Ion Selective Electrodes (ISE)

Background -- Ion Selective Electrodes (ISE) are membrane electrodes that respond selectively to ions in the presence of others. These include probes that measure specific ions and gases in solution. The most commonly used ISE is the pH probe. Other ions that can be measured include fluoride, bromide, cadmium, and gases in solution such as ammonia, carbon dioxide, and nitrogen oxide.

The use of Ion Selective Electrodes in environmental analysis offer several advantages over other methods of analysis. First, the cost of initial setup to make analysis is relatively low. The basic ISE setup includes a meter (capable of reading millivolts), a probe (selective for each analyte of interest), and various consumables used for pH or ionic strength adjustments.

Also see the newest ICE used with PDA.

The expense is considerably less than other methods, such as Atomic Adsorption Spectrophotometry or Ion Chromatography. ISE determinations are not subject to interferences such as color in the sample. There are few matrix modifications needed to conduct these analyses. This makes them ideal for clinical use (blood gas analysis) where they are most popular; however, they have found practical application in the analysis of environmental samples, often where in-situ determinations are needed and not practical with other methods. A large number of indicator electrodes with good selectivity for specific ions are based on the measurement of the potential generated across a membrane. Electrodes of this type are referred to as ion-selective electrodes. The membrane is usually attached to the end of a tube that contains an internal reference electrode. This membrane electrode and an external reference electrode are then immersed in the solution of interest. Since the potentials of the two reference electrodes are constant, any change in cell potential is due to change in potential across the membrane.

Different membrane materials have proved to give optimal responses for certain species. For example, a glass membrane is unsurpassed for measuring H⁺ activity, pH. This ISE may be referred to as a glass or pH electrode.

Liquid membrane electrodes have non-glass, solid-state crystals or pellets as the membrane component of the electrode. This approach has proved effective for numerous cations and anions. The most successful example is the excellent electrode for fluoride ion, which is based on a crystal of LaF₃ doped with Eu(II) to create crystal defects to improve conductivity.

Gas-sensing electrodes or combination electrodes that respond to the concentration of gases dissolved in aqueous solution. The electrodes consist of an ion-selective electrode, usually pH, in contact with a thin layer of solution that is held in place with a membrane permeable to the desired gas such as NH₃ or CO₂. When the gas passes through the membrane, the change of pH in the thin layer of solution is sensed by the glass membrane pH electrode.

Other combination electrodes consist of an enzyme immobilized on an ISE. The ISE is chosen to respond to a product of the enzyme-substrate reaction and the selectivity is provided by the enzyme.

How do they work or what is an Ion-Selective Electrode? An Ion Selective Electrode measures the potential of a specific ion in solution. (The pH electrode is an ISE for the Hydrogen ion.) This potential is measured against a stable reference electrode of constant potential. The potential difference between the two electrodes will depend upon the activity of the specific ion in solution. This activity is related to the concentration of that specific ion, therefore allowing the end-user to make an analytical measurement of that specific ion. Several ISE's have been developed for a variety of different ions.

How Does the mV Reading Correspond to the Concentration?  Standard solutions of known concentrations must be accurately prepared. These solutions are then measured with the pH/mV meter. The mV reading of each solution is noted and a graph of concentration vs. mV reading must be plotted. Now the unknown solution can be measured. The mV value of the unknown solution is then located on the graph and the corresponding solution concentration is determined.

Ion Selective Electrodes (including the most common pH electrode) work on the basic principal of the galvanic cell (Meyerhoff and Opdycke). By measuring the electric potential generated across a membrane by "selected" ions, and comparing it to a reference electrode, a net charge is determined. The strength of this charge is directly proportional to the concentration of the selected ion. The basic formula is given for the galvanic cell:  

Ecell = E_ise - E_ref

the potential for the cell is equivalent to the potential of the ISE minus the potential of the reference electrode.

Calibration -- Direct - The electric potentials are determined for a series of standards and a standard curve is developed. Additional analyses are fit to the standard curve in order to determine concentration. Direct calibration is the most common and easiest way to measure concentrations.

Standard Additions - The use of standard additions (the addition of known amounts of a standard) allows the use of the electrode in very complex matrices, without the need for direct calibration prior to measurement (Covington).

Titration's - ISEs have also been used as detectors for titration's (Orion). Titration methods use a titrant (such as EDTA) which will complex or react with the ion to be analyzed. The concentration of the ion in the sample is back calculated from the volume of the titrant used in the titration.

Membranes -- The nature of the membrane determines the selectivity of the electrode. A membrane is considered to be any material that separates two solutions. It is across this membrane that the charge develops. The term "membrane" is often confuse as implying permeability. While this is true in many cases, the term here is used denote any material which the charge can develop across (Covington).

Several types of sensing electrodes are commercially available. They are classified by the nature of the membrane material used to construct the electrode. It is this difference in membrane construction that makes an electrode selective for a particular ion.

1. Polymer Membrane Electrodes (Organic Ion Exchangers and Chelating Agents) -- Polymer membrane electrodes consist of various ion-exchange materials incorporated into an inert matrix such as PVC, polyethylene or silicone rubber. After the membrane is formed, it is sealed to the end of a PVC tube. The potential developed at the membrane surface is related to the concentration of the species of interest. Electrodes of this type include potassium, calcium, chloride, fluoroborate, nitrate, perchlorate, potassium, and water hardness.     

2. Solid State Electrodes (Insoluble Conductive Inorganic Salts) -- Solid state electrodes utilize relatively insoluble inorganic salts in a membrane. Solid state electrodes exist in homogeneous or heterogeneous forms. In both types, potentials are developed at the membrane surface due to the ion-exchange process. Examples include silver/sulphide, lead, copper(II), cyanide, thiocynate, chloride, and fluoride.

3. Gas Sensing Electrodes -- Gas sensing electrodes are available for the measurement of dissolved gas such as ammonia, carbon dioxide, nitrogen oxide, and sulfur dioxide. These electrodes have a gas permeable membrane and an internal buffer solution. Gas molecules diffuse across the membrane and react with a buffer solution, changing the pH of the buffer. The pH of the buffer solution changes as the gas reacts with it. The change is detected by a combination pH sensor within the housing. Due to their construction, gas sensing electrodes do not require an external reference electrode.

4. Glass Membrane Electrodes -- Glass membrane electrodes are formed by the doping of the silicon dioxide glass matrix with various chemicals. The most common of the glass membrane electrodes is the pH electrode. Glass membrane electrodes are also available for the measurement of sodium ions.

Sources of Error -- Diffusion - Orion Research points out that differences in the rates of diffusion of ions based on size can lead to some error. In the example of Sodium Iodide, sodium diffuses across the junction at a given rate. Iodide moves much slower due to its larger size. This difference creates an additional potential resulting in error. To compensate for this type of error it is important that a positive flow of filling solution move through the junction and that the junction not become clogged or fouled.

Sample Ionic Strength - Covington points out that the total ionic strength of a sample affects the activity coefficient and that it is important that this factor stay constant. In order accomplish this, the addition of an ionic strength adjuster is used. This adjustment is large, compared to the ionic strength of the sample, such that variation between samples becomes small and the potential for error is reduced.

Temperature - It is important that temperature be controlled as variation in this parameter can lead to significant measurement errors. A single degree (C) change in sample temperature can lead to measurement errors greater than 4%.

pH - Some samples may require conversion of the analyte to one form by adjusting the pH of the solution (e.g. ammonia). Failure to adjust the pH in these instances can lead to significant measurement errors.

Interferences - The background matrix can effect the accuracy of measurements taken using ISE's. Covington points out that some interferences may be eliminated by reacting the interfering ions prior to analysis.

(2) "Methods for Chemical Analysis of Water and Wastes," Environmental Protection Agency, Environmental Monitoring Systems Laboratory, EPA-600/4-79-020, 1983.

(3) "Standard Methods for Water and Wastewater Analysis," 18th Edition, 1994.

(4) Official Methods of Analysis of the Association of Official Analytical Chemists, Methods Manual, 15th edition (1990).

(5) Methods for Analysis of Inorganic Substances in Water and Fluvial Sediments, U.S. Dept. of the Interior, Techniques of Water Resource Investigations of the U.S. Geological Survey, Denver, CO, Revised 1989.

(6) "Test Methods for Evaluating Solid Waste, Physical/Chemical Methods", SW-846, Update III.

Agitation -- When carrying out selective ion measurements, it is important to have good agitation. This allows a fresh supply of ions to be exposed to the sensing portion of the ISE. It is best to select a speed that keeps a constant, smooth motion. A turbulent rate should be avoided.

pH Adjustment -- In many cases pH control is necessary for accurate, repeatable measurements. Certain ions exhibit different activity when different concentrations of hydrogen ions are present in solution. This occurrence will not only alter the potential due to the specific ion that is measured, it may also allow other ions in solution to become active that otherwise were not. This increased activity from the other ions will interfere with the ability to evaluate the ion of interest.

Response Time -- ISE's require a much longer time for the readings to stabilize. At least fifteen minutes should be allowed for equilibrium to be established when measuring standard solutions.

Establishing a Calibration Curve -- It is recommended to use three standard solutions when establishing a calibration curve. To choose the concentrations of the standard solutions it is helpful to know the approximate values of the unknown solutions. For example, if the unknown solutions are in the 100 ppm range, the choice of standards may include a 10 ppm, a 100 ppm, and a 1000 ppm solution.

Rinsing -- It is necessary to rinse the ISE between measurements to insure accurate readings. Use a steady stream of deionized or distilled water. Take care not to rub the electrode with a cloth to dry the probe. It is usually best to "shake off" any excess water. Take care not to hit the probe against anything while shaking the electrode.

Conditioning -- The ISE needs to remain moist at all times even when not in use. Consult the operator's manual that accompanies the electrode for details on cleaning, conditioning, and storing the ISE.

General Comments on Ion-Selective Electrodes:

What do some ISE look like?

Various ion selective electrodes by one manufacturer are shown at the left. The pHoenix Electrode Company. All of their ISE's are now available in both glass and plastic body combination electrodes. pHoenix Electrode Company is a manufacturer of electrochemical analytical sensors as well as Vernier and hand held data capture devices. They specialize in manufacturing pH, ORP, Conductivity, Oxygen and Ion Selective Electrodes designed for Laboratory, Industrial, Biotechnology and Medical applications.

Covington, AK. "Introduction: Basic Electrode Types, Classifications, and Selectivity Considerations." In. Covington, AK (ed.), Ion Selective Electrode Methodology. Volume 1. CRC Press. Boca Raton. 1-20.

Frant, MS. 1994. "History of the Early Commercialization of Ion-Selective Electrodes." Analyst 199, 2293-2301.

Meyerhoff, ME and WN Opdycke. 1986. "Ion Selective Electrodes." Advances in Clinical Chemistry 25, 1-47.

Orion Research, Inc. 1997. Web Site Index. http://www.orionres.com/. or ORION ISE Bibliography