E15a Ion-Selective Electrodes (ISEs) – Ammonium (NH4+)
Eureka’s Ammonium ion-selective electrode operates much like a pH probe except the pH glass
is replaced by a membrane that is magically selective for the ammonium ion. The electrode’s
filling solution contains an ammonium salt, and the difference between that electrode’s
ammonium concentration and the ammonium concentration in your water produces a charge
separation. That charge separation is measured, relative to the reference electrode, as a voltage
that changes predictably with changes in the ammonium concentration in the water adjacent the
membrane.
Ion-selective electrodes actually respond to ion activity, which is lower than the ion concentration
in high-conductivity waters (because all the other ions “shield” the ion of interest). Eureka uses
the conductivity of the sample water to estimate its ionic strength, and then uses the composition
of average river water to convert activity to concentration.
The Eureka ammonium sensor directly detects Ammonium ions. At higher pH (the pK
a for
Ammonium-Ammonia is 9.3) Ammonium is converted into ammonia gas (NH3). Eureka’s
software uses the pH, Conductivity, and Temperature of the sample water to calculate Ammonia
(as mg/L-N) and Ammonia (as mg/L-N). You can also display Total Ammonia; the sum of
Ammonia and Ammonium. It is common to use concentration units of mg/L-N (i.e., concentration
of total nitrogen present as ammonia or ammonium), since using this unit eliminates errors in
Total Ammonia when a pH change causes a shift in the equilibrium between Ammonium and
Ammonia.
Ammonium electrodes suffer interference from positive ions, especially potassium and sodium.
For instance, 1000 mg/L of sodium ions or 10 mg/L of potassium ions will cause at least a 1
mg/L-N increase in the Ammonium reading. In fresh water, potassium is the primary interference,
raising Ammonium values by 0.1-0.2 mg/L-N in average 200 µS river water. In brackish or
marine waters, sodium can become the primary interference because of its high concentration;
sea water reads up to 12 mg/l-N Ammonium even if there is no Ammonium present.
It’s best not to let your ISE dry out, so place a small amount of tap water in the storage cup to
ensure 100% humidity.
Ammonium requires a two-point calibration; we recommend calibration solutions of 4.63 mg/l – N
and 46.3 mg/l – N. The response of the Ammonium electrode is affected by temperature. While
Eureka uses generalized methods for temperature corrections, it’s best to calibrate near the
temperature of your water. Here’s the procedure:
1) Rinse your sensors several times with the standard you’ll use for calibration.
2) Fill the calibration cup with enough standard to cover both the ISE and reference
electrodes.
3) Follow the Manta 2 Control Software’s calibration instructions.
E15b Ion-Selective Electrodes (ISEs) – Nitrate (NO
3+)
The Nitrate ISE operates as does the Ammonium ISE (see F16a) except that its membrane is
selective for, you guessed it, Nitrate ions.
Eureka’s Nitrate ion-selective electrode operates much like a pH probe except the pH glass is
replaced by a membrane that is magically selective for the nitrate ion. The electrode’s filling
solution contains a nitrate salt, and the difference between that electrode’s nitrate concentration
and the nitrate concentration in your water produces a charge separation. That charge
separation is measured, relative to the reference electrode, as a voltage that changes predictably
with changes in the nitrate concentration in the water adjacent the membrane.
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