Full Paper Electrochemical Determination of Manganese Solubility in Mercury via Amalgamation and Stripping in the Room Temperature Ionic Liquid n-Hexyltriethylammonium Bis(trifluoromethanesulfonyl)imide, [N 6,2,2,2 ][NTf 2 ] Emma I. Rogers, a Biljana S ˇ ljukic´ , b Christopher Hardacre, c Richard G. Compton a * a Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK *e-mail: richard.compton@chem.ox.ac.uk b Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12, 11000 Belgrade, Serbia c School of Chemistry and Chemical Engineering/QUILL, Queen)s University Belfast, Belfast, Northern Ireland BT95AG, UK Received: September 19, 2008 Accepted: October 18, 2008 Abstract The solubility of manganese in mercury was determined electrochemically via amalgamation and stripping in the room temperature ionic liquid n-hexyltriethylammonium bis(trifluoromethanesulfonyl)imide, [N 6,2,2,2 ][NTf 2 ]. A hemispherical mercury electrode was made by electrodepositing mercury onto a planar platinum microelectrode. Cyclic voltammetry of Mn 2þ in [N 6,2,2,2 ][NTf 2 ] at the mercury microhemisphere electrode was investigated at temperatures of 298, 303 and 313 K. The solubility of Mn in Hg was determined on the basis of the charge under the reduction peak (Mn 2þ ! Mn 0 ) and the corresponding reoxidation. Keywords: Mercury microhemisphere electrode, Manganese(II) reduction, Room temperature ionic liquids, Managanese solubility in mercury DOI: 10.1002/elan.200804393 1. Introduction The manganese-mercury system has been explored and described in the literature. The Mn-Hg phase diagram at controlled pressure, sufficient to keep Hg as liquid, shows existence of four distinct phases: 1) liquid, 2) Mn (a, b and g Mn) with a minor Hg solubility in dMn, 3) two intermetallic compounds: Hg 5 Mn 2 and HgMn, and 4) Hg with a small solubility on Mn [1]. Some authors report the existence of Hg 4 Mn and Hg 4 Mn 3 intermetallic compounds, but there is no firm experimental evidence of their existence. The reported values for manganese solubility in mercury were obtained using different methods: chemical analysis of the amalgam filtrate [2 – 9], emf measurements [10 – 14] and electrooxidation of amalgam in polarographic [15, 16], voltammetric [17 – 20] or chronoamperometric [21] tech- niques. Despite these reports, the solubility of manganese in mercury is still not entirely certain as evidenced by the wide scattering of the reported experimental results, summarized by Moser and Guminiski [1]. The present study was under- taken to investigate the solubility of manganese in mercury using room temperature ionic liquids (RTILs) as a medium from which to deposit the manganese as an amalgam in mercury from Mn(II) and also to facilitate stripping from the amalgam. We believe that this is the first use of RTILs for the purpose of determining metal solubilities in mercury. The validation of the method in this paper should open up a wider use of RTILs for this purpose with the benefit of their wide potential windows allowing the study of many more different metals than hitherto possible. Room temperature ionic liquids are composed entirely of ions, and exist in the liquid state at around room temper- ature [22]. They are comprised of a bulky asymmetric organic cation, and a weakly-coordinating inorganic/organ- ic anion. The large number of combinations possible provides the opportunity to fabricate solvents with proper- ties for special applications [23]. RTILs have a number of interesting physical properties [24, 25], including low- volatility, high thermal stability, high viscosity, high polarity, high intrinsic conductivity and a wide electrochemical window (ca. 4.5 – 6 V) [26 – 28]. Their low volatility makes them (greener) alternatives to volatile organic solvents [27]. Their high intrinsic conductivity means that no supporting electrolyte is required in electrochemical experiments [22, 27, 28], and wide electrochemical window provides the possibility of studying molecules that give voltammetric signals at extreme potentials [28]. RTILs have also been employed as a supporting electrolyte extending the anodic window of a solvent, e.g., acetonitrile, compared to that obtained with tetra-n-butylammonium perchlorate (TBAP) as supporting electrolyte [26]. 2603 Electroanalysis 20, 2008, No. 24, 2603 – 2607 # 2008 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim