The addition of alkaline materials to supplement deficient "neutralization potential" (NP) of mine spoil, and thus to prevent or abate acid mine drainage, has riot been successful at most surface coal mines in Pennsylvania. A basic problem may have been improper accounting for acid‐production potential and thus inadequate addition rates of calcium carbonate (CaCO₃ ), calcium oxide (CaO) , or calcium hydroxide [ Ca (OH)₂ ] at many mines. The commonly used acid‐base accounting method is based on the following overall reaction:

FeS₂ + 2 CaCO₃ + 3.75 O₂ + 1.5 H₂O --> Fe(OH)₃ + 2 SO₄^-2 + 2 Ca⁺² +2 CO₂(g),

where the acidity from 1 mole of pyrite (FeS2) is neutralized by 2 moles of CaCO₃ . This method presumes that gaseous carbon dioxide (CO₂) will exsolve, and therefore may underestimate by up to a factor of 2 the quantity of CaCO₃ required to neutralize the "maximum potential acidity" (MPA) in the mine spoil. This paper reviews some geochemical reactions involving FeS₂ and various alkaline additives that support the argument that the acid‐base accounting method for computing MPA from overburden analyses should be revised. Considering the stoichiometry of the following overall reaction:

FeS₂ + 4 CaCO₃ + 3.75 O₂ + 3.5 H₂O --> Fe(OH)₃ + 2 SO₄^-1 + 4 Ca⁺² + 4 HCO₃^-,

4 moles of CaCO₃ are required to neutralize the maximum potential acidity produced by the oxidation of 1 mole of FeS₂ . Therefore, the multiplication factor for computing MPA from the overburden sulfur concentration, in weight percent, should be increased from 31.25 to 62.5.