Thermo Fisher Scientific Ion Selective Electrodes Nitrate - Manual (Page 6)

Instruction Manual Nitrate Ion Electrode
4
The pH range for the nitrate ion electrode is 2.5-11. Neutralize samples outside this range with acid or
base to bring them in range.
Units of Measurement
Nitrate concentrations are measured in units of ppm as sodium nitrate, ppm as nitrate, moles per liter, or
any other convenient concentration unit. Table 1 indicates some of the concentration units.
TABLE
1: Concentration Unit Conversion Factors
ppm NaNO
3
ppm NO
3
-1
moles/liter
850.0 620.0 1.0 X 10
-2
85.0 62.0 1.0 X 10
-3
8.5 6.2 1.0 X 10
-4
MEASUREMENT PROCEDURE
Direct Measurement
Direct measurement is a simple procedure for measuring a large number of samples. A single meter
reading is all that is required for each sample. The ionic strength of samples and standards should be
made the same by adjustment with ISA for all nitrate solutions. The temperature of both sample
solution and standard solutions should be the same.
Direct Measurement of Nitrate (using a pH/mV meter)
1. By serial dilution, prepare three standard solutions from the 0.1M or 1,000 ppm stock
standard. The resultant concentrations should be 10
-2
, 10
-3
, and 10
-4
M or 1,000, 100 and 10
ppm nitrate standards. Add 2 ml of ISA to each 100 ml of standard. Prepare standards with a
composition similar to the samples if the samples have an ionic strength above 0.1M.
2.
Place the most dilute solution (10
-4
M or 10 ppm) in a 150 ml beaker on the magnetic stirrer
and begin stirring at a constant rate. After assuring that the meter is in the mV mode, lower
the electrode tips into the solution. After the reading has stabilized, record the mV reading.
3.
Place the mid-range solution (10
-3
M or 100 ppm) in a 150 ml beaker on the magnetic stirrer
and begin stirring. After rinsing the electrodes with distilled water, blot dry and immerse the
electrode tips in the solution. When the reading has stabilized, record the mV value.
4.
Place the most concentrated solution (10
-2
M or 1,000 ppm) in a 150 ml beaker on the magnetic
stirrer and begin stirring. After rinsing the electrodes in distilled water, blot dry, and immerse
the electrode tips in the solution. When the reading has stabilized, record the mV reading.
5.
Using the semi-logarithmic graph paper, plot the mV reading (linear axis) against the
concentration (log axis). Extrapolate the calibration curve down to about 1.0X10
-5
M (1.0 ppm
NO
3
-1
). A typical calibration curve can be found in Figure 1.