Thermo Fisher Scientific Dioxide Ion Selective Electrodes Carbon - Manual (Page 11)

Instruction Manual Carbon Dioxide Electrode
11
Limits of Detection
The upper limit of detection in pure carbon dioxide solutions is 1X10
-2
M. Carbon dioxide is rapidly
lost to the air above a concentration of 1X10
-2
M. Dilution may be used if carbon dioxide
concentrations are above 1M. Also dilute samples between 1M and 10
-2
M or calibrate the electrode
at 4 or 5 intermediate points.
The lower limit of detection is around 1X10
-4
M. Refer to Figure 1 for a comparison of the
theoretical response to the actual response at low levels of carbon dioxide. Carbon dioxide
measurements below 10
-4
M CO
2
should employ low level procedures.
pH Effects
The carbon dioxide electrode can be used over the pH range 4.8 to 5.2. It is necessary to adjust the
sample pH using the recommended ISA to convert all carbonate and bicarbonate species in solution
to carbon dioxide.
Electrode Life
A carbon dioxide gas-sensing electrode will last six months in normal laboratory use. On-line
measurements might shorten operational lifetime to several months. In time, the response time will
increase and the calibration slope will decrease to the point calibration is difficult and membrane
replacement is required.
Electrode Storage
If storing the carbon dioxide electrode overnight or over the weekend, immerse the tip of the
electrode in the 0.1M NaCl storage solution. For longer periods of time, completely disassemble the
electrode, rinse the inner body, the outer body, and the cap with distilled water. After drying,
reassemble the electrode without filling solution.
ELECTRODE THEORY
Electrode Operation
A gas-permeable membrane is used to separate the sample solution from the electrode's internal
filling solution in the Eutech Carbon Dioxide Gas-Sensing Electrode. Dissolved carbon dioxide in
the sample solution permeates the membrane until an equilibrium is reached between the partial
pressure of the CO
2
in the internal filling solution and the partial pressure of the CO
2
in the sample
solution. The partial pressure of carbon dioxide in any given sample will be proportional to the
concentration of carbon dioxide.
Diffusion across the membrane affects the level of hydrogen ions in the internal filling solution:
CO
2
+ H
2
O
⇔ H +
+ HCO
3-
The relationship between the hydrogen ion, the bicarbonate ion, carbon dioxide, and water is given
by the equation: