Determination of the Redox Ratio of Iron in a Diamagnetic Matrix Using Low-Field NMR Relaxometry.

Proton nuclear magnetic resonance (NMR) relaxometry is proposed to determine the iron redox ratio of slag samples. It depends on the paramagnetic effect on bulk water relaxation times. Experiments have been carried out to calibrate the relationship between the concentration of Fe ³⁺ and Fe ²⁺ ions and the measured relaxation times on prepared standard samples. It is shown that Fe ²⁺ ions have very different effects on the relaxation times than Fe ³⁺ ions. For the solid slag samples, digestion in an acid was used to produce solutions with a pH of 1.0. Hydrogen peroxide (H ₂ O ₂ ) was then added to the digested samples to oxidize Fe ²⁺ ions to Fe ³⁺ ions. The concentration of Fe ²⁺ and Fe ³⁺ ions in solution can be calculated by measuring the relaxation times before and after oxidation. The Fe ³⁺ /∑Fe ratio of slag samples was also investigated by wet chemistry and ⁵⁷ Fe Mössbauer spectroscopy and calculated by FactSage at the equilibrium state. The results obtained from different methods are consistent, indicating that this NMR relaxometry method is reliable.