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The equivalence point is the point in a titration where the substance being titrated reacts completely with the sample. It can be determined using indicators such as pH, color change, conductivity, or precipitation. The titrant is dripped into the analyte solution until the equivalence point is reached. In acid-base titrations, pH indicators are often used. Other methods include measuring conductivity, pH directly, or the rate of change in temperature.
The equivalence point is the point in a chemical titration, i.e. a process of determining the concentration of a dissolved substance through an induced reaction, at which the amount of the substance being titrated is large enough to completely react with the sample. It is also sometimes called the stoichiometric point. The equivalence point can be determined in several ways using signs such as indicators of pH, color change, conductivity or precipitation. The concentration of a given reagent can be deduced by titrating a known substance in the reagent sample and finding the equivalence point of the reaction.
In the titration setup, a long, cylindrical piece of glass 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 is dripped, is set below. The titrant concentration is known, while the analyte concentration is unknown. Gradually, the burette is emptied into the analyte solution until the expected change in analyte occurs, signaling the equivalence point. At the equivalence point, the added amount of titrant has completely reacted with the reagent in the analyte, and the solution concentration of the analyte 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 color change to pink. Different pH indicators are used to indicate changes in different pH ranges as needed.
Sometimes, the color of the analyte changes during titration without the use of an indicator. The equivalence points of redox (redox) reactions can be determined in this way, since the oxidation states of the product and the reactant produce different colors. It is also possible that the titration reaction causes the formation of a precipitate, or solid. The precipitation can therefore be used as an indicator of the equivalence point, although the exact point of the precipitation can be difficult to determine.
Other methods that can be used to determine the equivalence point include measuring the conductivity of the solution, measuring the pH directly through a pH meter, and measuring the rate of change in temperature. The latter process is known as thermometric titrimetry. Thermometric titration takes advantage of the fact that the chemical reaction between the titrant and the analyte will be either exothermic – releasing heat – or endothermic – absorbing heat. Depending on the nature of the reaction, the temperature will either increase or decrease. The peak rate of temperature change in the reaction is the equivalence point.
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