**3. Expressions of dissolved oxygen**

Dissolved O2 is expressed in mm Hg, mL O2 per L, mg O2 per L, or mol per L (M). For conversion, a partial pressure of O2 (*P*O2) of 1.0 mm Hg = 0.03 mL O2 per L; 1.0 mL O2 per L = 1.4276 mg O2 per L; and 1.0 mg O2 per L = 1000/32 M. In *freshwater* at 760 mm Hg and 20°C, dissolved [O2] is 9.1 mg/L, or 284 M. Using a Clark electrode, *P*O2 of the reaction mixture phosphate-buffer saline (PBS), 10 mM glucose and 0.5% fat-free bovine serum albumin is 170.5 + 6.6 mm Hg (n = 4), or 228 + 9 M. The 56 mm Hg difference between [O2] in freshwater and the reaction solution reflects the effect of salinity on dissolved O2 (Weiss, 1970).
