A direct thermometric titration (TET) with 2 mol/L NaOH is used to determine the HF, NH4F, and maleic acid (C4H4O4) contents of acid cleaning solutions. Three endpoints (EPs) are obtained, which may be assigned as follows:
EP1: C4H4O4 (pKa1 = 1.9), HF (pKa = 3.17)
EP2: C4H4O4 (pKa2 = 6.07)
EP2: NH4F (pKa = 8.2)
The HF content is determined by subtracting the difference (EP2-EP1) from EP1.