Low-Level Measurements; Interferences - Hach ISENO3181 User Manual

6.3 Low-level measurements

Use the guidelines that follow for measurements at low concentrations (<1 mg/L NO
• 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 nitrate ion-selective electrodes are shown in
of the interferences, add the Nitrate ISA, which adjusts the pH to between pH 3 and 5. The
measurements are usually made at this pH range to remove carbonate and bicarbonate interference
and to decrease the organic acid interference.
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 CIO
0.1). The approximate selectivity coefficients for some ions with the Intellical Nitrate ISE are shown in
Table 1.
Interference
Perchlorate (CIO
Iodide (I
Bromide (Br
Chloride (Cl
Nitrite (NO
8 English
+ KN x × ax]
a a
Table 1 Interferences
–
)
4
–
)
–
)
–
)
–
)
2
Table
–
increases the nitrate concentration by
4
K (selectivity coefficient) value
1200 (interferes more)
10
0.1
0.006
0.001 (interferes less)
–
–N).
3
1. To remove most