Ion specific electrode meters are instruments which measure ionic activity within a sample.
Ion specific electrode (ISE) meters are millivolt meters which interface with ion selective electrodes (also known as specific ion electrodes [SIE]). These devices operate according to principles similar to those used by pH instruments; in fact, pH electrodes are simply ion selective electrodes which specifically respond to H+ ions within a solution.
Activity vs. Concentration
While ISE meters are considered to measure ion concentration, in reality they measure thermodynamic activity, which is sometimes referred to as "effective concentration." Activity is important when determining ion concentration because ion activity, more so than most molecules within a solution, is profoundly influenced by the surrounding solution. In overly simplistic terms, "activity" is used to describe the effective concentration of a molecule within a real substance, while true concentration can only be used when referring to ideal substances.
ISE meters and probes are designed to selectively detect specific ions, but other ions may also be inadvertently detected as well. These interfering ions are sometimes referred to as interferents, while the ions to be detected are referred to as determinants.
Manufacturers refer to the preference of an electrode for the determinant as selectivity; when expressed as a ratio it is referred to as the selectivity coefficient. For example, an electrode which is designed to sense potassium ions may have a selectivity coefficient of 385:1 over sodium ions, meaning the electrode is 385 times more selective to K+ than it is to Na+.
Measurement and Applications
Ion specific electrode meters may be configured to measure one of several different types of ion, depending on the electrode used. Commonly-measured elements include potassium, ammonium, calcium, sodium, lithium, nitrates/nitrites, chloride, and iodide, among others. Most manufacturers specify measurement ranges in parts per million (ppm), a common unit of concentration.
ISE meters are generally used in many different types of chemical testing applications, while measurement of fluoride, chloride, potassium, and other ions is common in water quality testing. Because ISE meters measure ion activity and can update in real time, they are often employed in biochemical and biophysical research.
Like many water quality instruments, ISE meters may also be configurable to measure other qualities using different probes, including pH, hardness, conductivity, oxygen reduction potential (ORP), and dissolved gases.
Meter Construction and Operation
Ion selective electrode meters are relatively simple voltmeters which accept inputs from two electrodes: a sensing electrode and a reference electrode. These two electrodes may be built into the same probe or use two separate probes.
The sensing electrode has a specialized membrane designed to be permeable only by the ion to be measured. Each electrode contains an internal element, typically made of silver, submerged within an electrolyte solution. The reference electrode is similarly-constructed, but does not include a permeable membrane in order to maintain a constant reference voltage. As ions are exchanged through the membrane, the silver element senses the ion activity and, in conjunction with the reference electrode voltage, outputs an electrical potential to the meter.
A sensing electrode (fluoride selective) and external reference electrode. The black arrows indicate voltages which are sent to the meter.
Image credit: University of Vermont
ISE meters may take on different form factors. Selection of these form factors depends upon the intended application of the meter.
Meters can be primarily classified into two form types: benchtop and portable. Benchtop units are designed to sit on a flat surface, while portable meters are handheld and designed to be easily carried for field testing. Both use external probes attached to the unit.
A benchtop meter (left) and a portable meter.
After receiving electrical potential data, an ISE meter may use one of several different calculation methods to determine ionic activity. Specific ions and solutions often respond better to certain methods in terms of accuracy and ease of use; example tests are listed below.
Direct potentiometry: ammonia in water, bromide, calcium in soil or water, chloride, cyanide, fluoride, nitrates, sulphides.
Incremental: most ions in wine and beer, potassium, chlorides in serum, sulphates, fluorides, silver.
- Titration: arsenic, cadmium, sulphates.
Direct potentiometry is the most common analytical method for determining ion concentration. This method relies on the Nernst equation, which governs the potential difference across an ion specific membrane and a reference electrode:
E = potential
E0 = ISE-specific constant
R = gas constant
T = temperature
n = charge of ion
F = Faraday constant
In simpler terms, the equation states that a measured potential difference is proportional to the logarithm of the ion concentration. By measuring a number of solutions with known ionic concentrations, the relationships between potential and concentration can be easily plotted, making it possible to determine the concentration of an unknown substance using this data.
Most modern microprocessor-based meters are preprogrammed with direct concentration data, thereby making manual plotting obsolete.
A typical ISE calibration curve for chloride. This data is preprogrammed into modern digital meters.
Image credit: San Jose State University
Incremental techniques are used when when a number of disparate measurements must be made with different electrodes. In this case, the multiple calibration attempts required to perform accurate direct potentiometry measurements would be cumbersome.
An example of incremental analysis is the known addition method. In this technique, the potential of a known volume of an unknown solution is measured. A small volume (around 10% of the unknown solution) of known solution is then added to the unknown one and the electrode potential is remeasured. The potential difference (ΔE) can then be found using the formula below.
CU = concentration of the unknown substance
CS = concentration of the known substance (standard)
VS = volume of the standard
VU = volume of the unknown substance
ΔE = change in electrode potential (in mV)
S = slope of the electrode
Other incremental methods include known subtraction, sample addition, and sample subtraction, all of which depend on the potential difference caused by manipulating known and unknown substances.
Because incremental methods require only one standard solution and two discrete measurements, they are useful for one-off measurements and for testing unstable substances. A major drawback, however, is the fact that the approximate concentrations of the unknown substance must be known in order to select an appropriate standard before undertaking any measurements.
ISE measurement using the two methods described above is relatively imprecise and typically results in accuracy of within 2-8%. For some measurements, titration techniques can shrink this value to around 1% accuracy.
The titrimetric method involves the addition of a reagent to the unknown substance. As the reaction progresses and the reagent bonds with the ions to be measured, the ions will fail to react with an ion specific electrode. At the endpoint of the reaction, a sudden change in electrical potential occurs which registers on the electrode. At this point, the volume of the reagent can be determined and applied to calculate the unknown concentration.
Potentiometric titration is fairly simple to automate and requires specialized electrodes for measurement.
A typical potentiometric titration curve for sodium hydroxide. Note the potential spike at the endpoint of titration.
Image credit: Periodni / E. Generalic
Standards are frequently employed when using or calibrating ion specific electrode meters. Example standards include:
ASTM D4127 - Standard terminology used with ion-selective electrodes
BS 7310 - Specification for ion-selective electrodes [...] and ion-selective electrode meters for determination of ions in solution
ISO 9517 - Iron ores - determination of water-soluble chloride by ion-selective electrode method
Chemical Sensors Research Group - Ion selective electrodes
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