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 V Ag/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 V Ag/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. The concentrations of lanthanum and other lanthanides are typically measured by depositing them as amalgams with mercury 1 or, more recently, mercury films 2 , followed by stripping. As early as 1915 3 , mercury pools were used as cathodes that replaced platinum and copper electrodes for the electrolysis of rare earth elements. The reason for this was the fouling of the cathode with insoluble hydroxides, in which the pH at the cathode surface was thought to play a critical role. Recently, similar concerns were raised for lanthanide deposition on mercury-filmed microelectrodes. 4 The authors claimed that pH was responsible for the stability of La deposition. The effect of pH is a consequence of electrolysis or electrochemistry in aqueous solution. Considering the generation of high currents (mA/cm 2 level) on small electrodes during metal deposition, it is expected that pH changes significantly near the electrode surface. Researchers generally control bulk pH, but the pH on an electrode surface has not previously been quantified because the technology for doing this was not available until the present day. Interest in lanthanides stems primarily from electroanalysis and the need for rapid methods of detection 2 and their chemical similarity to the actinides. 5In addition, the surface pH of deposited La films needs to be characterized in order for us to understand the deposition mechanism. Often, post-treatment analysis via inductively coupled plasma mass spectrometry is used successfully to identify the composition of electrochemically deposited films 6,7 or quantify the amount of material sequestered by the electrochemical method.2 However, transient processes such as the formation of pH gradients near an electrode surface during deposition limit the usefulness of post-treatm...

Electroprecipitation of Nanometer-Thick Films of Ln(OH)3 [Ln = La, Ce, and Lu] at Pt Microelectrodes and Their Effect on Electron-Transfer Reactions

“…Analogous to prior reports from Beyenal and co-workers, , we propose that the decrease in current for Ru­(NH 3 ) 6 3+ results from the production of OH – during H 2 O reduction at the electrode surface, leading to the precipitation of La 3+ ions in the form of La­(OH) 3 . The blockage of the reduction of Ru­(NH 3 ) 6 3+ is consistent with the formation of an electronically insulating La­(OH) 3 film .…”

“…The voltammetric results in Figure 1A,B, including the effect that the presence of La 3+ has in attenuating the reduction of Ru(NH 3 ) 6 3+ , are exceptionally reproducible using freshly prepared Pt microdisk electrodes. Additional examples of these responses using different Pt electrodes are presented in the Supporting Information Analogous to prior reports from Beyenal and co-workers, 29,30 we propose that the decrease in current for Ru(NH 3 ) 6 3+ results from the production of OH − during H 2 O reduction at the electrode surface, 35 leading to the precipitation of La 3+ ions in the form of La(OH) 3 . The blockage of the reduction of Ru(NH 3 ) 6 3+ is consistent with the formation of an electronically insulating La(OH) 3 film.…”

“…Electroprecipitation has proven to be a powerful tool to fabricate Ln­(OH) 3 nanostructures such as (La, Nd) nanowires, (La) nanotubes, nanospindles, nanorods, − nanocapsules, and (La, Gd) nanoparticles . Precipitation on the electrode surface occurs in acidic or neutral solutions when the local pH near the electrode surface becomes basic during base-generating reactions, for example, NO 3 – + H 2 O + 2e – → NO 2 – + 2OH – . , Beyenal and co-workers demonstrated the correlation between La­(OH) 3 precipitation with an increase in local pH, based on electrochemical quartz crystal microbalance (eQCM) and local pH measurements. , The formation of nanometer-thick Ln­(OH) 3 films was confirmed with scanning electron microscopy (SEM), atomic emission spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS) analyses. The removal of La­(OH) 3 films was found to occur during H 2 O oxidation.…”

“…27,28 Beyenal and coworkers demonstrated the correlation between La(OH) 3 precipitation with an increase in local pH, based on electrochemical quartz crystal microbalance (eQCM) and local pH measurements. 29,30 The formation of nanometerthick Ln(OH) 3 films was confirmed with scanning electron microscopy (SEM), atomic emission spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS) analyses. The removal of La(OH) 3 films was found to occur during H 2 O oxidation.…”

Flow Injection Electrochemical Quartz Crystal Microbalance with ICP-OES Detection: Electroprecipitation and Stripping of Lanthanum and Neodymium in a Flow Cell

“…22 More recently, our group has used EQCM to study electroprecipitation of f-elements on gold and carbon electrodes. [23][24][25][26] The cathodic electroprecipitation of felements in an EQCM micro flow cell or with the injection of submilliliter samples has not been explored yet. Still, it would allow rapid testing of different deposition conditions with very small samples.…”