Thermo Electron Orion 95-02, Инструкция по эксплуатации

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8
Sample Storage
If possible, samples should be measured at once, waiting only a
sufficient time for the sample to come to the temperature of the
electrode. In an open 150 mL beaker at 25° C, carbon dioxide
diffuses out of an acidic solution at a rate of about 3% per minute
with stirring and 0.5% without stirring. At higher temperatures the
rate of CO
2
loss increases. If solutions must be stored, make them
slightly alkaline (pH 8-9) by adding 10N NaOH* and storing them in a
tightly capped vessel to prevent pick-up of CO
2
from the air. Just
before measurement, acidify these stored samples with carbon
dioxide buffer.
* The amount of NaOH needed to adjust the pH will depend upon
the sample pH and buffering capacity. For unbuffered samples in
the slightly acidic range, 1 mL of 10N NaOH per 100 mL of
sample will be sufficient. NaOH should not be used to store
samples containing less than 100 ppm CO
2
since carbonate is
usually collected in a stoppered glass bottle. Fill the bottle
completely and cap tightly to prevent loss of CO
2
.
Measuring Hints
Minimize CO
2
loss from samples by:
- Measuring samples as soon as possible after collection.
- Storing samples according to Sample Storage .
- Minimizing the ratio of surface area to volume in the beaker.
- Keeping beakers containing standards and samples covered
between measurements according to Sample Storage .
- Adding carbon dioxide buffer just before measurement.
- Stir all standards and samples at a uniform rate during
measurement. Magnetic stirrers may generate sufficient heat to
change solution temperature. Place a piece of insulating
material such as cork, cardboard, or styrofoam between the
stirrer and beaker.
- Verify calibration every two hours by placing electrodes in the
first standard solution used for calibration. If the value has
changed, recalibrate.
- Always use fresh standards for calibration.