This paper presents the calculation and analysis of the important mononuclear complexes in the Se-H2O system.Based on the thermodynamic data of S0、ΔfG0、ΔfH0 and CP of each reaction substance at standard state,the dominant region distribution of the dissolved species in Se(-Ⅱ)-H2O,Se(Ⅳ)-H2O and Se(Ⅵ)-H2O systems were calculated at different pH values under the temperature range of 298～573 K.Combined with the equilibrium thermodynamic data of the oxidation and reduction reactions of Se containing substances at different temperatures,the potential-pH diagrams of the Se-H2O system at 298,373,473 and 573 K were constructed.It was found that both H2Se and H2SeO3 could exist stably in high acid content solution,while H2SeO4 could only exist in high-temperature and high acid content solution.At 298～573 K,Se could exist stably in aqueous solution.In the range of 298～473 K,H2SeO3,HSeO3- and SeO32-could be directly reduced to elemental Se.Therefore,by controlling a certain pH value and potential,Se could be removed from the solution by precipitation in the form of elemental Se.With the increase of temperature,the stable region of H2 O decreased,and the region of Se,which existed in the solid phase,also gradually decreased.In the liquid phase,the stable regions of H2 SeO3,HSeO3- and SeO32- with Se(+Ⅳ)continued to increase.With the increase of temperature,the oxidation potential of elemental selenium oxidized to Se(Ⅳ)and further to Se(Ⅵ)decreased slightly,and the oxidation reaction could be easier to carry out.
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