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3.2
The electrodes 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 too high temperature
and therefore replacement by another type of electrode is recommended.
Storage: When an electrode is to remain unused even for a small time, it is necessary to keep the
glass-membrane in the KCL storage liquid in the storage reservoir.
Faults: Generally, faults are caused by:
A)
Sensitivity decrement of the membrane. When this occurs the electrode must be cleaned with a
suitable detergent.
1) Depositions of hydroxides, lime iron hydroxide can be removed by immersing the electrode
in a diluted solution of hydrochloric acid and then rinsing with water.
2) Depositions of oil and fat can be removed by means of hot water together with 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 gr. pepsin and 1 ltr. 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 is used (benzene, ether, toluene) after treatment with a polar solvent
(methanol, acetone) is necessary.
B)
Leakage to earth: Fault indications can be expected when the resistance between reference
system and screening falls below 10/7 Ohm. When a fault occurs, first check if the electrode
cable is in good condition and if the connector contacts and the terminals in the Nieuwkoop pH-
meter or connection box are clean and dry.
C)
Short circuit by break or leakage: The pH-meter reading remains about pH-7 and is not
dependant on the pH-value of the liquid being measured. If this occurs, the electrode should be
replaced.
D)
Diaphragm poisoning: Strongly polluted liquids can cause the diaphragm to foul very quickly.
As a result, there will be a transition resistance over the diaphragm. When this resistance
exceeds 10/5 Ohm instability of reading and incorrect indication of the measured value will
occur. Fouled electrodes can be cleaned with hot water together with a domestic washing
solution if necessary. When fouling is caused by fat or hydroxides (lime) cleaning with an
organic solvent or diluted acid respectively is recommended. It may be necessary with this non-
flow electrode of the Nieuwkoop pH-meter, to place the electrode for a period in an electrolyte
solution at 80 o C and let them remain until the solution has completely cooled.
E)
Poisoning: An electrode can be poisoned by the penetration of the process liquid or by diffusion
of components of the process medium through the diaphragm. In addition, the inner liquid in
combination with the dust from the process medium may give deposits which block the
diaphragm (e.g. silver chloride + sulphide, deposits of silver sulphide. When poisoning occurs
replacement of the electrode will normally be necessary. It is recommended if poisoning of a
non-flow electrode occurs to use an alternate type of electrode.
F)
Poor conduction of the process liquid: When the electrical conduction of a process liquid is very
low, an instability of reading will occur. This problem may be cured either by adding electrolyte
solution to the process liquid or, if this is not possible, by making the reference input of the
measuring instrument high impedance.
G)
Degrading of the glass membrane: Degrading has one of the following effects:
-Decrease of the speed of response
-increase of the electrical resistance
-decrease of the sensitivity
-zero point shift.
The effect of ageing of the electrodes makes it necessary to carry out the re-calibration of the
electrode system regularly. The frequency depends on the required accuracy, the composition of the
sample to be measured and the temperature. E.g. if the lifetime of an electrode is 100% at the room
temperature it will be 20% at 80 o C and only 5% at 120 o C. It is possible to reactive an aged
electrode by immersing for 10 seconds in a PVC beaker containing a solution of vinegar (1 mol.) and
potassium fluoride (1 mol.) Ratio 1:1. After this the electrode must be cleaned carefully.
Warning: As a result of a strong etching proportion of the acid, the handling should be done carefully
and in consultation with a safety officer.
See further instructions of the pH-meter or combined pH/EC-meter.
Directions for use pH-electrodes
8
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