Thermo Fisher Scientific CyberScan 6000 Series Meters v.2 - Manual (Page 41)

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Ion Operation: Known Addition Method
PH6500, PC6500 and PCD6500 meters
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Measuring Concentration in Samples Using the Known Addition Method
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 are taken
before and after the addition of the standard. The concentration 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 millivolt 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 accumet research meter will prompt you through the method, permitting you to enter the
appropriate parameters at 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 result in a 15-30 mV change. For a divalent electrode, a 7-10 mV
change is adequate. If the millivolt 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 millivolt change is too small, readability errors are enhanced. See ISE
Theory on ‘Appendix: ISE Theory’ section.
Taking a Known Addition 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 KA to take the first mV
reading of sample.
4. Add a known volume of standard solution to the sample.
5. Wait for the reading to stabilize (STABLE will appear). Touch KA to take the second mV
reading of sample.
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 Addition 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.