Thermo Fisher Scientific CyberScan pH 5000 - Manual (Page 120)

Instruction Manual CyberScan pH5500/ 5000
11.4 Known Subtraction Method
The procedure for known subtraction is very similar to that for known
addition method. An aliquot of standard is added to a known volume
of the sample. The difference is that the standard does not contain
the same ionic species that you are trying to measure in the sample.
Instead, it contains an ion that will complex or precipitate the ion of
interest, removing it from the sample. Like known addition, the mV
readings are taken before and after the standard is added to the
sample and the difference in mV values can be used to calculate the
concentration of free ions of interest in the sample. This method will
also require you to enter values of certain parameters in order to
obtain an ion concentration. See ISE theory page 133.
To take a measurement
1. Immerse the ion selective electrode in a known volume of the
sample solution.
2. Stir the sample.
3. Touch
KS to take the first mV reading of sample. Wait for the
reading to stabilize (STABLE will appear).
OR
Touch
KS to accept the current reading.
4. Add a known volume of standard solution to the sample.
5. Touch
KS to take the second mV reading of sample. Wait for the
reading to stabilize (STABLE will appear).
OR
Touch
KS to accept the current reading.
6. Use the keypad to enter the sample volume.
7. Touch enter to accept the value.
8. Use the keypad to enter the standard volume.
9. Touch
enter to accept the value.
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