Quantitative electron spin resonance (ESR) determinations of forms and total amounts of Mn in aqueous environmental samples

An electron spin resonance (ESR) method for determining quantitatively amounts and forms of Mn in 0.2 ml of marine and lacustrine waters with no chemical processing is described and applied to various samples from the Puget Sound region of western Washington state. Results over the 50 ppb15 ppm (0.9–270 μM) total Mn concentration range are reproducible to within ±3–5% and average 8 ± 7 and 13 ± 7% lower than inductively coupled plasma (ICP) determinations on marine and lacustrine pore waters, respectively. Main features in profiles of total Mn versus depth in Pacific Northwest pore waters resemble those in profiles from other coastal regions.The method reveals that 72–77% of total dissolved Mn is present as free Mn⁺⁺ in most seawaters, 14–16% is complexed with chloride, and 10–12% is complexed with sulfate. Less than 5% is associated with carbonate, bicarbonate or organic ligands. These results should significantly reduce uncertainties in equilibrium calculations of state of saturation of seawaters with respect to precipitation of various Mn-solids. The method can be easily extended to various ionic strengths and temperatures.