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Electron Spin Resonance Investigation of Mn2+ Complexation in Natural and Synthetic Organics
Author(s) -
McBride M. B.
Publication year - 1982
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1982.03615995004600060004x
Subject(s) - chemistry , electron paramagnetic resonance , carboxylate , carboxylic acid , humic acid , sulfonate , inorganic chemistry , manganese , molecule , organic chemistry , sodium , fertilizer , physics , nuclear magnetic resonance
Mechanisms of Mn 2+ bonding in soil organics and pure organic compounds were compared by using electron spin resonance (ESR) spectroscopy. The observation that Mn 2+ in lower‐than‐cubic symmetry produces a very broad signal was used to quantitatively determine the degree of metal complexation by organics. As the pH was raised, multiligand molecules such as carboxymethylcellulose, citrate, and succinate formed complexes with Mn 2+ as shown by a decrease of ESR signal intensity. However, monocarboxylic acids such as benzoate and salicylate do not readily form complexes with Mn 2+ . A fulvic acid extracted from acid soil possessed a very limited number of strong complexation sites which became available to Mn 2+ only at pH values > 6.0. Polycarboxylic acids, including natural organics (peat, humic acid) and synthetic polymers (polymethacrylate, polygalacturonic acid) bond Mn 2+ electrostatically as the hydrated ion at low pH, but complex increasing amounts of Mn 2+ as the pH is raised. A comparison of the ESR characteristics of Mn 2+ bound to soluble carboxylic acids, polymeric carboxylic acids, and strong acid compounds such as sulfonate resin is used to characterize the bonding behavior of Mn 2+ in soil organics. The cooperative effort of several surface functional groups appears to be necessary to cause Mn 2+ to form an inner‐sphere complex with organic solids. Higher temperatures generally favor the complexation of Mn 2+ .