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Performance Criteria Proposed for
Automated Determination of Direct
Available P2O5 in Fertilizer
Rose A. Sweeney
University of Missouri-Columbia,
Experiment Station Chemical Laboratories,
Agriculture Building, Columbia, MO 65211
To determine available P2O5 in fertilizer
by the use of proposed automated
Generalized Method – serves as an example
of how performance criteria might be
described for an automated method/system.
Automated Method – measures P2O5 as
orthophosphate from a direct available
extract by the colorimetric vanadomolybdate
Steps for measuring P2O5 in the extract:
a. Hydrolysis of non-orthophosphate
► requires time, temperature and acid
b. Destruction of the coloring matter
► Both a and b can be accomplished by controlled
heating at 95°C with 4N HCLO4.
c. Digestion of Citrate
► Citrate – can interfere with color
► important step, can be accomplished
through masking
► done by controlling the concentration of
molybdate, vanadate and acd at color
d. Final colorimetric measurement
Limitations of the method:
► inability to measure some extractable organic
phosporus found in few specialty fertilizers
► the result of the direct available sample
preparation procedure which dictates that a
specific volume whether the sample contains 1
or 50% P2O5.
► Samples outside 7.5 to 17.5% P2O5 must be
diluted or spiked prior to analysis.
Materials and Equipments:
►Hydrolysis Apparatus
two 10.6 mL coils, positioned closely together
but each set in seperately controlled heating
blocks maintained at 95◦C.
►Residence Time
15 mins. at flow rates of 0.60 mL/min of acid,
0.23 mL/min of sample, and 0.32 mL/min of air.
40 ft x 1.6 mm id coil set in an AutoAnalyzer-stle
heating oil bath.
►Hydrolysis Acid
Perchloric acid at the strength of 4N – most
effective at digesting citrate and most
compatible with the color reaction.
► Concentrations of Molybdate and Vanadate
is required for adequate color
► Detector
any colorimeter or spectrophotometer may be
used which is capable of making absorbance
measurements at a wave length setting between
400 and 490 nm in a cell fixed or flowthrough,
whose path length may be varied between 0.5
and 2.0 cm, depending on the sensitivity needed
for measurement.
standard curve shall consist of at least 6 points
evenly distributed over the entire concentration
range. Standard points shall be made from NBS or
primary standard KH2PO4, previously dried 2 h at
105 ◦C
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