In situ color video observation of polarographic streaming phenomenon of an HMDE using electrochromic reaction

Abstract

In the present study, visualization of a characteristic upward streaming phenomenon of hanging mercury drop electrode (HMDE) on the cathodic branch of electrocapillary curve was successfully accomplished. The images of HMDE serving as a cathode for the electrochromic reaction of the BTD (2,1,3-benzothiadiazole)/BTD − (a one-electron reduced form: a radical anion of BTD) couple in dimethylsulfoxide solution were captured by a home-assembled microscopic video camera combined with an electrochemical measurement system. The electrogenerated BTD − of pink color was found to be gradually accumulated around the neck of HMDE as the potential was scanned to the negative direction. This inhomogeneous accumulation of BTD − was considered to be due to the well-known upward streaming phenomenon of HMDE. Expansion and shrinking phenomena of Hg drop were also noticed during the reduction process and were ascribed to the gradual increase of surface tension along the surface of HMDE from the bottom to the neck of it. These direct and visual observations evidence the existence of upward streaming phenomenon of HMDE for the first time since the streaming phenomena have been reported in 1966.