Titration determines the concentration of a specified chemical—the analyte—in a sample by using another chemical or reagent, called the titrant. The reagent reacts with the sample to produce a color, chemical reaction, or change in an electrical property that relates to the concentration of the analyte. The amount of reagent used is measured and converted to concentration.
Analyzing the analytes
“Modern instruments that indicate results of a titration use changes in electrical characteristics,” says Hank Levi, president at Scientific Gear. These instruments are known as potentiometric titrators.
To operate a potentiometric titrator, a sample is added to a container or flask. An electrode is inserted to measure the electrical characteristic, such as the voltage of the sample. The reagent is added from a burette until the electrical changes reach equilibrium or the endpoint. The amount of reagent required to reach that endpoint is measured and converted to concentration. “Scientists use potentiometric titrations for a wide range of chemical concentration analyses, including things like salt and acid/acidity,” Levi explains.
Measuring water content
In the 1930s, German chemist Karl Fischer developed a special reagent to analyze a sample’s water concentration. The methods that use these reagents are called Karl Fischer. A Karl Fischer reagent reacts with the water in a sample to create iodine, a non-electrical conductive chemical.
In volumetric Karl Fischer, the reagent is added to the sample, contained in a sealed container filled with an appropriate solvent, and fitted with an electrode. The reagent is metered into the container by a burette. The water concentration in a sample is calculated from the amount of reagent used to reach the endpoint.
In 1959, a modification of the Karl Fischer reagent was developed. This reagent combined the sample solvent and the water-reacting reagent in one chemical solution. Levi points out that “this technique uses an electrical current to release the active reagent chemical from the combined reagent/solvent solution.” The system uses two electrodes; one to detect the electrical condition of the sample and a second to provide the electrical current that releases the active reagent. This technique is called coulometric Karl Fischer.
To conduct a coulometric Karl Fischer test, a special sealed vessel, equipped with the two electrodes and a sample insertion port, is filled with coulometric Karl Fischer reagent. A sample is added through the insertion port, and current is automatically applied to separate the active reagent. Here, the amount of current needed to reach equilibrium is converted to the concentration of water in the sample.
Other equipment can be added to expand the application of a titrator. To increase throughput, for example, a sample changer can be used to automatically test multiple samples.
The sample itself can also require added equipment. In a Karl Fischer titration, for example, some samples—such as plastics—only release water at higher temperatures. In such cases, a Karl Fischer oven can be used to drive off water into the titration cell. The sample and the desired measurement both play a role in selecting the best titration platform for a particular task. For today’s scientists, many options are commercially available.