Data
74
Measuring principle
The measuring principle of the HR-200 is three-pole polarographic method. Gold is a material
of the cathode electrode, silver is used for the anode electrode and silver/silver chloride is
used for the reference electrode. The reference electrode regulates the potential to be
constant between the sample and cathode electrode (Fig. 2).
There are the other ways of measuring residual chlorine, galvanic method and 2-pole
polarographic method. The circuit applies no potential between the cathode and the anode
electrode for the galvanic method. The anode electrode is oxidized and produces electrons in
the anode metal. The electrons are carried to the cathode electrode to reduce residual
chlorine. Those electro chemical reactions occur at the same time. The circuit applies an
intentional potential to help chemical reaction between the cathode and the anode electrode
for the 2-pole polarographic method. These two methods have an disadvantage that the
reading is affected by the sample property (conductivity and residual chlorine concentration).
The 3-pole polarographic method reduces these problems by keeping potential between
sample and cathode constant. The electro chemical reactions on the cathode and anode
electrode with a polarographic circuit are expressed as follows.
Acting electrode: Cl
2
+ 2e
-
2Cl
-
Opposite electrode: Ag + Cl
-
AgCl + e
-
On the surface of the cathode electrode, residual chlorine is reduced to be chloride ion by the
electron given from the cathode electrode. The electron is produced in the anode electrode by
the oxidation of the anode metal. The residual chlorine concentration at the boundary of
cathode surface is zero because residual chlorine is reduced to be chloride on the surface.
The signal current is proportional to the concentration of the residual chlorine and diffusion
velocity toward the cathode surface. The signal current is not proportional to the bias potential
under optimum measuring condition. The signal current is proportional to the diffusion velocity
which is determined by temperature and concentration of residual chlorine. This current is
called diffusion current. The diffusion current is expressed as follows.
Id (diffusion current) = K (constant)
D [t] (diffusion coefficient)
C (concentration of reacting
substance)
Fig. 2 Circuit diagram of HR-200
0.1 V to 0.2 V
Reference
Cathode
Anode
h19136 | UTC 2020/03/30 3:34:51
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