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Thermo Scientific Orion VERSA STAR pH/ISE/Temperature Module
Chapter 9 Incremental Techniques
Known Addition and Subtraction
The Orion VERSA STAR meter has preset methods for single known addition, single known subtraction, double known
addition and double known subtraction.
• 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 guide 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.
