Electrochemical detection or accumulation of lanthanum and the lanthanides are typically investigated on modified electrodes such as carbon paste and mercury film electrodes.^1-4 However, little data exists on lanthanum’s interaction with unmodified electrodes. Lanthanum deposition was characterized using an electrochemical quartz crystal microbalance (eQCM) in aqueous solution on unmodified carbon electrodes. We used a home-made pH microelectrode to directly measure a 10 pH unit variation in the local pH ~ 100 µm above a flat carbon electrode. The eQCM frequency response during voltammetric sweeps in the presence of lanthanum showed that the background current masked the dependence of lanthanum deposition on local pH and applied electrode potential. Lanthanum deposition in unbuffered LiCl solutions exhibited a stable deposited film until the anodic scan reached nearly 1.8 V_Ag/AgCl. The addition of 1 mM α-hydroxyisobutyric acid, a complexing ligand, decreased the deposition onset potential on the first scan. Subsequent scans showed the frequency response returning to that without added HIBA. Similar experiments with acetate-buffered solutions resulted in film stripping at -2V_Ag/AgCl on the anodic scan. The large variation in local pH is likely due to water electrolysis at the surface of the working electrode, which can impact form of the electroactive species. The formation of insoluble lanthanum hydroxides at high pH values contributes to the stable film in pH-controlled anodic sweeps. The frequency response on gold electrodes is also shown.

Refernces:

1.         P. D. Schumacher, J. L. Doyle, J. O. Schenk and S. B. Clark, Rev. Anal. Chem., 2013, 32, 159-171.

2.         P. D. Schumacher, K. A. Fitzgerald, J. O. Schenk and S. B. Clark, Anal. Chem., 2011, 83, 1388-1393.

3.         P. D. Schumacher, N. A. Woods, J. L. Doyle, J. O. Schenk and S. B. Clark, Anal. Chem., 2011, 83, 4788-4793.

4.         P. D. Schumacher, N. A. Woods, J. O. Schenk and S. B. Clark, Anal. Chem., 2010, 82, 5663-5668.