eutech-instruments carbon dioxide gas User Manual

Instruction Manual 

 

Carbon Dioxide Electrode

 

 

6

All samples and standards should be at the same temperature for precise measurement. A difference 
of 1

o

C in temperature will result in approximately a 2% error. All samples must be aqueous. 

Always rinse the electrode with distilled water and blot dry between measurements.  Use a clean, 
dry tissue to prevent cross-contamination. 
Constant but not violent, stirring is necessary for accurate measurement. Magnetic stirrers can 
generate sufficient heat to change the solution temperature. To counteract this effect, place a piece 
of insulating material, such as gauze or styrofoam, between the stirrer and the beaker. 
 
Always check to see that the membrane is free from air bubbles after immersion into standard or 
sample. 
 

 
Samples should be measured immediately after preparation or collection, if possible. Wait only long 
enough for temperature equilibration between the sample and the electrode. At 25

o

C, in an open 

150 ml beaker, carbon dioxide diffuses out of an acidic solution at a rate of about 3% per minute 
with stirring and at a rate of about 0.5% per minute without stirring. The loss of CO

2

 increases with 

increasing temperature. 
 
If the samples cannot be measured immediately, add 10M NaOH to make them slightly alkaline (pH 
8-9) and store them in tightly capped vessels to prevent infusion of CO

2

 from the air. The amount of 

10M NaOH needed to adjust the pH to the alkaline range will depend on the sample's buffering 
capacity and the initial pH of the sample. If the samples contain less than 100 ppm CO

2

, collect in a 

stoppered glass bottle, filling completely and capping tightly to prevent CO

2

 from escaping. Do not 

add NaOH, since carbonate is usually present in the base as a contaminant. If the sample contains 
more than 100 ppm CO

2

, is slightly acidic and un-buffered, adding 1 ml of 10M NaOH per 100 ml 

of sample will suffice. Prior to measurement, acidify these stored samples with carbon dioxide 
buffer. 
 

 
Carbon dioxide buffer must be added to standards and samples before measurement. When the 
buffer solution is added, all standards and samples should be in the range of pH 4.8 to 5.2. In this 
range, all bicarbonate and carbonate species are converted to carbon dioxide and all interferences 
are minimized. Highly basic, highly acidic, or buffered samples must be adjusted to pH 4.8 to 5.2 
before the carbon dioxide buffer is added, since the buffering capacity of the acid buffer is limited. 
 
Adding the buffer solution adjusts the total level of dissolved species in solution to 0.4M. The 
sample should be diluted before measurement if the total level of dissolved species is greater than 
1M after the addition of the carbon dioxide buffer. For a further explanation, see the section entitled 

 

 
Measurement of carbon dioxide can be expressed in units of moles/liter, ppm carbon dioxide, ppm 
calcium carbonate, or other convenient concentration unit.  Table 1 lists conversion units. 
 
       TABLE 1:  Concentration Unit Conversion Factors 
 
         moles/liter      ppm CO

2

         ppm CaCO

3