Thermo Fisher Scientific Ion Selective Electrodes Ammonia - Manual (Page 12)

1. Measure out 100 ml of a pH 4 buffer solution, add it to a 150 ml beaker, place the beaker
on the magnetic stirrer, and begin stirring. Place the electrode tip in the solution for about 3
minutes.
2.
By serial dilution, prepare a 10-2M or 100 ppm ammonia standard by diluting the 0.1M or
1,000 ppm standard solution.
3.
To a 2500 ml erlenmeyer flask, add 1,000 ml of distilled water and 10 ml of 10M NaOH.
Place on magnetic stirrer and begin stirring. After rinsing the electrode, place the tip in this
calibration solution. Assure that the meter is in the mV mode.
4.
Using a 1 ml graduated pipet (A) and a 2 ml pipet (B), add increments of the 10-
2
M or 100
ppm dilution to the calibration solution using the steps outlined in Table 2. Allow the
reading to reach equilibrium and record the electrode potential after each step. Plot the
concentration (log axis) vs. the electrode potential (linear axis) on semi-logarithmic paper
to obtain the calibration curve.
5.
After rinsing the electrode, immerse the electrode tip in pH 4 buffer for 3 minutes. Be sure
to use the magnetic stirrer.
6.
Place 1,000 ml of the sample into a 1,000 ml beaker, add 10 ml of 10M NaOH, place the
beaker on the magnetic stirrer, and begin stirring. After rinsing the electrodes, immerse the
tip in the solution. When the reading has stabilized, record the mV potential.
7.
Determine concentration from the calibration curve. (Prepare a new low-level calibration
curve on a daily basis using freshly prepared solutions).
TABLE
2: Additions of 10-
2
M or 100 ppm Standard to 1,000 ml Distilled Water
and 10 ml 10M NaOH
Added Concentration
Step Pipet Volume(ml) M ppm
1 A 0.10 9.9 x 10-
7
0.01
2 A 0.10 2.0 x 10-
6
0.02
3 A 0.20 4.0 x 10-
6
0.04
4 A 0.20 5.9 x 10-
6
0.06
5 A 0.40 9.9 x 10-
6
0.10
6 B 2.00 3.0 x 10-
5
0.30
7 B 2.00 4.9 x 10-
5
0.49
Ammonia Measurements in Membrane Wetting Solutions
Since the membrane of the ammonia electrode is gas-permeable and hydrophobic, liquid water does
not penetrate the membrane holes and does not wet it. A non-aqueous solution, or a solution
containing a surfactant which wets the membrane, penetrates the membrane. Non-aqueous samples,
such as latex paints or nylon, and those containing surfactants, such as sewage, cause measurement
difficulties. The electrode must be suspended above the sample to measure ammonia in such
samples.