Improving pH Measurement in Drinking Water Monitoring

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Safe drinking water is critical for a population’s health and well-being, whether delivered through large municipal systems or community provision. Ensuring a reliable supply requires both the careful management of water sources and effective treatment and disinfection of water extracted for human consumption. While there are many stages in the water treatment process, each reliant on a range of analytical and measurement techniques, one of the most important operational water quality parameters is pH.¹ Whether pH measurement is conducted in the field, laboratory, or integrated on-line within the treatment process, the reliability and accuracy of measurements depend on the design and manufacturing quality of the electrodes used. With the right design features, electrodes can ensure fast, accurate pH response in a variety of sample measurements under many different conditions. This article examines how advances in electrode design are enhancing the speed, stability, and accuracy of pH measurement to support efficient water testing workflows.

Importance of accurate pH measurement

Although small changes in water pH usually have no direct impact on users, pH plays a crucial role in monitoring and maintaining the quality and safety of drinking water. The pH of water can alter its corrosivity and affect the solubility of contaminants. Careful attention to its control is therefore essential throughout the water treatment process to ensure satisfactory clarification and disinfection. Adjustments may be needed during treatment and prior to distribution² in order to:

• Ensure the pH value meets water quality standards
• Control corrosion
• Improve the effectiveness and efficiency of disinfection
• Facilitate iron and manganese removal
• Support chemical coagulation
• Remove hardness
• Enable removal of other contaminants.

The U.S. EPA has established national health-based standards that address both human-made and natural contamination of drinking water through the Safe Drinking Water Act.³ Compliance requires consistent, accurate measurement of a wide range of parameters, including pH. While many parameters are measured in the laboratory, key variables such as pH, conductivity, fluoride, and chlorine levels may also be measured on-line to enable real-time monitoring and provide instant alerts if limits are breached.

Limitations of conventional pH electrodes for water applications

Typical pH electrodes comprise a sensing electrode with a reference electrode built into the same electrode body (Figure 1). Of all the constituent parts, the reference element and composition of the sensing glass are the most important in terms of performance. Ideally, the sensing glass should be highly conductive and sensitive to H⁺ ions and insensitive to interfering ions, and the internal reference system should be minimally affected by temperature.

Figure 1 – Typical pH electrode components.

Electrode behavior is described by the Nernst equation: E = E⁰ + (2.3 RT/nF) log aH⁺, where E is the measured potential (in mV) determined by the sensing electrode, E⁰ is related to the potential of the reference electrode, (2.3 RT/nF) is the Nernst factor, and log aH⁺ is the pH. The Nernst factor, 2.3 RT/nF, includes the gas law constant (R), Faraday’s constant (F), temperature in degrees Kelvin (T), and charge of the ion (n). For the measurement of pH, where n = 1, the Nernst factor is 2.3 RT/F. Since R and F are constants, the Nernst factor and, therefore, electrode behavior, are dependent on temperature.

Standard pH electrodes rely on heterogeneous reference chemistry, typically silver in contact with a saturated solution of sparingly soluble silver chloride and a fixed concentration of chloride ions. While these electrodes are useful in many situations, the chemistry imposes certain limitations because small changes in temperature result in a large change in the solubility of silver chloride. The resulting slow solid–liquid equilibrium of the reference in response to changes in temperature may mean that such electrodes are prone to problems with accuracy, sluggish response, and drift.⁴

While temperature compensation is possible, silver/silver chloride electrodes may not conform to the assumptions that are necessary for these corrections to be valid. Figure 2 shows the change in the Nernstian slope of an electrode with temperature, and illustrates that all the mV/pH response curves intersect at the isopotential point (around pH 7), where the electrode potential is unaffected by temperature. Once equilibrium is attained, temperature compensation based on this model can be applied, but the slowness of metal/metal salt reference electrodes in reaching equilibrium may adversely affect the accuracy of these corrections.

Figure 2 – Change in Nernstian slope of an electrode with temperature.

A number of other issues limit the application of silver/silver chloride electrodes. The temperature-dependent solubility of silver chloride means that, as temperature increases, silver chloride can dissolve from the silver chloride-coated silver wire. However, this can fail to redeposit on the wire when temperature decreases. The resulting silver chloride particulates can precipitate in the electrode junction, causing a rapid deterioration in performance. Silver/silver chloride reference systems have also been found to contribute to contamination in certain types of samples, such as proteins, through complexation of silver ions with the sample or sample matrix.^5,6 Electrodes relying on silver/silver chloride reference systems often require complex engineering to avoid these problems.

Technological advances in pH measurement

Developments in pH electrode design include the use of reference systems that employ iodide/triiodide homogeneous chemistry. These designs are helping to overcome many of the limitations associated with silver/silver chloride reference systems.⁷ Iodide/triiodide ions are completely soluble over a wide temperature range and an ion pairing system can rapidly attain equilibrium. Since the temperature coefficient is minimal, electrodes based on this chemistry deliver improved stability and fast response, even under the widely varying temperature conditions often experienced at water measurement points. Unlike silver/silver chloride reference systems, iodine/triiodide ion systems do not produce precipitates. This reduces the risk of clogging the junction or contaminating samples, and improves overall durability and reliability. Electrodes using this chemistry tend to provide fast results without the need for frequent recalibration.

Figure 3 – Distinctive orange coil design of Thermo Scientific ROSS pH electrode (Thermo Fisher Scientific, Chelmsford, MA ).

In conjunction with the use of iodide/triiodide chemistry, further performance gains are being achieved through novel reference cell designs, such as those that incorporate a coil (Figure 3).⁸ Balancing performance with the design constraints imposed by the chemical and physical properties of an electrode’s construction materials is always a challenge. However, the application of reference chemistry that is much less sensitive to temperature changes enables new approaches to electrode design that help to protect the reference by eliminating the need for co-location of the reference and sensing wires. The ability to position the reference at a distance from the sensor makes it possible to design protection into the reference system. One approach is to use a reference coil, rather than straight wire, to increase the diffusion pathway and increase the amount of reference fill solution. Increasing the diffusion path minimizes changes near the reference wire that are the consequence of fill solution diffusing through the reference junction to the inner electrolyte. Together with the greater volume of reference fill solution present, this contributes to a longer-lasting reference system and more durable electrode.

Additional performance improvements can be made using iodide/triiodide reference systems in combination with a double-junction design. Double junctions in conventional silver/silver chloride electrodes are mainly employed to protect the sample from the metal salts generated by the reference system. However, in iodide/triiodide systems this is not an issue—instead, the double junction both isolates and protects the reference from the sample and allows users to adapt the outer fill solution to best suit their sample.

Conclusion

As a general indicator of water quality and the success of various treatment processes, pH measurement plays a critical role in water monitoring and quality management workflows. Fast, accurate, and drift-free measurement that is independent of temperature fluctuations is essential, whether in the laboratory, field, or on-line applications, where temperature may fluctuate widely at the point of measurement. Advances in pH electrode technology based on iodide/triiodide reference systems and novel electrode designs are improving the reliability, stability, and longevity of pH measurement systems. These advanced pH electrodes not only improve drinking water management, but also support reliable pH measurement throughout the environmental and water/wastewater treatment industries.

References

1. World Health Organization, Guidelines for Drinking Water Quality, 4th ed., pp 226–7; ISBN978 92 4 1548151; WHO 2011.
2. DEFRA. Manual on Treatment for Small Water Supply Systems. Updated Report, June 2015; http://dwi.defra.gov.uk/private-water-supply/installations/updated-manual-on-treatment-for-small-supplies.pdf
3. Government Publishing Office U.S. Title XIV of The Public Health Service Act: Safety of Public Water Systems (Safe Drinking Water Act); https://www.govinfo.gov/content/pkg/USCODE-2010-title42/pdf/USCODE-2010-title42-chap6A-subchapXII.pdf
4. The most common error in pH measurement; https://www.thermofisher.com/uk/en/home/life-science/lab-equipment/ph-electrochemistry/ph-measurement-testing.html#tempandpH
5. Klueh, U.; Wagner, V. et al. Efficacy of silver-coated fabric to prevent bacterial colonization and subsequent device-based biofilm formation. J. Biomed. Mater. Res. Part B. Appl. Biomater. 2000, 53(6), 621˗31.
6. Yamanaka M.; Hara, K. et al. Bactericidal actions of silver ion solution on Escherichia coli, studied by energy-filtering transmission electron microscopy and proteomic analysis. Appl. Environ. Microbiol. 2005, 71(11), 7589–93.
7. U.S. Patent Number 4,495,050: Temperature Insensitive Potentiometric Electrode System, Inventor: James W. Ross, Jr. (Cambridge, MA); Family ID: 26906071; appl. no.: 06/466,856. Filed: Feb 16, 1983.
8. Coil Design Trademark Registration Number: 3122603, Aug 1, 2006.

Ricki Hartwell is a senior product manager for Water Laboratory Products, Laboratory Products Division, Thermo Fisher Scientific, 22 Alpha Rd., Chelmsford, MA 01824, U.S.A.; tel.: 978-232-6000; e-mail: [email protected]; www.thermofisher.com