3.19 INCREMENTAL METHODS
Incremental methods are particularly useful for single sample analyses, as usually one standard solution and
two potential measurements are necessary. The effects of variable temperature and ionic strength are eliminated
by the large dilution effects. However, the volumes of the solutions must be accurately measured.
Known Standard or Sample Addition
The calculation for this technique has been programmed into the Model 3345.
In cases where the composition of the test solution is unknown or complex, the standard addition method is
employed. In this technique, the electrode is immersed in an known volume of the unknown solution and the
potential is recorded. A small measured volume of a standard solution is added and the potential measured.
The measured potential difference is used to solve the incremental technique equation to give concentration
of the unknown solution.
Alternatively, the electrode is immersed in a known volume of a standard solution and small measured
volumes of the unknown are added, following the same procedure. Appropriate when low sample volume or
high concentration. Results are more accurate if the effects of severe additions are averaged.
Known Standard or Sample Subtraction
The calculation for this technique has been programmed into the Model 3345.
In this method a small volume of reagent which complexes or precipitates the ion under investigation is added
to a known volume of the sample. Thus, there is a decrease in ion concentration and corresponding decrease
in measured potential. The measured difference potential is used to solve the incremental technique equation
to give concentration of the unknown solution. This method is preferred when standard solutions of the ion
are unstable, of unknown concentration or toxic. This method is also used when the sample ion concentration
is high and known addition procedures produce small changes in potential.
Alternatively, a small volume of unknown sample is added to a volume of standard solution which the test ion
will react with to form a precipitate or complex. The electrode used is sensitive to the ion in the standard
solution rather than the unknown solution ion. Therefore, measurements can be made for an ion for which no
electrode exisits, if a system of reaction can be found.
Multiple Sample Addition
This mode of operation enables a large number of samples to be analysed using only one standard solution.
In this technique, the electrode is immersed in a known volume of a standard solution and the potential is
recorded. A small measured volume of an unknown solution is added and the potential measured. The
measured potential difference is used to solve the incremental technique equation to give concentration of the
unknown solution. A further addition of a measured volume of an unknown solution is added, either a new
solution or second sample of the original solution. The concentration of the second unknown solution is
calculated, allowing for the change in volume using the change in potential to calculate the concentration of
the subsequent sample solution.
Titration
The principle of ion selective electrode titrations is based upon the reaction of two species in solution, the
endpoint being characterised by the total disappearance of one of the species or creation of a new reaction
product. Using an ion selective electrode as an indicator to monitor the reaction, an accuracy of 0.1 - 0.5%
can be achieved. Titrimetric methods can also be used not only to directly measure the presence of an ionic
species, but indirectly to measure an ion by the formation of complex with a measurable ionic species.
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