IM12B06J02-00EN-P
11
5. WIRING
Wiring combined pH/redox electrodes
depending on electrode type is done
using WU20 coax or triax cables. For
cable color coding, refer to IM 12B6W1-
01E-H. Depending on the chosen host,
refer to the respective IM.
6. GENERAL CALIBRATION &
MAINTENANCE PROCEDURE
6.1 Combined sensors SC21(C)
The isothermal point of intersection of
the electrode is at (pH 7 = 0 mV). Regu
-
larly check there is sufficient electrolyte
solution in the electrode or the reservoir.
The SC21C-AGC55 type sensor is filled
with a solution of 3.3 M KCI (i.e.246.9g
KCI/1000 g demi-water), which is avail
-
able as spare part nr: K1520VA.
The electrodes of the type SC21-...2. are
filled with a gelled electrolyte solution,
and refilling is not necessary. When
there is insufficient electrolyte solution,
this type of electrode has probably been
used at a temperature too high for this
electrode. Therefore, replacement with
a different kind of electrode is recom
-
mended.
6.1.1 C
ALIBRATION AND BuffERING
To calibrate a pH sensor, two buffer
solutions with known pH values are
required. It is recommended that one
buffer solution has a value near pH 7.00
(ITP). Depending on the process value to
be measured, the second buffer solution
should be either acidic (below 7.00) or
alkaline (above 7.00). Normally, the IEC
buffers (4.01, 6.87, and 9.18) are used.
The following is a very general 2-point
calibration procedure.
1. Refer to the appropriate Instrument
Instruction Manual for Calibration
procedures (Auto/Manual/Sample,
etc.)
2.
Clean the sensor (deposits may be
removed using a 5% (approximate)
solution of HCL).
3. Rinse the sensor thoroughly with
clean (demi) water.
4.
Immerse the sensor in the first buf
-
fer (6.87 pH is recommended).
5. Rinse the sensor thoroughly with
clean (demi) water.
6. Immerse the sensor in the second
buffer (4.01 or 9.18 recommended).
7. Repeat Step 4.
During calibration, the temperature
compensation should be active. After
the calibration is complete, replace or
re-install the sensor into the process.
NOTE:
During buffering, the liquid earth
and the temperature compensation
must be connected. The temperature of
the buffer solution must be within the
limits of the technical specifications as
indicated on the type plate.
6.1.2 T
ROuBLEsHOOTING
Generally, faults are caused by:
a.
Sensitivity decrease of the mem-
brane. When this occurs, the electrode
must be cleaned with a suitable deter-
gent.
1. Depositions of hydroxides lime,
iron, and hydroxide can be removed
by immersing the electrode in a
diluted solution of hydrochloric acid
and then rinsing it with water.
2. Depositions of oil and fat can be
removed by means of hot water
in conjunction with a domestic
washing solution. When the result is
unsatisfactory, a domestic abrasive
may be carefully used.
3. Albuminous depositions can be
removed by means of a solvent
containing: 8,5 ml concentrated
hydrochloric acid, 10 g pepsin, and
1 liter demi-water.
NOTE:
When polar solvents are used for
special cleaning purposes, it is necessary
to soak the electrode for some time after
cleaning, as the polar solvent influences
the gel layer.
When an a-polar solvent (benzene, ether,
toluene) is used, follow-up treatment
with a polar solvent (methanol, acetone)
is necessary.
