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57
• To return to normal Measure mode press
.
Note: If the reading is out of range, “----” will be displayed on the LCD.
KNOWN ADDITION
To measure the concentration of a sample using the
Known Addition incremental method:
• Press
and then to select ISE
Measure mode for the selected channel.
• Select the Known Addition method (see ISE Setup for
details).
• Prior to starting a KA procedure, the ISE sensor must
be calibrated with a minimum of two standards
containing ISA. The slope of the electrode will be
used in all calculations involved in KA.
• If following an established procedure: Press
,
Edit the method variables and follow the procedure
below.
• Press
to set the method parameters. Press
/ to select next/previous 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
Addition analysis it is important to determine what
sample volume, standard concentration and standard
volume will produce the best results. 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. Sodium, Potassium, Calcium), the
standard addition should increase the mV. For a negatively charged ion (i.e. Sulfide, Fluoride,
Chloride), the standard addition should decrease 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 into the sample. Put instrument in mV mode and record
the observed mV. Using a micropipette, add a volume of the highest ISE standard available
(i.e. 0.1M or 1000 ppm).
ISE MEASUREMENT (HI5222 only)
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