COMPLEXA TION AND HYDROLYSIS REACTIONS OF IRON(III) A thesis submitted to The University of New South Wales for the degree of Doctor of Philosophy by G.H.Khoe June 1987 I hereby declare that this thesis is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another person nor material which to a substantial extent has been accepted for the award of any other degree or diploma of a university or other institute of higher learning, except where due acknowledgement is made in the text of the thesis. iii Abstract The complexation and hydrolysis of iron(ill) has been investigated by potentiometric titration using a glass electrode at 25°C. The hydrolysis investigation was carried out in 1 mol dm-3 KN(¾, NaC1O4 and KCl medium. The results obtained for the stoichiometric formation constants of the species <Pspecies> which are defined as [ Fep(OH)fp-q>+] = Pspecies[ Fe3+] p[ w]-q are: KNO3, log PFeOH = -2.77 (0.01), log PFc(OH)2 = -6.61 (0.04), log PFei(OH)2 = -3.22 (0.01), log PFe3(QH)4 = -6.98 (0.04); NaC1O4, log PFeOH = -2.73 (0.01), log PFe(OH)2 = -6.29(0.01), log PFe2(oH) 2 = -3.20(0.01); KCl, log PFeOH = -3.21(0.02), log ~Fe(OH)2 = -6.73(0.03), log ~ez(OH) 2 = -4.09(0.04), log ~FCJ(OH)4 = -7.58(0.04), the estimated standard deviations being given in parentheses (charges on species omitted). The hydrolysis investigation was extended to include the ternary system iron(IIl)-water-sulphate in 1 mol dm-3 NaNO3 medium. The results obtained for the formation constants of the species, which are defined as [ FCp(OH)4_r(SO4)r] = ~species[ Fe] P[ HJ -q[ HSO4] r (charges omitted), are: log ~eOH = -3.01 (0.01), log ~Fez(OH)2 = -3.09 (0.01), log PFCJ(OH) 4 = -6.92 (0.03), log ~eSo4 = 0.41 (0.01), and log ~Fe3(QH)4so4 = -5.44 (0.02). These results required the independent determination of the second dissociation constant of sulphuric acid (pK2); this was found to be 1.104 (0.005). The complexation of iron(l11) with phosphate and arsenate ion has also been investigated in the same manner in 3.0 mol dm-3 NaNO3• The need for such a high iv ionic strength was unavoidable because of the low pH values involved (pH < 2.0). Intervention by early precipitation reactions limits the formation of hydroxo­ complexes of iron(III) and the hydroxo-phosphate or hydroxo-arsenate complexes. The results obtained for the formation constants of the species which are defined as [ Fepli3r-q(P()4)J = ~species[ Fe] P[ H] -q[ H3P04] r, are: log ~FeHPO,. = 1.28 (0.03) and log ~FeP04 = 0.78 (0.01). The corresponding results for the iron(III)-arsenate system are: log ~FeHAs04 = 0.11 (0.02) and log ~eAs04 = -1.34 (0.03). The first dissociation constants of phosphorus(V) and arsenic(V) acids were found to be 1.763 (0.002) and 2.128 (0.002) respectively. The titrations of the hydrolysed iron(III) solutions were concluded after pH relaxation (a downward drift in pH readings which precedes the appearance of turbidity) was observed. The onset of pH relaxation (at pH> 2.0) is probably caused by the commencement of the hydrolytic polymerisation reactions, and was used to determine precisely, within 0.002 pH units, the titration end point for the solution equilibrium region. Species distributions in solution at the onset of pH relaxation were calculated using the equilibrium models determined in the present work. Assuming that the composition of the polymers is [ Fe(OHh-jXj/k] n where xt- is the anion present in solution, the species distribution calculations show that the value of j in both 1 mol dm-3 KNO3 and 1 mol dm-3 NaC1O4 medium is 0.0, in 1 mol dm-3 KCl, 0.5, and in 1 mol dm-3 NaN~ in the presence of sulphate, 0.6 . These j values are constant over a range of total iron(Ill) concentration of 0.0002 to 0.020 mol dm-3. This suggests that c1- and SO4 2- are incorporated in the structure of the polymers. V Acknowledgements The work described in this thesis was undertaken as part of the research programme of the Environmental Chemistry Section of the Australian Atomic Energy Commission (now known as the Australian Nuclear Science and Technology Organisation, ANSTO). I would like to express my gratitude to Dr.J.V.Evans, Head of the Environmental Chemistry Section, Mr.D.RDavy, Chief of the Environmental Science Division, and Dr.D.G.Walker, Executive Director of ANSTO for allowing the work to be used for this thesis. I thank Associate Professor RO.Robins, Head of the Department of Mineral Processing and Extractive Metallurgy of the University of New South Wales, my supervisor, for his enlightening and ever-friendly advice and assistance before and throughout this project. My sincere appreciation is extended to my friend Dr.RN.Sylva who initiated this project, and guided me through the many complex paths of solution chemistry. To the many staff members of the Environmental Science Division, the Applied Mathematics and Computing Division and the Information Services Department of ANSTO, I also express my appreciation. vi Contents page Abstract (iii) Acknowledgements (v) Abbreviations and Glossaries (viii) 1. Introduction 1 2. Experimental 26 2.1. Reagents 26 2.2. Preparation of Solutions for Titrations 26 2.2.1 Preparation of Iron(IIO Solutions 27 2.3. Titration Equipment and Procedure 31 2.3.1 Titration Equipment 31 2.3.2 Microprocessor-controlled Titrator 34 2.3.3 Procedure 35 2.3.4 Validation of Equipment and Procedure 42 2.4 Numerical Analysis 44 3. Results 46 3.1. Hydrolysis in different media 55 3.1.1 Potassium Nitrate medium 56 3.1.2 Sodium Perchlorate Medium 62 3.1.3 Potassium Chloride Medium 66 3.2. Hydrolysis and Complexation in the Presence of Sulphate 70 vii 3.2.1 Second Dissociation Constant of Sulphuric Acid 71 3.2.2 lron(III)-sulphate Equilibria 73 3.3. Complexation with Phosphate 82 3.3.1 First Dissociation Constant of Phosphorus(V) Acid 84 3.3.2 Iron(III)-phosphate Equilibria 86 3.4. Complexation with Arsenate 94 3.4.1 First Dissociation Constant of Arsenic(V) Acid 95 3.4.2 lron(III)-arsenate Equilibria 97 4. Discussion 104 4.1. Iron(ill) Hydrolysis 106 4.2. Complexation in the Presence of Sulphate 123 4.3. Complexation in the Presence of Phosphate or Arsenate 135 4.4. Precipitation 145 5. Conclusions 161 6. References 164 Appendix A: Experimental Data 173 Appendix B: Precipitation Calculations 216 Appendix C: Publications 220 viii Abbreviations and Symbols AAEC Australian Atomic Energy Commission ex degree of formation as defined by equation (1.22) B base (titrant) added 13, 13p,q,r overall formation constant as defined by equation (1.19) 13' overall formation constant as defined by equation (1.21) 130 overall formation constant at zero ionic strength 13w overall formation constant as defined on page 16 'Y activity coefficient e.s.d. estimated standard deviation I ionic strength K step-wise equilibrium (formation) constant acid dissociation constant ionisation constant of water overall formation constant as defined on page 16 A empirical constant as defined by equation (1.1) L charged or uncharged unidentate ligand M metal ion; or concentration unit mol dm-3 m milli (10-3) mM milli mol dm-3 N total number of complexed species for a given metal ion n average number ofligands bound per total metal ion number of experimental data points for each titration ix number of titrations in an equilibrium system p 1°Iog ... , as in pH p number of metal ions in species (p,q,r) notation for complexed species as defined on page 15 q number of water molecules deprotonated in a complexation reaction r number of ligand ions or molecules in species R goodness-of-fit parameter of a model in numerical analyses tot as subscript, indicate total amount u error square sum of the residuals <I> degree of complex formation as defined by equation (1.23) z ionic charge sum of squares of charges of products of equilibrium reaction as defined minus sum of squares of charges of reactants Species and Notations (as defined on page 15) Fe3+ (1, 0,0) Oir (0,-1,0) FeOH2+ (1,-1,0) Fe(OH)i + (1,-2,0) Fei(OH)i 4+ (2,-2,0) Fe3(0H)4S+ (3,-4,0) X HS04- (0, 0,1) S042- (0,-1,1) FeS04 + (1,-1,1) FCJ(OH}4S04 3+ (3,-5, 1) H3P04 (0, 0,1) H2P04- (0,-1,1) FeHP04+ (1,-2,1) FeP04 (1,-3,1) H3AS04 (0, 0,1) H2As04- (0,-1,1) FeHAs04+ (1,-2,1) FeAs04 (1,-3,1) - 1 - 1. INTRODUCTION In aqueous environments, metal ions are present not only as aquated cations but also as complexes with other ligand ions and molecules. The stability of these complexes depends on the size, charge, and electronic configurations of the metal ions and ligands involved.1 The constitution of these coordinated species governs their geochemical mobility. Of the transition metal ions, iron(ill) has been chosen for the present work because it is of special interest to the uranium mining and milling industry and because of its ubiquity. The total iron content in soils has been reported to lie within the range 0.2 to 55 percent, with a median of 4 percent.