Titration

Titration is the process of determining the concentration of a chemical by introducing a solution of a known concentration. Acid-base and redox titrations are two of the most common methods.

The solution of known concentration is the standard solution, or reagent solution, also called the titrant. The solution of unknown concentration is the analyte. The ideal point for the completion of titration is the equivalence point. The endpoint is determined by an indicator at the end of the titration; the endpoint and equivalence point represent the same idea, which is that the titration should end when the indicator completes its process, such as changing color.

Titration is used in many industries. These include petrochemicals as well as food manufacturing and packaging—for example, measuring the maturation of cheese and wine. It is also used in the medical field to analyze fluids, including blood and urine, for the concentration of chemicals.

Background

Very rudimentary examples of titration have been recorded for centuries. During the seventeenth century, for example, instructions for making saltpetre involved nitric acid and potash, instructing the chemist to add potash drop by drop to the acid, until the addition of potash no longer caused bubbling in the mixture. The bubbling served as an indicator to measure when the mixture reached an equivalence point.

Ferenc Szabadvary provided a description of a 1729 process to determine the acidity of vinegar by slowly adding potash, and again determining how much was needed to reach the point at which the bubbling stopped—neutralization of the acid. Claude Joseph Geoffrey, who described his development of this method, pioneered the use of a standard solution for titration.

Other chemists also worked on similar volumetric methods of using a standard solution. Each measured the standard solution that was being added to the analyte in volume—grains, teaspoons, etc. Since most applications of these processes were meant for industrial use, the chemists were evaluating the substances for their suitability for a purpose. They wanted to know if a substance was strong enough for its needed use, or to determine the acidity of water, which would affect a product. Later methods replaced the volumetric method, adding the standard solution until neutrality was achieved and then weighing the solution to determine how much had been added.

Modern titrimetry developed primarily in France in the late eighteenth and early nineteenth centuries. Titre in French means the purity of noble metals, and the term was first used to refer to the measure of the purity of silver in 1835, when Gay-Lussac developed a volumetric method using a sodium chloride solution.

Titrimetric analysis was used to quickly assess quality of a substance, and developed primarily as industry became more important during the middle of the eighteenth century. Industrial chemistry also developed during this time, at first in manufacturing sulphuric acid for use in lead-chamber plants. The quality of substances used in manufacturing—primarily acids, alkali carbonates, and hypochlorites—had to be measured in some way. Scientists developed a number of methods, but these were slow, often taking weeks or months, in some cases relying on evaporating a liquid, then collecting the remaining chemical crystals and weighing them.

Near the end of the eighteenth century, Francois Antoine Henri Descroizilles developed redox titration in the development of a bleaching process using chlorine. His work led to the creation of a textile bleaching industry.

Overview

An acid-base titration uses one acid solution and one base solution. Acids are ionic compounds (compounds with a positive or negative charge) that break up in water to form a hydrogen ion, while bases are ionic compounds that break up in water to form a negatively charged hydroxide ion. The strength of acids and bases is measured on a pH scale ranging from 0 to 14, with pH of 7 being neutral. Numbers less than 7 are acidic (0 being the strongest acid) and greater than 7 are alkaline (14 being the strongest base).

When conducting a manual titration, the analyte, an acid of unknown concentration, is placed in a flask or beaker. A base of known concentration is placed in a calibrated buret (a graduated glass tube with a tap at one end) and positioned over the flask so it slowly drips into the analyte. The amount of base (the titrant) needed to neutralize the acid is measured. To obtain precise results, the process must be measured accurately and the equivalent point reached exactly, so the titrant must be measured precisely and added to the analyte very slowly. The pH can be measured using a pH meter in the flask, or by using an indicator. The amount of base needed to neutralize the sample is then determined.

Indicators are used when no visible reaction will occur to indicate when the titration reaches a 1:1 mixture—for example, a color indicator is an acid or base that changes color when it reaches a 1:1 mixture of the components.

Measurements gained during acid-base titration are plotted on a chart, with titrant volume on the x-axis and pH on the y-axis. The titration of a strong acid and strong base will have a sharp transition region. The point at which all of the solution being dripped into the flask has reacted with the solution is the equivalence point. The equation MiVi =Mf Vf, with Mi and Mf the initial and final molarities, and Vi and Vf the initial and final volumes, can be used to calculate the concentration of the unknown solution.

Redox titrations are also known as oxidizing-reducing titrations. They use either a reducing agent or an oxidizing agent as the titrant against the other agent. The oxidizing agent is added to the reducing agent. The equivalence point is reached when all of the oxidizing agent has reacted with all of the reducing agent. The redox titration method is used to determine the transfer of electrons from one agent to the other.

Redox indicators, which change color, can be used in this process to detect the endpoint of the titration. It can also be determined using a potentiometer, which measures variable potential, or voltage, in a circuit.

Combination reactions titrations include two types of titrations: elements of opposite ions or complex-formation. When elements of opposite ions are titrated against each other, one ion serves as the titrant, while the opposite ion acts as the analyte. Complex-formation titrations require ethylenediaminetetraacetic acid (EDTA) or a similar powerful complexing agent to be used as the titrant.

Bibliography

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