Building on pH meter calibration, this experiment uses the pH meter as a quantitative analytical tool to determine the strength of an unknown HCl solution by continuous pH monitoring during titration with standard N/20 NaOH. The experiment generates a pH versus volume curve, from which the equivalence point is identified as the inflection point — the point of steepest gradient — rather than a visual colour change. The shape of the S-curve is analysed in detail: the initial low-pH plateau (excess acid), the steep central climb spanning approximately 4 pH units per 0.1 mL near the equivalence point, and the high-pH plateau (excess base). The first and second derivative methods for locating the equivalence point precisely are demonstrated. The experiment also identifies the half-equivalence point (where pH = pKa) within the buffer region for weak-acid/strong-base systems, providing a direct experimental determination of the acid dissociation constant. The practical advantages of potentiometric over indicator-based end-point detection are discussed, and the normality and strength of the unknown HCl are calculated from the equivalence point volume.