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Total Phosphorus Determination

 Acid Persulfate Method.

Modifications by Ian Washbourne 8 Nov. 2002



Chemicals.                                                                          Amount  Composition.


Potassium persulfate solution                                                           < 5 ml ( 4.87 g / 100 ml, 0.18 M )

HCl(aq)                                                                                                     ~ 2 ml ( 12.1 N )

A/ Ammonium heptamolybdate tetrahydrate solution                  12.5 ml ( 2.5 g in 25 ml H2O )

B/ Potassium antimonyl tartrate solution                                         2 ml ( 0.5 g in 20 ml H2O )

C/ Sulfuric acid solution                                                                     85 ml ( 250 ml made up to 1 L with H2O )

Working reagent 1                                                                               100 ml ( Add 50 ml of C to 50 ml of 10g / 50ml Ascorbic acid solution.)

Working reagent 2                                                                               ~ 50 ml ( Add 35 ml of C to 12.5 ml of A, mix on the vortex and then add 2 ml of B. Re-mix )




 To each vial, sample ( 7.5 ml ) was added and then concentrated HCl was added in order to acidify to pH 1, ( 1 drop, 12.1 N ). The tubes were then vortexed to mix. Potassium persulfate reagent ( 187 ml, 0.18 M) ) was then added to each vial and vortexed to mix. Each tube was tightly capped with Teflon lined phenolic caps ready for autoclaving. A few of the tubes were marked with etching or pen in order to create a visual marker to check volume after autoclaving.

 The samples were then autoclaved at 105 C for 90 minutes.  After cooling, each sample was visually checked and taken note of if volume had reduced significantly.   Samples WERE NOT adjusted to account for volume loss, instead if two unchanged replicates of a sample did not remain then the sample was re-run with the next batch.


A 10000mM stock solution was made by adding dried KH2PO4(s) ( 1.36g ) to a 1 litre volumetric flask, dissolving and making up to the mark with de-ionised water

The 50mM stock solution was made by adding 10000mM ( 5ml ) stock solution to a 1 litre volumetric flask and making up to the mark with de-ionised water.


The KH2PO4 standards made were; 0.00 mM, 0.05 mM, 0.1 mM, 0.25 mM, 0.5 mM, 1.0 mM and 1.5 mM.

 KH2PO4 standard composition.

KH2PO4 Concentration

Volume of 50mM Stock ( made up to 250 ml with Deionised water )

0.00 mM

0.00 ml

0.05 mM

0.25 ml

0.10 mM

0.50 ml

0.25 mM

1.25 ml

0.50 mM

2.50 ml

1.00 mM

5.00 ml

1.50 mM

7.50 ml










Each standard was transferred to a scint vial ( 3 replicates, 5 ml in each ). To each standard tube working reagent 1 was added ( 100 mL ) with vortexing to mix. After this, working reagent 2 was added to each standard vial, again vortexing to mix. The standards were then left to stand for about 30 minutes, until colour formation was complete, and then re-vortexed and then run on the spec’ at 885 nm to detect for total phosphate concentrations.


 Once the standard curve was confirmed to be within acceptable parameters, the samples were reacted with the working reagents in the same way ( using 150 mL of each as opposed to 100 mL to account for the higher volume ) and then run on the spec’.


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