By Stuart R. Lynde


Table of Contents


Introduction

Ion Selective Electrodes (ISE) are membrane electrodes that respond selectively to ions in the presense of others. These include probes that measure specific ions and gasses in solution. The most commonly used ISE is the pH probe. Other ions that can be measured include metals (Floride, Bromide, Cadmium, and Cupric to name a few) and gasses in solution such as ammonia, carbon dioxide, nitrogen oxide, and oxygen.

The use of ISEs 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 (Figure 1). This is considerably less expensive than other methods, such as Atomic Adsorption Spectrophotometry (AAS) or Ion Chromatography (IC). 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.

Figure 1. The basic ISE setup requires only a probe, a meter capable of reading millivolts, and a few additional reagents for controling the ionic strength and pH of the sample.

The choice of methods should be dependant on your data needs. While ISEs do not typically have the same linear range, sensitivity, or detection limits as other methods, they are ideal in samples where matrix modifications or color are not practical. Many of these methods are suitable for field use, while others are not.

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History

Credit for the first glass sensing pH electrode is given to Cremer, who first described it in his 1906 paper (Meyerhoff and Opdeycke). In 1949, George Perley published an article on the relationship of glass composition to pH funtion (Frant). In the interim there were numerous papers dealing with various formulations of and several important contributions were made (Covington). The commercial development of ISE began when an engineer by the name of John Riseman thought he could develop a usefull bloodgas analyzer. He teamed up with Dr. James Ross, an electrochemist from MIT. Together they formed Orion Research (Frant). By the mid 1960s, the newly formed Orion Research Inc was producing Calcium electrodes for use in blood gas analyzers (Frant, 1994). Since then numerous probes have been developed for the analysis of samples containing many different ions.

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How do they work?

Ion Selective Electrodes (including the most common pH electrode) work on the basic pricipal 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 (Figure 2). 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 = Eise - Eref

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

Diagram of half-cell system

Figure 2. The above figure demonstrates the flow of current through the reference electrode to the ISE, completing the circuit. Modified and reproduced with permission from Orion Research.

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Calibration

Direct - The electric potentials are determined for a series of standards and a standard curve is developed. Additonal 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 (Orion), without the need for direct calibration prior to measurement (Covington).

Titrations - ISEs have also been used as detectors for titrations(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.

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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 (Orion Research). 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. Figure 3 shows different types of electrodes and some of the analytes they are used for.

Figure 3. This figure shows some of the analytes that can be measured and an image of the type of electrode used to measure it. Modified from images used with permission from Orion Research Inc.

Glass -- Hydrogen ion responsive glass is the earliest material used in ISEs (Covington). Probes made with this type of membrane are used for the determination of pH and Sodium.

Insoluable Inorganic Salts - This type of membrane is composed of a salt that has a low soluability and is electrically conductive (Covington). Probes using this type are commonly referred to as "solid state" electrodes and are typically used to measure ions like Flouride, Bromide, Cadmium, Chloride, Cupric, Cyanide, Iodide, Lead, Silver, Sulfate, and Thiocyanate (Orion).

Organic Ion Exchangers and Chelating Agents - These agents are incorporated into pvc membrane material (Covington). They selectively bind certain small ions from the aqueous phase and exchange them across the membrane creating the potential. Some of the analyte measured with probes using this type of membrane include Ammonium, Calcium, Chloride, Flourobarate, Nitrate, Perchlorate, Potassium, and Water hardness.

Gas Permeable -- This type of membrane is used to measure dissolved gasses such as Ammonia, Carbon Dioxide, Nitrogen Oxide and Dissolved Oxygen. Gas molecules diffuse across the membrane and react with a buffer solution, changing the pH of the buffer. This pH change is measured using an internal glass electrode (Orion). This is probably the second most popular probe type after the glass membrane pH probe.

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Sources of Error.

Diffusion - Orion Research points out that diferences 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 juction 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 juction and that the juction 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 controled 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% (Orion).

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 signifiacant measurement errors.

Interferances - The background matrix can effect the accuracy of measurements taken using ISEs (Orion). Covington points out that some interferences may be eliminated by reacting the interfering ions prior to analysis.

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Approved Methods for using ISE

Standard methods for sample analysis using Ion Selective Electrodes are published by several agencies. These include the American Society of Testing and Material (ASTM), United States Environmental Protection Agengcy (EPA) , American Public Health Association (APHA), Association of Analytical Chemists (AOAC), and the United States Geological Survey (USGS).

Table 1. Approved Methods for using Ion Selective Electrodes. (reproduced with permission from Orion Research, Inc)

Species Measured ASTM EPA APHA AOAC USGS
ACIDITY D1067-88 305.1, 305.2 2310B 984.24 I-1020
ALKALINITY D1067-88 310.1 2320B 973.43 I-1030
AMMONIA D1426-89 350.3 4500-NH3 (F), (G)   I-1524
BROMIDE D1246-88 60 FR 37974 (6)*      
CARBON DIOXIDE D513-88A        
CHLORIDE D512-89 60 FR 37974 (6)* 4500-CL (D) 971.27, 980.25  
CHLORIDE-
LEACHABLE
F1277-90        
CHLORIDE BY
TITRATION
      962.05, 962.07,
963.05, 966.10,
969.10
 
CHLORIDE TOTAL
IN COAL
D4208-88        
CHLORINE-RESIDUAL   59 FR 62456* 4500-CL (I)    
CHLORINE IN ORGANICS E256-91        
CONDUCTIVITY D1125-91 120.1 2510B    
CYANATE     4500-CN (L)    
CYANIDE D2036-89A 60 FR 37974 (6)* 4500-CN (E) (F)    
FLUORIDE D3868-79
D1179-88B
D1179-88A
59 FR 62456
60 FR 37974 (6)*
4500-F (C) 984.37
975.08
973.10
I-1327
I-2327
FLUORIDE IN AIR D3269
D3268
D4765
       
FLUORINE IN COAL D3761-91        
IODIDE D3869        
KJELDAHL NITROGEN D3590-89A 351.4 4500-Norg (A) (B)    
NITRATE   59 FR 62456
60 FR 37974 (6)*
4500-NO3 (D) (G) 986.31  
ORP D1498-76   2580    
OXYGEN D888-87D 360.1 4500-O (G)    
POTASSIUM     3500-K (E)    
SALINITY     2520B    
SODIUM D2791     976.25  
SULFIDE D4658 60 FR 37974 (6)*      
SULFUR IN COAL D4239-94        
pH D1293-84 150.1 4500-H 981.12, 943.02,
945.10, 950.07,
960.19, 962.19,
970.21, 973.04,
973.40, 973.41,
979.02, 981.12
I-1586
pH TITRATIONS       949.02, 955.01,
960.19
 
(1) "Annual Book of ASTM Standards, Water and Environmental Technology," American Society for Testing and Materials, (1992). ASTM Home Page

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

(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). AOAC Home Page

(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. USGS Home Page

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

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Links to Other ISE Sites

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More Information?

Follow this link to connect to an extensive bibliography of ISE related publications supplied by Orion Research.

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References

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/. Orion Research, Inc.

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Acknowledgements and Disclaimer

This page is the product of research conducted as part of an Environmental Analysis Class. All images were generated by the author or were modified by the author with permission from the source.


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Send comments or suggestions to:
Student Author: Stuart R. Lynde, slynde@vt.edu
Faculty Advisor: Andrea Dietrich, andread@vt.edu
Copyright © 1997 Daniel Gallagher
Last Modified: 09-10-1997