15-10-2016, 11:09 AM
1459157400-wrdotdoandbodanalysis4452657.ppt (Size: 4.68 MB / Downloads: 27)
MEMBRANE ELECTRODE METHOD
The membrane electrode is composed of two solid metal electrodes in contact with supporting electrolyte separated from the test solution by a gas permeable membrane. Oxygen dissolved in the sample diffuses through the membrane on the DO probe and is chemically reduced (accepts electrons), producing an electrical current between the anode and cathode in the probe. The amount of current is proportional to the concentration of DO. Following proper calibration, the meter relates this current to the concentration of DO.
Comparison with Winkler Titration
Fill two BOD bottles completely full of BOD dilution water, being very careful not to introduce air into either bottle.
2. Analyze one bottle for D.O. using the Winkler titration.
3. Insert the electrode into the second bottle, turn on the stirring mechanism, and wait for the reading to stabilize.
4. Calibrate the meter to the D.O. value obtained in the titration.
5. The meter is now ready for sample analysis
MEMBRANE ELECTRODE METHOD
This procedure varies considerably among the various instrument models available. Therefore, the procedure must be obtained from the instrument manual, but the following points should be noted.
1. Where possible with the specific equipment being used, compensation should be made during calibration for both ambient temperature and local atmospheric pressure. This pressure should be determined using a reliable onsite barometer. The oxygen solubility table following this procedure may be used.
2. Carefully blot any water droplets from the membrane using a soft tissue.
3. During calibration, be sure the membrane is exposed to fresh air. Laying the electrode on the bench for calibration is usually adequate.
4. Complete the calibration as soon as possible before the electrode membrane begins to dry.
5. The temperature registered on the meter should be checked against a trusted thermometer often.
6. Daily calibration of the D.O. meter is required. Calibration should also be verified after every five or six sample measurements.
7. Assure sufficient sample flow across membrane surface during analysis to overcome erratic response.