Hach 8450, Instrument Manual

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16116Intro.fm
Introduction
Section 1 Introduction
1.1 Introducing the Sigma Residual Chlorine Monitor Model 8450
Sigma Chlorine monitors are designed for on-line measurement of residual
chlorine concentrations in potable water, cooling towers, or wastewater. The
system consists of two main components: a programmable electronic monitor
housed within a NEMA-4X enclosure and an amperometric membrane probe.
The 8450 Monitor comes fully equipped with a variety of features. Monitor
operation and programming is accomplished via a magnet which, when
touched to the unit’s front panel icons, activates small internal switches and
permits adjustment of the various operation/set-up parameters. Each unit’s
alarm capabilities include two programmable high/low alarms and a fault
alarm. Four analog outputs are provided for operation of external alarm and
control devices. Chlorine levels from 0.00 to 20.0 ppm are displayed on an
auto-ranging LCD where concentrations may be viewed in either ppm or
mg/L. Other features include: password security, optional data logging, and an
optional RS232/RS485 communications port for remote monitoring and
system operation.
Sigma chlorine probes provide the means to directly measure chlorine. The
Model 8450 Monitor and its associated model 50 probes will respond to both
hypochlorous acid and the hypochlorite ion. Probes respond best in liquid
environments of up to a pH of 9; however, probes should be calibrated in the
pH at which measurements will be taken to obtain optimum accuracy. The
probe can be used in a constant-head flow assembly that provides better flow
control and enhances the operating environment for the probe.
1.2 Overview of Chlorine Characteristics
1.2.1 Free Chlorine
When chlorine (Cl
2
) is dissolved in water (H
2
O) it hydrolyzes (the reaction
between an ion and water in which the hydrogen–oxygen bond is broken) to
form hypochlorous acid or free chlorine:
Cl
2
+ H
2
O
→
HOCl + H
+
+ Cl
–
Hypochlorous acid is considered a “weak” acid and therefore tends to partially
dissociate and produce a hydrogen ion and a hypochlorite ion:
HOCl
↔
H
+
+ OCl
–
The combination of HOCl and OCl
–
is referred to as free chlorine. The extent
to which this dissociation occurs depends upon several factors, including the
temperature and pH of the solution. At pH levels between 6 and 9.5, the
dissociation of the hypochlorous acid is incomplete, thus both hypochlorous
acid and the hypochlorite ion will be present within the solution (see
Figure 1 ).
The Model 8450 Free Chlorine Monitor has been designed to measure these
two species of dissolved chlorine.
1.2.2 Probe Response and Effects of pH
The pH of the water has a definite effect on the response of Model 8450 Free
Chlorine probes. In general, probes will begin to exhibit a reduced response
as pH levels increase above 7.5.