Thermo Fisher Scientific Ion Selective Electrodes Fluoride - Manual (Page 18)

Instruction Manual Fluoride Electrode
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In the case of high and constant ionic strength relative to the sensed ion concentration, the activity
coefficient,
γ, is constant and the activity, X, is directly proportional to the concentration.
All samples and standards containing fluoride ions have TISAB added so that the background ionic
strength is high and constant relative to variable concentrations of fluoride. The recommended ISA
for the fluoride electrode is TISAB though similar solutions can be used as long as they do not
contain ions that would interfere with the electrode's response to fluoride.
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.
TROUBLESHOOTING GUIDE
The goal of troubleshooting is the isolation of a problem through checking each of the system
components in turn: the meter, the plastic-ware, the electrodes, the standards & reagents, the
sample, and the technique.
Meter
The meter may be checked by following the check-out procedure in the instrument instruction
manual.
Plastic-ware
Clean plastic-ware is essential for good measurement. Be sure to wash the plastic-ware well with a
mild detergent and rinse very well with distilled or deionized water.
Electrodes
The electrodes 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 the sections Measuring Hints and
Electrode Response
. Repeat the slope check.
3.
If the electrodes still fail to respond as expected, substitute another fluoride ion electrode
that is known to be in good working order for the questionable electrode. If the problem