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AOAC Official Method 2015.18
Phosphorus and Potassium in Commercial Inorganic Fertilizers
Inductively Coupled Plasma-Optical Emission Spectrometry
First Action 2015
[Applicable for the determination of both “citrate-EDTA soluble” phosphorus and potassium
(Alternative A) and “acid-soluble” phosphorus and potassium (Alternative B) in commercial
inorganic fertilizers by ICP-OES. Citrate-EDTA soluble phosphorus and potassium (Alternative A)
is directly synonymous with “available phosphate” and “soluble potash” respectively. Acid-
soluble is sometimes referred to as “total” phosphorus and potassium; however, Alternative B
may under-estimate the total phosphorus and potassium content if acid insoluble compounds
are present. Repeatability for citrate soluble P and K, expressed as % RSD, ranges from 0.28 to
1.30 for P and from 0.41 to 1.52% for K. Repeatability for acid soluble P and K, expressed as %
RSD, ranges from 0.71 to 1.13 for P and from 0.39 to 1.18% for K.
For liquid fertilizers containing
phosphite and for organic fertilizers, an alternative AOAC method such as
960.03
or
993.31
should be used as the ICP-OES will recover phosphorus that is not considered readily plant
available in these materials.
]
Alternative A: Neutral Ammonium Citrate–Disodium EDTA-Soluble P and K Using ICP-OES
A.
Apparatus
(a)
Analytical balance
.—Readability to 0.1 mg, AT 200 (Mettler Toledo,
Columbus, OH), or equivalent.
(b)
pH meter
.—Readability to pH 7.00, Model 8005 (VWR Scientific, Radnor, PA), or
equivalent.
(c)
pH combination electrode
.—Orion 9102BNWP (Thermo Fisher Scientific,
Waltham, MA), or equivalent.
(d)
Constant temperature water bath
.—Capable of maintaining bath temperatures of 65 ±
2°C, BK53 (Yamato Scientific, Santa Clara, CA), or equivalent.
(e)
Heated shaking water bath
.—
Capable of maintaining bath temperatures of 65 ± 2°C,
and set to approximately 200 reciprocations/min.
(f)
ICP-OES instrument
.—Vista Pro-axial view (Agilent Technologies, Santa Clara, CA),
or
equivalent.
(g)
Gated riffle splitter
.—Model 16-25X (Carpco Inc., Jacksonville, FL), or equivalent.
Candidates for 2016 Method of the Year
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