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Nitrate-nitrogen

Nitrate nitrogen is the most highly oxidized state of nitrogen found in natural waters. It is also usually more abundant than the other inorganic combined forms, namely ammonia and nitrite. Nitrate nitrogen is the form most easily taken up by aquatic green plants, and it represents the end product of aerobic decomposition of organic nitrogen containing molecules. Usually nitrate nitrogen occurs in relatively small concentrations in unpolluted surface waters, the world average being about 300 µg/l NO3-N.

High levels of nitrate in water can indicate biological wastes in the final stages of stabilization or run-off from heavily fertilized fields. Nitrate-rich waters often encourage excessive algal growths which may degrade surface waters. Nitrogen and phosphorus are the elements normally in highest demand by aquatic plants and algae relative to supply from drainage basins and the atmosphere.

Although “typical” nitrate-nitrogen values are difficult to identify, unproductive oligotrophic lakes will generally have inorganic nitrogen levels (which is primarily nitrate-nitrogen) of less than 200 µg/l, while productive eutrophic lakes will have inorganic nitrogen levels in excess of 500 µg/l.

The cadmium reduction method of nitrate determination is a procedure adequate for most survey work. In the low range nitrate test (less than 800 mg/l), cadmium metal is used to reduce the nitrates to the nitrite form. The nitrite ions react with sulfanilic acid to produce an intermediate diazonium salt which forms a red-orange color complex with chromotropic acid in direct proportion to the nitrate concentration in the sample.

Option A. Procedure (VACUettes)

1. Fill the 25 ml dilutor snapper cup to the top edge with nitrate free water.

2. Fill the 5 ml sample cup to the 5 ml mark with your sample.

3. Empty the contents of one cadmium foil pack into the sample cup. Cap the cup and shake it vigorously for exactly 3 minutes. Allow the sample to sit undisturbed for 2 minutes.

4. Make sure the VACUette tip is firmly attached to the ampoule tip.

5. Holding the VACUette almost horizontally, touch the tip to the contents of sample cup. The capillary tip should fill completely with sample.

6. Pull the VACUette into a vertical position. A small portion of the collected sample should fall into the sleeve of the VACUette tip.

7. Place the VACUette in the dilutor snapper cup and snap the tip. The ampoule will fill leaving a bubble to facilitate mixing.

8. Mix the contents of the ampoule by inverting it several times, allowing the bubble to travel from end to end. Dry the exterior of the ampoule and wait 10 minutes for color development.

9. Hold the comparator in a neatly horizontal position while standing directly beneath a bright source of light. Place the VACUette ampoule between the color standards moving it from left to right along the comparator until the best color match is found. If the color of the VACUette ampoule is between the two color standards, a concentration estimate can be made.

Option B. Procedure (Hach reagent pillows)

1. Prepare samples by filling a clean 50 ml Falcon tube to the 25 ml mark. Be sure to include duplicate samples from each site being tested. For best results, the test should be performed with the sample at room temperature.

2. Add the contents of one NitraVer 6 Nitrate Reagent packet to each sample. Immediately cap and shake for exactly 3 minutes. Allow the sample to stand undisturbed for 30 seconds. A deposit of unoxidized cadmium metal may remain after completion of this step.

3. Add the contents of one NitriVer 3 Nitrite Reagent packet to each sample, cap and shake for 30 seconds. A pink color will develop if nitrate is present, with the intensity of the color being proportional to the concentration of nitrate in the sample. Allow at least 10 minutes for proper color development, but do not wait more than 20 minutes before taking the reading.

4. Use a USB cable to connect the spectrometer to the computer. Start the LoggerPro software.

5. To calibrate the spectrometer, choose Calibrate → Spectrometer from the Experiment menu. The calibration dialog box will display the message “Waiting .. seconds for the lamp to warm up”. Allow the spectrometer to warm up for at least three minutes. Follow the instructions in the dialog box to complete calibration.

6. Click on the Choose Change Units ► Spectrometer from the Experiment menu and select % Transmission from the list.

7. Click on the “Configure Spectrometer” icon in the menu bar (rainbow looking icon third from the right). Set wavelength to 500 nm. Change units to μg/L. Click OK.

8. Click on the Data Collection icon in the menu bar (immediately to the right of the Configure Spectrometer icon). Select Events with Entry.

9. Insert the first sample into the spectrometer. Click the green COLLECT button. The spectrometer will take a few seconds to produce a reading. Select KEEP on the top right side of the menu bar. Enter the location of the sample. Then select STOP. Insert the next sample and COLLECT. In the dialog box, select Append To Latest to save the data. Continue for all samples.

10. To determine the amount of NO3-N in your samples, use the table. Average the duplicate samples for greater accuracy.

Concentration Nitrate-nitrogen (µg/l) vs. % Transmittance


%T

%T Units

Tens

0

1

2

3

4

5

6

7

8

9

10

813

782

753

727

702

680

658

638

619

601

20

585

568

553

538

524

511

498

485

473

462

30

451

440

429

419

409

400

390

381

373

364

40

356

347

340

332

324

317

309

302

295

289

50

282

275

269

263

257

250

244

239

233

227

60

222

216

211

206

200

195

190

185

180

176

70

171

166

161

157

152

148

144

139

135

131

80

127

122

117

112

107

101

97

91

85

80

90

74

67

60

53

47

39

32

25

17

8