The Intermediate Junction Reference System
Did your know that the most important factor affecting the accuracy of
pH measurement is the condition of the contact zone between the reference
electrolyte and the sample (reference junction)? Contamination or
blockage of this zone during use can lead to significant errors in sample
measurement, even though calibration indicates proper performance (1).
This is due to impedance of ion mobility at the contact zone, thus allowing
other ions in the sample to contribute more significantly to the junction
potential than they would at a more open junction. Consequently,
the voltage output of the electrode is a response to more than just the
hydrogen ion activity. Under these circumstances, correct calibration
is possible only because the calibrating buffers are at the same ionic
strength. Errors occur when the samples are at different ionic strengths
to the buffers.
The Intermediate Junction (IJ) system was basically designed to solve
this problem by considering two approaches:
1. Utilising a properly designed contact zone to allow reference
electrolyte to move freely to the sample, thus stabilising the junction
potential.
2. Allowing the contact zone to be easily renewed and cleaned when
it does become clogged.
This has led to our highly successful IJ42 pH electrode, with its double
junction design, concentric annular outlet, renewable electrolyte and removable
sleeve. Now the IJ44 reference system
has gone a step further with introduction of a new viscous gel electrolyte.
Intervals between electrolyte renewal have been considerably extended such
that IJ electrodes are even usable for process applications.
How the Intermediate Junction system works.
The electrode body is split across the middle with the lower section being
removable. The upper section houses the Ag/AgCl reference half cell.
The lower section contains the salt bridge between the reference half cell
and the sample. The reference half cell is permanent. The salt
bridge can be easily renewed by removing the sleeve, cleaning the ground
glass contact zone, filling the well with electrolyte and replacing the
sleeve. Replacing the sleeve displaces the electrolyte up the sleeve
and out the end.
1. Twist sleeve and remove.
2. Fill well with gel.
3. Replace sleeve. Gel is forced out top of electrode just
below the pH sensor.
A bottle of gel electrolyte is supplied with each IJ electrode, which allows
for about 20-30 fillings. Each filling can last as long as a month
between renewal depending on usage. Alternative liquid electrolytes
can be prepared by the user, although the time between renewal will not
be as long as with the gel. The IJ reference system is also available in
the IJ64 Redox electrode and the IJ14
Ag/AgCl Double Junction Reference half cell. Alternate gel electrolytes
are also available for cases where KCl cannot be used.
More information.
(1) Illingworth, J.A.; Biochem. J. (1981) 195, 259-262.