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59
• If following an established procedure: Press then edit the method variables and follow
the procedure below.
• Press to set the method parameters. Press / to select next/previ-
ous parameter to edit, then press and use or to set the desired
parameter value. Press to save the modified value and then press to exit
method parameters edit menu.
• If developing a procedure: Before attempting Known Subtraction analysis it is important to
determine what sample volume, standard reactant concentration and standard volume will
produce the best results and the way the reagent will react with the measured ion on a molar
basis (stoichiometric factor). As a general rule, the addition of standard should change the mV
value of the sample by 15-20 mV.
For a positively charged ion (i.e. Calcium), the reactant addition should decrease the mV. For
a negatively charged ion (i.e. Sulfide, Fluoride, Chloride), the reactant addition should increase
the mV. Start with a small trial. For example: Measure 50 mL of sample, add a magnetic stir
bar, place on a stirrer, add ISA (consult ISE manual) and place ISE electrode tip in to the
sample. Put instrument in mV mode and record the observed mV. Using a micropipette, add
a volume of the reactant standard. Start by adding 500 µL at a time (for example). Watch
the change in mV. When you have observed approximately a 15 mV change from the original
sample, calculate the total volume added. Adjust sample and standard volumes proportionally
to standard volumes that can be measured with accuracy. Use volumetric pipettes for standard,
ISA and reagent addition.
• Press
then edit the procedure variables to the volumes determined in the prior step.
Procedure:
• Press
to enter Known Subtraction mode.
• Volumetrically add sample to a clean beaker. Add a magnetic stir bar and place on a stir plate.
Stir sample. The method will prompt user to add ISA. Place ISE sensor tip into the solution and
a mV value will show on the display.
• Press
to take the first mV reading.
• When the reading is stable, press
to store the first mV reading. The second step of the
method will be displayed on the LCD in which the user is notified to add the volume of reagent
to the sample.
• Press
to take the second mV reading.
ISE MEASUREMENT (HI5222 only)
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