Omega FCLTX-100 instruction sheet Section Theory of Operation

PRODUCT INSTRUCTION SHEET

FCLTX-100 Series

SECTION 1.0

THEORY OF OPERATION

1.0FREE 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.

1.2SENSOR 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

2.1pH. 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

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

2.2FLOW. 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).

FIG. 1	Percent of free chlorine in protonated form (HClO)
100

90

80
70
60	OCl-
percent 50HOCl
40
30
20
10
0

5	5.5	6	6.5	7	7.5	8	8.5	9	9.5	10
					pH

TABLE 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

M4679/0708 page 1 of 6

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