Thermo Fisher Scientific Ion Selective Electrodes Iodide - Manual (Page 13)

Instruction Manual Iodide Electrode
13
The reference electrode must also be considered. When two solutions of different composition are
brought into contact with one another, liquid junction potentials arise. Millivolt potentials occur
from the inter-diffusion of ions in the two solutions. Electrode charge will be carried unequally
across the 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 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 electrodes 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 glass-ware, the electrodes, the standards and reagents, the
sample, and the technique.
Meter
The meter may be checked by following the check-out 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 electrode may be checked by using the procedure found in the sections entitled
Electrode
Slope Check.
1. Be sure to use distilled or deionized water when following the procedures given in
Electrode Slope Check
.
2.
If the electrode fails to respond as expected, see sections
Measuring Hints
and
Electrode
Response
. Repeat the slope check.
3.
If the electrode still fails to respond as expected, substitute another iodide ion electrode
that is known to be in good working order for the questionable electrode. If the problem