Analysis of Phosphate in Water

Introduction:

Phosphates are among the greatest contributors to phosphate content in lakes and rivers because phosphate containing compounds in the past have been used in detergent formulations as water softeners. Phosphate is not toxic to plants or animals. In fact, it is a plant nutrient which stimulates the growth of aquatic weeds and algae. This may cause lakes and rivers to become clogged and overrun with plants.

The principle of this method involves the formation of molybdophosphoric acid, which is reduced to the intensely colored complex, molybdenum blue. This analytical method is extremely sensitive and is reliable down to concentrations of 0.1mg of phosphorus per liter of water.

Apparatus:

The Spectronic 20 will be employed in the measurement of color intensity of the blue solutions. A wavelength of 650nm will be used for the analysis.

Reagents:

  1. Ammonium molybdate reagent
  2. Stannous chloride reagent
  3. 20mg/L stock phosphate solution

Procedure:

Note: Glassware should be washed thoroughly with hot water followed by rinsing with distilled water. Do not use phosphate containing detergents to clean equipment for this experiment.

Prepare the following standard phosphate solutions:

  1. 1.0mg/L standard: Place 2.0mL (use pipet) of 20.0mg/L phosphate solution in a 100mL graduated cylinder and dilute to 40mL with purified water. (Save exactly 25mL in an Erlenmeyer flask for analysis with the spectrophotometer.)
  2. 2.0mg/L standard: Repeat the directions for the 1.0mg/L standard using 4.0mL (instead of 2.0mL) of the 20.0mg/L standard. (Save 25mL.)
  3. 3.0mg/L standard: Repeat the directions for the 1.0mg/L standard using 6.0mL (instead of 2.0mL) of the 20.0mg/L standard. (Save 25mL.)
  4. 4.0mg/L standard: Repeat the directions for the 1.0mg/L standard using 8.0mL (instead of 2.0mL) of the 20.0mg/L standard. (Save 25mL.)
  5. 5.0mg/L standard: Repeat the directions for the 1.0mg/L standard using 10.0mL (instead of 2.0mL) of the 20.0mg/L standard. (Save 25mL.)
  6. Blank: Set aside 25mL of purified water which will be treated with the color developing reagent to serve as the blank.

These five standard solutions and the blank will now be treated according to the following “color development” procedure. After measuring the absorbances of these solutions , make a plot of absorbance versus concentration as described by your instructor.

Color Development in Sample

Follow this procedure for your five standard solutions, your blank, and your lake samples.

Place in an Erlenmeyer flask 25mL of the water sample to be analyzed. Put 1.0mL (with a pipet) of ammonium molybdate solution into a flask and swirl to mix. Before proceeding make sure you are familiar with the use of the Spec 20. Next, to the flask add two drops of stannous chloride solution and mix by swirling. If phosphate is present, a blue color will develop in a maximum of five minutes.

Note: The time period is somewhat critical. Measurements should be taken anywhere from 5 to 15 minutes after addition of stannous chloride.

While you are waiting for the blue color to develop, set the wavelength to 650nm. Use the blank to set the zero absorbance. Using the 650nm wavelength, measure the absorbance of the blue sample.

Waste disposal:

All solutions may be dumped down the drain.