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Section 1.6 Free Chlorine Sensor Description
3-Electrode Free Chlorine Sensor
As there must be an electrical connection between the counter electrode and the measurement
medium, the measurement medium must have a minimum conductivity of approx. 10 µ S/cm. This
means that the sensors are not suitable for use in highly-purified water, or similar.
2-Electrode Free Chlorine Sensor Description
The sensor for free chlorine is a potentiostatic 2-electrode sensor with a micro porous, Hydrophobic
(moisture repellent) PTFE membrane and special electrolyte.
Suitable chlorinating agents that can be measure include inorganic chlorine compounds such as
chlorine gas (Cl
2
), electrolytically generated chlorine, sodium hypochlorite (NaOCl), calcium
hypochlorite (Ca(OCl)
2)
, chlorinate lime CaCl(OCl). The sensor is not suitable for determining
organic chlorination agents such as cyanuric acid based products or combined chlorine.
The sensors are not suitable for detecting the absence of free chlorine.
The 2-electrode sensor can only be used in water of drinking or swimming pool water quality. Solid
materials in the media clog up the membrane and prevent the sensors from working correctly.
If the membrane comes into contact with tensides (detergents, surfactants) from washing, cleaning or
disinfecting agents, the hydrophobic properties of the membrane are lost, the membrane becomes
permeable, and therefore damaged. For this reason, contact with these substances must be strictly
avoided.
The membrane does not allow hypochlorite anions to penetrate the membrane to reach the
electrolyte chamber; so, the sensor does not measure OCl
-
, only hypochlorous acid.
The
working electrode
(cathode) is made of gold (Au). The anode, which performs the function of
a
combined reference and counter electrode,
is made of silver (Ag), and is provided with a coating of
silver chloride (AgCl).
A 2-electrode system has the disadvantage that the potential between ME and CE can vary. This
applies particularly in an open system, as the conditions at the electrodes can vary here, as a result
of fluctuations in the composition of the water. If the potential does vary, then other oxidizing
agents contained in the water having a polarization voltage in a similar range, can also react
and distort the result.
No zero adjustment is required. The electrode chamber contains a defined electrolyte, with which the
sensor does not exhibit a zero signal. Therefore, no zero adjustment using analyte-free water is
required.
