Competitive solvation and complexation of Cu(I), Cu(II), Pb(II), Zn(II), and Ag(I) in aqueous ethanol, acetonitrile, and
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I.
INTRODUCTION
MANY hydrometallurgical
processes apply the large changes in metal ion activity resulting from complexation with ligands such as CI-, CN-, or NH3 in water to modify reduction potentials, solubility, and equilibria. However, it is only recently that the changes in activity resulting from solvation with water-miscible solvents such as ethanol (EtOH), acetonitrile (AN), and dimethylsulfoxide (DMSO) have been quantified and applied to
Cell A:
The quantitative estimation of ionic activities in aqueous-organic solvent mixtures by various research groups have provided values with wide discrepancies, in some cases dependent upon which method and extrathermodynamic assumption is used. t8'9'1~ In our recent studies, tH-14] we have found the emf method to be the best and most convenient way of determining ionic activities in concentrated aqueous chloride salt solutions, and in aqueous solutions of EtOH, AN, or DMSO using cell A.
SCE
KC1 or
M"+(C1-). in C1-/H20 or
M
(reference
NaC104
M n + ( C l O 4 - ) n in S / H 2 0
test
electrode)
(salt bridge)
where S = EtOH, AN, or DMSO
electrode
Er~f
Ej
Etest
Ec~n --- Etest - Eref + Ej processes such as the removal of Au(CN)2- from activated carbon or resins or the refining of copper, tl-5] In mixed aqueous-organic solvents, the activity of an ion is dependent on various ion-solvent interactions. [2.3,6] In general, EtOH and AN are poor cation solvators and DMSO is a good cation solvator relative to water, but the specific solvation of Cu(I) and AgO) by AN tTl is unique. However, all three solvents significantly enhance the activity of simple anions, like CI-, while complex anions with low charge density are less affected.
G. SENANAYAKE, formerly Doctoral Student, Murdoch University, is Lecturer in Chemistry, University of Sri Jayawardenepura, Nugegoda, Sri Lanka. D.M. MUIR, Associate Professor in Mineral Chemistry, is with the Department of Mineral Science, School of Mathematical and Physical Sciences, Murdoch University, Perth WA 6150, Western Australia. Manuscript submitted November 17, 1988. METALLURGICAL TRANSACTIONS B
[11
The emf (E~a) is a direct measure of M "+ ion activity but its accuracy is dependent upon a knowledge of Es (Eq. [1]), where E j is the liquid junction potential which arises due to differences in ionic mobilities at the interface of the bridge electrolyte and the test electrolyte. In recent work, we have focused on establishing proper estimates of E j [12'15] in order to correct the measured emf data. This has enabled us to calculate and compare metal ion activities in these various test solutions tn-14] and to demonstrate that the solvent effects on metal ion activities in mixed-solvent perchlorate media are often larger than ionic strength effects or complexation effects in strong brine solutions, till Solvent effects on metal ion activities in perchlorate media, however, are of less value to hydrometallurgists than solvent effects in chloride media. This is because many metal ions can coordinate with both the solvent and
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