A titration curve is a graphical representation used in chemistry to track the progress of a controlled acid-base reaction. This visualization plots the change in acidity or basicity, measured as pH, as a solution of known concentration (the titrant) is gradually added to a solution of unknown concentration. This technique is fundamental for determining the precise concentration of a substance in a sample. The curve provides an observable record of the neutralization reaction as it moves toward completion.
What the Curve Represents
A titration curve is constructed on a two-dimensional plot, providing a visual of the chemical changes occurring. The X-axis tracks the volume of the titrant added to the sample solution. The Y-axis represents the measured pH of the solution in the flask.
The shape of the resulting curve is typically an elongated ‘S’ shape, illustrating the rate of change in pH throughout the reaction. Initially, the pH changes very slowly, resulting in a relatively flat segment. This slow change is followed by a dramatic, near-vertical rise or drop in pH as the neutralization nears completion. Finally, as the titrant is added in excess, the curve levels off once more, settling near the pH of the pure titrant solution.
Critical Points on the Curve
Understanding a titration curve requires recognizing specific regions and points that hold significant chemical meaning. The starting and ending pH values define the initial strength of the acid or base being analyzed. For example, a curve starting at pH 1.0 indicates a strong acid in the sample, while one ending at pH 13.0 indicates a strong base was the titrant.
The most important feature is the equivalence point, which is the point where the added moles of titrant are chemically equivalent to the moles of the substance being analyzed in the sample. This point is located at the center of the steep, vertical inflection of the curve. At the equivalence point, the reaction is stoichiometrically complete, and the solution contains only the salt produced and water.
Before the equivalence point, a flatter section of the curve is visible, known as the buffer region. In this region, the solution contains significant amounts of both the weak acid/base and its conjugate partner, allowing the solution to resist large changes in pH. The midpoint of this region, where half the titrant needed for neutralization has been added, is chemically significant because the pH equals the pKa of the weak acid or the pKb of the weak base.
How Acid and Base Strength Changes the Curve
The specific shape of the titration curve is highly dependent on the strength of the acid and base involved in the reaction. A titration of a strong acid with a strong base, such as hydrochloric acid with sodium hydroxide, produces a curve with a very sharp and symmetrical vertical jump. The equivalence point for this type of reaction is precisely at a neutral pH of 7.0, because the salt formed (e.g., NaCl) does not react with water to change the pH.
The curve changes significantly when a weak acid is titrated with a strong base, such as acetic acid with sodium hydroxide. This curve displays a more gradual slope in the buffer region and a shorter vertical inflection around the equivalence point. Because the conjugate base of the weak acid reacts slightly with water, the equivalence point is shifted to a basic pH, typically above 7.0.
Conversely, titrating a strong acid with a weak base results in an equivalence point that is acidic, falling below pH 7.0. In general, the weaker the acid or base being analyzed, the less pronounced the vertical steepness of the curve becomes. These differences in curve shape and equivalence point pH are fundamental to identifying the nature of the unknown substance.
Real World Uses
The data extracted from titration curves is widely applied in various industries for quality control and chemical analysis. The primary utility lies in accurately determining the unknown concentration of a substance, calculated directly from the volume of titrant required to reach the equivalence point. This method is used in pharmaceutical manufacturing to ensure the correct concentration and purity of active drug ingredients. In the food and beverage industry, titration curves help analyze the acidity of products like fruit juices and wines, which affects flavor and shelf life. Environmental science also relies on this technique to monitor pollutants, such as determining the alkalinity of water samples.