Fig. 8. Predicted vs. measured (Pt combination redox electrode) Eh based on soluble concentrations of Fe²⁺ (measured) and Fe³⁺ (estimated, in equilibrium with amorphous (am-)Fe(OH)₃ or -FeOOH solid phase) for Hanford soil–water suspensions. The E_H comparisons based on the SO₄^2––HS^– (measured) redox couple are also shown. Measured Fe²⁺ concentrations for highlighted (square-enclosed) samples were less than the detection limit (9.0 x 10^–6 M) and were estimated at 1.0 x 10^–6 M. Straight line indicates 1:1 correlation between measured and predicted E_H values.