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Plant Dry Ash/Acid Extraction for Total Phosphorus

Last revised July 22, 1997

Page contents (click to skip down):

* [Equipment]
* [Extractant solution]
* [Caveat: volatilization in dry ashing]
* [Procedure]
* [Post-analysis calculations]


1) 30 ml high-form crucibles and caps

2) Scintillation vials and caps

3) Laundry marker and graphite pencil

4) Plant standard material, e.g. NBS 1572 citrus leaves

5) Reservoir bottle of deionized water (diH2O) with 20 ml repipettor

6) Stock bottle of extraction acid with 10 ml repipettor

Aqua regia extraction acid
1) in 400 ml cylinder, add

2) transfer to 1000 ml volumetric

3) bring to volume with diH2O

4) invert to mix; allow to stand; recheck volume

Caveat: volatilization in dry ashing
" The literature contains contradictions regarding alleged losses mainly because the factors controlling the volatilization and retention mechanisms which lead to low recoveries are not always assessed.... All non-metals are potentially subject to volatilization losses which can be reduced by ashing at the minimum temperature required to ensure a reasonably complete combustion. Temperatures below 550 degrees C should be used...." (S. E. Allen et al. 1974 p.84.)

1) Dry sample 70 degrees C, 24 hours

2) Grind sample in Spex mill

3) Wash crucibles and caps in 10% HNO3 and muffle at 750 degrees C for 2 hours.

4) Label crucibles on bottom with graphite pencil

5) Weigh out ~0.5000 g of ground sample into each crucible using Mettler (four-place) balance. Record sample code and weight. 6) Place ~.5g samples in muffle furnace. Bring to ashing temperature (450 degrees C) slowly (90 minutes.) Ash forfour hours. Allow crucibles to cool.

7) Set out as many scintillation vials as there are crucibles of ash to extract. Rinse extraction vials (sciltillation vials) with ~3ml of extraction acid. Transfer ash pellet to extraction vial with 10.0 ml extraction acid as follows:

8) Set out a second set of scintillation vials, as many as the first.

9) To each vial add 200 microliters of extract solution and pipette in 25.0 ml diH2O using reservoir bottle repipettor.

(N.B. this dilution scheme works for, e.g., NBS 1572 citrus leaves containing 0.13% TP = 1.3 mg P per g plant material; yields a diluted sample for colorimetric analysis containing 0.5158 mg/l P as ortho-P. Modify dilution scheme for material thought to contain more or less TP than this.)

10) Measure diluted extract on colorimetric analyzer (ortho-PO4 manifold, EPA method 365.1. (automated Murphy-Riley). Calculate dry sample content from measured dilute extract value.

11) As recovery check, ash known NBS plant standards in parallel to unknowns.

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

Sample material: NBS 1572 citrus leaves

   Known P content:         0.13% TP

                            =  0.65 mg P per 0.5 g plant

   Weight extracted:        0.5 g = 500 mg

   Extraction acid volume:  10 ml

P Concentration in extract acid

  .5 g plant material in 10 ml extract

   =  0.65 mg P in 10 ml extract

           (0.65 * 100) mg P
   =  -------------------------
       ( 10 * 100 ) ml extract

   =       65 mg P
       1000 ml extract

   =  65 mg P per l extract

   =  65 mg/l P in undiluted extract.

Predicted concentration of diluted Alpkem sample

   200 ul sample extract added to 25 ml diH2O

   initial volume   =  200 ul  =  .2 ml original extract

   final volume     =  25.2 ml diluted extract

                         final volume        25.2
   dilution factor  =  ----------------  =  ------  =  126:1
                        initial volume        .2

      65 mg P          1              65 mg P
   -------------  *  -----  =  ---------------------
    1 L extract       126       126 L Alpkem sample

       =  .5158 mg/l predicted Alpkem raw determined value

Reverse calculation from determined value

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

   = P conc. of undiluted extract.

      .5158 mg Alp P         126
   --------------------  *  -----
    1 L diluted sample        1

   = 64.9908 mg/l P in undiluted extract

   = 0.65 mg P in our 10 ml of undiluted extract

Change of state, liquid extract to solid material

   Since 10 ml of undiluted extract should contain all the P
   from 0.5 g of plant material, a direct substitution is

   0.65 mg P in 10 ml undiluted extract

   = 0.65 mg P in 0.5 g plant material

          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 plant         1.0 g plant

             --- mg per g  =     X mg P per g plant

   = 1.3 mg P in 1.0 g plant material

   = 0.13% TP in NBS 1572 citrus leaves


Allen, S. E., et al. 1974.
Chemical Analysis of Ecological Materials. John Wiley and Sons, New York.

Jones, J. B. Jr., B. Wolf and H. A. Mills. 1990.
Organic matter destruction procedures. pp.195-6. In Plant Analysis Handbook. Micro-Macro Publishing, Inc., Athens, GA.

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