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Add buffer and indicator solution to a known volume of water.
After mixing, measure wavelength of light transmitted through sample
to determine colour change and thus, amount of free residual chlorine
in the sample.
High pH is not a limiting factor.
EPA approved DPD measurement.
No pH or temperature output.
Free Residual Chlorine— (DPD), The reaction takes place at a
buffered pH of 6.3 to 6.6.
Total Residual Chlorine— By adding potassium iodide to the
sample, chloramines in the sample oxidize iodide to iodine,
which then oxidizes the DPD indicator to the magenta color at
a buffered pH of 5.1.
A sensor consisting of a membrane (which allows HOCL to
migrate through it), two dissimilar metal electrodes, and an
electrolyte are submersed into the water sample.
A free residual chlorine reading is derived based on mV
changes experienced by the sensor.
Ease of use
No reagents or buffers added
How does the chlorine sensor work
Current is proportional to
diffusion rate, which is
proportional to concentration
HOCL + H+ + 2e- Cl- + H2O
2Ag + 2Cl - AgCl + 2e-
Amperometric sensor measures
FREE CHLORINE AND THE PH PROBLEM
Three ways to solve the
Restrict applications to
only those having
Use acid to adjust the
pH of the sample
Measure pH continuously
and perform automatic
7.0 7.5 8.0 8.5 9.0 9.5
pH at 25 C
The reagent solution...
Add an acid to the sample to lower
pH to about 6.
Acid converts OCl to HOCl, which
the sensor measures.
– requires sample conditioning
– requires reagents acid
HOCl + OCl HOCl
The Rosemount Analytical approach
Use continuous pH correction
Good between pH 6.0 and 9.5.
– Below pH 6.0, don’t need correction.
– Above pH 9.5, usually there is too little
HOCl available to measure.
If pH varies less than 0.2 peak-to-peak,
don’t need continuous pH correction.
If pH varies more than 0.2 peak-to-peak,
use continuous pH correction.
Free chlorine sensor - 499ACL-01
Easy to replace membrane - no tools
Range: 0 - 20 ppm
Linear range: 0 - 6 ppm
Linearity: 2% (typical). Following calibration
in the linear range, the measured
concentration of any point in the linear range
is within about ±0.1 ppm of the expected
Electrolyte life: about three months
Variopol quick disconnect option makes
replacing the sensor easy.
Determination of total chlorine
No single sensor can be used to determine all the different forms
of total chlorine.
The only way to measure total chlorine is to convert it into a form
the sensor can measure.
Requires a sample conditioning system. Most manufacturers use
an off-line system.
acetic acid buffer
with potassium iodide
total chlorine iodine
The concentration of iodine produced by the reaction between total chlorine
and potassium iodide is directly proportional to the concentra-tion of total
chlorine in the sample.
Can be used as disinfectant
Chloramines: effective vs. bacteria but NOT viruses.
Chloramines not powerful enough to form THMs.
Last a lot longer in the mains than free chlorine,
C + 2Cl2 + 2H2O → 4HCl + CO2
When dissolved in water, sodium bisulfite (SBS) is formed
from SMBS (Sodium metabisulfite) :
Na2S2O5 + H2O → 2 NaHSO3
2NaHSO3 + 2HOCl → H2SO4 + 2HCl + Na2SO4
Remember: You’re measuring the absence of chlorine, not chlorine.
chlorinated water sample
(for calibrating and checking
Linear velocity in basin is
typically not high enough
for a good measurement.
Basins and tanks:
Rarely is there enough flow
through a tank to provide
Best practice is to use an
extracted sample and the
FCL arrangement for flow
Loss of flow
Current depends on diffusion rate through
A dirty or fouled membrane blocks the passage of chlorine. The result is low
readings and sluggish response to concentration changes.
Diffusion rate (ie sensor current) depends on the concentration of
chlorine at the surface of the membrane.
As the sensor operates, it depletes the chlorine at the membrane
Diffusion from bulk liquid is not adequate to replace loss, so
Sample flow (cont’d)…
Flowing sample keeps the concentration of chlorine at the
surface equal to the bulk concentration.
Sensor current is proportional to the concentration of chlorine in
the bulk solution.
Sample flow is important
Normally, sensor current is a non-linear function of flow.
There is a minimum flow above which the sensor current is more or
less independent of flow.
Each type of sensor has an optimum flow.
minimum flow rate
Automatic cleaning systems
water or air line
spray cleaner activated
Commonly used with oxygen sensors.
Drinking water filter plant
backwash reclaim pond
Desalination/wastewater reclamation plant
pH Cl T
Waste treatment plant
to sludge system
Cl typically total chlorine