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5. Put the probe in
the sample with the
sensor fully in the
sample. Do not put
the probe on the
bottom or sides of
the beaker.
6.3 Low-level measurements
Use the guidelines that follow for measurements at low concentrations (<1 mg/L F
• Use plastic containers during calibration and measurements. Glass containers can cause
inaccurate measurements.
• Clean the probe regularly as specified in
• Soak the probe in the lowest concentration standard solution for 1 hour maximum before
calibration and measurement.
• Use the meter to set the stability criteria for the probe to a low value.
• Stir the standards and samples at a slow and steady rate to prevent the formation of a vortex.
• Use a dilute ionic strength adjustor (ISA) solution for calibration and measurements:
1. Dissolve the contents of one ionic strength adjustor powder pillow in 50 mL of deionized water.
2. Add 5 mL of the dilute (ISA) solution to each 25 mL of standard solution or sample.
Note: The ISA is not necessary when all of the conditions that follow apply:
• The sample does not contain interferences.
• The sample pH is within the range given in the specifications.
• Omission of the ISA is accepted by the regulatory reporting agency (if the measurement is for regulatory
reporting).
6.4 Interferences
The sensing element will measure some other ions that are known to interfere with the method. The
probe response to other ions usually increases the mV potential and causes a positive error. The
response to other ions can be semi-quantitatively calculated through the Nikolsky equation, an
extended Nernst equation:
E = Eº + (RT/(zF))ln[aN
Where
• ax = the activity of the interfering ion
• KN
x = the selectivity coefficient for the interfering ion relative to the primary ion
a
Cations and most anions do not interfere with the fluoride measurements. Anions commonly
associated with fluoride (chloride (Cl
3–
phosphate (PO
) and acetate (C
4
–
Hydroxide ions (OH
) interfere with the probe response when the pH is higher than pH 8. Some ions,
such as carbonate (CO
–
(OH
) interference, but the ions do not directly interfere. Hydroxide ions (OH
8 English
6. Shake the probe
from side to side to
refresh the reference
junction and remove
air bubbles.
Maintenance
+ KN
x × ax]
a
a
–
), bromide (Br
–
H
O
)) do not interfere with the probe operation.
2
3
2
2–
) or phosphate (PO
3
7. Stir for 30 to
60 seconds, then
read the fluoride
concentration of the
sample. The display
shows the value
when the reading is
stable.
on page 10.
–
2–
), sulfate (SO
4
3–
), increase the pH, which increases the hydroxide
4
–
).
–
), bicarbonate (HCO
3
–
) interfere with the
),
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