Optimizing the Chlor-Alkali Process: The Critical Role of pH, ORP, and Conductivity Control

Introduction

Chlor-alkali products are indispensable to modern society and make the chlor-alkali industry a cornerstone of the global chemical value chain. Owing to their widespread use, the three essential commodities produced in this industry—chlorine (Cl₂), caustic soda (NaOH), and hydrogen (H₂)—serve as critical building blocks for thousands of downstream processes. Industrial chlor-alkali production is based on the electrolysis of highly purified, saturated sodium chloride brine and has historically relied on three principal cell technologies: the mercury cell process, the diaphragm cell process, and the membrane cell process. Today, the membrane cell process is the dominant production method, having largely replaced older technologies such as mercury and diaphragm cells because of its superior energy efficiency and the elimination of environmentally hazardous materials such as mercury and asbestos. This industry-wide transition strongly aligns with the Best Available Techniques (BAT) under the European Union’s Industrial Emissions Directive.^[1]

Membrane Cell Process and the Importance of Process Analytics

The membrane cell process employs a highly selective ion-exchange membrane that physically separates the anode and cathode compartments of the electrolyzer. The process begins with the preparation of highly purified, saturated brine, which is subsequently fed into the electrolytic cells. In the anode compartment, chloride ions are oxidized to produce chlorine gas, while in the cathode compartment, water is electrolyzed to generate hydrogen gas and hydroxide ions. Sodium ions migrate through the membrane and combine with hydroxide ions to form caustic soda (NaOH).

To maintain stable operating conditions, the catholyte (electrolyte solution found at the cathode) is continuously recirculated and diluted with demineralized water, resulting in a caustic soda concentration typically ranging from 30–33 wt.-%,^[2] which is subsequently concentrated to the commercial standard of 50 wt.-% through evaporation.^[3] Meanwhile, depleted brine leaving the anode compartment is resaturated with salt and recycled back into the process. A simplified overview of this membrane-cell process and its main analytical control points is shown in Figure 1. 

Figure 1. Membrane-cell chlor-alkali process overview. Conceptual process flow showing brine purification, membrane electrolysis, caustic soda concentration, brine recycling, and the main analytical control points for pH, ORP, and conductivity.

An increasingly important development in chlor-alkali technology is the oxygen-depolarized cathode (ODC) process. In contrast to a conventional membrane cell, where water is reduced at the cathode and hydrogen is generated, an ODC cell supplies oxygen to a gas-diffusion cathode. The oxygen is reduced to hydroxide ions, thereby replacing the hydrogen-evolution reaction. As a result, hydrogen is not co-produced, and the electrical energy demand of electrolysis can be reduced substantially. The ODC process retains the fundamental requirements of membrane electrolysis, including highly purified brine, reliable membrane protection, stable chlorine production, and controlled caustic-soda generation. At the same time, it introduces additional requirements for a reliable oxygen supply and stable operation of the gas-diffusion cathode. The process configuration and the corresponding analytical control points are illustrated in Figure 2.

Despite its superior energy efficiency and product purity, membrane electrolysis places stringent demands on process control. The perfluorinated ion-exchange membranes are highly sensitive to impurities, and membrane fouling, scaling, or chemical contamination can significantly reduce performance and membrane lifetime while increasing operating costs.^[1] In addition, inadequate process conditions can promote the formation of undesirable by-products such as hypochlorite and chlorate, which negatively impact current efficiency and product quality.^[2] Consequently, reliable process analytics are essential throughout chlor-alkali production. Measurements of pH, oxidation-reduction potential (ORP), and conductivity are widely used to monitor brine quality, optimize chemical dosing, protect membrane assets, and ensure stable operation throughout brine purification, electrolysis, chlorine handling, and downstream dechlorination processes. 

Figure 2. Oxygen-depolarized cathode (ODC) chlor-alkali process overview. Conceptual process flow showing brine purification, ODC membrane electrolysis with oxygen supplied to the cathode, chlorine production, caustic soda concentration, brine recycling, and the main analytical control points for pH, ORP, and conductivity. 

​​​​​Key Control Points in the Chlor-Alkali Process

Raw Brine Purification

The chlor-alkali process begins with the dissolution of solid sodium chloride (NaCl) in water or recycled depleted brine to produce a saturated brine solution containing approximately 310–315 g/L NaCl.^[2] Because raw salt contains impurities such as Ca²+, Mg²+, and other trace metals, the brine must be purified before entering the membrane electrolysis cells. These impurities can block membrane transport pathways, increase cell voltage and electricity consumption, accelerate membrane aging, shorten membrane lifetime, and contribute to electrode damage. Over the operating life of a membrane, inadequate brine purification can result in economic losses amounting to millions of euros.

During primary purification, sodium hydroxide (NaOH) and soda ash (Na₂CO₃) are added while the pH is raised to approximately 10–12.^[1] Under these alkaline conditions, Mg²+ precipitates as Mg(OH)₂, Ca²+ precipitates as CaCO₃, and various metal ions precipitate as insoluble hydroxides. The resulting solids are removed by clarification, sedimentation, and filtration. Because precipitation efficiency is highly pH-dependent, accurate pH monitoring is essential for optimizing chemical dosing and maintaining consistent brine quality. If the pH is too low, impurity removal becomes incomplete, and contaminants may pass into the electrolyzer; if the pH is too high, chemical consumption increases and unwanted scaling can occur.^[4] A particular challenge in this application is the alkali error of pH electrodes.^[5] At pH values above approximately 10, especially in concentrated sodium chloride solutions and at elevated temperatures, sodium ions (Na+) interfere with the pH-sensitive glass membrane, causing the sensor to indicate a lower pH than the actual value. Consequently, pH sensors used in chlor-alkali brine purification must be specifically designed to minimize alkali error and ensure reliable process control.

After primary purification, trace levels of Ca²+ and Mg²+ may still remain. In this context, even very low concentrations of these ions can precipitate within or on the membrane, leading to fouling, increased cell voltage, reduced current efficiency, and shortened membrane lifetime. For this reason, primary purification is followed by secondary purification, which typically consists of polishing filtration and chelating ion-exchange resin treatment. This stage reduces residual Ca²+ and Mg²+ concentrations to the low μg/L range.^[1]

Electrolysis Cell (Anolyte Control) & Acidification

After primary and secondary purification, some membrane-cell chlor-alkali plants acidify the purified brine with hydrochloric acid (HCl) before it enters the electrolyzer. During electrolysis, hydroxide ions (OH) can migrate through the membrane from the cathode compartment into the anode compartment. These OH ions react with dissolved chlorine, forming hypochlorite and subsequently chlorate. The resulting side reactions consume electrical current without producing the desired chlorine product and can contribute to current-efficiency losses.^[6] The primary objective of this step is to suppress the formation of undesirable side products such as hypochlorite (ClO) and chlorate (ClO₃^-).^[1] By lowering the brine pH from around 4 to approximately 2, the formation of hypochlorite and chlorate can be significantly suppressed, reducing chlorate generation by up to a factor of two to three while also decreasing oxygen formation in the chlorine gas product.^[2] Accurate pH measurement and control are therefore essential to maximize chlorine yield, improve product quality, and minimize the generation of unwanted oxidizing by-products.

Although lower anolyte pH improves chlorine production efficiency, excessive acidification can negatively affect membrane stability and shorten membrane lifetime.^[2] Operators must therefore balance chlorate suppression against long-term membrane integrity. In practice, many plants operate within a compromise pH range of approximately 3–4,^[1] where hypochlorite and chlorate formation are sufficiently minimized while maintaining acceptable membrane durability.

In addition to pH control, conductivity measurement plays a critical role in maintaining the proper brine concentration entering the electrolyzer. Conductivity is directly proportional to NaCl concentration and is commonly used to control the addition of fresh saturated brine, ensuring that the operating concentration remains within the desired range, typically about 200–240 g/L NaCl in the depleted-brine circuit.^[7] Due to the highly corrosive environment containing hot concentrated brine, hydrochloric acid, chlorine, and caustic soda, inductive (toroidal) conductivity sensors are preferred over conventional contacting conductivity sensors.^[8] Unlike contacting sensors, inductive sensors contain no exposed metal electrodes and are therefore highly resistant to corrosion, scaling, fouling, and crystallization. They also perform reliably in the very high conductivity range characteristic of chlor-alkali brines, making them the preferred technology for robust concentration monitoring and process control in chlor-alkali applications.

Brine Recycling, Dechlorination, and Resaturation

The depleted brine leaving the electrolyzer is typically recycled and resaturated rather than discharged, thereby maximizing salt utilization, reducing raw material consumption, and improving overall process efficiency. Before recycling, residual active chlorine species, including dissolved chlorine (Cl₂), hypochlorous acid (HOCl), and hypochlorite (ClO^-), must be removed because they can damage the ion-exchange resins used in secondary purification, promote corrosion, and negatively affect process stability. The brine is therefore acidified to approximately pH 2 or even lower and treated by chlorine stripping, activated carbon, catalytic reduction, or reducing agents such as sulfite to achieve complete dechlorination.^[1] In some membrane-cell plants, accumulated chlorate (ClO₃^-) is additionally removed through acidic chlorate reduction beforehand, where a sidestream of brine is treated under strongly acidic conditions (pH ≈ 0) to convert chlorate back into recoverable chlorine, followed by treatment with reducing agents, activated carbon, or catalytic reduction.^[1]

Conventional pH electrodes often experience severe reliability problems in chlor-alkali dechlorination and brine-recycle circuits. One major challenge is chlorine attack on the reference system. Residual chlorine can diffuse through the reference junction and oxidize the Ag/AgCl reference element, resulting in zero-point drift, unstable measurements, frequent recalibration, and reduced sensor lifetime. These effects become particularly severe at elevated process temperatures.^[9] [10] A second challenge is diaphragm clogging, especially in purification and recycling stages where suspended solids, precipitates, and fine particles can block the reference junction.^[11] This leads to slow sensor response, inaccurate pH readings, and in some cases, irreversible sensor degradation even after cleaning. In addition, pH measurement under highly acidic conditions may be affected by acid error. At pH values below approximately 2, extremely high hydrogen ion concentrations can cause deviations as acid molecules penetrate the outer gel layer of the pH glass membrane.^[12] The resulting reduction in the effective hydrogen ion activity gradient causes the sensor to indicate a slightly higher pH than the true value. Although modern glass formulations exhibit negligible acid error down to approximately pH 1, this phenomenon should still be considered in applications such as dechlorination and acidic chlorate reduction where extremely low pH values are encountered.^[1] Consequently, robust pH sensors with chlorine-resistant reference systems, fouling-resistant junction designs, and optimized glass formulations are essential for reliable operation in chlor-alkali brine recycle systems.

In addition, ORP measurement is commonly used to verify treatment efficiency, as it provides a direct indication of the residual oxidizing chlorine species in the brine.^[13] A low ORP confirms that active chlorine has been effectively removed, ensuring that the recycled brine can safely pass through downstream purification units and be reused in the electrolysis process without damaging ion-exchange resins.

Caustic Soda Concentration Monitoring and Process Control

Sodium ions migrate through the membrane and combine with hydroxide ions to form caustic soda (NaOH). To maintain stable operating conditions, the catholyte is continuously recirculated and diluted with demineralized water, resulting in a caustic soda concentration typically ranging from 30–33 wt.-%, which is subsequently concentrated to the commercial standard of 50 wt.-% through evaporation.

Accurate monitoring of NaOH concentration is important to verify product specification and provide process feedback both downstream of the electrolyzer and after the evaporation stage.

Modern chlor-alkali facilities rely on inline analytical measurements rather than manual laboratory sampling to provide continuous real-time process control.

Several analytical technologies are available for NaOH concentration measurement. Density and ultrasonic measurements are generally preferred because they exhibit a strong and well-defined relationship with NaOH concentration across a wide operating range. Ultrasonic analyzers determine concentration from the velocity of sound in the liquid and are particularly attractive in chlor-alkali applications because they are independent of conductivity, color, and optical properties while offering excellent resistance to aggressive process conditions.^[14] Density analyzers are similarly well suited because the density of aqueous NaOH solutions correlates strongly with concentration, enabling accurate and stable continuous measurements.

Conductivity sensors normally cannot be used as standalone concentration measurements because the conductivity-concentration curve exhibits a reversal point at approximately wt.-% NaOH, where conductivity reaches a maximum and subsequently decreases with increasing concentration.^[14] Consequently, a single conductivity value may correspond to two different NaOH concentrations, making conductivity unsuitable as a standalone measurement across the entire concentration range. Despite this limitation, conductivity, normally measured using inductive (toroidal) conductivity sensors, remains valuable when combined with density or ultrasonic measurements in multivariable analytical systems. By combining multiple independent process parameters, operators can obtain reliable concentration measurements even in complex multi-component systems containing both NaOH and NaCl.

Process Analytics Beyond the Electrolyzer

Process analytics in chlor-alkali plants extend beyond the core electrolysis process to supporting systems such as cooling-water circuits and wastewater treatment. Sensors including pH, ORP, and conductivity are used to detect leaks, monitor dechlorination, control chemical dosing, and verify wastewater quality before discharge. 

Caustic Soda Cooling-Water Monitoring

Following electrolysis and evaporation, highly concentrated caustic soda must often be cooled in heat exchangers before storage or further processing. In these cooling systems, pH measurement is the primary monitoring technology. If a plate heat exchanger or shell-and-tube exchanger develops a leak, the higher-pressure caustic soda can enter the cooling-water circuit and rapidly increase the cooling-water pH. An inline pH sensor installed in the cooling-water return line immediately detects this alkaline excursion and triggers an alarm, allowing operators to isolate the leaking exchanger before the contamination spreads throughout the cooling tower system.

Wastewater Treatment and Brine Recycle Monitoring

Waste streams in chlor-alkali plants are generated primarily from brine purification, filter backwashing, dechlorination units, and occasional purge streams from the brine recycle circuit. Before these streams can be reused or discharged, residual oxidizing chlorine species such as dissolved chlorine (Cl₂), hypochlorous acid (HOCl), hypochlorite (ClO⁻), and, in some cases, chlorate (ClO₃⁻) must be removed. ORP sensors are commonly used to monitor the effectiveness of the dechlorination process by tracking the oxidizing strength of the solution before and after treatment, ensuring that residual chlorine is sufficiently removed and preventing highly corrosive chlorine from damaging downstream equipment. pH sensors are used simultaneously to control the addition of acid or reducing chemicals required for chlorine removal and wastewater neutralization. In addition, conductivity sensors monitor dissolved ion concentrations in both process streams and wastewater. These measurements help verify brine concentration, detect contamination, and confirm that wastewater treatment processes are effectively removing salts and inorganic impurities before final discharge or recycling.

Conclusion

In conclusion, the membrane-based chlor-alkali process depends on precise and reliable process analytics to maintain efficiency, product quality, and long-term equipment integrity. Proper process analytics approaches enable operators to optimize chemical dosing, protect sensitive ion-exchange membranes and resins, reduce unwanted by-product formation, and ensure safe, compliant, and economically sustainable chlor-alkali operation. The main measurement points, control targets, and operational purposes are summarized in Table 1.

Table 1. Key process analytics in membrane chlor-alkali production.

Summary of the main pH, ORP, and conductivity measurement points, including their control targets and operational purpose across brine purification, electrolysis, dechlorination, caustic soda handling, cooling-water monitoring, and wastewater treatment.

Reference