When the pH is 2 pH units or more below the pKs of the bases in the system the only species contributing significantly to ANC is the hydrogen ion (equation4.12) and thus the ANC is simply
4.13
For a sample containing only carbonates, if the pH is below 4 the ANC is approximately equal to -[H+] and no titration is necessary.
Titration Techniques
Operationally, the first few titrant volumes can be relatively large increments since the important data lies at pH values less than that of the equivalence point (approximately pH = 4.5 for an Alkalinity titration). As the pH is lowered by addition of acid the ionic strength of the solution increases and the activity of the hydrogen ion deviates from the hydrogen ion concentration This effect is significant below pH 3 and thus the effective linear range is generally between pH 4.5 and pH 3.0. The maximum incremental titrant volume (∆Va) that will yield n points in this linear region is obtained as follows.
If Vs » Vt then equation 4.3 reduces to
4.14
Let [H+]e be the concentration of hydrogen ions at the equivalence point and [H+]f be the final concentration of hydrogen ions at the end of the titration.