Metex 8760CLP Total Free Chlorine & pH Analyzer - Manual (Page 21)

AQUAMETRIX INC.
1-800-742-1413 www.aquametrix.com
21
In waters between 5 pH and 8.5 pH, the reaction is incomplete and both species are present to some
degree. Since H
+ is one of the ions that is formed and it's concentration is expressed as pH, it follows
that changing pH levels will influence the balance of this reaction and with it the availability of
hypochlorous acid for reaction.
In a water environment, the water pH will affect the chemistry of chlorine due to it's pH sensitivity.
This becomes important as pH rises.
6) H
2
O — H
1
OH
1
(preference is right-to-left)
Three things follow from this form of ionization:
1. Since the tendency of these two ions to react and form
H
2
O is much stronger than the tendency of water to
break down into the ions, it follows that as the pH rises
there are fewer H
+ ions and more OH - ions.
2. The H + released by the breakdown of HOCl (equation
5) react to form water (equation 6) and leave behind
residual OCl
- (hypochlorite) ions. Hypochlorite does
not react readily, so the chlorine is weaker.
3. If the pH goes down and H + ions become readily
available again, the OCl - ions revert to HOCl, which is
the killing form of chlorine. This pH change has been
known to cause surprise downstream fish kills.
Terminology
In the industry, there are a number of terms used to
indicate the various forms of chlorine that are of interest. These terms tend to be used rather loosely and
not necessarily consistently. For that reason, AquaMetrix will define the following terms for purposes
of this instruction manual and the 8760CLP system:
FREE AVAILABLE CHLORINE refers to the hypochlorous acid (HOCl) form of chlorine only. It is
said to be free available because it is the free, uncombined form of chlorine that is effective for killing.
TOTAL FREE CHLORINE refers to the sum of hypochlorous acid (HOCl) and hypochlorite ion
(OCl
- ). The hypochlorite ion is not effective for killing, but it is in a free form. All of the total free
chlorine would be in the form of hypochlorous acid if the pH is low enough.
COMBINED CHLORINE refers to chlorine which is not readily available, is not an effective
disinfectant and will not readily convert to hypochlorous acid or hypochlorite ion. For example,
chlorine combined as chloramines or organic nitrogen.
TOTAL RESIDUAL CHLORINE refers to the sum of total free chlorine and combined chlorine. In
environmental studies low total residual chlorine is of particular interest to ensure no downstream
consequences for aquatic life. Total residual chlorine is commonly monitored for final effluent.
Illustration 15: Chlorine concentration vs. pH