Thermo Fisher Scientific Ion Selective Electrodes Copper - Manual (Page 21)

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solution boundary resulting in a potential difference between the
two solutions, since ions diffuse at different rates. When
making measurements, it is important to remember that this
potential be the same when the reference is in the standardizing
solution as well as in the sample solution or the change in
liquid junction potential will appear as an error in the measured
electrode potential.
The composition of the liquid junction filling solution in the
reference electrode is most important. The speed with which the
positive and negative ions in the filling solution diffuse into
the sample should be as nearly equal as possible, that is, the
filling solution should be equitransferent. No junction
potential can result if the rate at which positive and negative
charge carried into the sample is equal.
Strongly acidic (pH = 0 - 2) and strongly basic (pH = 12 - 14)
solutions are particularly troublesome to measure. The high
mobility of hydrogen and hydroxide ions in samples make it
impossible to mask their effect on the junction potential with
any concentration of an equitransferent salt. One must either
calibrate the electrode(s) in the same pH range as the sample or
use a known increment method for ion measurement.
TROUBLESHOOTING GUIDE
The goal of troubleshooting is the isolation of a problem through
checking each of the system components in turn: the meter, the
glassware, the electrode(s), the reagents, the sample, and the
technique.
Meter
The meter may be checked by following the checkout procedure in
the instrument instruction manual.
Glassware
Clean glassware is essential for good measurement. Be sure to
wash the glassware well with a mild detergent and rinse very well
with distilled or deionized water. Clean glassware will drain
without leaving water droplets behind.
Electrodes
The electrodes may be checked by using the procedure found in the
sections entitled Electrode Slope Check.