Back titration

Back titration is a titration method used in analytical chemistry to determine the concentration of an unknown substance by measuring the amount of a known reactant that remains after the reaction has been completed. This method is often used when the unknown substance is not directly titratable.

Overview

In a typical titration, a known volume of a solution (the titrant) is added to a solution of the substance being analyzed (the analyte) until the reaction between them is just complete. The point at which this occurs is called the equivalence point. The amount of titrant required to reach the equivalence point is used to calculate the concentration of the analyte.

In a back titration, the process is reversed. A known excess of a reactant is added to the analyte, and the reaction is allowed to go to completion. Then, the remaining excess reactant is titrated with a second, known solution. The amount of this second titrant required to react with the excess reactant is used to calculate the amount of the reactant that reacted with the analyte, and from this, the concentration of the analyte.

Applications

Back titration is often used when the analyte is either volatile or insoluble in water, or when it has a particularly slow reaction rate. It is also used when the normal titration method would result in a weak acid-weak base titration, which would not produce a sharp end point.

Examples of applications of back titration include the determination of calcium carbonate in limestone and the analysis of antacid tablets for their acid neutralizing capacity.

Procedure

The procedure for a back titration is similar to that for a regular titration, with the addition of an extra step at the beginning. First, a known excess of a reactant is added to the analyte. The mixture is then allowed to react, usually with stirring and sometimes with heating, until the reaction is complete. This is often determined by observing a change in color or the cessation of gas production.

Once the reaction is complete, the remaining excess reactant is titrated with a second, known solution. The amount of this second titrant required to react with the excess reactant is used to calculate the amount of the reactant that reacted with the analyte, and from this, the concentration of the analyte.

See also

• Titration
• Analytical chemistry
• Acid-base titration
• Redox titration