Yokogawa FU20/PH20 All-in-One pH/ORP Sensor - Manual (Page 17)

IM 12B6J3-E-H
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7. GENERAL CALIBRATION &
MAINTENANCE PROCEDURE
7-1. Calibration for pH measurement
To calibrate a pH sensor, two buffer solu-
tions with known pH values are required. It
is recommended that one buffer solution
have a value near to pH 7.00. Depending
on the process value to be measured, the
second buffer solution should be either aci-
dic (below 7.00) or alkaline (above 7.00)
area. Normally, the IEC buffers (4.01, 6.87
and 9.18) are used.
The following is a very general 2-point cali-
bration procedure.
1. Clean the sensor (deposits may be
removed using a 5% (approximate) solu-
tion of HCL).
2. Rinse sensor thoroughly with clean
water.
3. Immerse the sensor in the first buffer
(6.87 pH is recommended).
4. Refer to appropriate Instrument
Instruction Manual for Calibration proce-
dures (Auto; Manual; Sample, etc.),
5. Rinse sensor thoroughly with clean
water.
6. Immerse the sensor in the second buffer
(4.01 or 9.18 recommended).
7. Repeat Step 4.
During calibration the temperature com-
pensation should be active. The instrument
automatically compensates for the sensitivi-
ty change of the pH sensor at different
temperatures. After calibration is complete
replace or re-install the sensor into the pro-
cess.
7-2. Calibration for ORP measurement
For ORP calibrations the same procedure
and buffer solutions pH 6.87 and pH 4.01
are to be used. When 1-20 grams (±1 tea-
spoon) quinhydrone is added to each
buffer, it will produce a stable redox poten-
tial of 96 mV and 265 mV respectively.
Mostly only 1-point calibrations are perfor-
med. Some analyzers also have the possi-
bility to adjust the slope (2-point calibra-
tion).
7-3. Maintenance
All pH sensors require routine maintenance
to keep the measuring elements clean and
functioning. Depending on the process,
different cleaning solutions may be requi-
red.
EXAMPLES:
1. Deposits of limes, hydroxides or carbo-
nates can be removed by immersing the
sensor in a solution containing dilute
hydrochloric acid (5% is recommended).
Afterwards rinse with water.
2. Deposits of oil and fat can be removed
with hot water in conjunction with a
detergent. When the results are unsatis-
factory, a mild (carbonate based) abrasi-
ve can be used.
3. Protein deposits should be removed with
a protein enzymatic solution. For instan-
ce a solution containing 8.5ml concen-
trated hydrochloric acid and 10 grams of
pepsin in 1 liter of water will do.
After cleaning the probe is reinserted into
the process by following the reverse proce-
dure.
NOTE:
Avoid using non-polar solvent like tri-chloro
ethylene, toluene or hexane. Cleaning with
iso-propanol or methanol is acceptable.
The non-polar solvents will break up the
gel-layer on the glass bulb and requires
that the electrode to be soaked in water for
at least 12 hours before functioning nor-
mally again.
The Teflon (PTFE) diaphragm of the sensor
can sometimes be regenerated by putting
it in hot (60 to 80°C) 3 molar Potassium
Chlorine (KCL) solution and letting it cool to
room temperature. This procedure clears
the junction of process material and then
"draws" the conductive (KCL) back into the
junction.