Accumet XL 20 User Manual page 139

Ion Operation: Known Subtraction Method
Measuring Concentration in Samples Using the Known Subtraction Method
The procedure for known subtraction is very similar to that for known addition method (see 'Ion
Operation: Known Addition Method' section). 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 on 'Appendix: ISE Theory' section.
Taking a Known Subtraction Measurement
1. Immerse the ion selective electrode in a known volume of the sample solution.
2. Stir the sample.
3. Wait for the reading to stabilize (STABLE will appear). Touch KS to take the first reading.
4. Add a known volume of standard solution to the sample.
5. Wait for the reading to stabilize (STABLE will appear). Touch KS to take the second
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.
10. Use the keypad to enter the volume of ionic strength adjuster used. If no ISA was used,
enter a value of zero.
11. Touch Enter to accept the value.
12. Use the keypad to enter the concentration of the standard solution added to the sample.
13. Touch Enter to accept the value. Your Known Subtraction result is now displayed.
14. Touch Print to send the data to a printer
OR
Touch OK to return to the Measure screen for a new measurement.
XL25, 50 and 60 meters
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