Hach-Lange ISENH418103 User Manual - Manual (Page 6)

• 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 ammonium 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 ammonium
For the ammonium ISE, the major interferences include perchlorate (ClO
4
- ), chlorate
(ClO
3
- ) and iodide (I - ). These ions must be avoided. Other interferences are listed in
Table 1 . To eliminate most of these interferences, add the Ammonium ISA to adjust the
sample pH.
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 K
+ raises the
ammonium concentration by 0.1). The approximate selectivity coefficients for some ions
with the IntelliCAL
®
Ammonium ISE are shown in Table 1 .
Table 1 Interferences
Interference Selectivity coefficient
Potassium (K + ) 0.1 (interferes more)
Sodium (Na + ) 0.002
Magnesium (Mg 2+ ) 0.00002
Lithium (Li + ) 0.00003
Calcium (Ca 2+ ) 0.00006 (interferes less)
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 ISENH4181 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.
5. If the display shows Check Standard Passed, the check standard measurement is
within the accepted limits set by the administrative user. Select Done to continue with
the sample measurement.
6. If the display shows Check Standard Failed, the measurement is outside of
accepted limits set by the administrative user and a recalibration is recommended. If
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