The definition of what constitutes free sulfide, as opposed to bound sulfide, should also be noted. Free sulfide is dissolved H2S gas, which is a weak acid and in solution exists in the equilibrium H2S ↔ HS− ↔ S2−. With the pKa1 ∼7.0 and the pKa2 >17, there is essentially no S2− in biological tissues, nearly equal amounts of H2S and HS− within the cell, and approximately a 20% H2S/80% HS− ratio in extracellular fluid and plasma at 37°C and pH 7.4. Due to the temperature sensitivity of pKa1, the H2S/HS− ratio remains nearly constant in blood and tissues of ectothermic vertebrates over a wide range of body temperatures. For example, in trout blood at 10°C, the ratio is 15% H2S/85% HS−.