Nitrate: Cadmium Reduction Method

Test preparation

Instrument-specific information

Table 1 details sample cell and orientation requirements for reagent addition tests, such as powder pillow or bulk reagent tests. Table 2 outlines sample cell and adapter requirements for AccuVac Ampul tests. Both tables list instruments compatible with this test program. To use the tables, select your instrument and find the applicable information.

Table 1: Instrument-specific information for reagent addition

Table 2: Instrument-specific information for AccuVac Ampuls

Before starting

Install the instrument cap on the DR 900 cell holder before ZERO or READ is pushed.

For optimal results, measure the reagent blank value for each new lot of reagent. Substitute deionized water for the sample in the test procedure to determine the reagent blank value. The instrument can automatically subtract the reagent blank value from sample results.

This method is technique-sensitive; shaking time and technique influence color development. For the most accurate results, use a standard solution within the test range and repeat the test several times. Adjust shaking time to achieve the expected result and use this adjusted time for sample measurements.

The reagents used in this test contain cadmium. Rinse the sample cell immediately after use to remove all cadmium particles. Collect reacted samples for proper disposal.

A deposit of unoxidized metal may remain at the bottom of the sample cell after reagent dissolution, but it will not affect results.

Review the Safety Data Sheets (MSDS/SDS) for all chemicals used and employ the recommended personal protective equipment.

Dispose of reacted solutions according to local, state, and federal regulations. Consult Safety Data Sheets for disposal information on unused reagents and refer to your facility's environmental, health, and safety staff or local regulatory agencies for further guidance.

Items to collect

Powder pillows

Refer to Consumables and replacement items on page 8 for order information.

AccuVac Ampuls

Refer to Consumables and replacement items on page 8 for order information.

Sample collection and storage

• Collect samples in clean glass or plastic bottles.
• Analyze samples as soon as possible for best results.
• If immediate analysis is not possible, filter samples and store at or below 6 °C (43 °F) for a maximum of 48 hours.
• To preserve samples for up to 28 days, adjust sample pH to 2 or less with concentrated sulfuric acid (approx. 2 mL per liter) and store at or below 6 °C (43 °F). This preserves both nitrate and nitrite.
• Allow sample temperature to reach room temperature before analysis.
• Before analysis, adjust pH to 7 with 5 N sodium hydroxide solution.
• Correct the test result for dilution caused by volume additions.

Powder pillow procedure

⚠ CAUTION: Hazardous waste exposure. Prepared samples contain cadmium. Refer to the SDS for safe handling and disposal instructions. Obey all local and regional disposal regulations.

1. Start program 355 N, Nitrate HR PP. For information about sample cells, adapters, or light shields, refer to Instrument-specific information on page 1. Note: Program names may vary between instruments, but the program number remains the same.
2. Prepare the sample: Fill a sample cell with 10 mL of sample. (Visual: Sample cell being filled with 10 mL of sample).
3. Add reagent: Add the contents of one NitraVer 5 Nitrate Reagent Powder Pillow. Stopper the sample cell. (Visual: Powder from a pillow being added to the cell, followed by a stopper).
4. Start reaction timer: Start the instrument timer. A 1-minute reaction time begins. (Visual: Timer displaying [01:00]).
5. Shake sample: Stopper the sample cell and shake vigorously until the timer expires. Some powder may not dissolve, but this will not affect results. (Visual: Two arrows indicating vigorous shaking).
6. Start reaction timer: Start the instrument timer. A 5-minute reaction time begins. An amber color indicates the presence of nitrate. (Visual: Timer displaying [05:00]).
7. Prepare the blank: When the second timer expires, fill a second sample cell with 10 mL of sample. (Visual: Sample cell being filled with 10 mL of sample).
8. Clean blank cell: Clean the blank sample cell. (Visual: Sample cell being cleaned).
9. Insert blank: Insert the blank into the cell holder. (Visual: Sample cell being inserted into an instrument cell holder).
10. Zero instrument: Push [Zero]. The display shows 0.0 mg/L NO₃^-N.
11. Clean sample cell: Clean the prepared sample cell. (Visual: Sample cell being cleaned).
12. Insert sample: Within 1 minute after the timer expires, insert the prepared sample into the cell holder. (Visual: Sample cell being inserted into an instrument cell holder).
13. Read result: Push [Read]. Results display in mg/L NO₃^-N.

AccuVac Ampul procedure

⚠ CAUTION: Hazardous waste exposure. Prepared samples contain cadmium. Refer to the SDS for safe handling and disposal instructions. Obey all local and regional disposal regulations.

1. Start program 361 N, Nitrate HR AV. For information about sample cells, adapters, or light shields, refer to Instrument-specific information on page 1. Note: Program names may vary between instruments, but the program number remains the same.
2. Prepare the sample: Collect at least 40 mL of sample in a 50-mL beaker. (Visual: 50-mL beaker).
3. Prepare ampul: Tap the bottom of a NitraVer 5 Nitrate AccuVac Ampul to dislodge the powder. Fill the Ampul with sample, keeping the tip immersed while it fills completely. (Visual: An ampul being tapped, then filled with sample while submerged).
4. Start reaction timer: Start the instrument timer. A 1-minute reaction time begins. (Visual: Timer displaying [01:00]).
5. Mix sample: Invert the Ampul 48 to 52 times as the timer counts down. (Visual: A bottle with circular arrows indicating inversion).
6. Start reaction timer: Start the instrument timer. A 5-minute reaction time begins. Keep the sample still while the timer counts down. An amber color indicates the presence of nitrate. (Visual: Timer displaying [05:00]).
7. Prepare the blank: When the second timer expires, fill a sample cell with 10 mL of sample. (Visual: Sample cell being filled with 10 mL of sample).
8. Clean blank cell: Clean the blank sample cell. (Visual: Sample cell being cleaned).
9. Insert blank: Insert the blank into the cell holder. (Visual: Sample cell being inserted into an instrument cell holder).
10. Zero instrument: Push [Zero]. The display shows 0.0 mg/L NO₃^-N.
11. Clean ampul: Clean the AccuVac Ampul. (Visual: An ampul being cleaned).
12. Insert sample: Within 1 minute after the timer expires, insert the prepared sample AccuVac Ampul into the cell holder. (Visual: An ampul being inserted into an instrument cell holder).
13. Read result: Push [Read]. Results display in mg/L NO₃^-N.

Interferences

Seawater calibration

Chloride concentrations above 100 mg/L can cause low results. To use this method for samples with high chloride concentrations, calibrate the instrument using nitrate standard solutions that match the chloride concentration of the samples. Prepare calibration standards containing chloride and 1.0, 3.0, 5.0, and 10.0 mg/L nitrate (as NO₃^-N) as follows:

1. Prepare 1 liter of chloride water with the same chloride concentration as the samples.
   a. Weigh the applicable amount of ACS-grade sodium chloride: (chloride concentration of samples in g/L) x (1.6485) = g of NaCl per liter. Note: 18.8 g/L is the typical chloride concentration of seawater.
   b. Add the sodium chloride to a 1-liter volumetric flask.
   c. Dilute to the mark with deionized water. Mix thoroughly. Use this water as the dilution water for preparing nitrate standard solutions.
2. Use a pipet to add 1.0, 3.0, 5.0, and 10.0 mL of a 100 mg/L nitrate-nitrogen (NO₃^-N) standard solution into four separate 100-mL Class A volumetric flasks.
3. Dilute to the mark with the prepared chloride water. Mix thoroughly.
4. Complete the test procedure for each standard solution and for the prepared chloride water (as a 0-mg/L standard solution).
5. Measure the absorbance of the standard solutions and enter a user calibration into the instrument.
6. Use the user program to measure samples with high chloride concentrations.

Accuracy check

Standard additions method (sample spike)

Use the standard additions method (for applicable instruments) to validate the test procedure, reagents, and instrument, and to identify sample interferences.

Items to collect:

• 1000 mg/L Nitrate Nitrogen (NO₃^-N) Standard Solution
• 100-mL volumetric flask, Class A
• 25-mL volumetric pipet, Class A and pipet filler
• Deionized water
• Pipet, TenSette®, 0.1–1.0 mL and tips

Procedure:

1. Prepare a 250 mg/L nitrate-nitrogen standard solution:
   a. Use a pipet to add 25 mL of a 1000 mg/L nitrate nitrogen standard solution into a 100-mL volumetric flask.
   b. Dilute to the mark with deionized water. Mix well. Prepare this solution daily.
2. Use the test procedure to measure the concentration of the sample, then keep the (unspiked) sample in the instrument.
3. Navigate to the Standard Additions option in the instrument menu.
4. Select values for standard concentration, sample volume, and spike volumes.
5. Prepare three spiked samples: use the TenSette pipet to add 0.1 mL, 0.2 mL, and 0.3 mL of the prepared standard solution, respectively, to three 10-mL portions of fresh sample. Mix well. (Note: For AccuVac® Ampuls, add 0.4 mL, 0.8 mL, and 1.2 mL of the prepared standard solution to three 50-mL portions of fresh sample.)
6. Use the test procedure to measure the concentration of each spiked sample, starting with the smallest spike. Measure each spiked sample in the instrument.
7. Select Graph to compare expected results to actual results.

Note: If actual results differ significantly from expected results, ensure sample volumes and spikes are measured accurately and match the standard additions menu selections. If results are outside acceptable limits, the sample may contain an interference.

Standard solution method

Use the standard solution method to validate the test procedure, reagents, and instrument.

Items to collect:

• Nitrate Nitrogen Standard, Solution, 10.0-mg/L NO₃^-N

Procedure:

1. Use the test procedure to measure the concentration of the standard solution.
2. Compare the expected result to the actual result.

Note: Factory calibration can be adjusted using the standard adjust option to match the instrument's expected value for the standard solution. This adjusted calibration is then used for all test results, improving accuracy with slight variations in reagents or instruments.

Method performance

Method performance data was derived from laboratory tests using a spectrophotometer under ideal conditions. Users may experience different results under varying test conditions.

Summary of method

Cadmium metal reduces nitrate in the sample to nitrite. The nitrite ion reacts in an acidic medium with sulfanilic acid to form an intermediate diazonium salt. This salt couples with gentisic acid to produce an amber-colored solution. The measurement wavelength is 500 nm for spectrophotometers or 520 nm for colorimeters.

Pollution prevention and waste management

Reacted samples contain cadmium and must be disposed of as hazardous waste. Dispose of reacted solutions according to local, state, and federal regulations.

Consumables and replacement items

Required reagents

Required apparatus (powder pillows)

Required apparatus (AccuVac)

Recommended standards

Optional reagents and apparatus