Sodium pyrophosphate (Na4P2O7)

0.1M Sodium pyrophosphate: Dissolve 44.6g per liter in deionized water and make to volume.
0.4% Superfloc: Dissolve 0.4g in 100mL of hot deionized water with vigorous and continuous stirring. Superfloc must be added very slowly to avoid clumping.

Sample Preparation and Procedure
Weigh, in duplicate, 2.000 (+/-) 0.002g soil into Erlenmeyer flasks; include blanks. Add 100mL 0.1M sodium pyrophosphate to each, stopper and load into shaker. Let shake for eight hours.

Remove from shaker. Add 5 drops 0.4% superfloc to each flask and shake vigorously. Transfer about 45mL solution to centrifuge tubes. Fill all tubes to same volume to balance the load. Centrifuge at 2000rpm for 20 minutes.

Remove centrifuge tube; inspect supernatant for clarity. Recentrifuge as necessary if colloids remain suspended. Transfer clear supernatant to numbered plastic bottles. Refrigerate.

Centrifuge additional portion of each sample. Pipette 10mL of each into 500mL Erlenmeyer flasks. Evaporate to dryness at about 70 degrees C. Determine extractable carbon by Walkey-Black method.

Sample Calculations
Percent Fe or Al = [(sample ppm * total sample d.f.) – (blank ppm * total blank d.f.)]divided by 10,000

Percent pyrophosphate = [(mL blank pyrophosphate-mL sample) divided by weight of sample] * N Fe(NH4)2(SO4)2 * (0.3/0.77) organic carbon
where the weight of sample is 0.200g.

Normality of Fe(NH4)2(SO4)2.6H2O = [(10mL)(1.0N K2Cr2O7)] divided by mL blank
NOTE: Use Fe(NH4)2(SO4)2.6H2O blank to standardize solution, not the pyrophosphate blank.

Soil Survey Laboratory Methods Manual. August 1992.  p.162.  U.S. Department of Agriculture.