Another factor affecting the availability of metal ions was their solubilities with HS. Hydrogen sulfide was abundant in the early sea giving rise to HS in the prebiotic acidic conditions and HS in the neutral (pH = 7.0) conditions. In the series of metal sulfides, insolubility increases at neutral pH following the Irving–Williams series:
* Mn(II) < Fe(II) < Co(II) ≤ Ni(II) < Cu(II) > Zn(II)
So in high amounts of HS, which was the prebiotic condition, only Fe was most prominently available in its ionic form due to its low insolubility with sulfides. The increasing oxidation of HS into SO leads to the later release of Co, Ni, Cu, and Zn since all of their sulfates are soluble.