Single Known Subtraction - Thermo Scientific Orion Versa Star Pro User Manual

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Chapter 4 pH, pH/ISE and pH/LogR Measurement Modules
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66 Orion Versa Star Pro Meter User Manual

Single Known Subtraction

In known subtraction, each addition of standard subtracts an increment of the species to be
measured. Enter the reaction ratio of the measured species to the added standard. For
example, if a sulfide (S ) sample will be subtracted by the addition of silver (Ag
2-
ions are required to subtract each sulfide ion and the reaction ratio is 0.5.
Sample concentration in Molarity or Normality should be known within an order of magnitude
and the total sample concentration should be approximately halved upon addition of the
standard. Concentrations are in Molarity or Normality due to the stoichiometric nature of the
subtraction. Refer to the following table to choose the correct standard concentration and
volume of addition.
Amount of standard to be added per
100 mL of sample
1 mL
5 mL
10 mL
1. Prepare the ion selective electrode (ISE) according to the instructions in the electrode user
manual. Prepare all required solutions (ionic strength adjusters, standards, etc) and review
any special requirements outlined in the electrode user manual. Connect the ISE, and any
other electrodes to be used, to the module inputs on the selected channel.
2. Determine the ISE slope as directed in the electrode user manual.
3. In the measurement mode, press the channel key until the channel with the pH/ISE
module with ISE connected is shown. If needed, press the mode key until ISE
Measurement is shown as the measure mode and use the
Electrode Type.
4. In the measurement mode, press the setup key.
5. Press the ,,  or  key to highlight Channel 1, Channel 2, Channel 3 or Channel 4
and press the f3 (Select) key.
a. Select the appropriate channel number that the ISE is connected.
6. Press the  or  key to highlight Incremental Techniques and press the f3 (Select) key.
7. Press the  or  key to highlight Single Known Subtraction and press the f3 (Select)
key.
8. Press the f3 (Clear) key as needed to delete the displayed slope value. Use the numeric
keypad to enter the ISE slope value determined in step 2 and press the f2 (Accept) key.
9. Enter the sample volume using the numeric keypad and press the f2 (Accept) key.
10. Enter the total volume (sample volume plus added ISA volume) using the numeric keypad
and press the f2 (Accept) key.
Standard concentration (in Molarity or Normality)
compared to expected sample concentration
50 times more concentrated
10 times more concentrated
5 times more concentrated
ISE Mode Menu
), two silver
+
to set the
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