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EUTECH INSTRUMENTS CARBON DIOXIDE GAS Скачать руководство пользователя страница 9

Instruction Manual

Carbon Dioxide Electrode

9

ELECTRODE CHARACTERISTICS

Reproducibility

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.

Interferences

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)

Effect of Dissolved Species

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.

Temperature Influences

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

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Содержание CARBON DIOXIDE GAS

Страница 1: ...Instruction Manual Carbon Dioxide Electrode 1 EUTECH INSTRUMENTS PTE LTD INSTRUCTION MANUAL CARBON DIOXIDE ION ELECTRODE ...

Страница 2: ...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 Co...

Страница 3: ...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 NaHCO3 as CO2 Code no EC SCS CO2 BT To prepare this solution from your own laboratory stock half fill a one liter volumetric flask with distilled water and a...

Страница 4: ...trode 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 ind...

Страница 5: ...egin 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 ...

Страница 6: ...ll 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 o...

Страница 7: ...agnetic stirrer add 10 ml of carbon dioxide buffer and begin stirring at a constant rate After assuring that the meter is in the mV mode lower the electrode tip into the solution After the reading has stabilized record the mV reading 3 Place 100 ml of the 10 3M 100 ppm standard into a 150 ml beaker on the magnetic stirrer add 10 ml of carbon dioxide buffer and begin stirring at a constant rate Aft...

Страница 8: ...are two standards whose concentration is near the expected sample concentration Add 10 ml of carbon dioxide buffer to each 100 ml of standard When calibrating assume that the added buffer has no effect on the standard concentration 2 Place the more dilute standard on the magnetic stirrer and begin stirring at a constant rate Assure that the meter is in the concentration mode Lower the electrode ti...

Страница 9: ...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...

Страница 10: ... mV potential against the carbon dioxide concentration on semi logarithmic paper results in a straight line with a slope of about 56 mV per decade over the range 1X10 4 M to 1X10 2 M Refer to Figure 1 For carbon dioxide concentrations above 1X10 4 M the electrode exhibits good time response 95 of total mV reading in two minutes or less Response times are longer below this value and carbon dioxide ...

Страница 11: ...ill 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 ...

Страница 12: ...ntration S electrode slope 56mV decade Because the hydrogen ion concentration is directly related to the carbon dioxide concentration electrode response to carbon dioxide is also Nernstian E E1 S log CO2 Carbon Dioxide Chemistry Carbon dioxide exists as bicarbonate and carbonate in basic solutions CO2 OH HCO3 CO2 2OH CO3 2 H2O The solution s pH governs the amount of carbon dioxide present in the f...

Страница 13: ...f the sample above 10 immediately when the sample container is opened 3 To a 100 ml volumetric flask quantitatively transfer 10 ml of the alkaline sample to the flask and fill to the mark with distilled water After agitating the flask to mix the contents transfer the solution to a 150 ml beaker 4 Place the beaker on the magnetic stirrer and begin stirring at a constant rate Add 10 ml of carbon dio...

Страница 14: ...etric flask with distilled water add 14 0 grams of reagent grade sodium bicarbonate NaHCO3 Swirl the flask to dissolve the solid Fill to the mark with distilled water cap and upend several times to mix the solution 2 3M sulfuric acid reagent To prepare this solution from your own laboratory stock add approximately 500 ml of distilled water to a one liter volumetric flask Using a hood and protectiv...

Страница 15: ...e very well with distilled or deionized water Clean glassware will drain without leaving water droplets behind Electrodes The electrodes may be checked by using the procedure found in the sections entitled Electrode Slope Check 1 Be sure to use distilled or deionized water when following the procedures given in Electrode Slope Check 2 If the electrode fails to response as expected see the sections...

Страница 16: ...hird standard and so on until the desired range of standards has been prepared Sample If the electrode works properly in standards but not in sample look for possible interferences complexing agents or substances which could affect response or physically damage the sensing electrode or the reference electrode If possible determine the composition of the samples and check for problems See Sample Re...

Страница 17: ...tion put electrode in solution Noisy or Unstable insufficient internal fill outer body of electrode Reading readings filling solution with proper amount of continuously or internal filling solution random changing defective meter perform meter checkout procedure see meter instruction manual bottom cap loose ensure that bottom cap is screwed on tight enough to close gap between bottom cap and body ...

Страница 18: ...ial magnetic stirrer between stirrer and beaker defective inner body refer to Checking the Electrode Inner Body electrode exposed to hold electrode by outer body air for extended period and pull on electrode cable Internal filling solution will flow under membrane and restore electrode response Low Slope or No Slope standards contaminated prepare fresh standards or incorrectly made buffer not used...

Страница 19: ... 2 9 grams of reagent grade NaCl to 500 ml of pH 7 buffer Dissolve the solid The solution may be stored for repeated use Disassemble the carbon dioxide electrode If the electrode is dry soak the glass tip of the inner body in Eutech Carbon Dioxide Electrode Filling Solution Code No EC ISA CO1 for two hours Rinse the electrode thoroughly with distilled water Put 100 ml of pH 7 buffer 0 1M NaCl adde...

Страница 20: ...eter 1 m cable length Storage Store electrodes in 0 1M NaCl ORDERING INFORMATION CODE NO DESCRIPTION EC CO2 01 Carbon Dioxide Gas Sensing Electrode EC MIS CD Carbon Dioxide Membrane Cartridge Kit box of 3 membrane cartridges and spare o rings EC SCS CO1 BT Carbon Dioxide Standard 0 1M NaHCO3 EC SCS CO2 BT Carbon Dioxide Standard 1 000 ppm as CO2 EC SCS CO3 BT Carbon Dioxide Standard 100 ppm as CaC...

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