Introduction
12
Waltron User Manual 101-046-B.3
9096 Degassed Cation Conductivity Analyzer
B) HCO
3
-
+ 2 H
2
O
CO
3
2-
pK = 10,3
Thia is a pH-dependent equilibrium. At a pH value of 5, 94% of the carbon dioxide is present as
CO
2
gas and 6% as carbonate ion HCO
3
-. Following the cation exchanger, the pH is about 5.5-6,
so there is almost exclusively CO
2
gas. The bicarbonate ion (HCO
3
-
)is practically non-existent.
These are the ionic components of the carbon dioxide but are much less corrosive than the ions
of the salt-containing components. In order to obtain a selective conductivity value for the salt
ions (maximum potential risk for corrosion), the carbon dioxide has to be removed from the water
sample. For this separation process, an inert gas (gas that does not contain CO
2
) passes through
the sample and the gas components are displaced by the gas components of the inert gas in the
sample. Thus, the inert gas drives the carbon dioxide from the sample. In this way, the CO
2
contribution is removed from the cation conductivity.
The removal of carbon dioxide is carried out in an exchange column using a counterflow
principle. The aqueous sample flows into the head space in the column and moves under gravity
downwards. Inert gas flows from the bottom upwards in the column and escapes from the
headspace to the outside. The inert gas is produced from decarbonised air. In the column an
exchange process takes place in which the carbon dioxide is driven off from the liquid. With the
circulating pump and a valve, a sample flow rate of 5-8 l / h is maintained . In order to achieve a
constant liquid level in the column, a constant head bypass is installed.which also prevents back
diffusion of carbon dioxide from the atmosphere into the column. A conductivity cell in the
sample output measures the degassed cation conductivity (DCC).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0
100
200
300
400
500
600
700
re
l. o
xy
ge
n
c
o
n
ze
n
tr
ation
[%
]
Time [sec]
Oxygen (as tracer) degasing; real reactor
Measuring values, real reactor
Fitting curve, ideal reactor
Eff. = 93,5 %
t
= 45 sec
