Chlorine dioxide therapy (CDT)
Chemical properties of chlorine dioxide (ClO2) molecule
About Chlorine Dioxide (ClO2 )
ClO2 history
The discovery of ClO2 •
Dates back to the early 1800's, when Sir Humphrey Davy created
the compound by mixing sulfuric acid with potassium chlorate.
Chemical properties of chlorine dioxide (ClO2 )
ClO2 is a chemical compound that consists of one
chlorine ion in the +4 oxidation state covalently (non-metal
to non-metal) bound to two oxygen ions.
its chlorine is in ionic form as part of a compound, such
as in sodium chloride (NaCl / common table salt). This is not the same as chlorine
in its elemental form. For perspective - chlorine is the most abundant dissolved
ion in ocean water.
ClO2 is a relatively small, volatile, highly
energetic molecule and a free radical even while in dilute aqueous solutions.
Although, it is stable in a dilute solution in a closed container
in the absence of light. i.e. diluted watery solutions
At high concentrations, it reacts violently with reducing agents ClO2 functions
as a highly selective oxidant ClO2 -
EPA
Guidance Manual 1999
Lennech Tech Water Treatment Solutions
ClO2 is a synthetic, green/yellowish gas with
a chlorine-like odor. ClO2 is a true gas (like oxygen)
and so retains its chemical structure and properties at all times. ClO2 gas is 2.4
times denser than air. At extremely high concentrations (>10%) in air it can
be explosive (this is nothing the therapeutic user needs to worry about).
Although it has an irritating chlorine-like odor and contains a chlorine atom
in its molecule, ClO2 exhibits physical and chemical properties that are
dramatically different from those of chlorine
ClO2 stability depends on environment:
• In the
absence of light. ClO2 is stable in a dilute
solution in a closed container
• In
AIR. ClO2 is an unstable gas that dissociates
into chlorine gas (Cl2 ), oxygen gas (O2 ) and
heat.
AWWA. 1990. Water Quality and Treatment, fourth edition. McGraw-Hill,
Inc., New York, NY.
•
Sunlight (photo-oxidation).
Qu ickly breaks ClO2 into Cl2 and O2 .
ClO2 can be removed by
aeration or carbon dioxide
ClO2 disinfects by oxidation, not chlorination
Differentiating factors between chlorine dioxide and chlorine
MOST IMPORTANTLY, ClO2 behaves differently
to chlorine in water
- ClO2 is
10 times more soluble than chlorine in water (3.01
g/L at 25°C). E specially in cold water;
when introduced into water
or other solvents it does not hydrolyze
(i.e. form another compound) and remains a dissolved gas in solution. Because of
this fact, it retains its structure and reactive
properties
- In contrast, chlorine
in water dissociates to form:
• Hypochlorous
acid. The main active biocide in solution, which with increasing
pH dissociates to form hypochlorite ion, having only 1/20 to 1/300 of the effect
of hypochlorous acid at controlling microbes. The efficacy of chlorine as an effective
biocide is reduced with increasing pH
• Hydrochloric
acids
- ClO2
is highly soluble in organic (hydrocarbon)
substances. E.g. oils, solvents, making its
oxidative / biocidal properties potentially useful in other applications
ClO2 reacts with other substances via oxidation, not
substitution (i.e. unlike chlorine, ClO2 does not chlorinate.
ClO2 contains no elemental free chlorine)
- ClO2 is more SELECTIVE
in its reactions with other substances than chlorine. Since ClO2
has lower oxidation strength (Oxidation/Reduction Potential or ORP) than chlorine,
but more than twice the oxidative capacity . ClO2 functions as a highly selective
oxidant due to its unique, one-electron transfer mechanism, where its reaction end-products
depend on pH
pH
Reaction
End product(s)
Low pH
(Acid)
With acidification,
ClO2 first reduced to chlorite ClO2 -
anion and finally to chloride Cl- anion after accepting
5 electrons
(ClO2 has an oxidation number of
+4). The chloride anion remains until stable chloride is formed
as a salt
Chloride anions (Cl- )
Neutral
or high pH
Sulphuric acid reduces ClO2 to chlorite
ions (ClO2-).
Chlorite anions (ClO2 -)
High pH
(Alkaline)
ClO2
is broken down to chlorite (ClO2 - ) and chlorate
(ClO3 - ): 2ClO2 + 2OH-
= H2 O
+ ClO3 - + ClO2 -
Catalyzed by hydrogen
(H+) ions
Chlorite anions (ClO2 -)
Chlorate anions (ClO3 - )
In drinking water, chlorite (ClO2 - ) is the predominant
reaction end-product, with approximately 50-70% of the chlorine dioxide converted
to chlorite and 30% to chlorate (ClO3 - ) and chloride (Cl)
Werdehoff, K.S, and P.C. Singer. 1987. "Chlorine
Dioxide Effects on THMFP, TOXFP and the Formation of Inorganic By-Products."J.
AWWA. 79(9):107.
http://www.epa.gov/ogwdw/mdbp/pdf/alter/chapt_4.pdf
Oxidation/Reduction Potential
(ORP):
• Oxidation strength
or tendency of a chemical to acquire
• Measured in volts
- the more positive volts, the greater affinity for electrons and tendency
to be reduced.
• The metabolism
of microorganisms and consequently their ability to survive and propagate
are influenced by the ORP of the medium in which they live
US EPA, 1996
Oxidation Capacity:
• Number of electrons
transferred during an oxidation/reduction reaction
Comparing CL2 and CLO2 oxidative strenghth
• The powerful oxidizer chlorine
(Cl2 ) tends to produce toxic by-products by reacting with
(and thus limiting its effect as a biocide):
(a) Hydrocarbons
(C-H) to produce toxic organic chlorides
(C-Cl) E.g trihalomethanes (THMs)
and (b) Various amines (C-NH2)
to produce toxic chloramines (C-NH-Cl) -amines
are derivatives of ammonia (NH3)
• In contrast, the weaker oxidizer
ClO2 does not react with ammonia and most organic substances and
does not generate any significant quantities of undesirable/toxic organochlorine
compounds.
Typically, ClO2 ONLY
(preferentially) reacts with:
(a) Compounds with activated carbonbonds
-E.g. phenols (ArOH); does not break carbon bond; this property to destroy phenols
is useful in water disinfection, since phenols can be responsible for an odor or
taste
(b) Reduced metals
-ferrous (Fe++), manganous (Mn++)
(c) Thiols (RSH)
(d) Aldehydes (RCHO)
(e) In aqueous solutions, tertiary amines (RNR'R")
(producing secondary amine and an aldehyde) and very slowly or not at all with primary
and secondary amines (RNHR') and other active compounds. E.g. sulfides,
cyanides
Hull LA, Davis GT, Rosenblatt DM, Williams HKR, Wegein RC,
Oxidation of amines, duality of mechanism in the reaction of amines and chlorine
dioxide . 1967. J. Am. Chem. Soc. 89, 1163,
(f) Other active compounds
-E.g. sulfides, cyanides
Oxidation/REDUCTION Potential (ORP) and OXIDATION
CApacity of Various Oxidants
Oxidant
Oxidation Strength
(Volts)
Oxidation Capacity
(how many e- s are transferred)
Available chlorine (%)
Per molecular weight
Ozone (O3 )
2.07
2 e-
Hydrogen peroxide (H2 O2 )
1.78
2 e-
Hypochlorous acid (HOCl)
1.49
2 e-
Chlorine (Cl2 )
1.36
100
Hypobromous acid (HOBr)
1.33
2 e-
Oxygen
1.28
0 e- (O2 Gas)
1 e- (peroxides)
2 e- (oxides)
Chlorine dioxide (ClO2 )
0.95
5 e-
263
Comparing CL2 and CLO2
oxidative capacity
Having an oxidation number of +4, the chlorine atom in
the ClO2 molecule accepts 5 electrons when reduced to chloride ion.
The oxidation capacity represents the "available chlorine"in the molecule,
so you will notice in the chartabove that ClO2contains 263% "available chlorine"compared
to 100% for chlorine i.e. > 2.5 times the oxidation capacity of chlorine. Having
greater oxidative capacity compared to other oxidizers, such as ozone or chlorine,
means that less chlorine dioxide is required to obtain an active residual concentration.
