Omega FCLTX-100 - Manual

P R O D U C T I N S T R U C T I O N S H E E T

Percent of free chlorine in protonated form (HClO)

0

10

20

30

40

50

60

70

80

90

100

5

5.5

6

6.5

7

7.5

8

8.5

9

9.5

10

pH

percent

SECTION 1.0

THEORY OF OPERATION

1.0 FREE CHLORINE DEFINED. Free Chlorine or "freely active chlorine"
is defined as the sum of molecular chlorine (Cl2), hypochlorous acid
(HOCl) and hypochlorite ions (OCl-). Molecular chlorine occurs at
pH values <pH4. Hypochlorus acid and hypochlorite ions are in pH
dependent equilibrium with one another as shown in FIG 1.

The graph shows % hypochlorous acid on the left of the curve.
Hypochlorous acid is a much stronger disinfecting agent (oxidizer) as
compared to hypochlorite ions.

1.2 SENSOR OPERATING PRINCIPLE. Both hypochlorous acid (HOCl)
and hypochlorite ion (OCl-) diffuse through the membrane between
the cathode and sample solution, even though the diffusion coeffi-
cients for each are different. At the applied potential, only hyphochlo-
rous acid is electrochemically reduced. HOCl is reduced to chloride
ion at the gold cathode. At the same time, the silver anode is oxidized
to form silver chloride (AgCl). When the concentration of HOCl at
the cathode is dramatically decreased by electrochemical reduction,
hypochlorite ion will be transformed into hypochlorous acid , to some
extent, by proton transfer. The release of electrons at the cathode
and acceptance at the anode creates a current flow, which under
constant conditions, is proportional to the free chlorine concentration
in the medium outside the sensor. The resulting low current output
is then conditioned to 4-20mA current by the sensor's onboard elec-
tronic circuitry.

SECTION 2.0

FACTORS INFLUENCING THE SENSOR

2.1 pH. Free Chlorine (FCL) exists as hypochlorous acid and hy-
pochlorite anion (FIG 1). The acid-base dissociation of FCL has a pKa
of approximately 7.5. The FCL sensor responds to hypochlorous
acid and hypochlorite anion with different sensitivity. In combina-
tion, an increase in pH reduces the measured FCL and decrease in pH
increases the measured FCL. The need for automatic pH compensa-
tion depends on the pH value and the variation range of pH ( Table
1). If pH variation of your sample is more than that listed in the table,
automatic pH compensation is required. pH compensation for the
sensors current (mA) reading is:

(sensor output in mA – 4)/(-0.0502pH3 + 0.867pH2 – 5.051pH + 12.43) + 4

2.2 Chemical Interferences. The sensors should not be used in water
containing surfactants, organic chlorine or stabilizers such as cyanuric
acid.

2.2 FLOW. The membrance covered free chlorine sensors (FCL series)
functions at any flow rate. To acheive reproducible measurements,
these free chlorine sensors require a specified constant flow rate. To
avoid complications (such as bubbles), it is best to operate the sen-
sors at a flow rate of 0.2-0.6 gpm if using flow cell FC72 or FC70 (old
version).

HOCl

OCl-

FIG. 1

pH Range

<6.5 6.5-7.5 7.5-8.3 8.3-9.0

pH Variation N/A + 0.35 + 0.20 + 0.05

TABLE 1

FCLTX-100 Series

Parts covered by this product instruction sheet include: FCLTX-100 Series

M4679/0708 page 1 of 6