The solubility of metallic selenium was measured in a mixture of 0.1M-NaCl and 0.05M-N2H4under anoxic conditions (O2 < 1 ppm) by both oversaturation and undersaturation methods. Equilibrium was attained in 40 days. The aqueous selenium species identified were HSe at pH between 5 and 8, and Se4
2at pH between 10 and 13, by UV-Vis absorption spectrometry. The solid phase was identified as Se (cr) by X-ray diffraction. The equilibrium constants of
Se(cr) + H+ + 2e- = HSe- logK0 = -6.5±0.5 and
4Se(cr) + 2e- = Se4
2- logK0 = -16.8±0.5
were determined. The standard molar free energy of formation of HSe- and Se4
2-was determined to be (37.1±2.9) and (95.9±2.9) kJ/mol, respectively.