In processes where disinfection is required, chlorine is the most common and economic disinfectant, followed by sodium hypochlorite (liquid bleach). It is important to disinfect the RO feedwater to avoid fouling of the membranes, which results in poor performance and frequent cleaning. If dechlorination is required, it’s common to continuously monitor the process to ensure proper dechlorination is achieved to avoid oxidative damage to the membranes. It is common to continuously monitor the process to ensure proper feed of the disinfectant. It has been an industry standard to use ORP to monitor chlorination/dechlorination of RO feedwater. Any species that affects the redox potential of the solution will affect the ORP mV value. Many end users, consultants, service providers and OEMs are employing chlorine analyzers rather than ORP.  

ORP

Oxidation reduction potential, commonly called ORP or redox potential, is a millivolt potentiometric measure of the potential of the water using a reference electrode and typically a platinum measuring electrode. Oxidation reduction potential is the measurement of a solution's oxidizing and reducing activity. There is no selectivity for specific ions. Oxidation and reduction reactions occur together until some equilibrium condition is reached. It takes time for the sensor to come to equilibrium. Typically a positive mV value is oxidizing, 0 mV is neither oxidizing nor reducing, and a negative mV value is reducing. The more positive or negative the ORP value, the more oxidizing or reducing is the solution. Anything that affects the redox potential affects the ORP value. Therefore it is not a direct measure of chlorine, only the redox potential of the solution in which chlorine will affect.

Advantages of ORP measurement of chlorine include:

• The cost of ORP monitoring systems
• No reagents are required

Disadvantages of ORP measurement of chlorine include:

• The electrode(s) must be replaced typically annually.
• There is no calibration because the redox potential is a relative value. Sensor performance can be checked on a solution with a known redox potential. 
  • If the electrode is an insertion type the system must be isolated or shut down to remove the sensor for calibration.
• Reliability of measurement versus direct measure.
• The redox potential includes all variables that influence a solution's oxidizing and reducing activity.
• Very slow to stabilize or reach equilibrium.
• Whereas a pH electrode will respond in seconds, a new or cleaned ORP electrode can take several hours (days) to initially equilibrate or re-equilibrate to a sample.
• Once equilibrated, an electrode's response time is measured in minutes, not seconds.
• No temperature compensation.

Direct measure with a chlorine analyzer

Chlorine is typically directly measured amperometrically or colorimetrically using N,N-diethyl-p-phenylenediamine (DPD). Continuous measurement rather than grab sample measurement may be employed. 

Amperometric Chlorine Measurement

Typical continuous amperometric chlorine measurement is for free available chlorine. The measurement is accomplished using a cathode and anode in which a voltage is applied at or slightly above the redox potential of chlorine to reduce the chlorine. This generates an mA signal which results in a parts per million chlorine value. To use amperometric chlorine measurement the sample must be clean and contain very little iron, copper, manganese or phosphates to avoid coating of the electrode. Advantages of the amperometric method include:

• Very stable zero point
• Excellent long-term stability
• Fast response to concentration changes
• Excellent life time
• No reagents needed

Disadvantages of the amperometric method include: 

• Amperometry measures mostly the hypochlorous acid — the signal depends on pH
• Bound and total chlorine cannot be measured
• Sensor coating can occur

Colorimetric Chlorine Measurement

The colorimetric measure of chlorine is an easy, fast, reliable and accurate direct measurement method. You can measure free available, monochloramine and total chlorine and calculate the combine chlorine. The measurement can be temperature and pH compensated for accuracy. It is a colorimetric method employing reagents to form a color, the intensity of which is proportional to the concentration of chlorine. 

Advantages of the colorimetric chlorine measurement method include:

• Direct measure of free, monochloramine and total chlorine
• Can measure the following species:
  • Free chlorine
  • Total chlorine
  • Free, monochloramine and total chlorine with a single analyzer
• Fast, accurate, reliable measurement
• Absolute measurement, no calibration necessary
• Excellent long-term stability

Disadvantages of the colorimetric chlorine measurement method include:

• Requires reagents
• Oxidized manganese and chromium can interfere

Monitoring the chlorination/dechlorination process to avoid membrane fouling and chemical oxidation is critical. Historically the industry standard has been to use ORP to monitor the chlorination/dechlorination process. Many end users and engineering firms are specifying direct measurement using either amperometric or colorimetric analysis methods to monitor the chlorination/dechlorination process. Direct measurement is more reliable and stable than ORP and requires less maintenance. There is no interpretation of the results due to indirect measurement thereby less risk of fouling or oxidation of the RO membranes.