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UCL Home  /  Geography  /  Resources  /  Laboratory  /  Laboratory Methods  /  Water Analysis  /  Total Phosphorus

Total Phosphorus

Total Phosphorus Digestion

Adapted from the American Public Health Association (1989) Standard Methods for the Examination of Water and Wastewater (17th ed.). American Public Health Association, Washington DC.


This method is for the determination of total P i.e. dissolved and particulate. The method therefore uses unfiltered water. Also applicable for total “dissolved” P if using filtered samples



Potassium persulphate (K2S2O8)

Standard phosphate solution (50 mg l-1)

Sulphuric acid (300 ml conc H2SO4 diluted to 1000 ml with DDW)

4 M Sodium hydroxide solution (Dissolve 160 g NaOH in 1000 ml DDW)

Aqueous phenolphthalein indicator


A set of standards must be treated simultaneously. Set up using the following dilutions of the standard solutions.

Take 1.00 ml of standard phosphate solution (50 mg l-1) and make up to 100 ml in a volumetric flask to produce a P stock sol’n of 500 µg l-1

(a) 10 ml of DDW in reaction vessel                                  (0 µg l-1 PO4-P)

(b) 9.5 ml DDW, 0.5 ml P stock                                           (25 µg l-1 PO4-P)

(c) 9.0 ml DDW, 1.0 ml P stock                                           (50 µg l-1 PO4-P)

(d) 8.0 ml DDW, 2.0 ml P stock                                           (100 µg l-1 PO4-P)

(e) 6.0 ml DDW, 4.0 ml P stock                                           (200 µg l-1 PO4-P)



1. Prepare oxidising reagent (must be fresh).

Dissolve 10 g K2S2O4 in 100 ml DDW in a beaker, using gentle heating to <40°C.

2. Pipette 10 ml of sample into each microwave reaction vessel

3. Add 2 ml of oxidising reagent to each standard and all samples

4. Add 0.2ml of the H2SO4 to each standard and all samples

5. Cap vessels, ensuring the white caps are clean and dry - this is extremely important in the control vessel to prevent overheating.

6. Place in turntable, ensuring all vessels are within a protective sleeve and vessels are distributed evenly in the carousel. Place in microwave.

7. Program a new method using “ramp to temperature” And assuming using 24 vessels:


Max. Power

% Power












Nb. If using <12 vessels set to 50% power, 12-20 set to 75%, >20 set to 100%

8. When the digestion run is completed allow samples to cool to room temperature before uncapping.

9. Transfer 8 ml of each of the digested standards and samples to centrifuge tubes and add one drop of aqueous phenolphthalein indicator.

10. Add a know volume of 4M NaOH solution to achieve a faint pink colour – record volume added (expect to add 2-4 ml)

11. Top tubes up with DDW so all samples are the same final volume

12. Add 1 ml of composite solution (see SRP method), cap and invert to mix, and leave for 1-2 hrs (<8hrs) for colour to develop and read absorbance at 885 nm using the DDW standard as a zero.

13. Digestion vessels and lids should be thoroughly rinsed with DDW prior to re-use or soaked in acid bath overnight, rinsed and dried for next user if not being re-used.


IMPORTANT: Check the microwave digester unit in first few minutes to ensure that temperature increases – the Mars microwave tends to over-read at the low temperatures so don’t be surprised if the samples are already 60C when they go in. (the CEM engineer assures us this is normal….). It is recommended that replicates of samples are done where possible and that a DDW blank is placed with each run. Standards MUST be used with every separate digestion to account for any variation between digestions.