How to Measure Phosphorus

Phosphorus is a plant nutrient that increases plant growth in fresh water. It is of great importance in the aquatic sciences and is often measured when studying freshwater lakes and other bodies of water. Phosphorus is a necessary element for aquatic algae and plant life. It changes from one form to another so quickly that scientists measure the concentration of phosphorus is fresh water constantly. There are three parts to phosphorus in fresh water. One part is soluble reactive phosphorus, the second is soluble unreactive phosphorus, and the third is particulate phosphorus.

Difficulty: Challenging

Instructions

Things You'll Need:

• Protective Gloves
• Acid Washed 50 ml Cylinder
• 125 ml Flask
• 1 ml Dropper
• Sulfuric Acid
• Spoon
• Ammonium Persulfate
• Stones for Boiling
• Sterno
• Deionized Water
• Phenolphthalein Indicator
• Sodium Hydroxide
• Test Tube
• Phosphate Acid Reagent
• Phosphate Comparator with Axial Reader

Measuring Phosphate in Fresh Water

1. Put on rubber gloves before beginning the measuring process and pour the water sample to the full line of the 50ml cylinder.

2. Pour this into the flask and add 1 ml of Sulfuric Acid to the water in the flask with a dropper. Do not shake, but swirl gently.

3. Add .05 of Ammonium Persulfate to the mixture, using a spoon. Gently dissolve.

4. Take the boiling stones and add them to the flask. Put the flask over the sterno and bring to a boil. Boil slowly for about a half an hour.

5. Add deionized water to the flask while boiling to keep the level at 10 -- 15ml. Make sure the water sample does not dry out or begin fuming.

6. Take off the sterno after thirty minutes and allow to cool.

7. Place one drop of Phenolphthalein Indicator to the sample after cooling.

8. Drop Sodium Hydroxide, drop by drop, swirling gently, to the mixture until it turns a pale pink.

9. Continue swirling and add Sulfuric Acid, using a dropper until the pink is gone.

10. Pour the mixture into a graduated cylinder. Take the deionized water and wash the flask with it. Add this water to the cylinder. If needed, add more water to bring the level to 50ml.

11. Pour the mixture into a test tube and add 1ml Phosphate Acid Reagent to the test tube. Place on the cap.

12. Add .1 gram of Phosphate Reducing Reagent to the test tube. Place on the cap and mix. Allow it to sit for 5 minutes.

13. Take off the cap to the test tube and put it into the Phosphate Comparator and Axial Reader.

14. Compare the color of the sample to the standards and record the findings in milligrams per Total Phosphate.

Tips & Warnings

• Wash all equipment that will be holding the water with acid wash for the most accurate readings. Unwashed glassware will compromise the findings.
• Work in a well ventilated area. Wear rubber gloves while handling chemicals.

Resources

• Plant Management in Florida
• Total Phosphate Test