Chlorine Theory & Measurement - IC Controls
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CHLORINE THEORY & MEASUREMENT  
Introduction Chlorine Chemistry
Chlorine, dissolved in liquid, is one of the most The different  forms of  chlorine  are  named as
effective  and  economical  germ-killers  for  the follows:
treatment of water to make it potable or safe to
Cl2 = chlorine
drink. Chlorine's powerful disinfectant qualities
come from its ability to bond with and destroy HOCl = hypochlorous acid
the  outer  surfaces  of  bacteria  and  viruses.
OCl- = hypochlorite ion
Drinking water chlorination is one of the most
widely  used  methods  to  safeguard  drinking
water supplies. In addition to water disinfection, NOTE: At atmospheric pressure and 20 °C, the
chlorine  and its  derivatives  are  used to  treat maximum solubility of chlorine is about 7,395
sewage  and  industrial  effluent,  and  as mg/L or 7.395 ppm (parts per million).
household  and  industrial  bleaches  (eg.,  to When  chlorine  gas  is  dissolved  in  water,  it
control  biological  agent  growth in  water  filled hydrolyzes  rapidly  according  to  equation  (1).
industrial  systems)  to  maintain  and  improve This reaction occurs very rapidly, in only a few
hygiene standards. tenths of a second at 18 °C (64.4 °F).
(1) Cl2  + H2O —> HOCl + HCl
What is chlorine?
Chlorine  (Cl)  is  among  the  most Since HCl,  hydrochloric  acid,  is  a strong acid,
abundant of nature's elements, and addition of gaseous chlorine to water results in
combines  with  other  elements  to
sustain  life  and  the  natural a  lowering  of  the  pH  from  the  acidic  HCl
processes  that  make  up  our byproduct.  The  important  product  of  reaction
environment.   Chlorine is  found in (1) is HOCl or hypochlorous acid. Hypochlorous
the Earth and, in the oceans. acid  is  the  killing  form  of  chlorine  in  water.
Chlorine  is  highly  reactive and,  as Hypochlorous  acid  is  unstable  because  the
such,  is  typically  found  in  nature
bonded  to  other  elements  like chlorine molecule is lightly bound and therefore
sodium,  potassium,  and will react quickly. Free available chlorine, or free
magnesium.   When  isolated  as  a chlorine, is hypochlorous acid and is taste free
free element, chlorine is a greenish and  aggressive  against  germs  and  organic
yellow  gas,  which  is  2.5  times compounds.  Chlorine  supplied  as  sodium
heavier than air.  It turns to a liquid
state  at  -34  °C  (-29  °F),  and  it hypochlorite, calcium hypochlorite, or bleach is
becomes  a  yellowish  crystalline in  alkaline  or  basic  form.  When  a  base  is
solid at -103 °C (-153 °F). present, a different reaction sequence occurs:
(2) NaOCl + H + -2O—> HOCl + Na  + OH
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(3) Ca(OCl)2 + 2H2O —> 2HOCl + Ca++ + 2OH- (6) H2O