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

Instruction Manual Sodium Electrode
11
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.
To adjust the background ionic strength to a high and constant value, ionic strength adjuster is
added to samples and standards. The recommended ISA for sodium is an ammonium
chloride/ammonium hydroxide buffer. Solutions other than this may be used as ionic strength
adjusters as long as ions that they contain do not interfere with the electrode's response to sodium
ions.
The reference electrode must also be considered. When two solutions of different compositions 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 solutions 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 in 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 glassware, 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.
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.
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 electrodes still fail to respond as expected, substitute another sodium ion electrode
that is known to be in good working order for the questionable electrode. If the problem