IM 50.560CA UA (B/4-03)
DEPOLOX
®
3
PLUS
RESIDUAL ANALYZER
50
c. Check flow and reassemble.
d. Perform calibration.
3.3.3
Bare Electrode Theory of Operation
The free chlorine that is measured in water for disinfection purposes
comes in three forms. Free chlorine consists of elemental chlorine (Cl
2
),
hypochlorous acid (HClO), and hypochlorite ions (CIO-).
Hypochlorous acid is found in the acidic to neutral pH range undissoci-
ated in water. Only when the pH value rises does it break down into H+
and ClO- ions.
To understand the mode of functioning of chlorine as a disinfectant, it is
necessary to know the chemical reactions that take place when water is
chlorinated. Firstly there is the hydrolysis of the chlorine. This is the
reaction of the chlorine with water. When chlorine gas is dissolved in
water the formation of hypochlorous acid and hydrochloric acid results.
All other disinfectants based on chlorine also react with water and form
hypochlorous acid. The hypochlorous acid is the active substance for the
disinfection process. Figure 3.6 shows the dependence of the form of
chlorine on the pH value of the water. With rising pH value, the share of
hypochlorous acid decreases by dissociating to form H3O+ and ClO-. In
other words, the hypochlorous acid needed as the active substance in
disinfection becomes less with increasing pH value while the share of
ClO- ions that are meaningless for disinfection rises.
The Free Chlorine (Bare Electrode) measuring cell consists of a three-
electrode system with external potentiostatic closed-loop control. The
working electrode and the counter electrode of the half-ring type are made
of a platinum alloy. A silver/silver-chloride electrode serves as reference
electrode; the contact between reference electrode and sample water is
established by two membranes. The reference electrode is mounted in
PVC brackets and completely submerged in electrolyte.
The electrolyte container is transparent, thus enabling a visual check to be
made of the amount of electrolyte in the reservoir. Replenishment of the
electrolyte is possible without shutting down the measuring cell system.
A membrane in the plug of the electrolyte reservoir provides the neces-
sary equalization of pressure.
The measuring cell is connected to a digital measuring amplifier which
maintains an adjustable constant potential (Upot) between working and
reference electrodes by means of potentiostatic closed-loop control.
