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Acid Persulfate Digest for Soil Total Phosphorus

[Principally from Nelson 1987]

Last revised July 22, 1997
-------------------------

Page contents (click to skip down):

* [Equipment]
* [Reagents]
* [Procedure]
* [Post-analysis calculations]

Equipment

1)15 ml screw-cap pyrex culture tubes (teflon-lined caps)

2) 1000 ul pipettor (blue tips)

3) 5000 ul pipettor (white tips)

4) 25 ml plastic scintillation vials

5) 125 ml Nalgene screw-cap mixing jars

6) Mettler 4-place balance

7) Weighing paper

8) Autoclave

Reagents
1) Deionized water (diH2O)

2) Potassium Persulfate K2S208 (e.g. Fisher P282-100)

3) 5.5M Sulfuric acid:

4) 8.0M Sulfuric acid: 5) Potassium sulfate/sulfuric acid solution: 6) N.B.S. soil standards, e.g. NBS 2710 Montana agricultural soil

Procedure
Day 1

1) Dry soil/sediment sample 60OC for 24 hours

2) Grind in Spex mill to correspond to NIST soil standard texture

3) Weigh approx. .0250 g (~25 mg) ground sample; record weight

4) Transfer sample to digest tube

5) Pipette 500 ul diH2O into digest tube

6) Pipette 500 ul 5.5M H2SO4 into digest tube (BEWARE of etching the pipettor; clean well!)

7) Weigh 0.200 g K2S2O8 and add to digest tube

8) Cap tube tightly and autoclave 1 hour (liquid cycle)

9) After cooling, rinse digest tube contents into scintillation vial using 15 ml diH2O

10) Cap vial, mix and allow particulates to settle overnight.

11) NOTE: it is necessary also to prepare digested reagent blanks by above process.

Day 2

1) Take 5.0 ml of clear supernatant solution from scint. vial; mix with 10.0 ml acid matrix (K2SO4-H2SO4 solution)

2) Analyze by Alpkem continuous-flow colorimetry: ortho-PO4 manifold, acid matrix (K2SO4-H2SO4 solution) as standard matrix and wash solution.

3) Alternatively to the manual dilution in 1), run scintillation vial contents directly on Alpkem using 1:3 dilution loop.

3) Calculate dry sample content from measured dilute extract values, taking reagent blank absorption into account of this analyzes as non-zero.

4) As recovery check, digest known NBS agricultural soil standards in parallel to unknowns and analyze as above.

Post-analysis calculations
Calculations for deriving soil solid-sample P content from a diluted extract: 

Sample material: NBS 2710 Montana agricultural soil

Known P content:      0.106% TP  =  .106 parts P in 100 parts soil
                                 =  0.0265 mg P per 0.0250 g soil

Weight extracted:      0.0250 g = 25.00 mg

Digest liquid volume:  0.5000 ml diH20      (500.0 ul)
                       0.5000 ml 5.5M H2SO4 (500.0 ul)
                      15.00   ml diH20      (2-place repipettor)
                     ------------
                      16.00   ml digest liquid


Concentration of digest liquid

  .025 g soil in 16.00 ml digest liquid

   =  0.0265 mg P in 16.00 ml digest liquid

   =  1.656 mg/L P in undiluted digest liquid


Predicted concentration of diluted Alpkem sample

  5.000 ml (5000 ul) sample digest liquid added to 10.00 ml acid matrix

  initial volume   =  5.000 ml original digest liquid

  final volume     =  15.0 ml diluted liquid sample

                        final volume
  dilution factor  =  ----------------
                       initial volume

                       15.00       3
                   =  -------  =  ---  =  3:1
                       5.000       1

    1.656 mg P        1         1.656 mg P
  --------------  *  ---  =  ----------------
   1 L extract        3       3 L Alp sample

                          =  0.5521 mg/l in digest


Reverse calculation from Alpkem determined value

P conc. of undiluted digest liquid =

(Alp determined P conc. of diluted sample) * dilution factor

    0.5521 mg Alp P         3
  --------------------  *  ----
   1 L diluted sample       1

  = 1.656 mg/l P in undiluted digest liquid

  = 0.0265 mg P in 16.00 ml undiluted digest liquid


Change of state, liquid extract to solid material

  Since 16.00 ml of undiluted extract should contain
  all the P from 0.0250 g (25.00 mg) of soil, a direct
  substitution is possible:

    0.0265 mg P in 16.00 ml undiluted digest liquid

  = 0.0265 mg P in 0.0250 g (25.00 mg) soil

         N.B. Weight of analyte (P) divided by actual weight (i.e.
         slightly more or less than 0.5g) of dry material extracted
         gives proportion (weight to weight) of analyte in the dry
         material:

           A mg P             X mg P
         ------------   =  -------------
          B g soil          1.0 g soil

           A
          --- mg per g  =     X mg P per g soil
           B

  = 1.060 mg P in 1.000 g soil

  = 0.106% TP in Montana soil
Bibliography

Nelson, N. S. 1987.
An acid-persulfate digestion procedure for determination of phosphorus in sediments. Commun. in Soil Sci. Plant Anal. v.18 no.4 p.359-69.

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