Ion selective electrodes, more commonly referred to as ISEs, are membrane electrodes that measure specific ions and gasses. This is done by converting the activity of a specific ion dissolved into a solution into electrical potential, typically measured in millivolts (mV). The sensing part of an electrode usually involves an ion-specific membrane and a reference electrode. A measurement of the electric potential created across the membrane by specific ions is compared to the reference electrode and a net charge is calculated. This net charge is directly proportional to the concentration of the specific ion.
History
The theory of ISEs dates back to Dr. Max Cremer, a botanist who first discovered an electrical potential proportional to the acid concentration difference across thin glass membranes in 1906. More than forty years later, George Perley published the relationship of glass composition to pH function in 1949.
Years later, John Riseman and Dr. James Ross of MIT teamed up to commercialize the development of ion selective electrodes. The team formed Orion Research Inc. in the 1960s and produced the first working ISE sensor, a calcium electrode used as a blood gas analyzer. Since then, a wide range of ISEs have been developed for analysis a wide range of ions.
Membrane Types
Membranes are the material which separates the two solutions. More importantly, it is the surface that the charge develops upon. Hydrogen ion responsive glass was the earliest membrane used for ISEs. Glass membranes are made from an ion-exchange type of glass. They have an excellent durability and work in aggressive mediums. This membrane is used in measuring pH, hydrogen, silver, lead, cadmium, and Sodium.
Insoluble Inorganic Salt Membranes (Solid State Membranes) are composed of salt with low solubility and are electrically conductive. These membranes are classified as homogeneous or heterogeneous structures. Homogeneous examples include single crystals, pressed pellets, and sintered materials. Heterogeneous structures are precipitates that are set into a matrix of polymeric materials. This type of membrane is used for measuring fluoride, bromide, cadmium, chloride, cupric, cyanide, iodide, lead, silver, sulfate, and thiocyanate.
With Organic Ion Exchangers and Chelating Agent Membranes, both are integrated into the PVC materials which comprise the membrane. Selectively, they bind specific ions from an aqueous state and are exchanged across the membrane which creates the electronic potential. Common parameters measured include ammonium, calcium, chloride, flourobarate, nitrate, perchlorate, potassium, and water hardness.
Gas Permeable Membranes are used in the measurement of dissolved gasses. Gas particles diffuse across the membrane and react to the buffer solution which alters acidity of the buffer solution. Examples of these measurable gasses include ammonia, carbon dioxide, nitrogen oxide, and dissolved oxygen.
Environmental Applications
Natural water is tested for levels of calcium, potassium, sulfide, carbonate, sodium, silver, and lead. Concentrations of copper, nitrogen, silver, cyanides, and ammonium are measured in wastewater. ISEs determine levels of nitrate, potassium, and sodium in sea water. Fluorides, nitrates, pH, chlorine, and water hardness are measured in drinking water.
In agriculture, ISEs are used on soil samples to determine the amount of calcium, nitrates, sodium, potassium, boron, and ammonium. Fodder is tested to measure contents of nitrogen, calcium, sodium, potassium, and nitrates for nutritional quality. ISEs are also used on fertilizers to analyze potassium, calcium, nitrogen and nitrate content.
Industrial Applications
Petroleum Refineries use ISEs in petroleum refining to determine levels of chloride in water for desalting crude oil, and measuring levels of ammonia, hydrogen chloride, hydrogen cyanide, hydrogen fluoride, and hydrogen sulfide.
ISEs are also used on soft drinks and juices to measure concentrations of fluorides, carbonates, and chlorides. Alcoholic beverages are tested to determine carbonate, sodium, fluorides, and bromide levels. Milk and dairy products are tested for fluoride levels to ensure safety from toxins. ISEs also measure nitrite and nitrate levels in meat processing, and check fluoride levels in fish to measure toxin concentration.
NexSens WQSensors
NexSens ISE WQSensors are double junction ion selective electrodes, proven to provide accurate data measurements with a long lasting life. The list of ion selective electrode sensors currently available includes bromide, calcium, chloride, fluoride, ammonium, and nitrate. An integral USB connector on the WQSensor cable offers a simple connection to the Windows-based PCs and Trimble Nomad handheld field computers. Replaceable sensors tips allow user to change the electrode without replacing the entire unit.