CHEMSCAN
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Total Chlorine

​Standard Chlorine Analysis

Chlorination of water or wastewater is performed to destroy microbiological organisms and also to oxidize nitrogen or sulphur compounds, metals and organic substances. Interactions between chlorine and various forms of nitrogen found in or intentionally added to water are complex and are generally classified as “combined” chlorine analysis. Other ChemScan Method Summaries address analysis of specific forms of “combined” chlorine (See ChemScan Method Summary #122, Chloramines in Water or Wastewater) or free chlorine (See ChemScan Method Summary #49, Free Available Chlorine in Water). This publication discusses analysis of chlorine under conditions where the total “residual” may be in the form of combined chlorine, free chlorine or both.

Upon introduction to water, chlorine gas will form hypochlorous acid (HOCl) and hypochlorite ion (OCl-). The relative percentage of these forms of chlorine is pH dependent, as shown in Figure 1. The term “free available chlorine” refers to the concentration of hypochlorous acid plus hypochlorite ion in the water.

Free chlorine readily reacts with free ammonia to form combined chlorine. The specific form of combined chlorine will depend on pH, reaction time and the chlorine to nitrogen ratio. Three types of chloramines are formed in aqueous solutions. Free chlorine is first converted to monochloramine at pH of 7 to 8 while the ratio of chlorine to ammonia is 5 to 1 by weight or less. As the ratio of chlorine to ammonia is increased, dichloramine and nitrogen trichloride (trichloramine) is formed. Thus, in water that contains substantial free ammonia, such as wastewater effluent, it is actually possible for an increased free chlorine dose to result in a reduced total chlorine residual, until the “breakpoint” has been surpassed. (See Figure 2)

Typical laboratory methods for total chlorine analysis are selected based on the tolerance for known interferences in the water to be analyzed. Specific methods include idiometric techniques (based on the liberation of free iodide from potassium iodide, measured using titration procedures or ion electrodes), amperometric titrations, DPD ferrous titrations and colorimetric procedures using DPD reagents. The DPD method may be subject to interference from monochloramine unless special precautions are taken. On-line methods are typically an automated version of one of the standard methods. Care should be taken to select a method of analysis that is appropriate for chlorine analysis in the presence of known or expected interference from other oxidizing agents, turbidity, color and organic contaminants as the above methods do not perform equally under all conditions.

ChemScan Analysis Methods

1. Amperometric Analysis

The method of choice for applications where the main issue is the absolute concentration of total chlorine (rather than the specific form of chlorine) is Amperometric analysis using the ChemScan A-2000 analyzer. This analyzer provides continuous analysis of a single sample line using polarographic principles and is not affected by turbidity, color or other oxidants in the sample. A sample flow is introduced into a mixing chamber together with a constant regulated flow of buffer solution in the form of white distilled vinegar. A small amount of potassium iodide is added with the vinegar for analysis of total chlorine. The mixing chamber contains a gold measurement electrode and a copper reference electrode. The buffered sample is continuously stirred within the mixing chamber, which also contains inert mixing spheres to aid agitation and to clean the electrodes. This method is an improved version of standard method 4500-Cl-C and D and is capable of analysis at total chlorine concentrations as low as 0.00l mg/l or as high as 50.0 mg/l.

2. Iodometric Analysis

This is the method of choice where the application requires control of the specific forms of combined chlorine, such as water chloramination monitoring and control (See ChemScan Application Summary # 86, Water Chloramination Process Control). This method is based on the principal that free and combined chlorine will proportionally liberate free iodine from potassium iodide. Free iodine has a strong UV absorbance signature, such that a full spectrum analyzer like the ChemScan UV-2150 can be used to detect the liberated iodine, plus compensate for other oxidizing or reducing agents or interferences in the background (including monochloramine - which can be independently detected), and calculate the resulting concentration of total chlorine. This is also the method of choice if multiple sample lines are being monitored or if other parameters are analyzed in addition to total chlorine. Typical range is 0.05 mg/l to 10.0 mg/l, but can be altered based on path length selection.

Monitoring System Requirements

​Sample extraction points are a function of the process being monitored. If the best control signal is desired, careful sample point selection, short sample lines and frequent measurement intervals are best.
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ChemScan, Inc.
2325 Parklawn Dr., Suite I, Waukesha, Wisconsin, USA 53186
​Phone: 262-717-9500
Toll Free: 800-665-7133
  • Applications
    • Municipal Water Monitoring >
      • Ammonia Feed
      • Chloramination Control
      • Chlorination
      • Color Analysis
      • Distribution Monitoring
      • Hardness Monitoring
      • Ion Exchange Breakthrough
      • Natural Organic Matter (NOM) Removal
      • Peak Point Chloramination Control
      • Phosphate Sequesterant Feed Control
      • Reservoir Monitoring
      • Security Monitoring
      • TOC Monitoring in Water
      • Well Blending
    • Wastewater Monitoring and Control >
      • Aeration Control and Energy Cost Reduction
      • Biological Nutrient Removal
      • Biological Phosphorus Removal
      • Chemical Phosphorus Removal
      • Chloramination /Breakpoint Avoidance
      • Chlorination/De-Chlorination
      • De-Nitrification Monitoring
      • Nitrification
      • Nutrient Deficiency Analysis
      • Lagoon Discharge
      • SBR Stage Control
      • TOC Monitoring
      • UV Disinfection Control
    • Industrial Water Treatment >
      • Boiler Water Analysis
      • Cooling Water Analysis (Closed Loop)
    • Industrial Process Monitoring >
      • Electroless Plating
    • ASA Technical Papers (ALL)
    • ChemScan Installation Photos
    • Success Stories
    • ChemScan Webinars
  • Service
    • Contact Service Request Form
    • ChemScan Service Team
    • MSDS
    • Communication Protocols
    • Software Utilities
    • Service Videos
  • Parts
  • Company
    • Terms and Conditions
    • Contact Form (General Inquiry)
    • Contact Service
    • Application Data Sheet
    • Employment Opportunities
    • Representatives and Distributors
  • Products
    • ChemScan Mini Analyzer >
      • mini oP (Ortho Phosphate)
      • mini oP (Ortho Phosphate) Extended Range
      • mini LoP (Low Ortho Phosphate)
      • mini UV-254
      • mini LowAm (Low Ammonia)
      • mini Mn (Manganese)
      • mini LowMn (Low Manganese)
      • mini LowChlor (Low Chlorine)
      • mini Sulfite
      • mini Mono (Monochloramine)
      • mini Ammonia
      • mini FreeAm (Free Ammonia)
      • mini Cu (Copper)
      • mini Low CrVI
      • mini Fe (Iron)
      • mini Ni (Nickel)
      • mini PAA (Peracetic Acid)
      • mini Silica
      • mini ChlorAm
    • ChemScan mini Accessories
    • ChemScan Process Analyzers >
      • ChemScan 2150/S Chloramination Analyzer
      • ChemScan 2150/DC Chem Feed Analyzer
      • ChemScan 2150/N Aeration Rate Analyzer
      • ChemScan 2150/NoP Chem Feed Analyzer
      • ChemScan 2150/NHoP Nutrient Analyzer
      • ChemScan 2150 Process Analyzer
      • ChemScan UV-3150 Process Analyzer
      • ChemScan 4100 Nutrient Control
      • ChemScan 6101 Nutrient Control
    • ChemScan Process Analyzers with HMI >
      • ChemScan 2250/S HMI Chloramination Analyzer
      • ChemScan 4200 HMI Analyzer
      • ChemScan 3250 HMI Analyzer
      • ChemScan 2250/NoP HMI Analyzer
      • ChemScan 2250/NHoP HMI Analyzer
      • ChemScan 2250/N HMI Analyzer
      • ChemScan 2250/DC HMI Analyzer
      • ChemScan 2250 HMI Analyzer
    • ChemScan Titrator/ISE Analyzer >
      • For Wastewater
      • For Water
    • ChemScan Analyzer Accessories
    • ChemScan Sensor Family