The Determination of the Amount of Phosphate in a Detergent

Purpose: In this experiment you will analyze a detergent for phosphate and compare your result with the value on the label. If you have some detergent at home that you would like to analyze, bring about 5 grams to lab. Record the name of the detergent, the amount of phosphate or phosphorus indicated on the label, the recommended amount for a load of wash. With this information we can determine which detergent analyzes for the highest and lowest phosphate (or phosphorus) levels and which uses the most and least phosphate (or phosphorus) per load.

Wastes: Most of the chemicals used in this experiment can be put down the drain. However, the color developed solutions contain the heavy metals, vanadium and molybdenum. Be sure to put them in the aqueous waste container, as directed by your instructor.

Procedure: There are two parts to this analysis. First the phosphate which is present as tripolyphosphate must be broken down (hydrolyzed) to phosphate. Then the resulting solution must be diluted and treated with a reagent, ammonium vanadomolybdate, which forms a yellow compound with phosphate. The intensity of the color depends on the concentration of the phosphate and the solutions are analyzed with a spectrophotometer by comparison to a calibration curve. More detailed instructions follow.

Preparation of the detergent sample:

  1. Place 1.0 g of detergent in a 125 ml Erlenmeyer flask. Add one drop of antifoaming agent and 25 ml of 15% sulfuric acid. Put excess H2SO4 in the aqueous waste container.
  2. Cover the flask with an inverted beaker or watch glass. Suspend the flask in a larger beaker of water, which contains a few boiling chips, on a hot plate in the hood.
  3. Boil the water for 30 minutes, replenishing evaporated water as needed. Allow to cool and transfer the contents to a 250 ml volumetric flask. Rinse out the flask several times with distilled water, pouring the rinses into the volumetric flask. Finally, dilute the solution to the mark with distilled water. Invert and mix thoroughly.
  4. Use a 2.0 ml pipette to transfer 2.0 ml of this solution to a 100 ml volumetric flask and fill to the mark with distilled water. This is the dilute phosphate solution to be used in the color development step. (Rinse the pipet with distilled water as soon as you are done with it!)

Color Development:

  1. Use a 10.0 ml pipet to measure out 10.0 ml of the dilute phosphate solution from the detergent. Put into a 50 ml beaker.
  2. Likewise, measure out 10.0 ml of each of the four standard solutions and put into 50 ml beakers.
  3. Use a 5.0 ml pipet to add 5.0 ml of ammonium vanadomolybdate solution to each beaker. Stir the solutions and allow 10 minutes for the color to develop. (It is very important that this pipet be rinsed with distilled water as soon as you are done with it. When the experiment is complete, these solutions should be disposed of in the aqueous waste container. Excess ammonium vanadomolybdate should also go into the aqueous waste container).
  4. The Analysis of the Solutions:

  1. Turn on a spectrophotometer and let it warm up for at least 15 minutes.
  2. Set the wavelength to 415 nm.
  3. With nothing in the sample chamber, adjust the meter to read 0% transmittance with the left-hand knob.
  4. Insert a cuvette with distilled water. Close the cover and set the meter to 100% Transmittance with the right- hand knob.
  5. Measure the absorbance of each of the four standard solutions and your unknown solution. Record these absorbances with the corresponding % phosphorus listed on the label. These percentages correspond directly to levels of phosphorus present in detergents.

In your Results section, include a plot on graph paper of the absorbance vs. % phosphorus for the standard solutions. Draw the best single straight line through these points. Use this calibration curve to determine the percentage (by mass) of phosphorus (not phosphate) in your detergent sample. In your Conclusion, compare the % you determine experimentally with the value listed on the box by the manufacturer. How many grams of phosphorus are added to each wash if the recommended amount of detergent is used? You will need the mass in grams of the recommended amount of detergent listed on the box to do this calculation.  Report this value in your conclusion as well.

This activity has been copied, with permission, from the California State University at Stanislaus server to ours, to allow faster access from our Web site. We encourage you to explore the original site

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