Hypochlorous Acid is a weak acid that partially dissociates into a Hydrogen Ion and a Hypochlorite
Ion as follows:
HOCl
↔
H
+
+ OCl
–
The degree of dissociation depends on the pH and the Temperature. Regardless of Temperature,
below a pH of 5 the dissociation of HOCl remains virtually zero and above a pH of 10 the dissociation
of HOCl is virtually 100%. Figure 1 shows this dissociation curve at several Temperatures. The sum
of Hypochlorous Acid and Hypochlorite Ion is referred to as Free Available Chlorine.
When Ammonia Nitrogen is present in the water, some or all of the Free Available Chlorine will be
converted into Chloramine compounds according to the following reactions:
NH
3
+ HOCl
→
H
2
O + NH
2
Cl (Monochloramine)
NH
3
+ 2HOCl
→
2H
2
O + NHCl
2
(Dichloramine)
NH
3
+ 3HOCl
→
3H
2
O + NCl
3
(Nitrogen Trichloride)
The sum of the Chloramine compounds is referred to as “Combined Available Chlorine”. Also, the
sum of Free Available and Combined Available Chlorine is referred to as “Total Available Chlorine”.
4. Measurement Chemistry:
Free Chlorine Measurements:
As discussed above, Free Chlorine is the sum of Hypochlorous
Acid and Hypochlorite Ion concentrations. Hypochlorous Acid is a reducible species in the Series
210 Residual Chlorine Analyzer. Therefore the measurement cell can be used to measure the
concentration of Hypochlorous Acid.
This measurement can be used to determine the concentration of Free Chlorine by one of two
methods. Consider Figure 1 in the discussion of both methods.
First, an acidic buff er solution can be injected into the water sample stream to reduce the pH below 5,
so that all of the Free Chlorine is in the form of Hypochlorous Acid.
Second, pH and Temperature measurements can be used to continuously determine the degree of
Hypochlorous Acid dissociation through software. The instantaneous degree of dissociation value
can then be used in conjunction with the Hypochlorous Acid concentration measurement to determine
the Free Chlorine concentration. This method will be referred to as “pH Compensation”.
The reaction at the cathode surface in this measurement is as follows:
HOCl + 2e
–
→
Cl
–
+ OH
–
Total Chlorine Measurements:
As discussed above, Total Chlorine is defi ned as the sum of Free
Available Chlorine and Combined Available Chlorine. Combined Available Chlorine species are not
reducible in the Series 210 measurement cell. Therefore, the following technique must be employed
to obtain a measurement.
First, Potassium Iodide (KI) is injected into the sample water so that all species comprising Total
Chlorine react to form Potassium Chloride (KCl). The measurement cell then measures KCl
concentration in the same fashion that it can measure HOCl concentration. Since KCl concentration
is proportional to Total Chlorine concentration, the measurement of KCl is also a measurement of
Total Chlorine concentration. The relevant reactions are as follows.
Free Chlorine Residual:
2H + 2HOCl + 2KI
↔
I
2
+ 2KCl + 2H
2
O
4
Содержание 210 Series
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