Standard ferric–ferrous potential and stability of FeCl2+ to 90°C. Thermodynamic properties of Fe(aq)3+ and ferric-chloride species

Abstract Standard electrode potentials ( E T 0 ) for the reaction Fe 3+ (aq) +0.5H 2(g) =Fe 2+ (aq) +H + (aq) were derived from electrochemical measurements in a non-isothermal cell at temperatures from 20.8° to 90°C in HClO 4 solutions with ionic strength of 0.11–1.31 m. A value of E 298 0 =0.770±0.002 V was obtained. An equation E T 0 =0.6000–3.0961×10 −3 T +6.4351×10 −4 T ×ln T , where T is temperature in K, based on the assumption of a constant heat capacity for the above reaction, Δ r C p 0 =14.84 cal/mol K [Hovey, J.K., 1988. Thermodynamics of aqueous solutions. PhD Thesis, Univ. Alberta, Edmonton, USA.], fits experimental values of E T 0 from the present study together with data from Whittemore and Langmuir [Whittemore, D.O., Langmuir, D., 1972. Standard electrode potential of Fe 3+ +e − =Fe 2+ from 5–35°C. J. Chem. Eng. Data 17, 288–290.] (5°–35°C) with an uncertainty better than ±0.002 V. The standard thermodynamic properties of ferric–ferrous reaction at 25°C were calculated: Δ r G 0 =−17.75±0.04 kcal/mol, Δ r H 0 =−9.4±0.5 kcal/mol, Δ r S 0 =28.0±1.5 cal/mol K. Using these values and thermodynamic data for Fe (aq) 2+ and H 2(g) from SUPCRT92 [Johnson, J.M., Oelkers, E.H., Helgeson, H.C., 1992. SUPCRT92: a software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species and reactions from 1 to 5000 bars and 0° to 1000°C. Computers and Geosciences 18, 899–947.], the standard thermodynamic properties for Fe (aq) 3+ ion at 25°C, 1 bar were generated: Δ f G 0 =−4.12 kcal/mol, Δ f H 0 =−12.65 kcal/mol, S 0 =−68.9 cal/mol K. The equilibrium stability constants for the reaction Fe 3+ +Cl − =FeCl 2+ were generated from the comparison of ferric–ferrous electrode potentials in a non-complexing medium (HClO 4 ) with those in HCl solutions from 25° to 90°C. A value of log β 0 =1.52±0.10 was obtained at 25°C, 1 bar. Experimental results were combined with the reaction enthalpies measured by Vasil'ev and Lobanov [Vasil'ev, V.P., Lobanov, G.A., 1967. Effect of temperature and ionic strength on heat of formation of hydrated FeCl 2+ in aqueous solution. Russ. J. Phys. Chem. 41, 1969–1973 [in Russian].] from 10° to 55°C, to describe the temperature dependence of the stability constant, enthalpy and heat capacity for this reaction: log β 1 0 =0.018 T +420.12/ T −5.23; Δ r H 0 (cal/mol)=−1918+0.082 T 2 ; Δ r C P 0 (cal/mol K)=0.163 T . This data allows calculation of the standard Gibbs energy (−37.61 kcal/mol), enthalpy (−47.21 kcal/mol), entropy (−30.2 cal/mol K) and heat capacity (−5.7 cal/mol K) for FeCl 2+ using thermodynamic properties for Fe 3+ and Cl − aqueous ions from this study and SUPCRT92, respectively. Thermodynamic properties of Fe 3+ and FeCl 2+ generated in this study together with data for FeCl 2 + complex derived from the literature data (Δ f G 0 =−71.27 kcal/mol, Δ f H 0 =−81.84 kcal/mol, S 0 =−5.9 cal/mol K, C p 0 =7.0 cal/mol K at 25°C, 1 bar) allowed calculation of hematite solubility in HCl solutions from 25° to 200°C.