Orion Benchtop pH/ISE Meter Instruction Manual
Special Features
44
Double Incremental Techniques
Double incremental techniques allow sample concentration and electrode
slope to be determined simultaneously. This eliminates the requirement of
performing a calibration or slope check before measurement as required
with direct measurement or single incremental techniques.
Double incremental techniques are useful for occasional samples or
samples with complex matrices. Another advantage of the double
incremental techniques is that the electrodes are not moved from solution
to solution during calibration and measurement. This eliminates errors
due to solution carryover and static charge disturbance of the
electrode membrane.
It is especially important with double incremental techniques to be sure that
any complexing agent is present in great excess or not at all. The ratio of
free ion to complexed ion must remain constant over both additions. An
indication that a complexing agent is present, but not in great enough
quantity to maintain a constant free/complexed ion ratio, is an abnormally
high or low slope. If this is a suspected problem, an excess of complexing
or decomplexing agent may be added to the original solution in the beaker
prior to the incremental additions.
Electrode interferences should be at a minimum, the effect of the
interference may change as the concentration of the ion of interest changes.
This situation may also result in high or low electrode slopes. It is possible
to reduce some electrode interferences. Consult the appropriate electrode
instruction manual for specific information.
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