Thermo Scientific AquaSensors™ DataStick™ Measurement System User Guide
107
7
Free Chlorine Concentration Calculation
This section explains how the free chlorine concentration of a solution can be calculated from the measurements provided by
a Chlorine and a pH DataStick. It also explains how the hypochlorous acid concentration can be calculated from an independent free
chlorine concentration measurement for use during DataStick calibration. Included are some guidelines for the PLC programmer.
Square brackets
are used to indicate concentration.
[ ]
7.1 Measurement
Free chlorine is composed of hypochlorous acid, HOCl, and hypochlorite ion, OCl
–
. The free chlorine concentration is the sum of the
hypochlorous acid concentration
and the hypochlorite ion concentration
[
HOCl
]
[
]
−
OCl
.
[
]
[
]
−
+
OCl
HOCl
(1)
pH is the negative of the decimal logarithm of the hydrogen ion concentration.
[ ]
+
−
=
H
pH
10
log
(2)
Or
[ ]
pH
H
−
+
=
10
(3)
According to the Handbook of Chlorination and Alternative Disinfectants, Fourth Edition, 1998, equation (4) relates the free chlorine
concentration in ppm to the hypochlorous acid concentration in ppm, the pH, and the temperature in °C.
[
]
[
]
[
]
[ ]
⎭
⎬
⎫
⎩
⎨
⎧
×
+
×
×
+
×
=
+
+
−
−
−
H
T
HOCl
OCl
HOCl
8
8
10
476
.
1
10
056714
.
0
1
(4)
The Chlorine DataStick measures the hypochlorous acid concentration
[
]
HOCl
and the temperature
T
of the solution. The pH
DataStick measures the
of the solution. With these measurements equation (4) can be used to calculate the free chlorine
concentration of the solution.
pH
For example, if the hypochlorous acid concentration is 4.99 ppm and the temperature is 27 °C as measured by the Chlorine
DataStick, and the pH is 7 as measured by the pH DataStick, then the free chlorine concentration in ppm will be
[
]
[
]
49
.
6
10
10
476
.
1
27
10
056714
.
0
1
99
.
4
7
8
8
=
⎭
⎬
⎫
⎩
⎨
⎧
×
+
×
×
+
×
=
+
−
−
−
−
OCl
HOCl
7.2 Calibration
The Chlorine DataStick requires a hypochlorous acid concentration
[
]
HOCl
value when it is calibrated. If the user has a free
chlorine concentration value obtained from a colorimeter, for example, then the calculation done in equation (4) must be reversed.
[
]
[
]
[
]
[ ]
+
−
−
−
×
+
×
×
+
+
=
H
T
OCl
HOCl
HOCl
8
8
10
476
.
1
10
056714
.
0
1
(5)
