19
109118-1 Rev. A,10/96
3.4
Problem Solving (cont)
3.4.3
Electrode Problem Solving (cont)
2.
The junction is designed to allow the fill solution to leak
out of the tip of the electrode at a slow controlled rate. To
test for proper flow, rinse the tip of the electrode, blot dry
and let the electrode hang in the open air for 15 minutes. If
it is flowing properly, salt crystals of the fill solution will
form on the junction. If they do not, refer to the
manufacturer's instructions for junction cleaning or
replacement.
Slow response or unstable readings in samples can be caused by
a number of things. Those listed below assume that everything
was fine in the calibration buffers.
1.
Low ionic strength samples will tend to be very drifty. You
are trying to take an electrical measurement in a solution
that is a very poor conductor. Try adding a drop of KCl fill
solution to about 5 mL of the sample. In most cases it will
not change the pH but will give the sample enough ionic
strength for the electrode to finish the reading.
2.
Non aqueous samples (organics) do not follow the normal
pH scale and can cause some measuring problems. These
solutions are usually low in ionic strength, may dehydrate
the pH membrane and often cause the junction to plug due
to incompatibility with the electrode fill solution. The most
reliable way to pH organics is to dilute them with water
until stable readings can be generated.
3.
Concentrated samples can cause pH measuring errors.
This is caused by problems with the pH sensing glass and
its ability to distinguish the Hydrogen ion activity over the
mass of other ions in the sample. The creation of new
junction potentials due to KCl dissociation problems can
also cause instability. The best solution is to dilute the
samples into measurable ranges.
For more information refer to the Corning Guide to pH
Measurement.
3