Automated Method for the Determination of the Phosphorus Content of Detergents

List of Figures

• Diagram: Automated Phosphorous

This method is applicable to any laundry detergent. Phosphorous levels from 0.1 to 50% P₂O₅ can be determined using appropriate standards and/or sample dilution.

This particular method is used to monitor phosphorus levels pursuant to Canada Water Act Regulations.

In this method, the detergent is dissolved in water and heated with ammonium persulphate and sulphuric acid to destroy a large part of the organic material and to hydrolyze the polyphosphates to orthophosphates. The solution is then diluted in a "Technicon Autoanalyzer" system, ammonium molybdate is added to form the heteropolymolybdophosphoric acid, and this complex is then reduced to molybdenum blue with stannous chloride. Because of the great dilution in the system and the high sensitivity of the stannous chloride-molybdenum, blue determination, it is possible to run the digested detergent solution directly without filtration or dialysis.

"The method has been tested for interference from materials commonly found in detergents, namely, silicate, synthetic surfactant, soap, NTA, sodium citrate, carbonate, optical brightener, sodium perborate and borax. No interference with the method was found from any of these materials at any concentration that is probable in a laundry detergent formulation."^[1]

The sample is normally represented by an unopened carton of laundry detergent. The carton is stored in a polyethylene bag, tied to prevent loss of moisture or other volatile material from the carton. It is acceptable to have a representative sample contained in a screw cap polyethylene bottle.

The carton is opened and the detergent riffled until it is approximately 250 ml in volume. If the original sample is approximately one pound or less in size, the working volume (i.e. 250 ml) is obtained by careful mixing and quartering.

This subsample is then ground in a Waring blender at high speed for two minutes. The blended sample is stored in an 8-oz. polystyrene container having a tight fitting lid.

1. Riffle Sampler
   
   
2. "Waring" Blender
   
   
3. Hot Plate (accommodating 40-50 beakers)
   
   
4. "Technicon" autoanalyser system with:
   • Sampler II
   • Manifold (see diagram)
   • CarloErba pump or Technicon pump II
   • Colorimeter containing a 15 mm flow cell and 660 nm filters
     
     
5. 150 ml pyrex beakers
   
   
6. 100 ml volumetric flasks
   
   
7. 5 ml volumetric pipettes

Diagram : Automated Phosphorous

Strong Acid Solution

Cautiously add 310 ml of concentrated sulphuric acid to 600 ml of distilled water. Cool and dilute to 1 litre.

Ammonium Molybdate Solution

Dissolve 25.0 g of (NH₄)₆Mo₇O₂₄·4H₂0 in 800 ml of distilled water. Carefully add 500 ml of strong acid solution. Cool and dilute to 2 litres. (Very stable.)

Stannous Chloride Solution

Dissolve 2.5 g of fresh SnCl₂·2H₂O in 20 ml of concentrated hydrochloric acid. Warming on a hot plate will aid dissolution. Dilute to 400 ml with distilled water. (Stable for three days at room temperature.)

Acid Wash Solution

Add 20 ml of strong acid solution (H₂SO₄) to 900 ml of distilled water, and dilute to 1 litre. Approximately 6-8 litres are required for one day of continuous sampling.

Stock Solution

Dissolve 4.3943 g of pre-dried (@105°C) KH₂PO₄ in distilled water and dilute to 1 litre.
Soln. = 1000 ppm P; 1 ml = 1 mg P.

Standard Solution

Pipette 100 ml of stock solution (1000 ppm P) and dilute to 1 litre.
Soln. = 100 ppm P; 1 ml = 0.1 mg P.

Working Standard Solutions

Prepare a series of standards by diluting suitable volumes of Standard Solution (100 ppm P) with distilled water. Suggested Standards: 25, 20, 15, 10, 5, 3, 0.5 ppm P.

Ammonium Persulphate

Reagent grade.

NaOH-EDTA Solution

Dissolve 65 g NaOH and 6 g EDTA in distilled water and dilute to 1 litre. (Used to clean autoanalyser system.)

1. Weigh duplicate 1.000 ± 0.001 g samples of blended detergent into dry 150 ml beakers.
   
   
2. Add approximately 90 ml distilled water.
   
   
3. Add 10 ml of strong acid (310 ml/l) and about 4 g of ammonium persulphate (measured with a scoop). Caution should be taken when adding the acid since some detergents contain large amounts of carbonate and there will be a strong evolution of CO₂.
   
   
4. Heat the beakers on the hot plate at approximately 180°C, for 1½ - 2 hours. To ensure complete and uniform digestion, it is best to digest all samples to a constant volume. (40 ml has been found to be sufficient.)
   
   
5. Allow the samples to cool and make up to 100 ml in a graduated cylinder.
   
   
6. Pipette 5 ml of the digested sample and dilute to 100 ml with distilled water in a volumetric flask.
   
   
7. Run the samples using the manifold shown in the diagram.
   
   
8. Sodium tripolyphosphate standards can be prepared and treated as above, and then checked against standards of potassium dihydrogen phosphate for complete digestion.

Prepare a calibration curve from the peak heights of potassium dihydrogen phosphate standards. Using this calibration curve, the peak heights of the samples, and the original weight of detergent, determine the phosphorous content of the detergent.

Report as % P₂O₅.

1. Goulden, P.D., W.J. Traversy and M. Comba, "The Determination of the Phosphorus Content of Detergents", Inland Waters Directorate, Technical Bulletin #45, Environment Canada, Ottawa.
   
   
2. U.S. Environmental Protection Agency, "Methods for Chemical Analysis of Water and Wastes", Cincinnati, Ohio, 1971.