Traditional titration methods generally consist of a solution containing the species to be analyzed (called the “analyte”) and a chemical referred to as the “titrant” loaded into a long glass cylinder with a stopcock at the bottom called a “buret.” The operator slowly adds the titrant to the analyte solution until the reaction is complete; this is called the “end point” of the titration. The end point is normally determinable when a chemical compound called an indicator (that was added to the analyte at the beginning of the titration) changes color. The operator then performs several calculations to determine the amount of analyte in the solution.
Potentiometric titrations operate on the same general principle, except that an electrode is inserted into the analyte solution and connected to a volt meter; the potential (voltage) of the analyte is then monitored as titrant is added. The chemist normally determines the end point later by constructing a graph of potential vs. volume of titrant. Although potentiometric titrations require specialized equipment, the method features numerous advantages over the traditional method involving colorimetric indicators.
Elimination of Indicators
Chemists often use colorimetric indicators that give a distinct color change when a titration reaction is complete. This method, however, becomes problematic when the solution being analyzed is turbid (cloudy) or darkly colored. Furthermore, colorimetric indicators do not necessarily exist for every conceivable analyte/titrant combination. With potentiometric titrations, which rely only on a voltage measured by an electrode, the color and transparency of the solution being analyzed become inconsequential.
Traditional titration methods generally rely on the operator to determine when the end point of the titration has been reached. Furthermore, if the operator even slightly misestimates the end point, the procedure must usually be repeated. Potentiometric titrations, however, are easily automated. Such devices, called “automatic titrators,” deliver small, fixed volumes (generally 0.1 milliliters or less) of titrant to the solution every few seconds while continuously monitoring the potential. The data may be plotted on an analog strip-chart recorder or digitally stored on a computer for later processing. Because the end points of the titration are generally determined mathematically, such devices cannot “overshoot” the end point.
Multiple Analyte Detection
Potentiometric titration methods, particularly pH titrations of acids, allow the determination of the concentrations multiple species that may be present in the analyte. Wine, for example, contains a mixture of citric, lactic, malic and tartaric acids. Conventional titration with a colorimetric indicator would now allow the chemist to determine the concentration of each wine, only the total concentration of all of the acids combined. Potentiometric titration, however, allows the chemist to determine the concentration of each acid simultaneously.