Low-Level Measurements; Interferences - Hach ISENH4181 User Manual

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 NH
• Fully rinse the probe with deionized water and blot dry between measurements.
• 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
The primary interferences for ammonium ion-selective electrodes are perchlorate (ClO
– –
(ClO ) and iodide (I ). There is no procedure to remove the interference from these ions. Other
3
interferences are shown in
of the interferences from
The selectivity coefficient is the approximate apparent increase in the measured concentration that is
caused by one unit of the interfering ion (e.g., 1 unit of K
0.1). The approximate selectivity coefficients for some ions with the Intellical Ammonium ISE are
shown in Table 1.
8 English
6. Shake the probe
from side to side to
refresh the reference
junction and remove
air bubbles.
+ KN x × ax]
a a
Table 1. Addition of the Ammonium ISA adjusts the pH and prevents most
Table 1.
7. Stir for 30 to
60 seconds, then
read the ammonium
concentration of the
sample. The display
shows the value
when the reading is
stable.
+
increases the ammonium concentration by
+
–N).
4
–
), chlorate
4