The complexation of iron(III) with sulphate, phosphate, or arsenate ion in sodium nitrate medium at 25 °C
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 [Fep(OH)q–r(SO4)r]=βspecies[Fe]p[H]–q[HSO4]r(charges ignored for brevity), expressed as log βspecies′ are: log βFe(OH)=–3.01 (0.01), log βFe2(OH)2=–3.09 (0.01), log βFe3(OH)4=–6.92 (0.03), log βFe(SO4)= 0.41 (0.01), and log βFe3(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 [FepH3r–q(PO4)r]=βspecies[Fe]p[H]–q[H3PO4]r, are: log βFe(HPO4)= 1.28 (0.03) and log βFe(PO4)= 0.78 (0.01). The corresponding results for the iron(III)–arsenate system are: log βFe(HAsO4)= 0.11 (0.02) and log βFe(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.