4.3.2 WINKLER TITRATION
An alternative to air calibration is to calibrate the probe to a method such as Winkler Titration as
follows:
1.
Draw a volume of nearly air saturated water from a single source and divide it into four
samples. Determine the oxygen in three samples using the Winkler Titration technique, and
average the three values. If one of the values differs from the other two by more than 0.5
mg/L, discard that value and average the remaining two.
2.
Place the probe in the fourth sample and stir.
3.
Set the
SALINITY
control to
0
(
FRESH
) or to the appropriate value of the sample.
4.
Readjust zero if necessary.
5.
Switch to the 0.1 mg/L mode and while continuing to stir the sample, allow the probe to
remain in the sample for at least two minutes; then set the
02 CALIB
control to the average
value determined above. Leave the probe in the sample for an additional two minutes to
verify stability. Readjust if necessary.
4.3.3 AIR-SATURATED WATER CALIBRATION
A third technique for calibration is by means of air-saturated water. This has proven an uncertain
method, as it is difficult to secure precise and stable saturation. Proceed as follows:
1.
Air saturate a volume of water by aerating for at least 15 minutes at a constant temperature.
2.
Place the probe in the sample and stir. Switch the function switch to
TEMP
from the
solubility of oxygen chart in Appendix E, note the temperature of the sample, and record the
mg/L value corresponding to the temperature indicated.
3.
Determine the local altitude or the “true” atmospheric pressure. Using the pressure/altitude
chart, determine the correct
CALIB VALUE
for your pressure or altitude
Note: True atmospheric pressure is as read on a mercury barometer. Weather Bureau reporting of
atmospheric pressure is corrected to sea level.
4.
Multiply the mg/L value from the solubility of oxygen table by the
CALIB VALUE
from
the pressure/altitude table and divide by 100 to determine the correct mg/L oxygen content of
the saturated sample.
EXAMPLE:
Temperature 21°C: oxygen value at sea level or 760 mm Hg
Pressure = 8.92 mg/L
Altitude 1400 feet: calibration value = 95
Corrected calibration value =
8.92 x 95
= 8.47 mg/L
100
5.
Readjust zero if necessary.
6.
Check that the
SALINITY
knob is set at zero. Adjust the
02 CALIB
control to the
calibration value determined in the foregoing step. Wait two minutes to verify stability;
readjust if necessary.
YSI Incorporated
YSI 58 DO Meter
13
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