EQCM and Surface pH Studies on Lanthanum Accumulation on Electrodes in Aqueous Solution

Abstract

Lanthanum can be detected using cathodic deposition followed by anodic stripping. The deposition process controls the detection limit. In this work, lanthanum deposition was characterized using an electrochemical quartz crystal microbalance (eQCM), and surface pH near the electrode surface was quantified using microelectrodes. The eQCM was used to correlate the lanthanum mass deposited on the electrode to the applied potential. Two different lanthanum responses were identified based on pH control. In the absence of pH control, the lanthanum film remained on the electrode surface until a large anodic potential was applied (1.8 VAg/AgCl or greater). In the presence of pH control, the lanthanum film was stripped off immediately after the applied potential was greater than −2.0 VAg/AgCl. When we measured surface pH near the electrode surface using needle-type microelectrodes, it was found that the pH near the surface increased from 4 to above the detection limit (>10), which indicated that the surface pH was responsible for the two different observed lanthanum responses despite the bulk pH being the same. This is the first study to use eQCM and needle-type microelectrodes to correlate lanthanum deposition with surface pH rather than bulk pH.