Chapter 4
pH, pH/ISE and pH/LogR Measurement Modules
68
Orion Versa Star Pro Meter User Manual
Thermo Scientific
Double Known Subtraction
Double Known Subtraction Recommendations
Ensure that the subtraction ability of the chosen standard concentration and volume added does not
exceed the concentration of the sample.
The sample volume should be 100 mL.
The concentration of the standard in Molarity or Normality should be 50 times the expected sample
concentration when the reaction ratio is 1.
Calculate the volume of standard required to halve the initial sample concentration and divide by six.
The volume of the first addition should be 5/6 the total standard mL and the volume of the second
addition should be 1/6 the total standard mL.
If available, consult a printout from a previous analysis of a similar sample.
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.
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
ISE Mode Menu
to set the
Electrode Type.
3.
In the measurement mode, press the setup key.
4.
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.
5.
Press the
or
key to highlight Incremental Techniques and press the f3 (Select) key.
6.
Press the
or
key to highlight Double Known Subtraction and press the f3 (Select)
key.
7.
Enter the sample volume using the numeric keypad and press the f2 (Accept) key.
8.
Enter the total volume (sample volume plus added ISA volume) using the numeric keypad
and press the f2 (Accept) key.
9.
Enter the standard concentration in Molarity or Normality using the numeric keypad and
press the f2 (Accept) key.
10.
Enter the reaction ratio using the numeric keypad and press the f2 (Accept) key.
11.
Rinse the ISE and any other electrodes in use with distilled water or rinse solution required
for that electrode. Blot the electrode dry with a lint-free tissue or shake the electrode, place
into the prepared sample and press the f2 (Next) key.
a.
Follow all electrode rinsing and drying instructions or exceptions in the ISE user
manual. Do not wipe or rub the sensing element of the electrode.
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