Recent CPCA Corners

Inline Chlorine Analysis – Colorometric vs. Ampreometric Methodology

Brett Smith
Author: Brett Smith
Published: June 2015

Chlorine is a strong oxidant used to disinfect drinking water, process wastewater and provide sanitation that is essential to protecting public health.

Utilizing Chlorine gas requires special storage, safety and handling considerations and is usually utilized in large municipalities. Hypochlorite solutions can be manufactured onsite, is safer to handle but may cost slightly more than utilizing chlorine gas. Chlorine is a member of the Halogen group of chemicals which include Bromine, Iodine and Ozone. These halogens are oxidizers and will interfere with the free chlorine readings picked up by the sensor and as such, a strong presence of these chemicals in solution could create false reading for the presence of Chlorine.

Infection agents can potentially exist in untreated water and can include Escherichia Coli, Leptospira, Salmonella, Vibrio Cholera, Hepatitis A and several others. Chlorine destroys water born germs by attacking the slime layer of a cell wall. This effect of chlorine will kill the cell or interrupt the ability of the cell to reproduce.

Chlorine when added to water produces to associative ions, HOCl – Hypochlorus acid and HCl – Hydrochloric acid. The HOCl disassociates in solution creating Hydrogen atoms and OCl – Hypochlorite ions. As the concentration of Hydrogen increases or decreases in solution, this affects the pH of the water solution. At pH values of 4.5 to 5 exclusively HOCl is present. At a pH >9.5 exclusively the hypochlorite OCl is predominantly present. At pH values of 7.5 even amount of HOCl and OCl is present and maintaining pH at 7.2 creates an optimum and stronger presence of HOCl which is a more effective disinfectant than OCl ion.

There are two major methods to determine chlorine content in water, Amperometric and Colorimetric.

  • Colorometric analysis utilizes the titration of a reagent into a water sample to determine percent chlorine or chlorine content expressed in parts per million (ppm) against known standards. The degree of color change is directly proportionate to the amount of chlorine in solution. DPD test kits which utilize a test strip on a grab sample is also a colorimetric method of analyzing chlorine in solution but not considered to be an “inline” analysis method.
  • In Amperometric analysis, the level of chlorine present is measured by the nano-amp change as free chlorine electrons in solution are consumed. This method utilizes a probe inserted into a flow cell and as the chlorinated water passes by the detection probe, the chlorine in solution permeates a one way membrane and reacts with a cathode submerged in an electrolyte solution. Every HOCl molecule that diffuses through the membrane is destroyed at the cathode and consumes 2 electrons. Cathode current (nano-amps) portional to the concentration of Hypochlorous acid- HOCl in the sample.

The advantages to an Amperometric analyzer are that the sensor can be very basic in design as well as uses simplified measurement electronics and they are highly reliable. The disadvantages are that flow is required across the sensor and because electrons are consumed, both cathode, and electrolyte need to be replaced.

Some Inline Chlorine analyzers utilize an optional pH sensor (that can read temperature as well) as well and there have been advancements in Chlorine probe technology that utilize an embedded memory chips that hold an assigned serial number, zero calibration, temperature calibration information and the total hours utilized (lifetime) data.