Metex 8760CLP Total Free Chlorine & pH Analyzer - Manual (Page 23)

AQUAMETRIX INC.
1-800-742-1413 www.aquametrix.com
23
8760CLP CHLORINE MEASUREMENT
Introduction
Chlorine in water is a measure of the amount of chlorine, usually thought of as a gas, that is dissolved in
the liquid. Chlorine is widely respected as a leading chemical for the treatment of water to make it
potable or safe to drink. In addition, free available chlorine is often used to control biological agent
growth in water filled industrial systems. The 8760CLP directly measures free available chlorine using
a model (P/N AM-A2104034) galvanic chlorine sensor.
Galvanic Measuring Cell
The chlorine measuring sensor, P/N AM-A2104034, is an electrochemical cell similar to a battery that
produces a current when chlorine is present. By using carefully selected electrodes, in contact with an
appropriate electrolyte, a chemical reaction occurs that uses electrons gained from chlorine molecules to
produce a galvanic current directly proportional to the concentration of chlorine present. Illustration 16
shows how such an electrode system works in a simple laboratory test. Illustration 17 shows how these
scientific principles can be implemented into a working chlorine electrode. Also, unlike an electrolytic
cell in which a flow of current produces the chemical reaction, there is no zero-current as galvanic
current is naturally zero when zero chlorine is present.
The chlorine sensor uses a galvanic cell separated from the sample by a chlorine permeable PTFE
membrane. The cell has a gold cathode in close contact with the PTFE membrane where chlorine gains
electrons (is reduced) to become chloride ions, and a silver anode that produces a fixed potential and
completes the reaction with the chloride to form silver chloride.
The chemical reactions within the cell are;
At the cathode: Cl
2
2 e 1 2 Cl 1
At the anode: 2 Ag 2 Ag 1 2 e 1
Overall: Cl
2
2 Ag 2 AgCl
Illustration 3: Galvanic chlorine sensor
Illustration 2: Basic galvanic cell