• View
• Edit
• History
• Print

Procedure

1. First, set up 5 containers of the pondwater and phosphorous solution. The concentration of the phosphorous is 5 mg/L: (Didn't we have 2 controls and 4 duckweed samples?)
   1. Fill up the 5 containers with 125 mL of pondwater.
   2. Then add 25 mL of the 50 mg/L phosphorous.
2. One of the containers will be the control, while the other 4 will have varying amounts of duckweed in them. Measure out the following amounts of duckweed into the following 4 containers: .05g, .065g, .085g, .130g. (<--These numbers aren't neccessary.The duckweed only needs to be different amounts. Measure out a small amount of duckweed and roughly double the amount until you have 4 samples of varying amounts of duckweed.Label them in order of the amount of duckweed they contain.
3. Now put the containers aside for a period of about 3 to 5 days, preferably in a cool place, giving the duckweed enough time to react with and reduce the concentration of Phosphorous.
4. Now remeasure the phosphorous concentrations of the solutions. However, before doing so, the pondwater and phosphorous solutions must diluted, as the spectrometer cannot be used for concentrations that contain more than 2 mg/L of phosphorous. To dilute the solutions:
   1. First, fill up a flask with 90 mL of distilled water.
   2. Then, add 10 mL of the solution that has the least duckweed.
   3. Repeat for all the solutions, including the control.
5. After diluting, you can now prepare the solution for the spectrophotometer:
   1. First, add 4 mL of Ammonium Molybdate to the first diluted solution created in the last step. Then, add 10 drops of stannous chloride.
   2. Cap the top of the flask and invert several times&&.
   3. After 10 minutes but before 12, pour some of the contents of the diluted solution in a test tube, and measure the concentration in a calibrated spectrophotometer. Record the data.
   4. Repeat for all the diluted solutions.
6. Finally, find the original undiluted concentrations by multiplying the data from the last step by the dilution factor. In this case, the dilution factor was 10.

Back