Potentiometry involves measuring an electrical potential that is related to a component in which someone is interested. For example, many communities fluoridate water supplies to enhance the durability of tooth enamel. One convenient way to measure fluoride concentration is to use an electrode where the potential depends upon the concentration of fluoride. Potentiometric electrodes are usually fast, portable, and do not require extensive training to operate.
Many strategies have been explored to create electrodes in which the electrical potential varies with the concentration of some species. The most common electrode in use is the glass membrane pH electrode. The heart of the glass membrane pH electrode is the glass membrane which ideally only allows H3O+ ions to become incorporated in its inner and outer layer. The inside is filled with a solution that has a fixed concentration of H3O+. The outside of the membrane is exposed to our solution in question. If there is much more H3O+ in the solution in question compared to the internal solution, the outer layer of the glass membrane will build up a positive charge relative to the inside. This difference in electrical charge across the glass membrane is the membrane potential that depends only on the concentration of H3O+ in the outer solution.
Many types of membranes have been developed (glass, solid state, and even liquid) to detect a wide variety of analytes, but the electrical potential across that membrane is the signal that must be measured. In order to measure this potential, a reference electrode and a meter are required. Most reasonable pH meters are capable of acquiring signals from different types of ion selective electrodes (ISEs).
KU's pH/Conductivity Meters
Fisher Accumet AR 20
The AR 20 meter is a research-grade pH meter capable of a variety of functions, including automatic temperature compensation and conductivity, and it can accept a newer type of electrode called the ion selective field effect transistor (ISFET).
The touchscreen panel provides access to all operating functions, as well as help screens to assist in operation.
Fisher Accumet AB15 Plus
The AB15 Plus is a recent addition to our labs. The AB15 plus has a variety of input and output connections, ISFET capabilities, and automated temperature compensation.
Corning Model 250 IonAnalyzer
We have 2 Corning Model 250 IonAnalyzers. These standard digital pH meters have all of the necessary features to use all of the ion-selective electrodes we have. They are not real fancy, but they are solid workhorses.
Ion Selective Electrodes
We currently have electrodes for the following ions:
Students may also construct their own liquid membrane electrode in an upper-level chemistry course.
Applications of Potentiometry (from the Nico2000, Ltd. site)
Pollution Monitoring: CN-, F-, S2-, Cl-, NO3- etc., in effluents and natural waters.
Agriculture: NO3-, Cl-, NH4+, K+, Ca2+, I-, CN- in soils, plant material, fertilizers, and feedstuffs.
Food Processing: NO3-, NO2- in meat preservatives.
Salt content of meat, fish, dairy products, fruit juices, and brewing solutions.
F- in drinking water and other drinks.
Ca2+ in dairy products and beer.
K+ in fruit juices and wine making.
Corrosive effect of NO3- in canned foods.
Detergent Manufacture: Ca2+, Ba2+, F- for studying effects on water quality.
Paper Manufacture: S2- and Cl- in pulping and recovery-cycle liquors.
Explosives: F-, Cl-, NO3- in explosive materials and combustion products.
Electroplating: F- and Cl- in etching baths; S in anodising baths.
Biomedical Laboratories: Ca2+, K+, Cl- in body fluids (blood, plasma, serum, sweat).
F- in skeletal and dental studies.