Thermo Fisher Scientific CyberScan pH 5000 - Manual (Page 118)

Instruction Manual CyberScan pH5500/ 5000
Measuring Ion Concentration in Samples Using the Incremental
Methods
When performing known addition, you will add an aliquot of standard
solution containing the ionic species of interest to a known volume of
your sample. The mV readings of the sample can then be calculated
based upon the difference in the mV readings. The use of the
incremental methods requires that certain parameters be identified. In
addition to the millivolts outputs of the electrode(s) in both the sample
and the sample plus the standard, it is necessary to enter into the
meter the volume of the sample, the volume of the standard, the
volume of the Ionic Strength Adjuster (ISA) if used, the concentration
of the standard and as previously stated the slope of the electrode in
use. Your CyberScan research meter will prompt you through the
method, permitting you to enter the appropriate parameters of the
appropriate time. When the meter has all of the data needed for the
calculation, it will display the ion concentration of the sample as well
as the parameters entered for the calculation.
The standard concentration should be high enough to nearly double
the sample ion concentration after its addition. However, the standard
volume should not significantly impact the sample volume. A 1:100
ratio of standard to sample ratio is optimal. For a monovalent
electrode, the standard addition should results in a 15-30 mV change.
For a divalent electrode, a 7-10 mV change is adequate. If the
millivolts change is too high, the addition of the standard may affect
the ionic strength and therefore the activity coefficients. This could
result in inaccurate measurements. If the millivolts change is too
small, readability errors are enhanced. See ISE Theory page 133.
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