55
Operation
Note: Alkaline buffers are easily altered by contact with atmospheric CO2. Considerable pH
variations are often found in opened bottles that have come into contact with the air. The more
alkaline the solution is, the more acute this phenomenon is. The manufacturer therefore
recommends pH 9.21 (at 25ºC) alkaline buffer which is much more stable than solutions pH 11, 12,
etc.
4.17.2 Redox standard solution 220 mV
These solutions are designed to check the redox measuring systems quickly. A mixture of
iron salts and pH 7 buffer.
4.17.3 Regenerating solutions
In some cases, these solutions can correct operating anomalies in pH and redox
electrodes. The most common symptoms of an electrode requiring regeneration are:
• Slow response, due to the ageing or coating of the membrane.
• High asymmetry potential, usually due to problems with the reference element, such
as contaminated or obstructed diaphragm.
• Low sensitivity, normally due to problems such as dirt on the diaphragm or membrane,
or loss of the hydrated layer of the membrane.
Electrode cleaner with pepsin.
This solution is indicated to regenerate electrodes which usually measure in samples with
high protein content, such as milk, serum, cheese, etc.
Diaphragm cleaner.
This solution removes the black deposit that appears on the ceramic diaphragm of the
electrode.
Membrane re-activator.
This a solution containing 1-2% HF. It is recommended for reactivating electrodes with
slow response or poor sensitivity. The hydrofluoric acid renews the hydrated gel layer of
the membrane.
Table 3 pH and mV values as a function of the temperature
ºC pH mV
0 2.01 4.01 7.12 9.52 11.45 —
10 2.01 4.00 7.06 9.38 11.20 245
20 2.00 4.00 7.02 9.26 11.00 228
25 2.00 4.01 7.00 9.21 10.90 220
30 2.00 4.01 6.99 9.16 10.81 212
40 2.00 4.03 6.97 9.06 10.64 195
50 2.00 4.06 6.97 8.99 10.48 178
60 2.00 4.10 6.98 8.93 10.23 160
70 2.01 4.16 7.00 8.88 10.19 142
80 2.01 4.22 7.04 8.83 10.06 —
90 2.01 4.30 7.09 8.79 9.93 —
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