Hach-Lange ISENO318103 User Manual - Manual (Page 6)

1. Dissolve the contents of one ionic strength adjustor powder pillow in 50 mL of
deionized water.
2. Add 5 mL of this solution to every 25 mL of standard or sample that is used.
Note: The ionic strength adjuster can be omitted only when all of the following conditions are
true:
• 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 nitrate 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 nitrate
For the nitrate ISE, the major interferences are shown in Table 1 . To eliminate most of
these interferences, add the Nitrate ISA and adjust the pH to between 3 and 5. The
measurements are usually made at this pH range to eliminate carbonate and bicarbonate
interference and reduce organic acid interference.
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 CIO
4
- raises the
nitrate concentration by 0.1). The approximate selectivity coefficients for some ions with
the IntelliCAL
®
Nitrate ISE are shown in Table 1 .
Table 1 Interferences
Interference K (selectivity coefficient) value
Perchlorate (CIO
4
- ) 1200 (interferes more)
Iodide (I - ) 10
Bromide (Br - ) 0.1
Chloride (Cl - ) 0.006
Nitrite (NO
2
- ) 0.001 (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 ISENO3181 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
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