Titration is a fundamental analytical chemistry technique used to determine the unknown concentration of a substance in a solution. This process involves the controlled addition of a solution with a precisely known concentration, called the titrant, to a measured volume of the solution with the unknown concentration, known as the analyte. The titrant is added until a complete chemical reaction occurs. By measuring the volume of titrant required, chemists can accurately calculate the analyte’s concentration.
What Titration Curves Show
Titration curves are graphical representations illustrating how a solution’s property changes throughout a titration. In acid-base titrations, they typically plot the pH of the solution on the y-axis against the volume of titrant added on the x-axis. The curve generally displays an S-shaped or sigmoidal pattern, reflecting pH changes as titrant is introduced. Initially, the pH changes slowly, then undergoes a rapid, steep change around the point of reaction completion, before slowing down again as more titrant is added.
Locating the Equivalence Point
The equivalence point is a theoretical concept representing the precise moment during a titration when chemically equivalent quantities of reactants have been mixed. For an acid-base reaction, this means the moles of acid exactly equal the moles of base according to the reaction’s stoichiometry. On a titration curve, the equivalence point is identified as the steepest part of the curve, often referred to as the inflection point, where the most significant pH change occurs with the smallest addition of titrant.
For strong acid-strong base titrations, the equivalence point consistently occurs at a pH of 7, indicating a neutral solution. However, for titrations involving weak acids or weak bases, the pH at the equivalence point will deviate from 7. For instance, in a weak acid-strong base titration, the equivalence point will be at a pH greater than 7, while for a strong acid-weak base titration, it will be less than 7.
Why the Equivalence Point Matters
The equivalence point holds importance in quantitative analysis as it serves as the theoretical ideal for determining an unknown concentration. Once the volume of titrant needed to reach this point is accurately known, along with its concentration and the initial volume of the analyte, the unknown concentration can be precisely calculated. This calculation relies on the stoichiometric relationship between the reacting substances. The equivalence point forms the foundation for various analytical applications, including quality control in manufacturing and environmental monitoring.
Equivalence Point Versus Endpoint
While often used interchangeably, the equivalence point and the endpoint in a titration are distinct concepts. The equivalence point signifies the theoretical point where reactants are stoichiometrically equal. The endpoint is the experimentally observed point, typically marked by a visible change, such as a color change of a chemical indicator. Indicators are substances added to the reaction mixture that change color within a specific pH range, signaling the completion of the reaction.
Chemists carefully select an indicator whose color change range is as close as possible to the expected pH of the equivalence point. While ideally the endpoint should coincide with the equivalence point, a slight difference can exist due to the indicator’s transition range. The endpoint serves as a practical, measurable approximation of the theoretical equivalence point in laboratory settings.