Voltammetry

Abstract

We report re-evaluated Sn(II)-Br- speciation by cyclic voltammetry on a glassy carbon (GC) electrode in concentrated Br- aqueous solutions. Contrary to the previous reports, electrochemical evidences of Sn(II)Br-5(3-), Sn(II)Br-6(4-), Sn(II)Br-7(5-), and Sn(II)Br-8(6-) were found in aqueous solutions containing different Br- concentrations with ionic strength, I=6. Formation constants (j) of Sn(II)-Br- speciation reactions and corresponding fractions of Sn(II)Br(j)(2-j)f(j) were evaluated. Compared to f(j) as a function of Br- concentrations based on the previous report by Fedorov etal., f(j) measured by our electrochemical measurements were dramatically different. In an aqueous solution with 6M Br-, 50% of Sn(II)Br-8(6-), 20% of each Sn(II)Br-6(4-) and Sn(II)Br-7(5-) exist in total Sn(II)-Br-. The diffusion coefficients, D of Sn(II)-Br- in aqueous solutions containing different Br- concentrations were estimated, and D decreased as a Br- concentration in the solution increased, which is mainly attributed to the enlarged molecular size of Sn(II)-Br- with more Br- coordination. A nucleation mechanism was also studied as a function of Sn(II)-Br- speciation. It was found that the nucleation mechanism of an electrodeposited Sn on a GC electrode gradually changed from instantaneous to progressive as j in Sn(II)Br-j(2-j) increases to 8, indicating the electrochemical Sn nucleation on a GC electrode is strongly dependent on Sn(II)-Br- speciation.