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Chlorine Analysers For Residual Chlorine Monitoring – HaloSense

HaloSense

Chlorine Analyser

Principle of Operation

The free and total residual chlorine sensors are three electrode amperometric sensors. Two of the electrodes (the gold working electrode and the silver/halide reference electrode) are behind a membrane that separates the sample from the electrodes and are submerged in an electrolyte. The membrane allows the movement of chlorine from the sample to inside the sensor where a low pH environment converts the vast majority of any OCl– present to HOCl. The HOCl is measured at the working electrode and a current proportional to the concentration of chlorine is produced which is reported back to the analyser.

  • Pressure flow cell with sensorLow purchase cost
  • Low cost of ownership
  • Reduced pH dependency (largely pH independent)
  • Stable and reliable
  • Bufferless
  • Reagentless

Many water companies want to measure free chlorine residuals without the need for chemical buffers traditionally associated with such measurements. Acetate and phosphate buffers are expensive and environmentally unfriendly. Buffer delivery systems are maintenance intensive and have fairly costly consumables and there are health and safety considerations in the handling of the acids and high disposal costs if the acid treated water is unable to be fed back into the water supply.

Amperometric cells and most polarographic probes only respond to hypochlorous acid, (HOCl). HOCl dissociates into hypochlorite (OCl) in a pH dependent manner. This is why most chlorine monitors need acid buffers in most applications. The typical pH of water measured on a water treatment works may range from 7 to 9.2. Chemical buffering reduces the pH to between 5 and 6 and ensures that the majority of the residual chlorine is present as HOCl (see graph below).

 
Typical Probe Response to pH

The HaloSense Free Chlorine Sensor measures all the HOCl and the majority of the OCl present (blue line on graph). This results in a vastly reduced pH effect and means that most chlorine monitoring applications require no buffer and no pH compensation.

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