Acid-base titration

A titration measures the concentration of an unknown acid or base by reacting it with a solution of known concentration, called the standard solution. The standard solution is added slowly from a buret, a long, marked glass tube with a valve at the bottom, into a measured volume of the unknown until the reaction is just complete. That moment is the equivalence point, where the acid and base have reacted in exactly the amounts the balanced equation requires; for a strong acid and strong base like hydrochloric acid and sodium hydroxide, the moles of hydrogen ions equal the moles of hydroxide ions. An indicator is chosen so that it changes color close to the equivalence point; that visible color change is called the endpoint. Phenolphthalein, which turns from colorless to pink as a solution becomes basic, works well when titrating a strong acid with a strong base. The volume of standard solution used then gives the unknown's concentration: for an acid and base that react one-to-one, M_acid x V_acid = M_base x V_base. For example, if 20.0 mL of 0.100 M sodium hydroxide neutralizes 25.0 mL of hydrochloric acid, the acid's concentration is 0.0800 M. A titration curve, which plots pH against the volume added, shows a sharp jump in pH near the equivalence point for a strong acid and a strong base; when the acid or base is weak, the jump is smaller.

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