EUTECH INSTRUMENTS CARBON DIOXIDE ION ELECTRODE Instruction Manual

Instruction Manual Carbon Dioxide Electrode

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EUTECH INSTRUMENTS PTE LTD.

INSTRUCTION MANUAL

CARBON DIOXIDE ION ELECTRODE

Instruction Manual Carbon Dioxide Electrode

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TABLE OF CONTENTS

GENERAL INSTRUCTIONS.........................................................................................................................................3

Introduction.............................................................................................................................................................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

Interferences............................................................................................................................................................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.....................................................................................................................

......................................... 15

Standards...............................................................................................................................................................16

Sample...................................................................................................................................................................16

Technique..............................................................................................................................................................16

TROUBLESHOOTING HINTS.................................................................................................................................... 17

Checking the Electrode Inner Body ...................................................................................................................... 19

SPECIFICATIONS........................................................................................................................................................ 20

ORDERING INFORMATION ..................................................................................................................................... 20

Instruction Manual Carbon Dioxide Electrode

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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

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as CO2, Code no. EC-SCS-CO2­BT. 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

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. 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.

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

Instruction Manual Carbon Dioxide Electrode

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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.

Instruction Manual Carbon Dioxide Electrode

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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

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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
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

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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 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