eutech-instruments carbon dioxide gas User Manual

Instruction Manual 

 

Carbon Dioxide Electrode

 

 

9

ELECTRODE CHARACTERISTICS 
 

 
Electrode measurements reproducible to 

±2% can be obtained if the electrode is calibrated every 

hour. Factors like temperature fluctuation, drift, and noise limit reproducibility. Reproducibility is 
independent of concentration within the electrode's operating range. 
 

 
Certain volatile weak acids are potential electrode interferences. Concentrations of these interfering 
species that cause a 10% error at 10

-3

M CO

2 

(100 ppm CaCO

3

 or 44 ppm CO

2

), at pH 4 and 5, are 

listed in Table 2. 
 
 

TABLE 2: Interference Levels - 10% Error at 10

-3

M CO

2 

 

Interferences pH 

4 pH 

5 

HCOOH  
(formic acid)      

7.5 x 10

-3

M 

(345 ppm) 

2.0 x 10

-2

M 

(1,840 ppm) 

CH

3

COOH  

(acetic acid) 

3.6 x 10

-3

M 

(216 ppm) 

6.2 x 10

-3

M 

(372 ppm) 

HSO

3-

(SO

2

)    

(sulfur dioxide) 

7.5 x 10

-4

M 

(48 ppm) 

5.0 x 10

-3

M 

(320 ppm) 

NO

2-

(NO

2

) 

(nitrogen dioxide) 

5.3 x 10

-4

M 

(24 ppm) 

3.5 x 10

-3

M 

(160 ppm) 

 

 
One common substance that is a potential electrode interference is water vapor. The concentration 
of the internal filling solution under the membrane is changed when water, in the form of water 
vapor, moves across the electrode membrane. These changes will be seen as electrode drift. If the 
total level of dissolved species in solution - the osmotic strength - is approximately equal to that of 
the internal filling solution and the sample and electrode temperatures are the same, water vapor 
transport is not a problem. 
 
Samples of low osmotic strength are automatically adjusted to the correct level through addition of 
carbon dioxide buffer. If samples have osmotic strengths greater than 1M, they should be diluted 
before measurement. However, this dilution should not reduce the carbon dioxide level below 10

-

4

M. If dilution is not possible, for the reason mentioned, the sample can be measured by adjusting 

the osmotic strength of the electrode filling solution. The total level of dissolved species in the 
electrode filling solution may be adjusted by adding 0.425 grams of reagent-grade sodium nitrate 
(NaNO

3

) to 10 ml of electrode filling solution. 

 
Samples and standards should be within 

±1

o

C of each other, since electrode potentials are 

influenced by changes in temperature. Because of solubility equilibrium on which the electrode 
depends, the absolute potential of the reference electrode changes slowly with temperature. The 
slope of the electrode, as indicated by the factor "S" in the Nernst equation, also varies with