Although silica is one of the most common adsorbents, the position of its isoelectric point (IEP) on the pH scale remains an open question. Different values from 1 to 4 have been published, and there is even a controversy about the very existence of the IEP. The discrepancies in the literature are at least partially due to application of different instruments. Different zetameters produce different zeta potentials of silica in the acidic range and different IEP.