The Go Direct Nitrate Ion-Selective Electrode BNC is used to measure the potential (mV) and calculate the concentration of nitrate (NO₃^–) ions in aqueous samples. It is designed to be used with the Go Direct Ion-Selective Electrode Amplifier (order code GDX-ISEA).

Note: Vernier products are designed for educational use. Our products are not designed nor are they recommended for any industrial, medical, or commercial process such as life support, patient diagnosis, control of a manufacturing process, or industrial testing of any kind.

What's Included

Using the Product

To prepare the electrode to make measurements, follow this procedure:

• Connect the Ion-Selective Electrode BNC to the Go Direct Ion-Selective Electrode Amplifier. Push the BNC connector of the electrode onto the connector on the amplifier, then turn the BNC connector about one-half turn clockwise.
• Connect the amplifier to your computer, Chromebook™, LabQuest^®, or mobile device and run the data-collection software. Change the sensor channel to the appropriate ion or Potential, if necessary.
• Your ISE needs to be prepared before use. This includes a 30-minute soak in the High Standard solution.
• If you plan to use the electrode outside the range of the standards provided, you will need to prepare your own standards and use those for soaking and calibration.
• The ISE should not rest on the bottom of the container.
• The small white reference contacts near the tip of the electrode should be immersed.
• Make sure no air bubbles are trapped below the ISE.
• Do not leave the ISE soaking for more than 24 hours.
• Do not completely submerge the sensor. The BNC connection is not waterproof.

Preparing the Nitrate ISE for Use

Soak the electrode in the High Standard solution (included with the ISE) for approximately 30 minutes. The ISE should not rest on the bottom of the container, and the small white reference contacts near the tip of the electrode should be immersed. Make sure no air bubbles are trapped below the ISE. Important: Do not leave the ISE soaking for more than 24 hours. Important: If you plan to use the electrode outside the range of the standards provided, you will need to prepare your own standards and use those for soaking.

Note: If the ISE needs to be transported to the field during the soaking process, use the Short-Term ISE Soaking Bottle. Remove the cap from the bottle and fill it 3/4 full with High Standard. Slide the bottle’s cap onto the ISE, insert it into the bottle, and tighten.

For long-term storage, greater than 24 hours, make sure the sensor is stored in its storage bottle with the sponge slightly damp.

Collecting Data

1. Remove the storage bottle from the soaking solution (high standard). Thoroughly rinse the lower section of the probe, especially around the tip, using distilled or deionized water. Blot dry with a paper towel.
2. 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.
3. 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.

Note: Readings are reported in mV by default. If you choose to change the sensor channel to nitrate concentration, you may want to calibrate the sensor. If you wish to convert the mV reading to concentration manually, refer to the How the Sensor Works section of this user manual.

Calibration

A calibration is stored on each Go Direct Ion-Selective Electrode Amplifier before it is shipped that is appropriate for an electrode. As the electrode’s membrane ages, this factory calibration may become inadequate. For best results, we recommend performing a two-point calibration.

Note: If you plan to use the electrode outside the range of the standards provided, you will need to prepare your own standards and use those for soaking and calibration. Standards should be two decades apart (e.g., 5 mg/L and 500 mg/L).

For additional calibration information, see www.vernier.com/til/4011

Specifications

Care and Maintenance

How the Sensor Works

Combination Ion-Selective Electrodes consist of an ion-specific (sensing) half-cell and a reference half-cell. The ion-specific half-cell produces a potential that is measured against the reference half-cell depending on the activity of the target ion in the measured sample. The ion activity and the potential reading change as the target ion concentration of the sample changes. The relationship between the potential measured with the ISE and the ion activity, and thereby the ion concentration in the sample, is described by the Nernst equation:

• E = measured potential (mV) between the ion-selective and the reference electrode
• E_o = standard potential (mV) between the ion-selective and reference electrodes
• R = universal gas constant (R = 8.314 J mol^-1 K^-1)
• T = temperature in K (Kelvin), with T (K) = 273.15 + t °C where t is the temperature of the measured solution in °C.
• F = Faraday constant (96485 C mol^-1)
• n = valence of the ion
• C = concentration of ion to be measured
• C_o = detection limit

Since R and F are constant, they will not change. The electrical charge of the ion (valence) to be measured is also known. Therefore, this equation can be simplified as:

E = E_o –S • log(C + C_o)

where is the ideal slope of the ISE.

The following table describes ideal behavior:

Assuming C₀ is near zero, the equation can be rewritten as:

C = 10^˄[(E – E_o) / S]

allowing for the calculation of the ion concentration.

It is very important to note that this table reflects ideal behavior. Ion-selective electrodes have slopes that are typically lower than ideal. It is generally accepted that an ISE slope from 88–101% of ideal is allowable. The slope (S) is an indicator of ISE performance. If the slope changes significantly over time, it may indicate that it is necessary to replace the ISE sensor tip.

Convert Potential to Concentration (Optional)

To measure the mV readings from an aqueous sample, calibration is not required. To convert mV readings to concentration (mg/L or ppm), you will use a modified version of the Nernst Equation:

C = 10^˄[(E – E_o) / S_m]

C = concentration of ion to be measured (mg/L or ppm)

E = measured potential of sample (mV)

E_o = measured potential (mV) at a C = 1 mg/L NO₃^––N concentration

S_m = measured electrode slope in mV/decade

The value of S_m, the measured electrode slope, is determined by measuring the potential of two standard solutions, and solving the equation below:

S_m = – [(Low Standard – High Standard) / # of decades*]

*A decade is defined as the factor of the difference between the two standard solutions. For example, the difference between a 1 mg/L standard and a 100 mg/L standard is 2 decades (a factor of 100 difference, or 1 × 10²).

Example Calculation, converting mV to mg/L

For this example, the measured quantities are shown in the chart below:

C = 10^[(50 mV – 160 mV)/ –58 mV/decade] = 79 ppm NO₃^––N

Units of 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:

Troubleshooting

Nitrate in the Environment

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^–.

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).

For additional troubleshooting and FAQs, see www.vernier.com/til/665

Repair Information

Accessories/Replacements

Warranty

Warranty information for this product can be found on the Support tab at www.vernier.com/gdx-no3-bnc/#support

General warranty information can be found at www.vernier.com/warranty