Using Custom Sensors and Electrodes for Water Quality Monitoring

What is the Environment Act 2021 and what is Section 82?

The 2021 Environment Act is aimed at improving how water quality is measured, monitored, and managed in real-time, particularly in rivers, lakes, estuaries, and coastal waters. The Environment Act 2021 introduced several measures to improve environmental protections, and continuous water quality monitoring is a critical part of these efforts, especially for the management of wastewater and storm overflows. Section 82 of the Environment Act 2021 focuses on improving monitoring regarding water quality, specifically targeting how sewerage undertakers (i.e., water companies responsible for managing public sewer systems) handle wastewater discharges, including storm overflows, into rivers, streams, and coastal waters.

Sewerage undertakers are required to install and maintain monitoring equipment on storm overflows and combined sewer overflows to track how often they discharge untreated sewage into water bodies. Sewerage undertakers must ensure that any discharge from their sewer systems does not breach water quality standards, particularly in sensitive areas such as bathing waters and protected ecosystems.

The government guidance details the types of technologies and methodologies that sewerage undertakers should use to monitor water quality effectively. This includes specifications for:

• Flow meters and sensors for measuring discharge volumes.
• Water quality sensors for detecting pollutants (e.g., ammonia, pH, DO2, etc.).
• Data collection and management systems for storing and reporting monitoring data.

The legislation in place dictates that sewage undertakers must monitor their waste output upstream and downstream, of their asset being monitored, once per hour or every 15 minutes during a high-risk event. These changes are required to be rolled out as soon as practicably feasible and have started by 2025 with complete monitoring completed by 2035.

How can Sentek Help?

Electrochemical sensors play a significant role in supporting the Continuous Water Quality Monitoring Programme and compliance with the Environment Act 2021, particularly for sewerage undertakers in implementing Section 82. These sensors are designed to measure various chemical properties of water in real-time, offering precise and reliable data essential for maintaining water quality and detecting pollution. Sentek can provide a range of electrochemical sensors suitable for measuring common sewage works contaminants and key indicators of a water courses health:

• pH Levels
• Temperature
• Dissolved Oxygen
• Turbidity/Conductivity
• Ammonia – An increase in Ammonia is a common indicator of sewage contamination.
• Chlorine – Chlorine sensors can detect the presence of disinfectants that may be released from wastewater treatment plants.

Nitrate sensors help track nutrient levels, which can cause issues like eutrophication (excessive nutrient enrichment in water bodies leading to algal blooms and oxygen depletion).
As one of the leading UK sensor manufacturers, Sentek can support clients with design of custom electrochemical sensors, probes and sondes to meet their specific water quality monitoring requirements. These can be installed at combined sewer overflows and other wastewater discharge points to monitor when and how often untreated wastewater is being released into rivers or coastal waters. Sensors provide early detection of changes in water quality. For example, if a sensor detects a sudden rise in ammonia or a drop in dissolved oxygen, this could indicate a pollution event such as a sewage spill. Early detection allows companies to respond quickly and mitigate the environmental impact before it escalates.

Sentek electrochemical sensors have been used for long-term deployment across the globe, providing continuous monitoring that ensures companies remain compliant with the environmental regulations over time. By tracking trends and long-term changes in water quality, sewerage undertakers can identify problem areas, manage wastewater systems more effectively, and make informed decisions on infrastructure improvements.

Sensors from Sentek can be integrated with remote monitoring systems and data platforms, enabling seamless collection and transmission of water quality data. This integration is particularly useful for large-scale, continuous monitoring across multiple locations, such as river systems or coastal areas.

The sensors can also be incorporated into smart monitoring networks, where data is aggregated and analysed to provide insights into overall water quality trends and potential sources of pollution.

Sentek electrochemical sensors could significantly enhance water quality monitoring efforts by providing reliable, real-time data that helps sewerage undertakers meet their obligations under the Environment Act 2021 Section 82. These sensors enable proactive management of water resources, ensuring timely detection of pollution and helping to maintain high environmental standards.

Ammonia vs Ammonium

Ammonia is just one component in sewage that can be particularly harmful to aquatic life, it can be very challenging to monitor, particularly in the field. Ammonia ISE are available but presently are only practically useable in a lab-based setting.

Ammonia is only present as the deprotonated form NH3 in a basic pH greater than 7, below this it is mostly in the less harmful ammonium ion (NH4+). Ammonium is unable to cross cellular membranes so is therefore much less toxic than ammonia. As pH and temperature increases the proportion of ammonium being deprotonated to the smaller and more harmful ammonia increases so by monitoring NH4+, pH and temperature we can relatively accurately infer the ammonia concentration in the water. The table below shows the percentage of ammonia vs ammonium at varying pH and temperature.

This can be plotted onto a curve also shown here.

Schematic model of ammonium ion (NH4 + ) and neutral ammonia (NH3) proportions in function of the pH. At acidic and neutral pHs, NH4 + is the most abundant species. However, NH3 is dominant in highly alkaline solutions. Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Schematic-model-of-ammonium-ion-NH4-and-neutral-ammonia-NH3-proportions-in_fig2_358973331

Calibration and Maintenance

Sensors or sondes can be pre calibrated prior to installing but will need periodic recalibration to maintain a high degree of accuracy. Over time the sensor will begin to drift away from the calibration point. Each sensor type will drift at a different rate as they age – the longer the sensors go without calibration the greater degree of uncertainty the measurement will have.

Regular maintenance will also be required to ensure that the sensors or sondes are working at their best, giving that monitoring a water way will naturally lead to an amount of biological fouling to the sensors. Contamination of the sensor junctions or plates will lead to increased resistance which in turn will skew measurements, so it is important that these are periodically cleaned off and recalibrated.

For more information on the Environment Act 2021 Section 82 and sensors used to measure chemical properties of water, contact a member of the team.

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The Anatomy of pH Electrodes

In this article, we delve into the anatomy of pH electrodes. We will explore key elements; including the reference system, electrolytes, membrane, and the electrode body. Understanding the inner workings of a pH electrode is beneficial to achieve precise and accurate pH measurements for your specific application.

Firstly, what is pH and why do we measure it?

A pH probe is one example of many ion selective electrodes (ISE) that are produced at Sentek. ISE and pH measurement are used from lab applications to work in the field. Sentek manufactures probes for a variety of customers with applications including engineering projects, process control, agricultural, environmental, medical, pharmaceuticals and food and the brewing industry.

pH is a quantitative measurement of the acidity or basicity of an aqueous solution; this is directly related to the concentration of hydrogen ions (H⁺) or hydroxide ions (OH^–) in the solution. When the concentration of OH^– is equal to that of H⁺ the solution is considered neutral which occurs at pH 7. As the amount of H⁺ in the solution increases the pH of the solution decreases and as the OH^– increases the pH increases.

Figure 1 Showing an example pH scale and the logarithmic relationship between the pH scale and the H+ concentration. Source Microsoft Bing images

pH is a logarithmic scale defined by pH =-_Log10 (aH⁺). This means that at pH 6 the activity of the hydrogen ion is ten times greater than that at pH 7, and pH 5 is 100 times greater than pH 7. A pH electrode uses potentiometric techniques to determine the activity of hydrogen ions in a given solution using a potentiostat. The measured potential describes hydrogen activity by the Nernst equation:

In order to measure the potential difference a pH probe requires not only an H⁺ sensitive membrane but also a non sensitive stable reference to compare against.

Reference

When looking at the anatomy of a pH electrode, the most common type of pH electrode in use today uses the silver/silver chloride (Ag/AgCl) reference electrode.  In these cells a reversible redox reaction is occurring between solid AgCl and solid Ag and dissolved Cl^–. A redox reaction describes how a reaction progresses via either the loss or gain of an electron.

Half-cell equation of Ag/AgCl:

The Ag/AgCl electrode will offer a stable and reproducible potential regardless of pH which makes it perfect for a reference electrode. The most common type of reference electrode used today is the silver/silver chloride due to its stable reading and non-toxic materials. Mercury/mercurous chloride (Hg/Hg₂Cl₂) is another type of reference electrode used; Calomel electrodes are less prone to blockages and are considered to be extremely stable, however due to the toxicity of mercury, it is generally avoided for the much safer silver/silver chloride (Ag/AgCl) reference.

Half-cell equation of Hg/Hg₂Cl₂:

The half-cell electrochemical potentials(E⁰) above for AgCl and the Calomel only show half the picture. In order to measure the potential difference another electrode is required to measure against. The half cells in this scenario would have been measured against the standard hydrogen electrode (SHE):

The SHE is an arbitrary zero point to measure all other potentials against. Due to the impracticality of using hydrogen gas the SHE is only very rarely used. In a normal pH electrode, the other half cell is normally a silver/silver chloride electrode behind a H⁺ sensitive membrane.

Once the pH membrane and the reference electrode are in the test solution the circuit is complete, with the only varying potential being the working electrode inside the membrane glass. The change in H⁺ on the outside of the membrane glass will change the potential of the working electrode which can then be compared to the reference electrode. This is why reference electrodes have to be completely insensitive to a change in pH and have a stable potential.

The anatomy of pH electrodes can change for many reasons, one being that electrodes can come as a mono or part of a combination for convenience depending on the application. The reference is in contact with the sample solution via some type of porous material such as cotton, teflon and ceramic. Each of the junction types have variable characteristics depending on the requirements of the electrode.

Ceramics are a robust junction material that have low flow rates but high junction resistance. Ceramics have very stable readings this makes them a great general purpose junction material. Due to the small holes and low flow rate ceramic junctions are prone to blockages and need to be kept clean, this can be combatted by using multiple ceramic junctions.

Teflon junctions have a higher flow rate and are therefore less prone to blocking than a ceramic junction. Due to the lower maintenance of teflon junctions these are easier to handle.

The simplest type of junction would simply be a small hole called a ground joint, this would offer the lowest junction potential and the highest flow rate. These are simple to clean but will require a lot of re-filling, great for viscous samples or samples with lots of suspended solids.

A double junction or even a triple junction could extend the life of an electrode in harsh environments. An additional cavity filled with electrolyte and another junction is used to further separate the reference from the sample. Double junctions are often used when a sample is particularly hostile to the reference in use, however, the junction potential of all of the internal junctions used will have to be considered as well the external junction and the types of electrolyte(s) used.  Using an electrolyte similar to the sample medium in the outer chambers can decrease the amount of overall junction potential.

Sentek has also developed an iodine/iodide pH and reference system which offers a fast response time and a low temperature sensitivity.

Figure 2 shows a diagram of Sentek electrode with labels pointing to all of the key components of a pH electrode

Electrolytes

Potassium chloride (KCl) solution is the most common type of electrolyte used in pH probes, it allows for a connection i.e. a salt bridge between the reference electrode and the membrane glass. When reviewing the anatomy of pH electrodes for your experiments, consider that different types of electrolytes are available for different applications, which can be key for decreasing the junction potential for more precise measurements. An advantage of KCl is that the diffusion rates of the anion and cation are very similar which helps to establish a stable junction potential. The electrolyte can be made into varying types of solutions and gels to fit any requirements. The combination of junction material and electrolyte will determine the flow of the electrode. Standard KCl solution is a free-flowing liquid that will pass quickly through a junction, this will keep the junction wetted through and allow for good connection from the electrode to the sample, this type of electrolyte is well suited to the controlled lab environment where high precision is key. Gelled electrolytes are used more in the field, these restrict the flow of the electrolyte whilst still allowing for a good connection from the electrode to the sample solution, having a thicker gel will not only last longer between re-fills but will also help protect the reference from contamination, these are well suited to work in the field and industrial applications. Sentek has developed pH electrodes and reference electrodes that enable pH measurement in a variety of difficult samples e.g. non-aqueous samples, soil samples, high pH, high salt concentrations as well as slurries and viscous samples.

Membrane Glass

For a pH probe to make a potentiometric measurement two electrodes are required, one stable pH insensitive electrode and another behind an ion selective membrane. In this case our target ion is H⁺ and the membrane glass provides us with our selectivity. All membrane glass is specially made to be a good ionic conductor to allow the transfer of charge from the outside of the pH bulb to the inside. As a pH bulb is immersed in an aqueous solution the outer layers of Si-O groups become protonated by H⁺, this is the formation of what is known as the leached layer. The ionic equilibrium shown below is the key element in the selectivity of the membrane:

There are two glass membrane/solution interfaces, one outside of the membrane and one on the inside of opposing polarity. The internal solution is stable as it is completely sealed so the difference in potential arises from the internal glass membrane/solution interface. The difference of the inner glass leached layer and the outer glass leached layer.

Figure 3 A diagram showing a not to scale illustration of how the charge is transferred via alkali metals to the inner solution from outside of the electrode. pH Electrodes – Chemistry LibreTexts

Sentek can produce a variety of different types of pH sensitive glass to suit the needs of the application, with a selection of relatively low resistance glass for quick response times or high resistance glass for extremely hostile environments. The temperature of a sample can not only greatly change the resistance of the glass membrane but can also change the activity of the ions in the solution, this is why adding temperature compensation into probes is becoming increasingly popular. We have a selection of temperature compensation selections for all available pH meters.

External Shell

It’s impossible to explore the anatomy of pH electrodes without considering the external shell. Most electrodes have a glass external body. Glass has very good chemical resistance even for most acids and bases. Glass also allows the user to have a clear view of the inside of their electrode, very useful when the electrode is refillable or to see any contamination. The obvious draw back being is that glass is brittle, so whilst useful in some applications, such as the lab, it may not be well suited to more rugged work in the field. Electrode bodies can be made from durable types of plastic and have even been made into specially machined metal bodies for maximum protection.

pH Electrodes from Sentek Ltd

Sentek offers a vast range of standard pH electrodes, from the P11 Protected Bulb, to the P11/DJ/NaF for Non-Aqueous Samples. Here at Sentek we can also offer the option to design and build electrodes to your exact specifications. Your unique applications may require specialised solutions, and we have the expertise to tailor our products to your precise requirements.

We understand that selecting the right sensor for your application can be complex, so our dedicated team are ready to assist with any questions you may have.

Get in touch today to discuss your requirements.

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Disposable and Non-Disposable pH Sensors in Biopharmaceutical Production

The term “biopharmaceuticals” was coined in the 1980s and refers to pharmaceuticals produced using molecular biology techniques as opposed to synthetic drugs that are the products of chemical processes.

Biopharmaceutical processing systems have traditionally employed stainless steel piping and bioreactors. However, for some years the industry has moved towards single-use production methods which rely on disposable elements such as the bioreactor vessels themselves.

Single-use or disposable bioprocessing equipment is now increasingly being adopted for small scale commercial manufacturing replacing classic fixed stainless steel equipment based bioprocessing facilities. Generally, single-use equipment is composed primarily of plastic components that have been sealed and pre-sterilised with gamma radiation. They frequently have a long shelf life of up to 2 years.

While single-use technology offers several advantages including flexibility, it also introduces new measurement and monitoring challenges that need to be met by sensors designed for these applications.

Monitoring and controlling pH is critical to biopharmaceutical manufacturing to ensure the quality and yield of the final product. Requirements for the pH electrodes used in these single‐use bioreactors are different from those used in conventional stainless steel bioreactors where cleaning and sterilisation of the bioreactors means that the pH electrode must be resistant to these procedures or removed or protected. There are cost implications with these requirements which mean that the pH sensor should have a long lifetime. With single use bioreactors, however, long in use lifetime or resistance to steam and cleaning procedures are less crucial factors but a low sensor cost on a per‐use basis is important as well as a long shelf life.

Potentiometric glass electrodes are the most common pH sensors for bioprocess monitoring and control. They are well established, reliable and robust but are generally too expensive to be disposed of after a single run. Therefore, single‐use glass pH electrodes have been developed that can be pre-sterilised with gamma radiation. There are 2 types that are available – wet or dry stored pH electrodes. The wet storage electrode is kept in its calibration buffer thus providing a one-point calibration capability.

The dry stored electrodes have been developed to be integrated directly into the bioreactor during manufacturing including a gamma irradiation process and can be stored dry. The challenge for these is that they may not be possible to calibrate them until the bioreactor has been set up and where failure is now an expensive result. Some dry stored electrodes are claimed to be pre-calibrated before committing to storage.

Meeting The Needs of The Market

At Sentek, we have been producing electrodes, OEM sensors, and other accessories for over 30 years. As the leading manufacturer of bespoke electrochemical sensors in the UK, we follow all the current trends. We have a wide range of custom sensors including pH electrodes that can be dry stored and biotechnology solutions to suit your needs. Whether you need a pharma pH probe, accessories, or a custom solution to a problem, we will work closely with you to fulfil your needs. Check our website today to view our products online, or contact us about a solution to suit you.

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What is a pH probe?

A pH probe is a device that measures the pH level of a substance or solution. It does this by measuring the amount of hydrogen (H+) or hydroxyl (OH-) ions in a substance. If there are more H+ than OH- ions in a solution, then the acidity goes up. On the other hand, alkaline solutions will have more OH- ions than H+ ions. We represent the acidity or alkalinity of a substance on a pH scale. A pH level of 0 is highly acidic, 14 is highly alkaline, and 7 is neutral.

Measuring the pH level of a substance is crucial in many industries including food production, cosmetics, and pharmaceuticals. So, to answer the question, “what is a pH probe?”, we need to look at its components, how it works, and what industries use it.

Structure of a pH probe

When looking at the question, “what is a pH probe?”, there are several main components of the structure to look at:

1. Electrode shaft materials can be made from materials such as glass, epoxy, PVC or Ryton, used depending on the application. The glass body is strong and durable and resistant to cracking even in extreme conditions, for example some steam sterilisable and autoclavable electrodes are able to withstand temperatures of 135°C and 10 BARS of pressure. Epoxy bodies are also very durable and can be used when the sample has the potential to attach or etch the glass body.
2. A glass membrane, which is specially formulated to be sensitive to hydrogen ions, is built onto the probe’s body and contains a buffered solution with a known pH level. The difference between the interior known solution and the exterior solution creates a potential that the probe measures against the reference electrode. Membranes can take different shapes depending on their application, for example;
   • a bulb membrane shape is used for aqueous samples with high purity where high accuracy is needed in laboratory, beer and blood applications for example,
   • a flat membrane shape is best for surface measurements in gels, cosmetics, dairy and viscous samples,
   • a rod membrane shape is designed for medium viscosity colloidal samples with lower purity like liquid food products and cement,
   • and a spear tip membrane is used for semi solids and medium viscosity samples like bread, dough, fats, meat, soil and slurries.
3. The reference junction allows a pathway to form, which links the reference electrode with the sample. The membrane measures the potential of the sample, against the stable potential of the reference to determine the pH level.

How does a pH probe work?

An electrode requires two ‘cells’ in order to complete an electrical circuit and give a reading from a pH meter. These are the pH cell and the reference cell, and in most electrodes they are both within the same casing to form a combination electrode. When the two cells are used in the form of two separate electrodes, they are called half cells. The pH cell is made up of a glass membrane, which is specially formulated to be sensitive to hydrogen ions. When the glass membrane comes into contact with an aqueous sample, a hydrated layer forms on the inside and outside of the membrane. Hydrogen ions will transfer across this membrane, and the direction of the transfer depends on whether the sample is acidic or alkaline. The difference between the inner and outer charge gives a membrane potential which can be compared to the stable reference potential to measure the pH value. The reference cell is stable as it is in contact with the sample through a junction rather than a hydrogen sensitive membrane. A meter calculates the difference in the pH and reference cell potential and gives a value in millivolts or is converted to pH units.

The composition of most pH electrodes will consist of a pH sensitive membrane fused to a glass stem. The stem is filled with a neutral conductive electrolyte in which a silver/silver chloride wire is placed to form an electrical connection. The reference consists of a silver/ silver chloride wire encased in a potassium chloride electrolyte, and a junction. The junction is the communication channel between the reference cell and the sample, and can be made out of different materials depending on the application and type of sample.

Industries that use pH probes

Many industries need to use pH probes so that they can measure the quality of their products. The food and beverage industry needs to measure the quality of dairy, beer and meat products to avoid the risk of selling spoiled, unsafe, or poor tasting products. When it comes to construction, measuring the pH level of metals helps find ways to prevent materials from wearing down or corroding. Likewise, measuring pH levels in wastewater treatment helps ensure the water does not harm equipment or the environment. pH levels also need to be measured in swimming pools and Jacuzzis to reduce the risk of eye and skin irritation, corrosion, and ensure that chlorine in the water is effective.

Sentek

Here at Sentek we are a leading UK manufacturer of glass pH probes for all industries. To find out more about our range of pH electrodes, please browse our range online or speak with our expert team today.

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