Chapter 2 Rainwater Analysis (Phân tích nước mưa) - Practical Environmental Analysis, 2nd Edition - M. Radojevic, V. Bashkin

62 Chapter 2
meter, carry out the measurements as follows: Place the working KCl standard
solution in a small beaker and suspend the conductivity cell in the solution
holding it approximately 1.5 cm above the bottom of the beaker and
make sure that it is not in contact with any of the beaker walls. Adjust the
conductivity reading to 100 µmho cm 1 . Rinse the cell with pure water, and
carry out the measurement on your sample in the same way. You do not need
to calibrate the meter between each sample, but you must rinse the cell
between each reading. Also analyse the water blanks. These should have conductivities
no higher than your pure water. Always measure the conductivity
before the pH. Leakage of electrolyte from the pH electrode can cause a significant
change in the conductivity of highly pure, low conductivity samples.
When not in use store the electrode by immersing in laboratory water.
2.4.5 pH
2.4.5.1 Materials.
● Buffer solutions, pH 4 and 7
● pH meter, preferably with a combination of pH electrode and temperature
compensation probe.
● Magnetic stirrer and stirring bar
2.4.5.2 Experimental Procedure. Ideally, the pH measurement should be
made immediately after sample collection but this may not always be possible
in practice. Use the pH meter and electrode as follows. First, calibrate the
combination pH electrode and meter using a two-point calibration with
buffer solutions of pH 7 and 4. A technician or demonstrator will show you
how to operate the pH meter; otherwise follow the instructions in the instrument
manual. After calibration, immerse the pH electrode in the rainwater
sample and take a measurement. You may swirl the solution but allow it to
come to rest before taking the measurement. Alternatively, you may place a
magnetic stirring bar in the solution and stir for 15 s on a magnetic stirrer.
After turning the stirrer off, allow the pH to stabilise (ca. 2 min) before taking
the reading. You may notice that in some solutions, the pH value drifts.
In case of drift, allow the reading to stabilise before recording the pH value.
If the reading does not stabilise, and the electrode performance is satisfactory,
record the pH value reached after 5 min and report this. The drift could
be due to chemical changes going on in the sample and not due to a faulty
electrode. You can test the performance of the electrode by measuring the pH
of a dilute acid solution (e.g. HCl or H 2 SO 4 ) of pH 4 and noting the pH measured
and the response time. Report the pH to two decimal places. Rinse the
electrode with laboratory water and swab it dry between each reading. When
taking readings, suspend the electrode in the test solution making sure that it