Hach 8450, Instrument Manual

Page: 10 / 66

Page 10
Overview of Chlorine Characteristics
16116Intro.fm
Section 1
The effect of pH on the Sigma Free Chlorine probe is illustrated in Figure 1 .
The Sigma Free Chlorine probe has a response that goes beyond the HOCl
species, and that it has a usable response to the hypochlorite ion (OCl
–
). This
enables the Sigma probe to be used at higher pH values than if it only
responded to hypochlorous acid. Depending upon expected pH swings, the
Free Chlorine probe response is not significantly affected below a pH of about
8. If the pH is constant, the probe can be calibrated and used at up to pH 9;
however, large pH swings centered at about pH 9 can cause errors. If the pH
is to be kept between 5 and 6 (i.e. 100% HOCl residual), a Sigma CO
2
Sparging system is recommended.
Figure 1 HOCl–OCl
–
Equilibrium and Sigma Free Chlorine Probe Outputs vs. pH
Sigma Standard Free Cl Sensor Response
2
Point (a) — Equal Parts HOCl and OCl
Point (b) — 80% HOCl and 20% OCl (example)
–
–
Greater
HOCl
Greater
OCl
Equal Parts
HOCl & OCl
–
a
b
-
-