Thermo Fisher Scientific Ion Selective Electrodes Ammonia - Manual (Page 11)

2. Place the more dilute solution on the magnetic stirrer and begin stirring at a constant rate.
Assure that the meter is in the concentration mode.
3. Lower the electrode tip into the solution.
4.
Adjust the meter to the concentration of the ammonia standard and fix the value in the
memory according to the meter manufacturer's instructions after stabilization of the
reading.
5.
Rinse the electrode with distilled water and blot dry.
6.
Place the most concentrated solution on the magnetic stirrer and begin stirring at a constant
rate.
7.
Lower the electrode tip into the solution. Adjust the meter to the concentration of the
ammonia standard and fix the value in the memory according to the meter manufacturer's
instructions after stabilization of the reading.
8.
For low level measurements, place the rinsed, dried electrodes into a solution containing
100 ml of distilled water and 1 ml of ISA. After stabilization, fix the blank value in the
meter according to the meter manufacturer's instructions.
9.
Place 100 ml of the sample and 1 ml of ISA in a 150 ml beaker. Place the beaker on the
magnetic stirrer and begin stirring.
10. Lower the electrode tip in solution. When the reading has stabilized, read the concentration
directly from the meter display.
11. The calibration should be checked every 2 hours. Assuming no change in ambient
temperature, place the electrode tip in the first ammonia standard. After the reading has
stabilized, compare it to the original reading in step 3 above. A reading differing by more
than 0.5 units or a change in ambient temperature will necessitate the repetition of steps 2-
8 above. The meter should be re-calibrated daily.
Low Level Ammonia Determination (using a pH/mV meter)
As the concentration of ammonia decreases, the rate of ammonia diffusion through the membrane is
slow, the rate of equilibrium between the ammonium in the internal filling solution and ammonia is
slow, and thus the response time increases. If the internal filling solution is diluted with ammonia-
free distilled water (1:10), response at low levels can improve. Measurements can be speeded up by
first placing the electrode tip in an ammonia-free pH 4 buffer, then into the sample. Always keep
standards and samples covered. Work with large solution volumes to minimize surface-area-to-
volume ratio, thereby avoiding ammonia absorption from air. Allow 5-10 minutes for a stable
reading in the pH 4 buffer or a low-level solution.
Use the following low level ammonia measurement procedure in the non-linear portion of the
calibration curve. (See Figure 1.) This procedure is used for ammonia samples containing less than
1.0x10-
5
M ammonia.