How do ion selective electrodes work?

Ion selective electrodes allow us to measure the activity of a specific ion in a sample, by measuring the potential of a measuring cell which has a membrane that is specific to a certain ion, against the potential of a reference cell. They allow fast and accurate measurements, and are practical across a range of applications. The oldest ISE available is the pH electrode, which is selective for the hydrogen ion and is widely used across the world. Other ISEs later became available and these ISEs can provide us with accurate information about the concentration of ions present in our sample.

The membrane of each ion selective electrode is different, as they are all selective to a specific ion. When placed into a sample which contains this ion, a proportional voltage develops in relation to the stable voltage of the Ag/AgCl reference. In order to get a measurement from an ISE, the sample to be tested should be water based and within the measuring range given by the specification. There are certain other criteria that should be met, such as pH range and absence of interfering ions.

For more specification and criteria information, take a look out our Combination and Mono ISE data table or download the manual for the ISE of interest.

What equipment is needed to make a measurement?
Ion Selective Electrode - How Do Ion Selective Electrodes Work?

Sentek offer a range of ISEs, which can be found in Combination and Mono ISE data table. They are available as monos, which would require a separate reference electrode, or combination electrodes which house the measuring cell and reference cell in one body. Our electrodes need connecting a conventional pH meter with mV mode, and the measurement can be calculated manually, or alternatively an ion meter can be used which converts the mV value into concentration.
The following methodologies are often used in ISE analysis:

  • Direct Potiometry: This is the simplest method, involving using standard(s) of known value to form a calibration curve (log of concentration vs. millivolt), and reading the mV potential of the sample from this graph to give a concentration. This method is simplified if an ion meter is used.

For instances in which the matrix is difficult or the sample is not entirely clean or aqueous, the following incremental techniques can be used:

  • Known addition: The potential of the sample solution is measured, followed by the addition of a small volume of a higher concentration standard solution. The new potential is measured and using the known electrode slope, the unknown value is determined from the difference between the two values.
  • Sample addition: Similar to known addition, however the potential of a standard solution is first measured, and then a small volume of sample is added.
  • End point titration: ISEs can show a significant change in potential at the equivalence point of titration. The ion being measured must be contained in the titrand or titrant and must therefore be in excess of absence at the end point.

More information about these methods can be found in the manual for each ISE.

To make a measurement, calibration standards and ISAB are usually required. The standards are of a known value, usually a decade apart (e.g., 10ppm and 100ppm). The ISAB is used to ensure the standards and sample have similar ionic strength.

What are ISEs used for?

ISEs have a range of uses in many industries, including water analysis, agriculture, food production and waste control. Common examples include determination of fluoride in medical applications, or measurement of ammonia in waste water so that regulations are met. Widely used ISEs include Chloride, Fluoride, Nitrate, Sodium, Sulphide, Ammonia and Ammonium, and these are used in a vast range of both laboratory and industrial settings.

Advantages of ISEs

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. Our solid state ISEs can be left dry for long periods of time and don’t require refilling by the user.

Considerations

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. For full details of interferents for each sensor see our  Combination and Mono ISE data table. There are a few ways to deal with interferences, which include chemical elimination (e.g. adding Barium Chloride to remove sulphate ions). Also, incremental techniques such as known addition can be used.

Here at Sentek we’ve been producing and supplying a range of electrochemical sensors since 1991. 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 9001:2015.

Our dedicated team at Sentek understand the importance of accurate and efficient measurements. That’s why we employ skilled professionals and adhere to stringent quality standards to deliver a quality product, globally. Contact us for more technical information or specialist advice.