Acid neutralizing capacity

The relation between pH and ANC in natural waters depends on three conditions: Carbon dioxide, organic acids and aluminium solubility.

Surface waters with high DOC are typically found in areas where there is a lot of peat and bogs in the catchment.

Aluminium solubility is a bit tricky and there are several curve fit variants used in modelling, one of the more common being: In the illustration to the right, the relation between pH and ANC is shown for four different solutions.

The red line has a high amount of dissolved carbon dioxide (pCO2=20 times ambient), a level that is not uncommon in ground water.

The reason why ANC is often defined as the difference between cations of strong bases and anions of strong acids is that ANC is derived from a charge balance: If we for simplicity consider a solution with only a few species and use the fact that a water solution is electrically neutral we get where R− denote an anion of an organic acid.