YSI 600DW-B Sonde - Manual (Page 292)

Frequently Asked Questions About YSI Drinking Water Systems Appendix H
FREE CHLORINE MEASUREMENT
What is the chemical formula of “free chlorine”?
When chlorine gas (Cl
2
) is added to water, a reaction (called disproportionation) occurs to form
hypochlorous acid (HOCl) and hydrochloric acid (HCl). The hypochlorous acid is a strong oxidant and the
active species in most drinking water disinfection processes rather than gaseous chlorine (Cl
2
). One
molecule of HOCl has the same oxidizing capacity as a molecule of Cl
2
. Thus, the formula for “free
chlorine” is really HOCl rather than Cl
2
. (A portion of the free chlorine will also be in the form of
hypochlorite ion OCl
-
, with the amount depending on the pH of the sample.) However, convention
dictates that free chorine be reported in units of “mg Cl
2
per liter” even though the active compounds in
solution are HOCl and OCl - . Thus, YSI free chlorine readings are expressed in units of “Cl2 mg/L” rather
than “HOCl/OCl - mg/L” to be consistent with standard reporting convention.
How does the YSI sensor measure free chlorine?
The free chlorine sensor operates on the same amperometric principles as the YSI Rapid Pulse dissolved
oxygen sensor which is used in all YSI 6-series sondes designed for surface- and ground-water studies.
Basically, the hypochlorous acid (HOCl) and hypochlorite ion (OCl
- ) formed when gaseous chlorine is
dissolved in water diffuses through a permeable membrane which covers the electrodes of the sensor. The
HOCl/OCl
-
is then reduced at the cathode of the sensor producing a current which is proportional to the
amount of free chlorine present in the solution outside the membrane. As the free chlorine is reduced at
the cathode, the silver anode is oxidized to silver chloride (AgCl). The electrochemistry of the chlorine
sensor is carried out in a “pulsed mode”, with the cathode polarized for only 1/100 of the time (40 mS) and
depolarized for 99/100 of the time (3960 mS) in order to (a) use the existing Rapid Pulse DO circuitry and
software, (b) reduce the flow dependence of the free chlorine sensor, and (c) minimize the deposition of
AgCl on the anode, the removal of which requires a maintenance protocol. One major difference between
the Rapid Pulse oxygen and chlorine sensors is that the reducing and “hold” potentials are set to much more
positive values where chlorine is reduced, but oxygen is not. The second major difference is that a
hydrophobic Teflon membrane is used to cover the electrodes of the dissolved oxygen sensor while a
hydrophilic polyvinylidene fluoride membrane is used with the free chlorine sensor.
Is the extent of the flow dependence reduction from the Rapid Pulse system the same for dissolved
oxygen and chlorine?
No. Under continuous pulsing conditions, the Rapid Pulse DO system exhibits a flow dependence of about
3 % while the Free Chlorine system shows a flow dependence of up to 25 %. The greater flow dependence
of the chlorine system under the same pulse conditions (40 mS “on”/3960 mS “off”) is likely due to the
different diffusion properties exhibited by HOCl with a hydrophilic membrane versus oxygen gas through a
hydrophobic membrane. In any case, the flow dependence of the free chlorine sensor is of great enough
magnitude that a flow of about 500 mL per minute is required to overcome it under continuous pulsing
conditions and hence the requirement for a flow cell for most studies involving Drinking Water sondes with
Free Chlorine sensors.
What do the terms “Total Chlorine” and “Combined Chlorine” refer to?
There are two categories of “chlorine” in most treated drinking water – free chlorine (HOCl + OCl
-
) and
combined chlorine which is made up of species formed in the reaction of free chlorine with ammonia
which is present in the water. Combined Chlorine is thus made up of a general class of compounds called
chloramines with formulae such as NH
2
Cl, NHCl
2
, and NCl
3
. In water which is treated just with chlorine,
there will be a small amount of combined chlorine from the natural ammonia in the water and a large
amount of free chlorine; in water which is treated with both free chlorine and ammonia there will be a small
YSI Incorporated Drinking Water Monitoring Systems Operations Manual H-6