Yokogawa FU24 All-in-One pH/ORP Sensor - Manual (Page 15)

IM 12B6J3-03E-E
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6.2 Calibration for ORP and rH measurement
For calibration of ORP and rH, the procedure for MANUAL CALIBRATION can be used as
described in the Instruction Manual of the analyzer.
The rH value is a function of the reference system and the pH value of the buffer solution.
The FU24 sensor has a reference system of saturated Silver/Silver Chloride (Ag/AgCl).
The commonly used standards for ORP and rH calibration are made from Chinhydron
(Quinhydrone) powder dissolved in pH buffer solutions (1 g / 200 ml). In Table 3 the
measurement values are given as function of the used pH buffer solution with Chinhydron
powder. The accuracy of the standards is approximately ± 10 mV.
Table 3: ORP, pH compensated ORP and rH as function of pH buffer solution with
Chinhydron powder.
pH buffer
ORP (mV)
pH compensated rH
ORP (mV)
1.68 403 88 23.6
4.01 265 88 23.6
6.87 96 88 23.6
7.00 88 88 23.6
6.3 Maintenance of the FU24 sensor
A pH sensor requires routine maintenance to keep the measuring elements clean and
functioning. Depending on the process, different cleaning solutions may be required.
Avoid cleaning the complete sensor with solution. Some cleaning solutions will
damage the modelcode sticker and connector which are placed on top of the
sensor. Only clean the measuring elements at the bottom side of the sensor.
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.