We present a new technique for preparing and characterizing brushes consisting of weak acidic monomers. The brushes consist of polystyrene-polyacrylic acid block copolymers. The block copolymers are deposited from the air/water interface on a hydrophobically modified Si wafer via the Langmuir-Blodgett technique. The titration of such brushes at low ionic strength is measured with reflectometry. It is shown that the titration curve of the grafted PAA is shifted toward higher pH with increasing grafting density, as predicted by theoretical models for weak acidic brushes.