Hach-Lange ISEF12103 User Manual - Manual (Page 6)

• The sample does not contain interferences.
• The sample pH is in the range that is given in the specifications.
• Omission of the ISA is accepted by the regulatory reporting agency (if the measurement is
for regulatory reporting).
Interferences
The sensing element responds to fluoride as well as other ions. Typically, probe response
to another ion increases the potential, and causes a positive error. The response to other
ions can be semi-quantitatively determined through the Nikolsky equation, an extended
Nernst equation:
E = Eº + (RT/(zF))ln[aN
a
+ KN
a
x × ax]
Where
• ax—the activity of the interfering ion
• KN
a
x—the selectivity coefficient for the interfering ion relative to fluoride
Cations and most anions do not interfere with the ISEF121 probe response to fluoride
ions. Anions commonly associated with fluoride (Chloride (Cl
- ), Bromide (Br - ), Sulfate
(SO
4
2- ), Bicarbonate (HCO
3
- ), Phosphate (PO
4
3- ) and acetate) do not interfere with probe
operation.
Hydroxyl ions (OH
- ) do interfere with probe response above pH 8. Some ions, such as
Carbonate (CO
3
2- ) or Phosphate (PO
4
3- ), make the sample more basic, which increases
Hydroxyl (OH - ) interference, but do not directly interfere with the probe operation.
Hydroxide (OH - ) ions interfere with the probe response to fluoride when the level of
hydroxide is greater than 10% of the level of fluoride present. At pH 8 or less, there are
no hydroxide ions to interfere. The error increases when the pH is increased and the
fluoride levels are decreased.
If Fluoride ISA is added to standards and samples, the pH is buffered between 5.0 and
5.5, which prevents hydroxide ion interferences.
The selectivity coefficient is the approximate apparent increase in the measured
concentration caused by one unit of the interfering ion (e.g., 1 unit of OH
- raises 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
Hydroxyl (OH - ) 0.1 (below pH 8)
Run a check standard
The run check standard feature validates instrument performance between sample
measurements. Use the run check standard feature for periodic or user-defined interval
measurements of a traceable standard solution. Set the criteria for check standards from
the ISEF121 Settings menu.
Note: Access control must be off or a valid password must be entered before any of the check
standard method options can be changed.
1. Push . The Full Access Options menu is shown.
2. Select Run Check Standard.
Note: Select the correct probe if two probes are connected to the meter.
3. Prepare the standard solution shown on the display. Add one powder pillow per
25 mL of standard solution.
4. Put the probe in the standard solution and push Read. The display will show
"Stabilizing" and a progress bar as the reading stabilizes. The display shows the
value of the check standard and either Check Standard Passed or Check Standard
Failed.
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