MNII-FULVIC ACID COMPLEXING EQUILIBRIUM MEASUREMENTS BY ELECTRON SPIN RESONANCE SPECTROMETRY

Electron spin resonance (ESR) spectrometry was used to determine weighted average equilibrium functions [Formula: see text] for water-soluble complexes formed between fulvic acid and divalent manganese. [Formula: see text] values ranged from 0.2 × 10−2 (at pH 6.15) to 0.5 × 10−3 (at pH 6.45) and were in excellent agreement with [Formula: see text] values determined previously for Mn–fulvic acid complexes by an ion-exchange method. The ESR method was found to be more sensitive, convenient and faster than the ion exchange procedure. From the data presented in this and in earlier publications it appears that MnII is bound simultaneously electrostatically and by hydrogen-bonding as Mn(OH2)62+ to fulvic acid donor groups in outer sphere complexing sites. These complexes are likely to be unsymmetrical.