Nitrate ions, NO3–, may be found in freshwater samples from a variety of sources. Sewage is often the primary source. Sometimes nitrates are present due to runoff from fertilized fields. Nitrates can also result from the runoff from cattle feedlots and barnyards. In all of these cases, as plant and animal organisms die, bacterial action breaks down the protein into ammonia, NH3. Some ammonia is converted into ammonium ions, NH4+. Other bacterial action converts some of the ammonia and ammonium ions into nitrite ions, NO2–, and then into nitrate ions, NO3–.
Units of Nitrate Concentration
Nitrate ion concentration is usually expressed in units of mg/L of NO3– as N, also known as “nitrate-nitrogen.” This means that the concentration of nitrate is expressed as if the nitrate were only in the form of nitrogen itself. The standards that are included with your Nitrate ISE have concentrations of 1 and 100 mg/L of NO3– as N. Here is the calculation for making a 100 mg/L NO3– as N standard starting with solid NaNO3 (as shown in Table 1). Notice that the atomic weight of N, 14.0, is used instead of the atomic weight of NO3–, 62.0.
Unpolluted waters usually have nitrate-nitrogen (NO3– as N) levels below
1 mg/L. Nitrate-nitrogen levels above 10 mg/L are considered unsafe for drinking water.
Test results are sometimes published in units of mg/L NO3– instead of NO3– as N. To convert 100 mg/L NO3– as N to mg/L NO3–, you would perform this conversion:
Sampling Freshwater Samples for Nitrate Concentration
For best results, calibrate the Nitrate ISE, using the 1 mg/L and 100 mg/L standards.
How Can I Have My ISE Read mV Output Instead of mg/L?
The amplification equation is: V=0.00727*mV + 1.223
Therefore, the reverse amplification equation, solving for mV, would be:
mV = 137.55*V – 0.1682
- Make sure the sensor is properly calibrated. If the meter has a reading of
1.0 mg/L and the sensor is not in a 1.0 mg/L solution, you need to calibrate. After calibration, rinse off the tip of the ISE and blot it dry with a paper towel.
- Insert the tip of the ISE into the aqueous sample to be tested. Important: Make sure the ISE is not resting on the bottom of the container, the white reference contacts near the tip of the electrode are immersed, and no air bubbles are trapped below the ISE. Note: Do not completely submerge the sensor. The handle is not waterproof.
- Hold the ISE still until the reading stabilizes and record the displayed reading. Note: With some aqueous samples, especially those at high concentrations, it could take several minutes for the reading of the Nitrate ISE to stabilize. If you know the approximate concentrations of your samples, it is best to analyze them from lowest concentration to highest.
Using the Nitrate ISE with Other Vernier Sensors
Some combinations of sensors interfere with each other when placed in the same solution. The degree of interference depends on many factors. For more information, see www.vernier.com/til/638
Using Ionic Strength Adjuster (ISA) Solution to Improve Accuracy
For optimal results at low concentrations of nitrate ions, a standard method for taking measurements with the Nitrate Ion-Selective Electrode (ISE) is to add ionic strength adjuster (ISA) solutions to each of your standard solutions and samples.
Adding an ISA ensures that the total ion activity in each solution being measured is nearly equal, regardless of the specific ion concentration. This is especially important when measuring very low concentrations of specific ions. The ISA contains no ions common to the Nitrate ISE itself. Note: The additions of ISA to samples or standards described below do not need to have a high level of accuracy— combining the ISA solution and sample solution counting drops using a disposable Beral pipet works fine. The following are instructions for using ISA solutions with Vernier Ion-Selective Electrodes.
Use an ISA with the Nitrate ISE by adding 2.0 M (NH4)2SO4 ISA solution (26.42 g (NH4) 2SO4 / 100 mL solution) to the NO3– standard or to the solution being measured, in a ratio of 1 part of ISA (by volume) to 50 parts of total solution (e.g., 1 mL of ISA to 50 mL of total solution, or 2 drops of ISA to 5 mL of total solution).