The measurement and control of pH covers a diverse range of applications, from checking and maintaining product quality in the chemical, pharmaceutical, food and beverage industries through to helping water industry users ensure they meet regulatory limits for acidity and alkalinity when discharging water.
With many factors that can affect the accuracy and operation of pH measurement equipment, it is important to select the right equipment to arrive at a solution that will offer the best performance that while also being able to withstand the inherent characteristics of the medium being measured.
pH describes the concentration of hydrogen ions in a solution. It is expressed using the pH scale, on which solutions are graded from 0 to 14. A solution with a pH of less than 0-6 (that is, large number of hydrogen ions) is considered acidic, while a pH of 8-14 (that is, a low number of hydrogen ions) is basic. A solution with a pH of 7, equivalent to pure water at 25°C, is considered neutral on the scale.
The accurate measurement of pH is important because the scale is logarithmic. A small change in pH value can equate to a large variation in practice. For instance, a solution with a pH4 is ten times more acidic than a solution at pH5, and 100 times more acidic than pH6. A strong acid or base at either end of the pH scale is highly corrosive to metals and also to skin cells.
For this reason, keeping a close eye on pH values is important; a small drift can have a significant impact on product quality, as well as process efficiency and safety.
OPTIMISING PRODUCT QUALITY
Treatment of potable water and wastewater is one of the most common applications where pH measurement is used. Water at the lower end of the pH scale can degrade pipes and taps, allowing toxic materials to seep into the water supply. Water with a pH level that is too high can have an unpleasant taste. In wastewater treatment, pH levels must be controlled to ensure optimal conditions to achieve the required chemical or microbial reactions, and ensure that the process runs efficiently.
In food and beverage applications, pH levels are vital in creating the right physical and chemical reactions to ensure that the taste of products is palatable and consistent. Managing ingredients such as yeasts and moulds, which behave differently at different pH levels, is crucial to ensuring final product quality, while also ensuring that undesirable pathogens cannot develop. The pH levels of the water supply used for washdown duties must also be managed to ensure its safety.
In the textiles and pulp and paper industries, specific pH values affect the colour and fading qualities of dyes. In the oil and gas industry, pH measurements are used to ensure that the separation process used in desalting is optimised, while also reducing the likelihood of corrosion in pipework transporting wastewater.
In agriculture, careful management of pH is an important factor in ensuring that soil is at the right conditions for specific plant growth. If pH falls outside the optimal range, it must be altered by adding acidic (such as native sulphur) or alkaline (such as lime) material. The optimal conditions will depend on the type of crop being grown.
As well as helping to ensure precise control in industrial applications, accurate pH measurement is also vitally important for ensuring regulatory compliance. This is especially the case when it comes to monitoring the quality of water discharged to the environment. As international environmental standards are becoming more stringent, both water utilities and industrial operators are expected to be able to keep a tight control on the pH levels of effluent flows released into waterways. Failure to do so can result in often catastrophic damage to aquatic life, with polluters facing potentially severe financial penalties where cuplability can be established.
CHOOSING THE RIGHT SENSOR
When it comes to pH measurement, there is no single one-size-fits-all solution that can be applied to every situation. Instead, the choice of which sensor to use will be guided by the characteristics of the process being measured, including factors such as temperature, variations in pH and the nature of the process medium, such as whether it is corrosive or whether abrasive particles are present that could cause wear and tear.
pH-measuring electrodes are constructed from special glass composed of alkali metal ions. These ions react with the hydrogen ions in the solution to generate a potential difference that in turn generates the pH measurement. Different glassware is available for low or high temperatures, or with increased resistance to acid and/or fibrous build-up.
pH sensors are generally fragile and have a finite lifespan. The lifetime of a sensor electrode is difficult to predict. Sensor failures can occur slowly, due to gradual poisoning of the reference electrode, or they can occur suddenly through factors such as breakage. Modern sensors now include intelligent diagnostics to maximise electrode service life, delivering early warning of electrode poisoning or other issues to operators before they escalate into a problem.
Varying temperatures in samples is one of the most common causes of pH measurement errors. A sensor with temperature compensation can improve accuracy. In ABB’s latest generation of digital pH sensors, this is achieved by locating the pH electrode, reference electrode and in-built temperature sensor together at the electrode tip, improving the quality of measurements for applications with variable sample temperatures, while also reducing calibration costs.
Lastly, analysing the data produced is a key part of optimising product quality. Developments such as plug-and-play sensor connection may help to simplify the process of connecting and configuring the sensor to the transmitter, as well as reducing the level of skills needed to carry out the work. A digital output also removes the need for a high-impedance cable, allowing longer distances between sensor and transmitter without compromising accuracy.
For simple instruments, pH sensors can have a big impact on both process efficiency and the final quality of the end product being treated or produced. It is becoming easier to be able to make sure that they are offering the highest levels of performance, with developments in both sensor diagnostics and analyser technologies.