Nutrients in Aquatic Ecosystems

1. Measure Nitrogen in Sample

1. On the spectrophotometer, find the program for nitrate (with user manual or instrument menu) and enter the program number.  
2. Pipette 10 mL of the water sample into one of the sample tubes. Pour this into one of the sample tubes.
3. Repeat for a second sample tube.
4. Add the contents of one nitrate reagent powder pillow to one sample tube.
5. Cap both sample tubes.
6. On the spectrophotometer, press timer and enter to start a reaction period for the reagent. Shake the sample vigorously until the reaction time is over and timer beeps. Sample will begin to turn amber.
7. Press enter. A second 5-min reaction period will begin.
8. After the timer beeps the second time, wipe off the outside of the two sample tubes with a lint-free paper towel.
9. Place the sample tube without reagent (blank) tube into the spectrophotometer.
10. Tightly cover the cell with the instrument cap to ensure ambient light is blocked.
11. Zero the spectrophotometer for a reading of 0.0 mg/L NO₃-N.
12. Remove the blank cell and place the sample cell with reagent into the cell holder. Tightly cover the sample cell with the instrument cap.
13. Press READ. The cursor will move to the right, then the results in mg/L NO₃-N will be displayed.

2. Measure Phosphorus in Sample

1. Measure out 5.0 mL of the water sample using a pipette.
2. Pour measured water into a sample tube.
3. Add the contents of one potassium persulfate powder pillow for phosphonate to the sample tube. 
4. Cap the tube tightly and shake to dissolve.
5. Label the top of the tube cap and place the tube in a COD reactor (in a chemical hood) and heat for 30 min. 
6. Place it in a test tube rack and allow to cool to room temperature.
7. Using a graduated cylinder, measure out 2 mL of 1.54 N sodium hydroxide.
8. Pour this into the sample tube. Cap and mix.
9. On the spectrophotometer, find the program number for phosphate (with user manual or instrument menu) and enter the program number.
10. Clean the outside of the sample tube with a lint-free paper towel.
11. Place the test tube so that it is facing the front of the instrument.
12. Place the cover on the test tube.
13. Take out test tube and add the contents of the purchased reagent powder pillow for the ascorbic acid method.
14. Cap tightly and shake for 10-15 s.
15. Press timer and then enter. A 2-min waiting period will begin.
16. After the timer beeps, clean the outside of the test tube with a lint-free paper towel.
17. Place the test tube into the instrument with the logo facing the front of the instrument.
18. Place the cover over the test tube.
19. Press read. The display will show the results in mg/L.