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CARBON DIOXIDE GAS
User Manual
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Eutech Instruments CARBON DIOXIDE GAS User Manual

Instruction Manual Carbon Dioxide Electrode
EUTECH INSTRUMENTS PTE LTD.
INSTRUCTION MANUAL
CARBON DIOXIDE ION ELECTRODE
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Instruction Manual Carbon Dioxide Electrode
TABLE OF CONTENTS
GENERAL INSTRUCTIONS.........................................................................................................................................3
Required Equipment................................................................................................................................................ 3
Required Solutions.................................................................................................................................................. 3
GENERAL PREPARATION.......................................................................................................................................... 4
Electrode Preparation.............................................................................................................................................. 4
Checking Membrane ............................................................................................................................................... 4
Changing the Membrane......................................................................................................................................... 4
Connecting the Electrode to the Meter....................................................................................................................4
Electrode Slope Check (with pH/mV meter)........................................................................................................... 4
Electrode Slope Check (with ion meter) ................................................................................................................. 5
MEASUREMENT............................................................................................................................................................ 5
Measuring Hints...................................................................................................................................................... 5
Sample Storage........................................................................................................................................................ 6
Sample Requirements.............................................................................................................................................. 6
Units of Measurement............................................................................................................................................. 6
MEASUREMENT PROCEDURE..................................................................................................................................7
Direct Measurement................................................................................................................................................ 7
Direct Measurement of Carbon Dioxide (using a pH/mV meter)........................................................................... 7
Direct Measurement of Carbon Dioxide (using an ion meter)................................................................................ 8
ELECTRODE CHARACTERISTICS ........................................................................................................................... 9
Reproducibility........................................................................................................................................................ 9
Effect of Dissolved Species..................................................................................................................................... 9
Temperature Influences...........................................................................................................................................9
Electrode Response ............................................................................................................................................... 10
Limits of Detection................................................................................................................................................ 11
pH Effects.............................................................................................................................................................. 11
Electrode Life........................................................................................................................................................11
Electrode Storage .................................................................................................................................................. 11
ELECTRODE THEORY...............................................................................................................................................11
Electrode Operation............................................................................................................................................... 11
Carbon Dioxide Chemistry.................................................................................................................................... 12
ELECTRODE APPLICATION.................................................................................................................................... 13
Carbon Dioxide Content of Wines and Carbonated Beverages ............................................................................ 13
Carbonate Content in Groundwater....................................................................................................................... 14
Carbonate Measurements in Concentrated Ammonium Hydroxide...................................................................... 14
TROUBLESHOOTING GUIDE...................................................................................................................................15
Meter ..................................................................................................................................................................... 15
Glassware.............................................................................................................................................................. 15
Electrodes.....................................................................................................................
Standards...............................................................................................................................................................16
Sample...................................................................................................................................................................16
Technique..............................................................................................................................................................16
TROUBLESHOOTING HINTS.................................................................................................................................... 17
Checking the Electrode Inner Body ...................................................................................................................... 19
SPECIFICATIONS........................................................................................................................................................ 20
......................................... 15
ORDERING INFORMATION ..................................................................................................................................... 20
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Instruction Manual Carbon Dioxide Electrode
EUTECH INSTRUMENTS PTE LTD.
CARBON DIOXIDE GAS-SENSING ELECTRODE
INSTRUCTION MANUAL
GENERAL INSTRUCTIONS
Introduction
Eutech Carbon Dioxide Gas-Sensing Electrode is used to measure carbon dioxide, carbonate, and bicarbonate in aqueous solutions quickly, simply, accurately, and economically. With a flow­through cap, the electrode can be used in flow-through applications.
Required Equipment
1. A pH/mV meter or an ion meter, either line operated or portable.
2. Semi-logarithmic 4-cycle graph paper for preparing calibration curves when using the meter in the mV mode.
3. A magnetic stirrer.
4. Eutech Carbon Dioxide Gas-sensing Electrode, Code no. EC-C02-01.
Required Solutions
1. Deionized or distilled water for solution preparation.
2. Eutech Carbon Dioxide Standard, 0.1M NaHCO3, Code no. EC-SCS-CO1-BT. To prepare this solution from your own laboratory stock, half fill a 1 liter volumetric flask with distilled water and add 8.40 grams of reagent-grade NaHCO3. Swirl the flask gently to dissolve the solid. Fill the flask to the mark with distilled water, cap, and upend several times to mix the solution.
3. Eutech Carbon Dioxide Standard, 1,000 ppm NaHCO BT. To prepare this solution from your own laboratory stock, half fill a one liter volumetric flask with distilled water and add 1.91 grams of reagent-grade NaHCO the flask gently to dissolve the solid. Fill the flask to the mark with distilled water, cap, and upend several times to mix the solution.
4. Eutech Carbon Dioxide Standard, 100 ppm NaHCO3 as CaCO3, Code no. EC-SCS-CO3­BT. To prepare this solution from your own laboratory stock, half fill a one liter volumetric flask with distilled water and add 0.084 grams of reagent-grade NaHCO3. Swirl the flask gently to dissolve the solid. Fill the flask to the mark with distilled water, cap, and upend several times to mix the solution.
5. Eutech Carbon Dioxide Buffer Solution, Code no. EC-ISA-CO1-BT. To prepare this solution from your own laboratory stock, half fill a 1 liter volumetric flask with distilled
as CO2, Code no. EC-SCS-CO2-
3
. Swirl
3
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Instruction Manual Carbon Dioxide Electrode
water, add 294 grams of Na3C6H5O7.2H2O (sodium citrate dihydrate). Swirl the flask gently to dissolve the solid. Fill the flask to the mark with distilled water. Stir the solution and adjust the pH of the solution to 4.5 with concentrated HCl. This buffer is used to adjust the pH of the solution to the operating range of the electrode. To each 100 ml of sample and standard solution, add 10 ml of carbon dioxide buffer.
GENERAL PREPARATION
Electrode Preparation
This electrode is shipped dry. Before using, unscrew the large cap (See Figure 5), and remove the inner glass electrode from the outer body. Fill the outer body with 2 to 3 ml of internal filling solution. Place inner glass electrode into the outer body, and screw on the large cap until finger tight. Place the assembled electrode in an electrode holder with a 20o angle from the vertical to avoid trapping air bubbles at the bottom of the electrode.
Checking Membrane
A small hole of any size on the membrane or breakage of the membrane causes failure of the electrode. It is recommended to check the membrane on every newly assembled electrode.
1. Connect a newly assembled electrode to a pH/mV meter.
2. Lower the electrode tip in distilled water.
3. Record the reading after stirring the distilled water for about 15 minutes.
4. Add proper buffer solution (see section Required Solutions) to the distilled water. A drastic change in the reading in a positive direction indicates damage of the membrane.
Changing the Membrane
Unscrew the small cap from the outer body and remove the old membrane cartridge from the small cap. Insert the new membrane cartridge into place, and re-assemble the electrode. (See Figure 4.)
Connecting the Electrode to the Meter
Connect the electrode to the meter according to the meter manufacturer's instructions. No external reference electrode is required. To prevent air entrapment, mount the electrode at a 20
o
angle from
the vertical.
Electrode Slope Check (with pH/mV meter)
(Check electrodes each day)
1. To a clean, dry, 150 ml beaker, add 90 ml of distilled water and 10 ml of carbon dioxide buffer. Place the beaker on a magnetic stirrer and begin stirring at a constant rate. After assuring that the meter is in the millivolt mode, lower the electrode tip into the solution. Remove air bubbles by redipping probe.
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Instruction Manual Carbon Dioxide Electrode
2. Using a pipet, add 1 ml of 0.1M or 1,000 ppm standard into the solution. When the reading has stabilized, record the mV value.
3. Using a pipet, add 10 ml of the same CO2 standard used above to the beaker. When the reading has stabilized, record the mV value.
4. Determine the difference between the two readings. The electrode is operating correctly if a difference of 56 ± 3 mV is found, assuming the solution temperature is between 20o and 25oC. See the TROUBLESHOOTING section, if the change in potential is not within this range.
Slope is defined as the change in potential observed when the concentration changes by a factor of
10.
Electrode Slope Check (with ion meter)
(Check electrodes each day)
1. Prepare standard carbon dioxide solutions whose concentrations vary by tenfold. Use either the 0.1M or 1,000 ppm carbon dioxide standard. Use the serial dilution method for this preparation.
2. To a 150 ml beaker, add 100 ml of the lower value standard and 1 ml of ISA. Place the beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the electrode tips into the solution. Assure that the meter is in the concentration mode.
3. Adjust the meter to the concentration of the standard and fix the value in the memory according to the meter manufacturer's instructions. Rinse the electrodes with distilled water and blot dry.
4. To another 150 ml beaker, add 100 ml of the higher value standard and 2 ml of ISA. Place the beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the electrode tips into the solution.
5. Adjust the meter to the concentration of the standard and fix the value in the memory. Read the electrode slope according to the meter manufacturer's instructions. Correct electrode operation is indicated by a slope of 90-100%. See the
TROUBLESHOOTING
section if the slope is not within this range.
MEASUREMENT
Measuring Hints
Samples should be measured immediately after collection. Samples should be stored according to the directions given in Sample Storage if immediate measurement is not possible.
The ratio of surface area to volume in the beaker should be minimized. Beakers containing the samples or the standard should be kept covered between measurements.
Carbon dioxide buffer should be added just before measurement
.
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Instruction Manual Carbon Dioxide Electrode
All samples and standards should be at the same temperature for precise measurement. A difference of 1oC 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.
Sample Storage
Samples should be measured immediately after preparation or collection, if possible. Wait only long
o
enough for temperature equilibration between the sample and the electrode. At 25
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
increases with
2
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 CO2 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 CO2, collect in a stoppered glass bottle, filling completely and capping tightly to prevent CO2 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 CO2, 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.
Sample Requirements
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
Effect of Dissolved Species.
Units of Measurement
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 CO2 ppm CaCO3
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