We have referred to the pH changes of water owing to changes in the content of CO2, in different forms. Increase in free CO2 (H2CO3) cannot make the water pH lower than 4.5, which is the end point of methyl orange indicator i.e. when you titrate acid (say 1/100 N NCL) against a basic solution (say hard water) the penolphthalein end point is reached at a pH of 8.3, and when you titrate further down the methyl orange end point is reached. The reverse titration with base as titrant would indicate the same and give acidity values. If the pH of a water sample is less than 4.5 this must be caused by a strong mineral acid (cf H2SO4 in water on acid sulphate soils in Buguma). On oxidation of pyrites in acid sulphate soils sulfuric acid results:
4 Fe S2 + 15 O2 + 2H2O 2 Fe2 (SO4)3 + 2 H2SO4
(see also discussion on “acid sulphate soil”).
Soil from marine sediments will have sulfide compounds (cat's clay) and sulfide in such soils would oxidize on exposure to air producing sulfuric acid and water in ponds in cat's clay formation can have a pH of 2 and a mineral acidity of 350 mg/L (equivalent CaCO3) as observed by Boyd (1979).