Keeping an Electrochemical Eye on Your Chlorine

Amperometry, an electrochemical method for water utilities to continuously measure free chlorine in drinking water, helps to ensure the proper amount of chlorine is being used in the disinfection process

Water utilities have been using free chlorine to disinfect drinking water for more than 100 years and have been measuring chlorine residuals for almost as long. Monitoring the product water ensures adequate chlorine is present for disinfection. It also helps the utility prevent taste and odor problems arising from overchlorination. Overchlorination also can increase the formation of harmful disinfection byproducts. Initially, utilities relied on laboratory testing to monitor residuals; but in the last 40 years, continuous chlorine analyzers have become a standard feature in the plant. Presently, there are four ways of continuously measuring free chlorine.

One common method, called colorimetry, involves treating the sample with chemicals that react with free chlorine to produce a colored solution. The darker the color, the greater the concentration of chlorine. The sample enters the analyzer where it mixes with the reagents. After allowing time for color development, the analyzer measures the intensity of the color and converts the result into a parts-per-million (ppm) chlorine reading. This method is straightforward, but has some disadvantages. Colorimetric analyzers require reagents, which must be replaced regularly, and the treated sample stream must be sent to waste.

The other three methods are electrochemical. The sensor produces a signal, either current or voltage, proportional to the amount of free chlorine in the sample. Amperometric (current) methods are more common than potentiometric (voltage) methods. Potentiometric methods and many of the amperometric methods require reagents. A few manufacturers offer reagent-free amperometric systems. Besides eliminating the expense of reagents, reagent-free systems have the further advantage of allowing the sample to be returned to the product water.

This article is a brief introduction to the determination of free chlorine using amperometry. It describes the principles of operation of the sensor, but is limited primarily to membrane-covered, two-electrode sensors. Bare electrode sensors, in which the sensing electrode is in direct contact with the sample, and three-electrode sensors also exist. The article discusses why some amperometric free chlorine systems use reagents and others do not. Finally, the article considers some practical aspects of measuring free chlorine using amperometric sensors.

Chemistry of Aqueous Chlorine Solutions
Before describing the operation of the sensor, it is useful to review some of the chemistry of aqueous chlorine solutions. Free chlorine forms when chlorine gas (Cl2) or sodium hypochlorite (NaOCl) is added to fresh water. Upon dissolving in water, Cl2 and NaOCl form a mixture of hypochlorous acid (HOCl) and hypochlorite ion (OCl- ). The relative amount of each depends on pH. High pH favors OCl- at the expense of HOCl. (See Figure 1.) A pH change does not affect the sum of the concentrations of HOCl and OCl- . The sum (HOCl plus OCl- ) is defined as free chlorine. Therefore, free chlorine concentration is independent of pH.

Figure 1. Free chlorine (HOCl + OCl- ) concentration as a function of pH at 25 degrees Celsius. As pH increases, the percent of free chlorine existing as HOCl decreases and the percent as OCl- increases. The total (HOCl + OCl- ) remains constant. The distribution also depends on temperature. At any given pH, lower temperature increases the percentage of HOCl.

Figure 2. Internal construction of a typical membrane-covered free chlorine sensor. The polarizing voltage causes electrons to flow from the anode to the cathode and keeps the concentration of HOCl at the cathode equal to zero. Driven by the concentration difference, HOCl in the sample continuously diffuses through the membrane to be consumed at the cathode.

Sensor Internals and Operation
Figure 2
shows the internal structure of a membrane-covered, two-electrode free chlorine sensor. The sensor consists of a hydrophilic membrane stretched over a metal cathode, typically gold, but sometimes platinum. Hydrophilic means the membrane is permeable to water and anything dissolved in it. HOCl and OCl- in the sample can pass through the membrane to reach the cathode. A voltage, called the polarizing voltage, applied to the cathode causes the following reaction to occur:

Equation 1

HOCl + 2e- = Cl- + OH-

Equation 1 reads: At the cathode, HOCl combines with two electrons and forms chloride (Cl- ) and hydroxide (OH- ). Although both HOCl and OCl- cross the membrane, only HOCl reacts at the cathode. The electrons in Equation 1 are important. They flow to the cathode, causing a current, which (as will be shown later) can be used to measure free chlorine. For the sensor to operate properly, the concentration of HOCl at the cathode must be zero. The value of the polarizing voltage is chosen to ensure zero concentration at the cathode.

The electrons consumed in the above reaction come from a second electrode in the sensor called the anode. The anode is a piece of silver (Ag) metal immersed in potassium chloride (KCl) solution. The anode reaction is as follows:

Equation 2

Ag + Cl- = AgCl + e-

The anode and cathode reactions are simultaneous. For every two electrons consumed by HOCl at the cathode, the anode must give up two electrons. As the anode reaction occurs, the silver metal becomes a silver ion and combines with chloride from the KCl solution to form solid silver chloride (AgCl).

The sensor is filled with potassium chloride (KCl) solution. KCl serves three purposes. It provides Cl- for the anode reaction, and it keeps the potential of the anode at a fixed value. It also completes the circuit in the sensor by providing electrical contact between the anode and cathode. A thin film of KCl solution fills the space between the membrane and cathode.

In a two-electrode sensor, the anode has two functions. It provides electrons for the cathode reaction, and it provides a stable reference point for the polarizing voltage. The potential of the Ag/AgCl depends on the concentration of Cl- at the surface of the silver metal. Because the anode provides current (i.e., electrons), the surface Cl- concentration will change as the electron flow changes. Variable Cl- concentration causes the electrode potential to change. However, using an anode with a large surface area lowers the current density and keeps the electrode potential essentially unaffected by the current. The mass of the metal also is important. Because silver is constantly being lost, the mass of silver must be large enough to ensure a reasonably long sensor operating life. Three-electrode sensors have a separate reference electrode and anode. The anode provides the electrons consumed at the cathode, and no current is drawn from the reference.

Sensor Current Depends on Membrane Diffusion Rate
In an amperometric sensor, electron flow is used to measure free chlorine. As Equation 1 shows, every time an HOCl molecule is destroyed, two electrons flow to the cathode. If one HOCl is destroyed every second, the electron flow must be two electrons per second. The current (electrons/second) is proportional to the rate of destruction of HOCl at the cathode. To reach the cathode and consume electrons, the HOCl had to diffuse through the membrane. Therefore, sensor current depends on diffusion rate. The greater the diffusion rate, the higher the current. Diffusion is driven by concentration difference. The greater the difference between the concentration of HOCl in the sample and at the cathode, the higher the diffusion rate. Because the polarizing voltage keeps HOCl at the cathode equal to zero, the diffusion rate and current depend on the concentration of HOCl in the sample. A typical free chlorine sensor generates between 100 nanoamperes (nA) and 400 nA (1 nA = 10-9 A) per ppm of free chlorine. Typical free chlorine levels in drinking water are 0.2 ppm to 1.2 ppm.

Current also depends on sensor design and sample temperature. The material and surface area of the cathode and the structure and thickness of the membrane all affect sensor current. Temperature affects current because it alters the permeability of the membrane, or the ease with which HOCl and OCl- pass through it. As temperature increases, permeability increases. Therefore, an increase in temperature will cause an increase in current even though the concentration of HOCl (and OCl- ) remained constant. If the sensor is calibrated at one temperature and measurements are made at a different temperature, measurements will be in error. A 1 degree Celsius change in temperature can cause as much as a 5 percent change in current, so a small temperature change can introduce a substantial error. A correction equation in the analyzer software allows it to automatically compensate the raw sensor current for changes caused by temperature. To measure temperature, a resistance thermometer (RTD) is incorporated into the sensor.

Calibration Is Important
The design factors influencing sensor current cannot be perfectly controlled. Sensor-to-sensor variation is unavoidable, so sensitivity (nA/ppm) to HOCl is not the same for each sensor. Sensors must be calibrated. Because dilute solutions of free chlorine are unstable, bottled calibration standards are not practical. Instead, chlorine sensors are calibrated against the results of a laboratory test run on a grab sample. Several manufacturers offer easy-to-use, portable, battery-powered instruments for measuring free chlorine. Prompt, on-site testing is important. Chlorine solutions are unstable. The concentration of chlorine can change substantially during the trip to the laboratory and the wait for someone to test the sample, making the calibration invalid. Even a delay of a few minutes can cause an error.

Entering calibration data is straightforward. The user enters the test results in ppm. The analyzer stores the sensor current and calculates a calibration factor. The analyzer saves the factor and uses it to convert subsequent current readings into ppm chlorine values. Because current also depends on temperature-induced changes in membrane permeability, the analyzer software corrects current to a reference temperature before doing the concentration calculation.

Even when no HOCl is present, amperometric free chlorine sensors generate a small current called the zero or residual current. When a sensor is first placed in service, its zero current should be measured. The analyzer will subtract the zero current from the sample current before converting the result to ppm chlorine. Deionized water makes a good zero standard.

Reagents or No Reagents
Every manufacturer of amperometric free chlorine sensors faces a problem. Sensor current depends on HOCl, but free chlorine is a pH-dependent mixture of HOCl and OCl- . There are two issues involved. First, at high pH the fraction of free chlorine existing as HOCl can be quite small, placing a practical upper pH limit on the sensor. Second, because a pH change alters the amount of HOCl, a sensor calibrated at one pH can be used only at that pH. The error is not minor, either. A glance at the graph in Figure 1 suggests that a 50 percent error will result if the sensor is calibrated at pH 7.5 and used at pH 8.0.

One way to solve the problem is to acidify the sample before it reaches the sensor. Acid lowers the pH and converts OCl- to HOCl, the form of free chlorine the sensor measures. As Figure 1 shows, lowering the pH to about 5.5 almost completely converts OCl- to HOCl. (The actual amount of OCl- converted to HOCl at a given pH depends on temperature.) Any acid will work. Cost, hazard properties and ease of handling are key considerations. Acetic acid, white vinegar and carbon dioxide gas are commonly used.

There are advantages and disadvantages to sample pretreatment. The major advantage is chemical simplicity. If the pH is less than 5.5, only HOCl is present, and the sensor will measure it. The measurement is simple and straightforward; nothing is inferred. There are, however, some disadvantages. A sample conditioning system is required. If acetic acid or vinegar is used, a reagent injection pump and a mixing system are needed. Peristaltic pumps, commonly used for liquid addition, are reliable, but the pump tubing needs periodic replacement. Reagents don't last forever, either. Ordering, storing and replacing reagents is an unavoidable part of using a sample pretreatment system.

An alternative solution is to measure the pH of the sample and use pH to correct the raw chlorine sensor reading. This approach is limited by the amount of sensor current available at high pH. Because the concentration of HOCl decreases as pH increases, there is a limit above which the sensor current is too small to be reliably measured. The sensitivity drops as temperature decreases, so the upper pH limit is also temperature sensitive.

Fortunately, the decrease in sensor current as pH increases is not as much as the graph in Figure 1 suggests. For example, the graph shows that at pH 9 about 3 percent of the total chlorine is HOCl. Therefore, the sensor current at pH 9 should be about 3 percent of the value at pH 6. In fact, in at least one manufacturer?s sensor, the current at pH 9 is 15 percent of the value at pH 6. The higher than expected current is the result of conversion of OCl- to HOCl in the sensor. HOCl and OCl- exist in a dynamic equilibrium represented by the equation:

Equation 3

OCl- + H2O = HOCl + OH-

The concentrations of the reactants and products obey the rule shown below, where K is a constant.

Equation 4

The brackets denote concentration. At the cathode, HOCl is consumed, so its concentration drops. To keep the ratio constant, OCl- must decrease. As Equation 3 shows, if OCl- decreases, HOCl and OH- must increase. The HOCl thus created from OCl- is consumed at the cathode and the current is higher than expected. The process is self-limiting. As Equation 4 shows, an increase in OH- (from the sample and the cathode reaction) limits the amount of HOCl that can form.

There are advantages and disadvantages to using pH correction. A disadvantage is lack of simplicity. The free chlorine concentration is calculated by correcting the raw chlorine signal for pH variation. The accuracy of the correction depends on the accuracy of the correction algorithm and the accuracy of the pH measurement. Another disadvantage is an upper pH limit, which depending on the manufacturer can be between 8.2 and 9.5. The major advantage to continuous pH correction is the elimination of reagents and sample conditioning systems. This not only reduces costs, but also eliminates the need to track, reorder and store reagents. Finally, with reagent-free systems, the sample can be returned to the product water, keeping waste to a minimum.

Sampling and Keeping the Sensor Clean
Sidestream sampling, where the sensor is installed in a flow cell and small diameter tubing brings sample to it, is almost always recommended for amperometric sensors. It is unavoidable if the measurement system uses sample pretreatment. Once the sample leaves the sensor flow cell, it can be returned to the process line or it can be sent to waste. In areas where water is scarce or at remote sites where a sewer is not available, there is a big advantage in reusing sample or at least in producing as little waste as possible. Installing a reagent-free sensor directly in the process line is rarely a good idea. At some point the sensor will need to be removed for maintenance. This means shutting down the process line and possibly reducing plant output.

Amperometric chlorine sensors need a flowing sample. Only HOCl molecules adjacent to the membrane can diffuse through it. Therefore, the diffusion rate and sensor current really depend on the concentration of HOCl at the membrane surface, not the concentration in the bulk liquid. Diffusion of HOCl through the membrane lowers the surface concentration. As the surface concentration drops, the diffusion rate drops and sensor current falls. In a flowing sample, HOCl in the bulk solution constantly replaces HOCl lost by diffusion, so the sensor current remains constant. Low flow can cause a 50 percent or more reduction in sensitivity. Always consult the manufacturer?s data sheet to determine the recommended minimum flow. Pay particular attention to flow requirements if the sensor is to be installed in a tank. The flow may not be adequate for the sensor to work properly. Also, if reusing the sample is not practical, consider how much wastewater will be generated at the minimum recommended flow.

Maintaining proper sample flow can sometimes be a problem. If source pressure is variable, a pressure regulator in addition to a valve is necessary to control flow. It is also useful to install a rotameter, so operators can tell at a glance whether the flow is correct. Constant head flow controllers eliminate the need for valves and regulators. They use head pressure alone to control flow. If the inlet flow is too high, the excess water simply drains to waste. Constant head flow controllers have the disadvantage of being at atmospheric pressure, which can make returning sample to the process a problem.

It is important that the membrane stays clean. If the membrane gets dirty, passage of HOCl is hindered and the diffusion rate drops. Reduced diffusion rate causes the sensor current to drop. Maintaining cleanliness in drinking water is not a major problem. Potable water is highly filtered, so the level of suspended solids is low. However, in time, solids can build up on the membrane. Maintaining cleanliness is a bigger problem outside the filter plant, where rust and deposits accumulated in the piping provide a source of solids that can foul the membrane. Typically, a membrane should not need cleaning more than about every three months. Mechanical cleaners, although useful with bare electrode sensors, cannot be used with membrane-covered sensors.

Membrane-covered amperometric sensors, whether used in reagent-based or reagent-free systems, provide reliable continuous determination of free chlorine in drinking water. Sensors do require periodic calibration and cleaning, but maintenance requirements are generally minor when sensors are used in drinking water. Because they consume chlorine as they operate, amperometric sensors require a flowing sample.

This article originally appeared in the 11/01/2003 issue of Environmental Protection.

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