Hach ISEF121 Series, User Manual

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probe response when the level of hydroxide is more than 10% of the level of fluoride. The error
increases with an increase in pH and a decrease in fluoride.
When the Fluoride ISA is added to standards and samples, the pH is buffered between 5.0 and 5.5,
which prevents interference from hydroxide ion.
The selectivity coefficient is the approximate apparent increase in the measured concentration that is
caused by one unit of the interfering ion (e.g., 1 unit of OH
–
increases the fluoride concentration by
0.1). The approximate selectivity coefficients for some ions with the Intellical Fluoride ISE are shown
in
Table 1 .
Table 1 Interferences
Interference Selectivity coefficient
Hydroxide (OH
–
) 0.1 (below pH 8)
Section 7 Verify the calibration
Measure the value of a fresh standard solution at regular intervals to make sure the result is
accurate. The meter compares the expected standard solution value to the measured value and
accepts or rejects the measurement. The user can change the standard solution and acceptance
criteria for verification in the probe-specific settings.
Note: Password protection may prevent access to the acceptance criteria.
7.1 Verification procedure
1. Go to the
verification menu.
The display shows
the standard solution
to use for
verification.
Note: Menu name for
HQd meters: Run check
standard.
2. Pour 25 mL of the
standard solution
into a 50
‑
mL plastic
beaker.
3. Add one Fluoride
Ionic Strength
Adjustor (ISA)
Powder Pillow.
Note: As an alternative,
add 5 mL of Fluoride
ISA Solution.
4. Add a stir bar.
Put the beaker on an
electromagnetic
stirrer. Stir at a
moderate rate.
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