KU Chemistry Potentiometry

About Potentiometry

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.

Fisher Accumet AR 20

The AR 20 meter is a research-grade pH meter capable of a variety of functions including automatic temperature compensation, conductivity, and 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.

Homemade pH meters

In an upper level chemistry lab, students have the capability of constructing their own pH meter and interfacing it to a computer. This homemade meter is based on a high impedance operational amplifier wired as a non-inverting amplifier.

Ion Selective Electrodes

We currently have electrodes for the following ions:
Fluoride
Chloride
Nitrate
Nitrite
Ammonium
Lead
Copper
Mercury
Barium
Cadmium

Students may also construct there own liquid membrane electrode in an upper level chemistry course.

Applications of Potentiometry (from the Nico2000, Ltd. site)

Pollution Monitoring: CN, F, S, Cl, NO3 etc., in effluents, and natural waters.
Agriculture: NO3, Cl, NH4, K, Ca, 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, brewing solutions.
F in drinking water and other drinks.
Ca in dairy products and beer.
K in fruit juices and wine making.
Corrosive effect of NO3 in canned foods.
Detergent Manufacture: Ca, Ba, F for studying effects on water quality.
Paper Manufacture: S 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: Ca, K, Cl in body fluids (blood, plasma, serum, sweat).
F in skeletal and dental studies.

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 (you guessed it) 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 concnetration 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).