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6. GENERAL CALIBRATION & MAINTENANCE PROCEDURE
6.1 Calibration for pH measurement
To calibrate the SC25V sensor, two buffer solutions with known pH values are required. It
is recommended that one buffer solution has a value near to pH 7.00. Depending on the
process value to be measured, the second buffer solution should be either acidic (below
pH 7.00) or alkaline (above pH 7.00). Normally the IEC buffers (pH 4.01, 6.87 and 9.18)
are used.
The following is a very general 2-point calibration procedure:
1. Clean the sensor using a 5% solution of HCl;
2. Rinse sensor thoroughly with clean water;
3. Immerse the sensor in the first buffer (pH 6.87 is recommended) and
execute calibration as described in the Instruction Manual of the analyzer.
4. Rinse sensor thoroughly with clean water;
5. Immerse the sensor in the second buffer (pH 4.01 or 9.18 is recommended)
and execute calibration as described in the Instruction Manual of the analyzer.
6. Rinse sensor thoroughly with clean water.
During calibration, the temperature compensation should be active. The EXA/FLXA
analyzer automatically compensates for the sensitivity change of the pH sensor at different
temperatures.
After calibration, replace or re-install the sensor into the process.
6.2 Maintenance of the SC25V sensor
A pH sensor requires routine maintenance to keep the measuring elements clean and
functioning. When the sensitivity of the electrode has decreased or the response has
slowed down, the electrode should be cleaned. Depending on the process, different
cleaning solutions may be required.
In most cases cleaning with water, iso-propanol or methanol is sufficient. In other cases,
the measuring elements of the sensor have to be cleaned with specific solutions.
Examples:
1. Deposits of limes, hydroxides or carbonates can be removed by immersing the bottom
part of the sensor in a solution containing dilute hydrochloric acid (5% is recommended).
Afterwards rinse the sensor with water.
2. Deposits of oil and fat can be removed with hot water with a detergent.
When the results are unsatisfactory, a mild (carbonate based) abrasive can be used.
3. Protein deposits should be removed with a protein enzymatic solution, for instance a
solution containing 8.5 mL concentrated hydrochloric acid and 10 grams of pepsin in 1
liter of water.
Note: Avoid cleaning with non-polar solvent like tri-chloro ethylene, toluene or hexane.
The non-polar solvents will break up the gel-layer on the pH glass bulb and requires
that the sensor has to be soaked in water for at least 12 hours before it will function
again.
The Teflon diaphragm of the sensor can be regenerated by putting it in hot
(± 70 °C, 158 °F) 3 molar Potassium Chlorine (KCl) solution and letting it cool down to
room temperature. This procedure clears the diaphragm and will soak the diaphragm with
conductive KCl again.
IM 12B06J01-40EN-P
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