The complexation of iron(III) with sulphate, phosphate, or arsenate ion in sodium nitrate medium at 25 °C

Abstract

The complexation and hydrolysis of iron(III) has been investigated potentiometrically using a glass electrode at 25 °C in the presence of sulphate ion in 1.0 mol dm^–3 sodium nitrate. The formation constants of the species which are defined as [Fe_p(OH)_q–r(SO₄)_r]=β_species[Fe]^p[H]^–q[HSO₄]^r(charges ignored for brevity), expressed as log β_species′ are: log β_Fe(OH)=–3.01 (0.01), log β_Fe2(OH)2=–3.09 (0.01), log β_Fe3(OH)4=–6.92 (0.03), log β_Fe(SO4)= 0.41 (0.01), and log β_{Fe3(OH)4(SO4)}=–5.44 (0.02). These results required the independent determination of the second dissociation constant of sulphuric acid (pK₂), which was found to be 1.104 (0.005). The complexation of iron(III) with phosphate and arsenate ion has also been investigated in the same manner at 25 °C in 3.0 mol dm^–3 NaNO₃. No significant hydrolysis was observed because of the low pH values (<2.0) involved. The formation constants of the species which are defined as [Fe_pH_3r–q(PO₄)_r]=β_species[Fe]^p[H]^–q[H₃PO₄]^r, are: log β_Fe(HPO4)= 1.28 (0.03) and log β_Fe(PO4)= 0.78 (0.01). The corresponding results for the iron(III)–arsenate system are: log β_Fe(HAsO4)= 0.11 (0.02) and log β_Fe(AsO4)=–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.