Equivalence Point Definition

The equivalence point is a chemistry term you'll encounter when you do a titration. However, it technically applies to any acid-base or neutralization reaction. Here's its definition and a look at methods used to identify it.

Equivalence Point Definition

The equivalence point is the point in a titration where the amount of titrant added is enough to completely neutralize the analyte solution. The moles of titrant (standard solution) equal the moles of the solution with unknown concentration. This is also known as the stoichiometric point because it is where the moles of acid are equal to the amount needed to neutralize the equivalent moles of base. Note this does not necessarily mean the acid to base ratio is 1:1. The ratio is determined by the balanced acid-base chemical equation.

The equivalence point is not the same as the endpoint of a titration. The endpoint refers to the point at which an indicator changes color. More often than not, the color change occurs after the equivalence point has already been reached. Using the endpoint to calculate equivalence naturally introduces error.

Methods of Finding the Equivalence Point

There are several different ways to identify the equivalence point of a titration:

Color Change - Some reactions naturally change color at the equivalence point. This may be seen in redox titration, particularly involving transition metals, where the oxidation states have different colors.

pH Indicator - A colored pH indicator may be used, which changes color according to pH. The indicator dye is added at the beginning of the titration. The color change at the endpoint is an approximation of the equivalence point.

Precipitation - If an insoluble precipitate forms as a result of the reaction, it can be used to determine the equivalence point. For example, the silver cation and chloride anion react to form silver chloride, which is insoluble in water. However, it can be difficult to determine precipitation because the particle size, color, and sedimentation rate may make it difficult to see.

Conductance - Ions affect the electrical conductivity of a solution, so when they react with each other, the conductivity changes. Conductance may be a difficult method to use, especially if other ions are present in the solution that can contribute to its conductivity. Conductance is used for some acid-base reactions.

Isothermal Calorimetry - The equivalence point may be determined by measuring the amount of heat that is produced or absorbed using a device called an isothermal titration calorimeter. This method is often used in titrations involving biochemical reactions, such as enzyme binding.

Spectroscopy - Spectroscopy can be used to find the equivalence point if the spectrum of the reactant, product, or titrant is known. This method is used to detect etching of semiconductors.

Thermometric Titrimetry - In thermometric titrimetry, the equivalence point is determined by measuring the rate of temperature change produced by a chemical reaction. In this case, the inflection point indicates the equivalence point of an exothermic or endothermic reaction.

Amperometry - In an ampometric titration, the equivalence point is seen as a change in the measured current. Amperometry is used when the excess titrant is able to be reduced. The method is useful, for example, when titrating a halide with Ag⁺ because it isn't affected by precipitate formation.

Sources

• Khopkar, S.M. (1998). Basic Concepts of Analytical Chemistry (2nd ed.). New Age International. pp. 63–76. ISBN 81-224-1159-2.
• Patnaik, P. (2004). Dean's Analytical Chemistry Handbook (2nd ed.). McGraw-Hill Prof Med/Tech. pp. 2.11–2.16. ISBN 0-07-141060-0.
• Skoog, D.A.; West, D.M.; Holler, F.J. (2000). Analytical Chemistry: An Introduction, 7th ed. Emily Barrosse. pp. 265–305. ISBN 0-03-020293-0.
• Spellman, F.R. (2009). Handbook of Water and Wastewater Treatment Plant Operations (2 ed.). CRC Press. p. 545. ISBN 1-4200-7530-6.
• Vogel, A.I.; J. Mendham (2000). Vogel's Textbook of Quantitative Chemical Analysis (6th ed.). Prentice Hall. p. 423. ISBN 0-582-22628-7.