A method for the definition of the carbon oxidation number in the gases dissolved in waters and the redox variations using an elemental analyser (FlashEA 1112). Preliminary data from a stratified lake

Obtaining meaningful redox potential determinations is questionable in disequilibrium solutions, like surface waters and groundwater. However, identifying redox processes in a natural ecosystem is a key factor to characterize the chemical quality and, consequently, the sustainability of that environment. To do that, the determination of an exact redox potential is not needed and on many cases it is more important to describe how and if the redox state of a system changes. In particular, the redox status of aqueous systems is assigned based on analysis of samples for redox-sensitive compounds. In general, these analyses do not include measurements of gases dissolved in the waters. Here, we present an innovative method to underline in an easy way changes in redox state of a system using the elemental composition (C, H, S) measured in the headspace gas after the equilibration with water phase. The results allow defining the empirical formula of the gas, the mean number of oxidation of the carbon and the molar ratio between C:H:S. The method is applied to investigate the redox condition in a stratified lake, San Rocchino (Northern Tuscany, Italy) using an instrument (Thermo Scientific FlashEA 1112) equipped for headspace gas analysis. Samples of waters and dissolved gases were collected at five different depths along a vertical profile. Such analytical system is used to determine the mean oxidation number of the carbon gaseous phase in the headspace, which allows distinguishing relatively oxidizing from relatively reducing condition. Further information on the redox state is reached considering the molar ratio C/H and C/S. This innovative method is simple and rapid, and the instrument is robust and it needs low maintenance. Moreover, it could be applied to other research and exploratory fields, such as geothermic and volcanic monitoring, polluted aquifer and contaminated sites. ► Definition of the empirical formula of a gas ► Determination of the mean oxidation number of carbon gaseous phase in the headspace ► Information on the redox state is reached considering the molar ratio C/H and C/S.