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SD 300_pH_1 01/2013
5. Set-up
Connect the electrode, switch on the instrument with the
button.
After the segment test
the instrument briefly shows information about its configuration:
If a zero point or slope adjustment was made
(See chapter 7. Instrument Configuration.)
Remove the protective cap from the electrode. (Caution: The cap should contain KCL 3 M
or a storage solution)
Then the instrument is ready for measurement.
6. Principles of Measurement
6.1 pH measurement
The pH value describes the acidic or alkaline behaviour of an aqueous solution.
pH values below 7 are acidic (the smaller the greater the acidity); values above 7 are alkaline;
pH 7 = neutral.
From the negative decadic logarithm of the hydrogen ion activity (often this is approximately
equal to the concentration of hydrogen ions) the following is calculated:
pH value = -log
10
(
c(H
+
) ∙ f(H
+
)
)
with c(H
+
): Hydrogen ion concentration in mol/l
f(H
+
): Activity coefficient of the hydrogen ions
(usually less than 1)
The abbreviation "pH" stands for pondus Hydrogenii (Latin pondus: "weight"; hydrogenium:
"hydrogen").
In order to register the pH value of a solution, it should always be recorded together with the
temperature measurement, e.g.:
pH 5.87; 22.8 °C.
Reason: The pH value of most liquids changes with the temperature.
The pH measurement is a very precise, but very sensitive measurement. The measured signals
are very weak (high-ohmic), especially when measured in low ionic solutions. Therefore, it
must be ensured that
- faults (electrostatic charging, etc.) are prevented,
- a stable measurement value is achieved through slow stirring,
- the plug contacts are kept clean and dry,
- electrodes (except for special, waterproof designs) are , as far as possible, not immersed
below the shaft,
- the electrode is calibrated sufficiently often (see below) (The calibration frequency depends on
the electrode and usage and may vary between every hour and several weeks.),
- an appropriate electrode is used.
(1 mol/l)
Summary of Contents for SD 300 pH
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