Thermo Scientific Orion Versa Star Pro, Руководство пользователя

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Chapter 4
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pH, pH/ISE and pH/LogR Measurement Modules
Thermo Scientific Orion Versa Star Pro Meter User Manual
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61
Incremental Techniques (pH/ISE Module Only)
The Versa Star Pro meter with pH/ISE measurement module offers the option for incremental
techniques when in the ISE measure mode. Preset methods for single known addition, single
known subtraction, double known addition, double known subtraction, single analate addition
and single analate subtraction are available.

When performing incremental techniques, the sample concentration must be known within
an order of magnitude so the increment may be correctly chosen and the sample and
standard must be volumetrically determined.

It is very important that any complexing agent is present in excess (at least 50 to 100
times) or not at all. The ratio of free to complexed ions must remain constant over the
addition. An indication that a complexing agent is present, but not in great enough quantity
to maintain a constant free ion to complexed ion ratio, is an abnormally high or low slope. If
this is a suspected problem, add an excess amount of complexing or decomplexing agent
to the original solution in the beaker.

Electrode interferences should be at a minimum, since the effect of the interference might
change as the concentration of the ion of interest changes. This situation may also result in
high or low electrode slopes. Consult the electrode user manual for information on reducing
or eliminating some electrode interferences.

Known addition is a useful method for measuring dilute samples, occasional samples or
samples that contain an excess of complexing agent. Known addition can also be used to
verify the results of a direct measurement.

Known subtraction is useful when measuring ions for which stable standards do not exist.
Known subtraction can also be used as a quick version of a titration. When performing a
known subtraction, it is necessary to know the stoichiometric ratio between the standard
and sample.

Double incremental techniques calculate the electrode slope and sample concentration
simultaneously. The electrode slope is determined directly in the sample, which results in
greater accuracy for samples with complex matrices and greater analysis speed.

The Electrode Type must be set for the Ion Selective Electrode (ISE) that will be used for
the analysis, since the selected Electrode Type determines the expected slope range and
slope direction (positive or negative). The Electrode Type is set using the
ISE Mode Menu .