Which region of the titration curve is characterized by a relatively flat slope at the beginning?

• A. Equivalence Point
• B. Buffer region
• C. Endpoint
• D. Titration

Answer: (B)

The main idea here is how buffers resist pH changes during the early stage of a titration. At the start, the solution contains both the weak acid (or weak base) and its conjugate base (or conjugate acid). When you add titrant, the added H+ or OH− is consumed by this pair, so the pH doesn't move much. For a weak acid being titrated with a strong base, the reaction HA + OH− → A− + H2O shifts the balance between HA and A− slowly, so the pH stays roughly constant. The Henderson–Hasselbalch relationship, pH ≈ pKa + log([A−]/[HA]), shows that as long as the ratio [A−]/[HA] doesn't change much, the pH changes little, producing a relatively flat slope. This region is what people call the buffer region, because the mixture acts as a buffer against added titrant. As you add more titrant and approach the equivalence point, the buffer is overwhelmed and the pH begins to change rapidly, giving a steep slope.