Thermo Fisher Scientific Ion Selective Electrodes Calcium - Manual (Page 15)

Instruction Manual Calcium Electrode
15
Solutions other than this may be used as ISA's as long as ions that they contain do not interfere with
the electrode's response to calcium ions.
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 into 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 glass-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.
Glass-ware
Clean glass-ware will drain clean. That is, when rinsed with distilled or deionized water, the water
does not bead on the inside walls of the glass-ware.
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 section
Measuring Hints
. Repeat the
slope check.
3.
If the electrodes still fail to respond as expected, substitute another calcium electrode that
is known to be in good working order for the questionable electrode. If the problem
persists and you are using an electrode pair, try the same routine with a working reference
electrode.
4.
If the problem persists, the standards and/or reagents may be of poor quality, interferences
in the sample may be present or the technique may be faulty. (See
Standards and
Reagents, Sample,
and
Technique
sections below.)