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Lewis and Clark College Department of Chemistry
 
An Electrochemical Study Of The Fe3+ Ion As A Function Of Silicate Melt Composition
 
 
Jim Haury
May 25,1994
 
 
 

Primary References

Seinkow, K.W. ; Rizzo, R.W. ; Haskin, L.A. ; Lindstrom, DJ. ; An Electrochemical Study of Ni2+, Co2+, and Zn2+ Ions in Melts of Composition CaMgSi2O6; Geochimica et Cosmochimica Acta, Vol 46; Pergamon Press Ltd.,1982

Keedy, C.R. and Hart J. ; Electrochemical Behavior of Multivalent Ions as a Function of Silicate Melt Composition; Murdock Summer Research Project, 1993
 
 

 

Abstract
 

    Electroanalytical measurements made in molten silicate salts can provide useful information on the behavior of trace elements in silicate melts which model geochemical phenomena. Electroanalytical methods are conveniently divided into two categories: steady state and transient.1 Steady state methods entail measurements of the potential difference at zero current where time is effectively eliminated as a variable. On the other hand voltammetry, considered a transient method, is concerned with the current-potential relationship in an electrochemical cell. Voltammetry represents a wide range of electroanalytical techniques. These techniques can be used to study the solution composition and structure through the current-potential relationship in an electrochemical cell.

    Cyclic voltammetry, also known as linear sweep voltammetry, consists of cycling the potential of a stationary electrode immersed in a quiescent solution and measuring the resulting current (versus that of a reference electrode).1,4 The controlling potential which is applied across these two electrodes can be considered an excitation signal. The excitation signal is a linear potential scan with a triangular waveform.4

    Using cyclic voltammetry, the standard state reduction potential, E*, of say Fe2O3 in a silicate melt as a function of melt composition can be determined.2 From this study the diffusion coefficient of Fe3+ and its variation with melt composition can also be determined. The melt composition has been varied from Diopside (CaO.MgO.2SiO2) to Anorthite (CaO.Al203.2SiO2) in a systematic manner.3,5

    Cyclic voltammetry is usually done in aqueous solutions at 25 degrees C using a Platinum working electrode, a reference electrode such as Ag/AgCl, and a counter electrode with a supporting electrolyte such as KCl.1,4 These aqueous studies and the results of low melting silicates and their trends cannot be simply extrapolated to studies of melts of geochemical interest. 1 But if the temperature of the silicate composition is increased to approximately 1500 degrees C which happens to be in the vicinity of its melting temperature, the composite takes on the fluid properties of an aqueous solution. With this information, geochemists are able to postulate the environmental conditions present when the silicate melt solidifies by looking at the migration of a multivalent ion of interest, Fe3+ for example. To withstand the high temperatures, however, all the electrodes must be made from Pt and all the chemicals must be in the oxide form since oxygen is not excluded from the system.2,6

    In previous studies, reduction potentials of metal ions by cyclic voltammetry were measured as a function of the varying amount of SiO2 in the composition.2 In this study, the MgO found in Diopside was replaced incrementally with portions of Al203, thus it was expected that the better the match of ionic charge and size found in the melt, the more difficult it would be for reduction to occur.3 In the pure Diopside melt, the Mg2+ and the Fe3+ have different charges but identical ionic sizes. So when the Fe3+, which occupies the site where the Mg2+ used to be, sees an electron it will easily migrate over to it and become reduced to Fe2+ state. As the composition changes toward Anorthite, the A13+ and the Fe3+ have identical charges but the ionic sizes are considerably different. Now when the Fe3+, which occupies the site where the Al3+ used to be, sees an electron it is electronically more stable as Fe3+ and is not as easily reduced to the 2+ state.

    A problem previously encountered was that some metal ions are eventually reduced to the metal atom.3 This was true for chromium  in the 3+ state but was not a factor in this study. As a check, for a reversible reduction at the working electrode, the following equation shouldbe obeyed:7

Ipeak = 1.O87nFD01/2C0(nFv/RT)1/2

where n is the number of electrons transferred at the electrode, F is the Faraday constant, C0 is the initial concentration of the oxidized species, v is the sweep rate in volts/sec., and R and T are the  ideal gas constant and temperature in Kelvin, respectively. A plot of Ipeak vs v1/2 is not only a test of reversibility of the reaction electrode, but also a method for obtaining the diffusion coefficient. The slope of the plot is proportional to D01/2 which is the diffusion coefficient of the oxidized species.2 The Ipeak and v data were obtained from the PAR data acquisition system and analyzed using Quatro Pro spreadsheet with the aid of a linear regression technique to obtain the diffusion coefficient at the various compositions and different temperatures.3

    Some of the data from this study, fitted to an ideal curve for a one and two electron transfer7, was found to match very well to the curve for the one electron transfer especially when the spectra had been conditioned using IR compensation to account for the IR drop between the reference electrode and the working electrode.1 This was hypothesized and now there is data that supports this idea of a reversible one electron transfer.
 
 
 

Bibliography

1. Willard,H.H. , Merritt,L.L., Dean,J.A., Settle,F.A. , Instrumental Methods Of Analysis, Seventh Edition, Wadsworth, Inc. Belmont, Ca., 1988, pp.697-728.

2.  Semkow,K.W., Rizzo,R.A. , Haskin,L.A., Lindstrom1D.J., "An electrochemical study of Ni2+, Co2+ ,and Zn2+ ions in melts of composition CaMbSi2O6", Geochimica et Cosmochimica Acta Vol.46, Pergamon Press Ltd., 1982, pp.1879-1889.

3. Keedy,C.R. , Hart,J. ,"Electrochamical Behavior of Multivalent Ions as a Function of Silicate Melt Composition" , Murdock Summer Research Project, 1993.

4. Kissinger,P.T. , Heinman,W.R. , "Cyclic Voltammetry" , J. Chem. Educ. Vol 60, 1983, pp.702-706.

5. Sanders,H.J., "Chemistry and the Solid Earth" , Chemical & Engineering News Vol 45, Number 42, Oct. 2, 1967, pp. 1A-49A.

6. Hoare,J.P. , The Electrochemistry of Oxygen , Intersdence Publishers, J. Wiley & Sons, 1968, pp.13-80.

7. Nicholson,R.S. & Shain,I. , "Theory of Stationary Electrode Polarography", Analytical Chemistry Vol 36, Number 4, 1964, pp.706-723.