Thermo Fisher Scientific Ion Selective Electrodes Nitrate - Manual (Page 8)

Instruction Manual Nitrate Ion Electrode
6
Direct Measurement of Nitrate (using an ion meter)
1. By serial dilution of the 0.1M or 1,000 ppm nitrate standard, prepare two nitrate standards
whose concentration is near the expected sample concentration. Measure out 100 ml of each
standard into individual 150 ml beakers and add 2 ml of ISA to each.
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. Lower the electrode tips into the solution.
3. Adjust the meter to the concentration of the nitrate standard and fix the value in the memory
according to the meter manufacturer's instructions after stabilization of the reading.
4.
Rinse the electrodes with distilled water and blot dry.
5.
Place the more concentrated solution on the magnetic stirrer and begin stirring at a constant
rate. Lower the electrode tips into the solution.
6.
Adjust the meter to the concentration of the nitrate standard and fix the value in the memory
according to the meter manufacturer's instructions after stabilization of the reading.
7.
For low level measurements, place the rinsed, dried electrodes into a solution containing 100
ml of distilled water and 2 ml of ISA. After stabilization, fix the blank value in the meter
according to the meter manufacturer's instructions.
8.
Place 100 ml of the sample and 2 ml of ISA in a 150 ml beaker. Place the beaker on the
magnetic stirrer and begin stirring.
9.
Immerse the electrode tips in the solution and wait for the reading to stabilize. Read the
concentration directly from the meter display.
10. The calibration should be checked every two hours. Assuming no change in ambient
temperature, place the electrode tips in the first nitrate standard. After the reading has
stabilized, compare it to the original reading in Step 3 above. A reading differing by more
than 0.5 mV or a change in ambient temperature will necessitate the repetition of Steps 2-6
above. The meter should be re-calibrated daily.
Low Level Nitrate Measurements (using a pH/mV meter)
This procedure is recommended for solutions with ionic strengths less than 1.0X10
-2
M. If the solution
is high in ionic strength, but low in nitrate, use the same procedure, but prepare a calibration solution
with a composition similar to the sample.
1. Using 20 ml of standard ISA, dilute to 100 ml with distilled water. This low level ISA [0.4M
(NH
4
)
2
SO
4
] is added at the rate of 1 ml low level ISA to each 100 ml of sample. The
background ionic strength will be 4.0X 10
-3
M.
2.
Dilute 1 ml of 0.1M standard to 100 ml to prepare a 1.0X10
-3
M NO
3
-1
solution for
measurements in moles per liter. Use the 1,000 ppm standard for preparing a 100 ppm NO
3
-1
standard by diluting 10 ml of the 1,000 ppm standard to 100 ml. Standards should be prepared
fresh daily.