So I've performed soil extractions with KCl 1M to determine exchangable acidity.

I titrated an aliquot of the extract with NaOH. Then I added NaF to another aliquot and titrated with NaOH. It is my understanding that when adding NaF, I will be substituting OH- for F- in Al(OH)n formed after hydrolisis of Al3+ extracted by the KCl 1M; thus, KCl + NaF should require less NaOH to reach phenolphthaleine equivalence point.

The problem is that this did not consistently happen, in some cases I required more NaOH after adding NaF. For example, when titrating my KCl blanks, I required 0.1 mL for pure KCl but 0.25 mL for KCl + NaF.

Any ideas as to what the problem might be? I thinking it's just poor execution of the technique but I worry there might be something fundamentally wrong which I have been missing.