An ion-selective electrode (ISE) is a type of sensor used to determine the concentration of specific ions in an aqueous solution by converting the activity of that ion into an electrical potential which can be measured by a voltmeter. ISE is an important element of water quality analysis. Depending upon the electrode, ISE meters are capable of identifying and measuring the concentration of a wide variety of chemical elements or compounds including fluoride, bromide, cadmium, and gases in solution such as ammonia, carbon dioxide, and nitrogen oxide.
The use of ion-selective electrodes offer several advantages over other methods of analysis such as photometry. When compared to other analytical techniques, ion-selective electrodes are inexpensive, fast, portable and easy to use. A complete system consists of a meter, ion-specific probes and an assortment of solutions including reference solutions, ionic strength adjusters and storage solutions. These can all typically fit in an easy-to-carry kit for portability. Plastic-bodied solid-state or gel-filled electrodes are robust and durable and testing can consist of simply lowering the probe into a lake, river or other sample. ISE meters excel in applications requiring constant monitoring.
That is not to say ion-specific electrodes don’t have limitations. Though with careful use they can achieve level accuracy levels of 2-3%, they can be prone to interference from other ions in the solution. Additionally, ISE meters are prone to drift during a sequence of measurements and may also be exposed to the vagaries of ionic strength which can result in loss of accuracy unless conditioners are used.
ISE meters are an outstanding choice for a wide range of applications including pollution monitoring, agriculture, food processing, manufacturing (for studying effects on water quality), electroplating, biomedical laboratories, education and research.
ISE Meter Technology
Ion selective electrodes operate on the principle of measuring the electrical potential of specific ions in solution as they move from an area of high concentration to an area of low concentration across a selectively permeable membrane.
ISE meters consist of two electrodes, one sensing electrode-- often called a half-cell-- and one reference electrode. It is very common for the sensing and reference electrodes be combined into a single combination electrode. The reference electrode is filled with an electrolyte of a known value which keeps its ionic activity constant.
The sensing electrode includes a specialized membrane that only allows specific ions to pass through. The sensing electrode of a pH meter, for example, only allows hydrogen ions to pass. Once immersed in a solution, the selected ions pass through the membrane until both side of the membrane have reached equilibrium. The concentration of ions that have passed through the membrane is compared to the constant in the reference electrode. The difference between the sensing electrode and the reference electrode is converted into a milliamp signal. According to the Nernst equation, the concentration gradient is proportional to the electrical gradient.
It’s a bit of a misnomer to refer to “ionic concentration” as ISEs measure ionic activity NOT concentration. Ionic activity depends upon ionic strength. If ionic strength is high then concentration is proportional to activity. Ionic strength adjusters are used to boost the strength of low ionic activity solutions making the test results better mirror concentration.
Ion selective electrodes are available in a wide variety of styles with each manufacturer incorporating their own elements of construction or composition. There are, however, certain features found in all ISEs. All are cylindrical tubes, usually made of plastic. The ion selective membrane fixed at one end so that the external solution can only come into contact with the outer surface, and the other end is fitted with a connection to the millivolt measuring device. Internal connection are complete with the use of a liquid or gel electrolyte or a solid-state system.
The range of ISE types is wide and the specifics of how they work is very complex. Most ISEs, however, are one of the following styles:
Glass Electrodes
Glass electrode ISEs feature a doped glass membrane that is sensitive to a specific ion. Commonly used for pH measurements, glass electrodes can also be formulated for use with other ions including lithium, sodium, ammonium, and others.
Solid State Electrodes
Solid state electrodes can be found as both a half-cell or as a combination electrodes including the reference electrode. These electrodes incorporate a solid sensing surface made of compressed silver halides, or solid crystalline material.
Liquid Membrane Electrodes
Liquid membrane electrodes can be found as both a half-cell or as a combination electrodes including the reference electrode. The sensing surfaces of these electrodes are comprised of a homogeneous polymer matrix containing organic ion-exchangers selective for the determined ion. These sensors incorporate easily replaceable membrane modules and are available for measurements of nitrate, potassium and calcium.
Methods of ISE Analysis
ISE can be analyzed by way of three different methods. The actual steps are not overly important as they vary somewhat by manufacturer and each meter will walk you through the process during testing.
Direct Potentiometry
Direct potentiometry is the simplest and most widely used method of using ISEs. This method is very effective when large batches of samples covering a wide range of concentrations need to be measured. Using direct potentiometry this can be done without having to change range, recalibrate or make any complicated calculations.
Direct potentiometry involves calibrating the instrument with two or more standards and using an ionic strength adjuster to both the sample and the standards. Beyond that, it is simply measuring the electrode response to the sample and reading the concentration directly from the calibration graph or meter display.
Depending upon the application, direct potentiometry can produce acceptable results by simply dangling the electrodes into a river or pond or effluent outflow without the need to take samples back to the lab.
Incremental Methods
Incremental methods are four techniques useful for quickly determining the ion concentration in samples with variable or concentrated constituents. The four techniques: known addition, known subtraction, analyte addition, and analyte subtraction all involve either adding a standard to the sample, or a sample to the standard. The ISE then calculates the sample ion concentration of the test sample.
Incremental measurement methods have some inherent advantages over direct potentiometry. During the testing process, for example, the electrodes remain immersed in the test solution. This prevents change in the liquid junction potential of the reference electrode, which can vary be several millivolts when moved from one solution to another. Cross contamination between samples is also eliminated.
Calibration and sample measurement are both made essentially at the same time and in the same solution so that ionic strength, errors due to viscosity, and temperature differences between standard and sample are not significant and potential errors stemming from these conditions are reduced.
Potentiometric Titration
Titration is the slow addition of one solution of a known concentration (called a titrant) to a known volume of another solution of unknown concentration until the reaction reaches neutralization. It is a technique to determine the concentration of an unknown solution and can be used to increase the precision of ISE measurements.
Titration can also be used to extend the range of ions measurable by ISEs which can’t be measured by other methods such as direct potentiometry. It is also a valuable tool when handling unstable or toxic substances. In these instances, a dangerous or immeasurable solution is mixed with a titrant which renders the solution safe or measurable.
Care and Maintenance of ISEs
The nature of ISEs requires they be handled with some care. Depending upon the type of electrode, the membrane surface may be delicate and should be treated with care to avoid damage. After extensive use membranes can become coated with residue or covered with fine scratches which adversely affect the performance of the electrode.
After use, the electrode should be rinsed and allowed to dry. For prolonged storage a protective cap should be used to protect the membrane. Cleaning solutions are available which can remove built-up deposits. Depending upon the ISE, the reference electrode may include an electrolyte which needs to be refilled periodically.
All glass electrodes have a bulb which must be kept hydrated and a reference junction which must be kept wet to prevent excess leakage of the internal electrolyte solution from the reference junction. It is recommended keeping the electrodes in a storage solution between uses.
If you have any questions regarding ISE meters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
