Free
Chlorine
or
Total
Chlorine?
a Pyxis Lab® Inc. Technical Overview
by Dr. Caibin Xiao
The concepts of free chlorine and total chlorine are misleading and the cause for significant con-
fusion in the water treatment industry. In this short paper, we would like to provide an overview
of the underlying chemistry to understand the use of the terms related to the measurement and
control of chlorine in a real-world process.
Sodium Hypchlorite as the Chlorine Source
Chlorine exists as hypochlorous acid and hypochlorite ion in waters that do not contain any organic
or inorganic nitrogen compounds. The ratio of these two species is dependent on the water pH and
temperature. Above pH 8.5, hypochlorite ion is the dominant species.
HClO(aq) > H+(aq) + ClO-(aq)
The free chlorine concentration is the stoichiometric total of these two species regardless of the
water pH and temperature. Some people may think that free chlorine should only include hypo-
chlorous acid because the biocidal power of hypochlorite ion is believed to be less than hypochlor-
ous acid.
Figure 1. Chlorine & Bromine speciation as a function of pH
Monochloramine
Sodium hypochlorite reacts with ammonium ions rapidly in the pH range common in the municipal
and industrial waters. When the chlorine to ammonium molar ratio is less than 1:1, or the chlorine
to nitrogen ppm ratio (Cl2/N, ppm/ppm) is less than 5:1, monochloramine, NH2Cl, is the only chlo-
ramine species is produced. Monochloramine is a chemically well-defined species although its pure
form is not stable to be separated.
Beyond Monochloramine & the Breakpoint
When the chlorine to ammonium ratio is greater than 1, dichloramine starts to form. This is com-
monly referred to as the breakpoint chlorination process. As dichloramine is forming, a series of re-
actions happen leading to dichloramine decomposition to nitrogen gas and non-oxidizing chloride.
This is the reason why the breakpoint curve has a dip – as more chlorine is pumped to a system, the
monitored ORP or total chlorine declines rather an increase. This decline is not an artifact of any
measurement method used. It is a real decrease of the total oxidizing species due to the decompo-
sition reaction below. When this happens in a water stream containing above 10 ppm total chlorine,
nitrogen bubbles are generated and visible to the naked eye.
NH2Cl + NHCl2 N2 + 3HCl
Figure 2. Breakpoint curve
DPD Free & Total Methods
DPD stands for N,N-diethyl-p-phenylenediamine. It reacts with chlorine to form a red colored spe-
cies at pH between 5.0 - 6.5 that can be analyzed with a spectrophotometer or a LED-based colorim-
eter at wavelength around 560nm. The DPD reagent containing the necessary buffer can be formu-
lated as a single solid mixture or a liquid. The liquid reagent is less stable than the solid mixture. For
inline applications, two DPD reagents are used. The DPD containing liquid reagent is acidified to
very low pH to prevent DPD from being oxidized by oxygen. The second liquid reagent is a buffer.
The free chlorine concentration determined by the free DPD method is the stoichiometric total of
both hypochlorous acid and hypochlorite ion regardless of the sample pH.
The free chlorine concentration determined by the free DPD method is the stoichiometric total of
both hypochlorous acid and hypochlorite ion regardless of the sample pH.
The DPD free chlorine method can determine as low as 0.01 ppm chlorine. The challenge for ul-
tra-low range chlorine analysis is variation caused by the reagent color background.
Total chlorine is the total account of free chlorine and all chloramines, including monochlora-
mine, dichloramine, and other organic chloramines. DPD can react with iodide in the same way
as the chlorine iodide reaction. Most chloramines can react with iodide to form iodine. The total
DPD reagent pack contains iodide to determine the total concentration of chloramines including
monochloramine and dichloramine via iodine chemistry.
Figure 3. Converting chloramines to iodine
DPD without iodide also reacts slowly with some chloramines including monochloramine, especial-
ly when the sample temperature is above 70 Fahrenheit (or 21 oC). For this reason, the DPD free
method is not entirely selective to only free chlorine. If the chloramine concentration is high, one
can easily have a detectable amount of free chlorine with the free chlorine reagent. DPD can be
oxidized by almost all commonly used oxidizing biocides and other oxidizers such as oxygen and
MnO2.
The Indophenol Method for Monochloramine
The indophenol method for monochloramine is a monochloramine specific colorimetric method.
The reagent selectively reacts with monochloramine to form a green-color indophenol. If the water
does not contain other chloramines, the total DPD chlorine concentration should agree with the
concentration by the indophenol method. Since the total DPD method can pick up dichloramine and
the indophenol does not, the total chlorine concentration will be greater than the monochloramine
concentration if beyond breakpoint chlorination occurs and dichloramine starts to form.
Another way to detect the onset of dichloramine formation is to test free chlorine in the monochlo-
ramine treated water. The DPD free chlorine reagent does not react with monochloramine in a
significant amount at sample temperature below 80 ºF within 60 seconds. For municipal waters
treated with monochloramine, less than 0.1 ppm chlorine is commonly determined by the free DPD
reagent. If the free chlorine concentration determined by the free DPD reagent is above 0.1 ppm,
it could be an indication of dichloramine formation. In a well-treated monochloramine water, total
chlorine concentration should be in close agreement with the monochloramine concentration, and
the free chlorine concentration should be less than 0.1 ppm. Adding to the confusion, when we talk
about ppm monochloramine we are talking about ppm monochloramine as ppm chlorine rather
than ppm NH2Cl.
Industrial Cooling Water
Sodium hypochlorite and, less commonly, chlorine gas are used for biological growth control in
the industrial cooling water systems. Inevitably, cooling water in the cooling systems contains
intentionally added nitrogen containing treatment chemicals, such as azoles, and natural occur-
ring organic nitrogen containing compounds from the makeup. For some cooling systems, process
chemicals leaking into the cooling water is another source of nitrogen containing contaminants.
Thus, the cooling water contains a range of oxidative chlorine species, including free chlorine and
chloramines.
Tri & Di-Chloroisocyanuric Acid
Trichlor and dichlor isocyanurate are chloramines. Through a series of hydrolysis reactions when
these chlorine precursors are dissolved in water, more than 10 oxidative species including hypo-
chlorous acid and hypochlorite are formed. The free chlorine to the total chlorine ratio is a function
of cyanurate concentration, pH, and the system demand on chlorine. The free chlorine DPD method
may not be a consistent way to monitor the chlorine dose. Instead, the total chlorine DPD method
offers more consistency.
Figure 4. Chlorine Precursor
Bromine & Bromine Precursors
Many small and medium sized cooling towers are treated with bromine. Bromine is added to the
cooling water system either from a bromine precursor such BCDMH (bromochlorodimethylhydan-
toin) or a sulfamate-stabilized liquid bromine formulation. These bromine precursors are bromoam-
ines by definition. They hydrolyze in water to form free hypobromite. In water without demand,
a molar BCDMH can produce one molar bromine and one molar chlorine. The latter hydrolyzing
reaction is slow. The DPD free method will pick up the bromine portion while the total method will
pick up the total including the contribution from CDMH.
It is unclear if the bromine precursors completely hydrolyze to form hypobromous acid and hypo-
bromite. Bromoamines are more reactive toward DPD. The DPD free method will certainly detect
portion of total bromoamine while the total DPD method will detect the total concentration includ-
ing hypobromous acid, hypobromite, and unhydrolzed bromine-sulfamate. For dosing control pur-
poses, either free bromine by the free DPD method or the total DPD method can be used. Like the
situation for chlorinated systems with heavy nitrogen containing contaminants, the total bromine
by the total DPD method should be periodically checked to prevent excess amount of bromoam-
ines.
Figure 5. Sulfamate stabilized bromine hydrolyzing to bromine
Figure 6. BCDMH and CDMH
Figure 7. BCDMH hydrolizing to bromine
Methods to Measure Free-Total Chlorine & Monochloramine other than DPD
Methods for free chlorine analysis can be roughly categorized into three:
1. Regeaent based spectrophotometric methods. DPD is the most widely used reagent for the
spectrophotometric determination of chlorine.
2. Direct spectrophotometric method. Hypochlorous acid has a distinct absorption band around
280 nm. The monochloramine band is at 245 nm while the dichloramine has a strong band at 203
nm and a weak band at 290 nm. These UV absorption bands can be used for the direct spectro-
photometric determination of free, monochloramine and dichloramine. It is especially useful to
simultaneously monitor both monochloramine and dichloramine in monochloramine generators.
3. Electrochemical based methods.
The electrochemical method is based on the property that hypochlorous acid can be reduced at
gold or platinum electrode. The sensors for chlorine can be further divided into two categories. The
membrane based amperometry sensors have a porous membrane separating the sensing electrode
and the inner electrolyte from the sample water containing chlorine to be determined. The bare
gold type of amperometric sensors do not have the inner electrolyte. The sensing electrode is in
direct contact with the chlorine species to be determined in the water sample.
In membrane based amperometric sensing, the sensor signal is proportional to the diffusion rates
of hypochlorous acid and hypochlorite ions through the sensor membrane. The diffusion rates of
these two ions are quite different. Thus, the sensor signal is a function of the sample pH. To report
the free chlorine concentration as the total of hypochlorous acid and hypochlorite ion, comparable
with the DPD free method, a pH compensation must be implemented in the aperometric sensor.
In non-membrane amperometric sensing, the sensor signal is proportional to the diffusion rates of
hypochlorous acid and hypochlorite ions from the bulk water directly to the sensor electrode sur-
face. Like the membrane-based sensor, a pH compensation is required for the free chlorine concen-
tration.
Membrane based amperometric sensors offer some selectivity towards different oxidative species
by selecting the membrane material and inner electrolyte. This type of sensor cannot distinguish
free chlorine from chloramines and will detect a mix signal reflecting the compound contribution
from all oxidizing species that can penetrate the sensor membrane. Since the sensor has a different
sensitivity to different chloramine agents, the compound signal may not be equivalent to the total
chlorine concentration determined by the total DPD method. The total chlorine sensor can be only
used in the system with one chloramine, or the system is constant with regarding the chloramine
speciation. In this situation whether the sensor is free or total chlorine depends on the concentra-
tion value being calibrated against.
The membrane less bare gold amperometric sensor does not rely on the membrane diffusion mech-
anism. Monochloramine and other chloramines exhibit a small but significant amount of response
on the electrode. This is beneficious for obtaining a signal closer to the DPD total for the compli-
cated industrial system. If properly calibrated, the bare gold electrode sensors can be ideal for the
monitoring and dosing control of chlorine and bromine.
What Should We Measure, Free or Total?
To answer this question, we must understand our goal of measuring chlorine. We want to measure
chlorine for the purpose of disinfection. For drinking water, national governments have a chlorine
range as a requirement. For industrial systems, guidelines from a variety of organizations or local
experience have chlorine targets. A source of confusion is that free chlorine is believed to be a
more affective biocide than chloramines. For this belief, people want to achieve a higher measur-
able free chlorine as the target rather than a reasonable total chlorine target.
For municipal drinking water treated with sodium hypochlorite or chlorine gas, we just need to
measure free chlorine. The DPD colorimetric method is a government accepted standard method.
Inline amperometric sensors can be calibrated to the free chlorine value measured by the DPD
method. Largely, the chlorine concentrations measured by the DPD free method and total method
are in close agreement. Some water may exhibit slightly higher chlorine value by the total method.
For industrial systems containing a range of oxidizing species, one can measure either free or total
chlorine as the control target. However, if free chlorine is selected to be a routine monitoring tar-
get, total chlorine must be measured periodically to access the system total demand to avoid two
following scenarios.
1. Dosing more and getting less. Like the ammonium – chlorine breakpoint phenomenon, the
presence of other organic or inorganic nitrogen compounds in the industrial water could make
chlorination process complicated. After all nitrogen containing compounds being converted to
mono-chloramines, further adding chlorine will result in a decrease in total chlorine or net oxi-
dizing power loss. By comparing the relative trend and ratio of free chlorine and total chlorine
concentrations, over chlorination could be avoided.
2. To achieve a given amount of free chlorine results in an excess amount of total chloramine.
Systems containing a large amount of nitrogen containing compounds can have a large amount
of chlorine demand. If the chlorine control target is solely based on the free chlorine concentra-
tion, a large amount of chlorine will be consumed to form chloramines to achieve the desired
free target. For this type of system, the fundamental question is whether chloramines are ef-
fective for the purpose of disinfection and how much chloramines as total chlorine is needed
to achieve the disinfection goal. I have encountered cooling systems that reached 10 ppm total
chlorine to have 0.5 ppm free by the free DPD method. The excess amount of chlorine dosed
does not only result in wasting chemicals but also causing an increasing amount of corrosion.
In many large municipal water systems, monochloramine is the only chlorine species. Little
free chlorine (less than 0.05 ppm) is detected with the free DPD method. The concentration
of monochloramine determined by the total DPD should agree with that determined by the
monochloramine specific method that is based on the reaction of monochloramine with indophenol
to form a blue compound. If the concentration by the total DPD method is greater than that by the
indophenol method, it is likely that the water sample contains both monochloramine and dichlora-
mine. Dichloramine can be detected by the iodide containing total DPD method. The discrepancy
between these concentrations is an indication of the chlorination approaching the breakpoint.
An Example
A Pyxis Lab® OXIPANEL™ IK-765SS-B Series self-brushing panel containing the Pyxis Lab® ST-765SS
series bare-gold sensor was installed in a data center cooling tower. The tower was treated with
stabilized bromine. The following chart is a comparison of the sensor reading with offline DPD
total values. Since 2019, the ST-765SS Series bare-gold sensor platform has been deployed globally
for a wide variety of different applications, including municipal water treated with free chlorine,
chlorine dioxide, and monochloramine, simultaneously measuring sulfite and chlorine for RO feed-
water and wastewater effluent dichlorination and ozone on bottling water process.
Figure 8. Pyxis Lab® OXIPANEL™ Bare-Gold Sensor vs. DPD Wet Chemistry Test Results
Conclusion
For complicated industrial waters, both free chlorine and total chlorine are empirical terms. The
free DPD method can pick up contribution non-free chlorine oxidizers. The total DPD method mea-
sured all oxidizers including chloramines that can convert iodide to iodine. Because different chlo-
ramines have different reaction kinetics and reactivity to iodide, the total chlorine certainly does
not account for all chloramines stoichiometrically. For dosing control, one could choose either free
or total as the primary control target and check the second periodically to prevent grossly over or
under biocidal feeding. The bare gold electrode sensor measures a mix of signals from all oxidizers
that can be electrochemically reducing on the gold electrode, with appropriate calibration to either
free or total DPD chlorine, the sensor can be a reliable way for monitoring and dosing oxidizing bio-
cide to a range of applications including industrial cooling towers.
Pyxis Lab, Inc.
1729 Majestic Drive (Suite 5)
Lafayette, CO 80026
www.pyxis-lab.com
