Hach-Lange ISENH318103 User Manual - Manual (Page 6)

Interferences
The sensing element responds to ammonia 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 chloride
Volatile amines interfere with Ammonia ISE measurement. Most gases do not interfere as
they are converted to ionic form in basic solutions. Ionic species cannot cross the gas-
permeable membrane and are not direct electrode interferences. However, the level of
ions in solution can change the solubility of ammonia. Standards and samples should
have about the same level of ions and dissolved species.
Ammonia forms metal complexes with a number of metal ions: mercury, silver, copper,
gold, nickel, cobalt, cadmium and zinc. At pH >11, most of these metals form hydroxide
complexes or precipitate. The Ammonia ISA adjusts the pH to >11. When hydroxide is
present at the 0.1 M level and the ammonia concentration is below 10
-3 M, only mercury
will appreciably complex ammonia. The total ammonia level of the sample will be
measured if the mercury in the sample is preferentially bound to some other species.
Iodide is recommended for this purpose, since it forms a soluble mercury complex at all
pH levels. Use of Ammonia ISA inhibits the formation of some common metal complexes
in the sample because it contains a high concentration of hydroxide ion.
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 ISENH3181 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
the acceptance criteria is set to Cal Expires on Failure: Yes, the display shows the
calibration icon and a question mark until the probe is recalibrated. To correct the
probe calibration and status indicator, calibrate the probe (refer to
Calibration
on page 2).
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