An ion selective electrode (ISE) is a type of membrane-based potentiometric device that’s designed to measure ion activity in a solution. This analytical technique allows you to measure the electrical potential of the solution. There are many reasons why ISE is preferred over other methods, which include the following:
- It’s easy to operate and somewhat affordable
- The concentration measurement range is wide enough to obtain many different readings in an aqueous solution
- You’ll receive a real-time measurement, which allows you to identify any activity changes as they occur
- Unlike many sensors, it measures activity instead of concentration
- You can use this device to measure negatively and positively charged ions
ISE devices are effective when paired with medical, agricultural, and industrial applications. Measuring ion activity is essential in numerous fields. In this guide, you’ll learn about ion selective electrodes and what these devices are typically used for.
How Ion Selective Electrodes Work
An ion selective electrode is a device that measures ion activity via electrical potential. Once an ion dissolves in a solution, the change in the concentration can be measured by this device. ISE devices come with an input signal as well as an output signal. The output signal is a specific quantity that’s registered by the device. You’ll receive a potentiometric signal when measuring ions in a solution. Keep in mind that the voltage depends on the logarithm of the specific ionic activity, which is based on the Nernst equation.
The Nernst equation indicates that the voltage across the membrane is a function of the electrode potential. The voltage depends on everything from the temperature in Kelvin and the universal gas constant to Faraday’s constant. Some of the additional factors to consider include the ionic activity and the charge number of the specific ion.
ISEs can be used to perform analysis in fields like chemistry, biology, agriculture, and environmental science. They are also paired with biochemical and analytical chemistry applications. The main components in an ISE include the ion-selective membrane as well as internal and external reference electrodes.
Types of Ion Selective Electrodes and Membranes
There are four basic types of ion selective membranes that are used in ISE devices, which include everything from glass to crystalline membranes.
Glass Membranes
A glass membrane is made to be used with an ion-exchange version of glass, which can be either chalcogenide or silicate. This ISE comes with good selectivity for single-charged cations like Ag+, H+, and Na+. If you select chalcogenide glass, it has selectivity for certain double-charged metal ions, which include Cd2+ and Pb2+. The membrane should offer high durability regardless of the chemicals it’s paired with. It can also be used in aggressive media.
When using a glass membrane, you must account for the alkali error range and the acidic error range. When the solution contains a low concentration of hydrogen ions, it will have a high pH value. Any contributions to measurements from interfering alkali ions like Li+ and Na+ are comparable with a single hydrogen ion. This effect becomes less noticeable when the pH is higher than 12 and the sodium or lithium concentration is lower than 0.1 mol/L.
The acidic error range occurs when the solution consists of a high concentration of hydrogen ions. In this scenario, the electrode’s dependence on the pH reading becomes non-linear. These effects are most noticeable when the pH is less than 1.0.
Crystalline Membranes
Crystalline membranes are made with poly- or monocrystallines of one substance. These membranes offer good selectivity. The only ions that can interfere with the response from the electrode are the ones that can enter the crystal structure. Unlike glass membrane electrodes, crystalline membranes don’t have an internal solution that lessens the potential junctions. It’s possible for crystalline membrane selectivity to apply to the anion and cation of the substance that forms the membrane. For example, the fluoride selective electrode is based on LaF3 crystals.
Ion-Exchange Resin Membranes
Ion-exchange resins use special organic polymer membranes that consist of an ion-exchange substance. This is the most common ISE on the market. By using specific resins in the design of the membrane, it’s possible to prepare selective electrodes for a wide range of different ions, which include multi-atom, and single-atom options. They also offer anionic selectivity. Keep in mind, however, that anionic electrodes have lower physical and chemical durability.
Enzyme Electrodes
Enzyme electrodes are used less than the other options because they aren’t true ISEs. They fall under the ISE category because of how the device is set up. This type of electrode comes with a double-reaction mechanism, which is an enzyme that reacts directly with a specific substance.
The result of this reaction will be detected by the true ISE, which could be a PH-selective electrode. These reactions take place in a special membrane that covers the ISE, which is why enzyme electrodes can still be considered ion selective. A glucose-selective electrode is a type of enzyme electrode.
Components of an Ion Selective Electrode Setup
The basic setup of ISE devices involves a voltmeter, internal and external reference electrodes, and an analyte solution. The ISE consists of an internal reference electrode that’s made from silver wire covered with solid silver chloride. It’s also embedded in a potassium chloride solution that has been saturated with silver chloride. The solution will contain the ions that need to be measured.
As for the reference electrode, it’s similar to the ion selective one. The main difference is that the selective membrane is replaced by fritted glass, which is a porous substance. The internal filling solution slowly passes through the porous glass to form a liquid junction with an external solution. The reference electrode connects to the ion selective one with a milli-voltmeter. You can measure the ionic activity by immersing the electrodes in the same solution.
How the Signal is Measured
When you use ion selective electrodes, you’ll need to compare the electrochemical potential that the ISE shows to the signal of an analyte. The ion selective membrane of your choice will only allow certain types of ions to pass through, which makes the membrane selectively permeable.
Every ISE measurement is made by comparing the internal reference electrode with a concentration of the analyte that’s being measured. The potential is identified with the formula that’s displayed below:
Ecell = Eise – Eref
Eise is the potential of the ion-selective membrane and the internal reference electrode. The ion-selective membrane is displayed as “Em”. The Eise is directly governed by the analyte activity that occurs in the internal solution. In comparison, the Em is controlled by the analyte’s activity on both sides of the selective membrane. The external reference part of the cell depends on the liquid junction potential as well as the half-reaction that the electrode has.
Applications of Ion Selective Electrodes
Ion selective electrodes have many applications. These devices are essential when measuring the activity of sodium, nitrate, and chloride, which is why they are regularly used in analytical chemistry. These electrodes will deliver accurate and selective measurements of the ions within specific solutions.
Let’s consider the chloride ISE. This device uses a solid-state membrane to detect chloride ions in many different solutions. When searching for this device, it’s also available as a polymer membrane that contains an ionophore selective to be used with chloride ions.
Chloride ISEs are utilized for clinical, environmental, and industrial applications. For example, this type of ISE can monitor chloride levels in urine and blood. You can also measure chloride in wastewater, soil, and water bodies, which means that these ISEs have environmental applications. Consider using this device to analyze chloride in food and beverage production or similar types of industrial processes.
A nitrate ion selective electrode is built with a polymer membrane that contains an ionophore selective. The ionophore selective is designed for nitrate ions. Once you immerse the device in a solution, the membrane will interact with surrounding nitrate ions and produce a voltage.
Nitrate ion selective electrodes are regularly paired with agriculture, water quality, and environmental applications. They can measure the amount of nitrate pollution that has impacted wastewater and other bodies of water. They can also monitor nitrate levels in fertilizer solutions and soil. These measurements can help you optimize plant growth as well.
Sodium ion selective electrodes use a glass membrane to effectively identify the concentration of sodium in a solution. In medical applications, these electrodes can monitor sodium levels in urine and blood, which helps with diagnosing health conditions. In industrial applications, you can use this device to measure sodium content in chemical manufacturing and food and beverage production.
There are also some very specific applications that rely on using ion selective electrodes, which include everything from biomedical science labs to fluoride detection. In biomedical science labs, ISEs are routinely used to detect quantifiable signals in skin samples, body fluids, and blood cultures. Most biomedical applications involve enzyme electrodes.
These electrodes are also crucial to medical researchers. ISEs help transfer ionic currents into the natural flow that occurs in the human body. Medical researchers can use this data to improve detection processes. For example, it’s possible to identify concentrations of mercury and lead with ISEs.
In environmental analysis, these electrodes can selectively identify the type and quantity of a pollutant in soil and other substances. The most advanced selective membranes allow experts to measure toxic compounds that can damage the environment. The results of these measurements allow researchers to develop effective and comprehensive environmental action plans.
Modern drinking water municipal systems commonly use fluoride to ensure people can protect their gums and teeth. Fluoride comes with strong antibacterial properties that help fight against many oral and dental issues. You can measure fluoride concentration with an ISE. This practice often occurs in medical lab tests with blood serum samples. The best aspect of using ISE devices with blood serum samples is that it’s a nondestructive process. The samples can be reused after the testing takes place.
ISEs are effective at identifying changes to pollutant concentrations, which is why they are often installed in groundwater and agricultural monitoring applications. Experts use these devices to measure ions in groundwater runoff to ensure that no harmful contaminants get into natural systems.
Calibration and Accuracy of Ion Selective Electrodes
When you use ion selective electrodes to measure a solution, it’s important that you calibrate it with known ion concentrations. Calibration curves can help you determine unknown concentrations.
To understand how calibration occurs, consider how fluoride is added to toothpaste and drinking water. At the right amount, fluoride can help prevent cavities and other dental issues, which is why it’s important to determine the concentration of the substance.
Based on the Nernst equation, you can measure the fluoride concentration by establishing a calibration curve. You’ll need to use an equation that looks like E = K + SlogC. The millivolt reading is represented by “E” in this equation. C is shown as milligrams per liter (mg/L).
At 200 mg/L, the log C is 2.301, which results in a millivolt (mV) reading of -35.6. Based on the calibration curve, a 25 mg/L solution will lead to a log C measurement of 1.396 and an mV reading of 16.8. A 1.563 mg/L solution returns an mV reading of 89.3. Use the calibration curve to ensure you obtain accurate readings.
There are many potential errors that can impact ion selective electrode measurements, which include the following:
- Membrane effects
- pH fluctuations
- The presence of different liquid junction potentials
- Buffer stability
- Interfering species
- Electrostatic forces
- Fluoride contamination
- Electrode temperature dependence
You can mitigate these errors by taking some precautions. For example, the severity of these effects can increase if the membrane deteriorates. Keeping the membrane in good condition can help you prevent these issues. It’s also important that you avoid chemical interference when taking measurements.
Limitations of Ion Selective Electrodes
While ion selective electrodes are useful for many applications, there are some limitations associated with these devices. Since the ion membrane is selective, it only allows the measured ions to pass through. In this scenario, the potential will only be determined by those ions. However, ISEs don’t always work as intended. Even though the ISE membrane is selective, it can allow multiple types of ions to pass through. The measured potential will be affected by the unwanted ions.
Since these devices are dependent on ion selective membranes, you can only measure one type of ion with an ISE. For example, a sodium membrane can’t detect any other ions. Ion selective electrodes are also limited by measuring the concentration of ions at the surface of the membrane, which might be a problem at higher ion concentrations.
In this situation, the ions might become less mobile, which means that the concentration near the surface of the membrane would be lower than in other locations. Let’s delve further into some of the problems that can impact ion selective electrodes and how to mitigate them.
Ionic Interference
Even though ISEs are meant to be used for one ion at a time, they are still sensitive to some other types of ions. In many applications, ionic interference is insignificant and doesn’t materially change the results. However, there are times when the electrode will be considerably more sensitive to the unwanted ion. When this occurs, you may need to reduce the concentration of the interfering ion until it’s absent or only present in trace amounts.
The interfering ion could be removed with chemical treatments like complexing or precipitation. However, both methods are time-consuming and should be avoided if possible. You may be tasked with increasing the primary ion in the solution to account for the heightened sensitivity of a secondary ion.
Activity and Ionic Strength Coefficients
You’ll also need to consider the activity and ionic strength coefficients. As mentioned previously, ion selective electrodes measure the concentration of a specific ion at the membrane surface. When using a dilute solution, this measurement will directly relate to the total number of ions that are present in the solution.
However, at higher concentrations, it’s possible for inter-ionic interactions between different ions in the solution to reduce mobility, which means that there will be fewer measured ions near the membrane surface. When this occurs, you’ll receive a lower measured voltage that doesn’t accurately reflect the number of ions in the solution.
Ionic strength measures the effect that all ions have on the solution. This measurement requires you to multiply the square of the valency of all ions by the total sum of the molar concentration. The concentration that’s measured at the head of the electrode is referred to as the activity of the specific ion.
As for the activity coefficient, you can obtain it by dividing the ratio of the activity by the concentration. Keep in mind that this factor varies and depends on the measured ion’s valency and ionic radius. It also changes based on the solution’s ionic strength.
When measuring the activity coefficient, you should receive a result that’s lower than 1.0. It will become smaller when the ionic strength increases. At high ion concentrations, the difference between the actual concentration and the measured activity becomes higher.
This effect makes ISE measurements less accurate in a couple of ways. For example, if you make a calibration graph with concentration units, the line will become less linear when the concentration increases. To account for this error, you must measure more calibration points to precisely define the curve.
The sample solution might also contain additional ions that you don’t want to measure, which can increase the solution’s ionic strength. If the sample’s ionic strength is much higher than the standard, the calibration line and measured samples could be incompatible.
You could overcome this issue by calculating the main ion’s activity coefficient in a pure solution where you know the relative concentration and composition of all ions. You can then convert the measured activity into concentration. However, this isn’t possible for most practical applications.
Conclusion
From food and beverage production to wastewater treatment, ISEs can be paired with many applications. They are highly useful in multiple fields, which is why many industrial and municipal facilities continue to use these devices despite their limitations.
Before you begin capturing ion activity and concentrations with ISEs, you must understand how to properly use them. Knowing the limitations of these devices should help you obtain accurate readings. While there are some drawbacks to using ion selective electrodes, the technology should improve in the coming years.
Source link: https://sensorex.com/ion-selective-electrodes/ by Joshua Samp at sensorex.com