The equivalence point is the point in a chemical titration — meaning a process of determining the concentration of a dissolved substance through an induced reaction — at which the amount of the substance being titrated is just large enough to fully react with the sample. It is also sometimes referred to as the stoichiometric point. The equivalence point can be determined in a number of different ways using signs such as pH indicators, color change, conductivity, or precipitation. The concentration of a given reactant can be deduced by titrating a known substance into the reactant sample and finding the reaction’s equivalence point.
In the titration setup, a long, cylindrical piece of glassware known as a burette is filled to a calibrated mark with a specific volume of the titrant, or solution to be titrated. A specific volume of the analyte, or solution into which the titrant will drip, is set up below. The concentration of the titrant is known, while the concentration of the analyte is unknown. Gradually, the burette is emptied into the analyte solution until the expected change in the analyte occurs, signalling the equivalence point. At the equivalence point, the amount of titrant added has completely reacted with the reactant in the analyte, and the concentration of the analyte solution can now be determined.
In acid-base titrations, where an acid is titrated into a base or vice versa, a pH indicator compound is often added to the analyte solution so that the equivalence point can be determined visually. Phenolphthalein is a commonly used pH indicator that is colorless at low pH levels but turns bright pink at high pH. The equivalence point in an acid-base titration involving phenolphthalein would be signaled by a rapid change of the color to pink. Different pH indicators are used for indicating changes across different pH ranges as needed.
Sometimes, the color of the analyte will change during titration without the use of an indicator. Equivalence points of oxidation-reduction (redox) reactions can be determined this way, since the oxidation states of the product and reactant produce different colors. It is also possible that the titration reaction will cause a precipitate, or solid, to form. Precipitation can therefore be used as an equivalence point indicator, though the exact point of precipitation can be difficult to determine.
Other methods that can be used to determine the equivalence point include measuring the conductivity of the solution, directly measuring pH through a pH meter, and measuring rate of temperature change. This last process is known as thermometric titrimetry. Thermometric titrimetry makes use of the fact that the chemical reaction between the titrant and analyte will either be exothermic — releasing heat — or endothermic — absorbing heat. Depending on the nature of the reaction, the temperature will either rise or fall. The peak rate of temperature change in the reaction is the equivalence point.