Ionode IJ pH V5.1 - Manual (Page 17)

Ionode IJ Instruction Manual 17
Response Time
This varies according to the pH glass
composition, membrane wall thickness, pH
meter filtering, buffering capacity of the sample,
measurement temperature (longest at <10C),
lack of thermal equilibrium between the
electrode and buffers/samples, cleanliness of
the junction, and the age, chemical exposure
history and cleanliness of the membrane. Most
increases in response time for new electrodes
are due to the latter (see 4.4.1
Cleaning ).
The IJ44C is a balance between chemical
resistance (see
Sodium Error ), electrical
resistance and robustness. This results in a
long life membrane of moderate response time.
The response time can be reduced by using a
different composition of pH glass (IJ44A). This
has good resistance to strong acids but
reduced resistance to alkali, and should be
used sparingly above pH 12.
Impedance
pH membranes typically have resistances of
100
– 1000MΩ. This requires the pH meter to
have an input impedance of at least 10
12 Ω in
order to avoid signal loss and polarisation of the
electrode. The latter occurs when a significant
current flows through the electrode. This alters
the Ag/AgCl/sat KCl half cell potentials and
permanently
damages the membrane.
Polarisation can be caused by damp or shorted
connectors, poor connector insulation, a low
impedance meter or meter circuit malfunction.