Acid-Base Titrations Explained
A titration is a controlled reaction between two solutions that lets you find the concentration of one of them, provided you already know the concentration of the other. For acid-base chemistry, it is the standard laboratory technique for answering the deceptively simple question: exactly how strong is this solution?
The Basic Setup
A known volume of the solution with unknown concentration (the analyte) is placed in a flask, usually with a few drops of indicator added. A solution of known, precise concentration (the titrant) is loaded into a burette — a long graduated tube with a stopcock — positioned above the flask. The titrant is added slowly, drop by drop near the endpoint, while the flask is swirled continuously, until the indicator signals that the reaction is complete.
The Equivalence Point
The equivalence point is the exact moment when the moles of acid added equal the moles of base present (or vice versa), based on the reaction's stoichiometry. At this point, the acid and base have completely neutralized each other according to the balanced equation. Locating the equivalence point precisely is the entire purpose of the titration, because once you know the volume of titrant used to reach it, you can calculate the unknown concentration using the relationship moles of acid = moles of base (adjusted for the stoichiometric ratio in the balanced equation).
A Worked Calculation
Suppose 25.0 mL of hydrochloric acid of unknown concentration is titrated with 0.100 mol/L sodium hydroxide, and 32.5 mL of the NaOH is required to reach the equivalence point. Since HCl and NaOH react in a 1:1 ratio (HCl + NaOH → NaCl + H2O), moles of NaOH used = 0.0325 L × 0.100 mol/L = 0.00325 mol. Because the ratio is 1:1, moles of HCl = 0.00325 mol as well. Dividing by the original acid volume gives the concentration: 0.00325 mol ÷ 0.0250 L = 0.130 mol/L.
Choosing the Right Indicator
An indicator must change color at a pH close to the equivalence point of the specific reaction being run — not simply at pH 7. A strong acid-strong base titration has an equivalence point at pH 7, and either phenolphthalein or methyl orange works well. A weak acid-strong base titration has an equivalence point above pH 7 (because the salt formed is itself weakly basic), so phenolphthalein, which changes color around pH 8.2–10, is the appropriate choice. A strong acid-weak base titration has an equivalence point below pH 7, better matched by methyl orange, which changes around pH 3.1–4.4.
Reading a Titration Curve
Plotting pH against volume of titrant added produces a titration curve with a characteristic steep, near-vertical jump right at the equivalence point — a small addition of titrant causes a large pH swing there, which is exactly why a few drops can make the difference between an accurate result and an overshoot. The midpoint of that steep vertical section marks the equivalence point graphically, and its exact pH value depends on whether the acid and base involved are strong or weak, since weak acids and bases produce curves that are shifted and less symmetrical than the strong-acid-strong-base case.
Sources of Error and Good Practice
Rinsing the burette with the titrant solution (not just water) before filling it prevents dilution errors. Reading the meniscus at eye level avoids parallax error in the volume reading. Adding titrant dropwise, and even sub-dropwise using a wash bottle to rinse droplets down from the flask walls, right as the color change approaches keeps the measured endpoint as close as possible to the true equivalence point, since overshooting by even one extra drop can measurably skew the calculated concentration in a small-scale titration.
Summary
An acid-base titration uses a solution of known concentration to determine an unknown one, by carefully tracking the volume needed to reach the equivalence point — the moment stoichiometrically equivalent amounts of acid and base have reacted. Choosing an indicator whose color change matches the expected pH of that equivalence point, rather than defaulting to pH 7, is essential for an accurate result. This technique builds directly on the concepts in acids, bases, and pH and the calculation skills from stoichiometry and mole calculations.