In the United States alone, nearly million measurements are made annually of blood potassium levels using this electrode. For all of the ISEs described above, the same equations can be used to predict the relationship between potential and analyte activity A or concentration. The potential measured, E meas , is the potential difference between the analyte E outer side of the membrane and the reference E inner side of the membrane:. The potential of each side is related to activity A or concentration as described by the Nernst equation, where z is the charge of the ion of interest:.
Since A ref and E o are both constant, E inner equation 3 is constant. If equations 2 and 3 are plugged into equation 1, they combine to give the following:. You can see that ISEs should exhibit a Nernstian response, as you previously learned for the direct indicator electrodes.
You have learned that one of the most important analytical characteristics of ISEs is their selectivity; i. In reality, ion-selective electrodes can experience interferences by responding to the presence of other ions.
Equation 4 assumes that all of the electrode response E meas is due to one ion. We will call the interfering ion j with corresponding activity A j and charge z j. This new equation is called the Nikolskii-Eisenman equation:.
The selectivity coefficient is a numerical measure of how well the membrane can discriminate against the interfering ion. As you can see from the equation, the smaller the k ij values, the less impact the interfering ion will have on the measured potential. When k ij values are less than 1, the ISE is more responsive to the analyte ion and when k ij values are greater than 1, the ISE is more responsive to the interfering ion.
For example, a k ij value of 0. Selectivity coefficients can be experimentally determined. Selectivity coefficients for some of the electrodes previously discussed are listed below. For some help with this problem, open up the following spreadsheet. Make sure that you are on the F - tab.
For further reading on ISEs, refer to references 8 through 13 and the following online resources:. ISE products from GlobalSpec an engineering search engine with free registration. Ion-exchange process. Figure 3. Development of a potential at an ISE. Common examples include determination of fluoride in medical applications, or measurement of ammonia in waste water so that regulations are met.
ISEs are relatively user friendly, and can be used both in the laboratory and field. They can tell us important information about the quality of a sample, and also have implications on safety and process control. ISEs are not perfect, and other ions can sometimes act as interferents by either giving a similar response to the ion being measured, interacting with the membrane or interacting with the ion being measured thus decreasing its activity.
There are a few ways to deal with interferences, which include chemical elimination e. Also, incremental techniques such as known addition can be used. Over that time, we have built up an enviable reputation for quality and customer service. Our sensors are made at our UK based manufacturing facility, which is fully certified under ISO Our dedicated team at Sentek understand the importance of accurate and efficient measurements.
Contact us for more technical information or specialist advice. The measured slope can be used as an indication of the proper functioning of an ISE. The Function of the Reference Electrode The membrane potential cannot be measured directly. It needs a Metal-Liquid interface or a metal-solid solution interface in modern "all-solid-state" ISEs on both sides of the membrane. Theoretically these could just be metal wires immersed in the solutions. But the electrical potential on many simple metal-liquid junctions is not stable; thus the need for a so-called reference system on both sides of the ISE membrane, with a particular metal-liquid interface which is known to have a stable potential.
The magnitude of this potential need not be known because it is the same for all measurements of standards and samples and is thus eliminated during the calibration process.
Nevertheless, it must be noted that this potential, plus any others that may be generated at any or all of the metal-liquid or liquid-liquid junctions in the circuit, is the value which is seen when the electrodes are immersed in pure water or any other solution which does not contain the target ion.
This explains why the measured voltage is not expected to be zero when no target ion is present and also why it is not necessarily always positive when the target ion is present - it all depends on the difference between the ISE voltage and the sum of all the other voltages in the circuit. Reversing the charges above would describe the situation for a monovalent negative ion.
It should be noted here that immersion in pure water should be avoided because it tends to leach out the target ion from the ISE membrane. This, together with the inherent instability of the liquid junction potential of the reference electrode, will cause an unstable voltage to be measured in pure water and require the ISE membrane to be re-equilibrated in a high concentration "pre-conditioning" solution before it will give stable readings again.
In practice, the most common reference system is a silver wire coated with solid silver chloride and immersed in a concentrated solution known as the "filling solution" of potassium chloride saturated with silver chloride. These devices consist of ISEs and are created to detect a quantifiable signal derived from biological signatures and behavior.
Biosensors are operated through a wide variety of lab components including skin samples, blood culture, and body fluids. Enzyme electrodes are most often used in this aspect, since biomedical processes often include enzymes as one of the most common recognition elements. Electrodes remain an essential part of medical research. These devices transfer ionic currents into the natural current flow within the human body. The strengthened currents can help medical practitioners and researchers to improve their detection processes.
This makes it easier to locate concentrations of lead, mercury and other potentially harmful substances within the body. Since ISE is effective at picking up pollutants such as ammonium concentrations, they are commonly installed to monitor groundwater purity. Experts can use these devices to monitor agricultural run-off to help ensure that there are no harmful soil contaminants being leeched into natural systems.
Researchers apply the use of electrodes to selectively detect the quantity and type of pollutants that can be found in soil and other environmental compositions.
Modern selective membranes make it easier for experts to detect specific compounds that are highly toxic to the environment while reducing interference, which allows them to effectively formulate subsequent environmental action plans.
Many modern drinking water treatment systems include the use of fluoride. Fluoride is known to offer strong protection to teeth and gums due to its antibacterial properties which make it effective against a series of dental and oral issues. Fluoride detection through ISE is also practiced in medical laboratory tests via blood serum samples, which can offer insights on the breakdown of constituents, which provide an assessment of overall health.
ISE remains as one of the most effective methods of fluoride testing since it is nondestructive and allows samples to be reused upon testing. AG Scientific is a leading provider of biochemical components catered to the advancement of the life sciences.
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