Hach-Lange 1200-S sc - Manual (Page 7)

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Section 2 General information
2.1 General handling instructions
CAUTION!
The sensor will only work correctly when the tip of the probe is completely
immersed in liquid. The measuring probe must not be stored dry for more
than approx. 10 minutes or protrude from the medium to be measured. Use
the transport cap during the installation of the sensor, during maintenance
and when transporting, and fill the cap with 3 mol. KCl or pH 4 buffer
solution.
CAUTION!
Potential danger with contact with chemical/biological substances. Working
with chemical samples, standards and reagents can be dangerous. Make
yourself familiar with the necessary safety procedures and the correct
handling of the chemicals before use and read and follow all relevant safety
data sheets.
2.2 Applications
The sensor makes it possible to simply and exactly measure pH directly in
aqueous solutions.
Typical applications include
• Inlet and / or outlet of a sewage treatment plant
• Activated sludge tanks or
• Surface water (rivers, streams or lakes).
Various different possible installations enable the system to be adapted to a very
wide range of conditions.
2.3 Measuring principle
2.3.1 pH measurement
pH is the negative logarithm of the hydrogen ion activity as well as a
measurement of the acidic or alkaline content of a solution:
pH = –log10 [aH+]
The pH value is normally measured using a glass electrode and a reference
electrode. In this case the glass electrode acts as the transducer that converts the
chemical energy (hydrogen ion activity) into electrical energy (measured in
millivolts). The reaction is symmetrical and the electrical circuit is closed by the ion
flow from the reference solution to the test solution.
Together with the reference solution, the electrode generates a voltage (EMF).
The magnitude of the voltage is dependent on the type of reference electrode, the
internal design of the glass electrode, the pH value of the solution as well as the
temperature.