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AOAC962 02

AOAC Official Method 962.02
Phosphorus (Total) in Fertilizers
Gravimetric Quinolinium Molybdophosphate Method
First Action 1962
Final Action 1965
A. Reagents
(Store solutions in polyethylene bottles.)
(a) Citric–molybdic acid reagent.—Dissolve 54 g 100%
molybdic anhydride (MoO3) and 12 g NaOH with stirring in 400 mL
hot H2O, and cool. Dissolve 60 g citric acid in mixture of 140 mL
HCl and 200 mL H2O, and cool. Gradually add molybdic solution to
citric acid solution with stirring. Cool, filter, and dilute to 1 L. (Solution may be green or blue; color deepens on exposure to light.) If
necessary, add 0.5% KBrO3 solution dropwise until green color
pales. Store in dark.
(b) Quinoline solution.—Dissolve 50 mL synthetic quinoline,
with stirring, in mixture of 60 mL HCl and 300 mL H2O. Cool, dilute
to 1 L, and filter.
(c) Quimociac reagent.—Dissolve 70 g sodium
molybdate⋅2H2O in 150 mL H2O. Dissolve 60 g citric acid in mixture of 85 mL HNO3 and 150 mL H2O, and cool. Gradually add
molybdate solution to citric acid–HNO3 mixture with stirring. Dissolve 5 mL synthetic quinoline in mixture of 35 mL HNO3 and
100 mL H2O. Gradually add this solution to molybdate–citric
acid–HNO3 solution, mix, and let stand 24 h. Filter, add 280 mL acetone, dilute to 1 L with H2O, and mix.
B. Preparation of Test Solution
Treat 1 g test sample as in 957.02B (see 2.3.01), diluting to
200 mL.
C. Determination
Pipet, into 500 mL Erlenmeyer, aliquot containing ≤25 mg P2O5
and dilute to ca 100 mL with H2O. Continue by one of the following
(a) Add 30 mL citric–molybdic acid reagent and boil gently
3 min. (Solution must be precipitate-free at this time.) Remove from
heat and swirl carefully. Immediately add 10 mL quinoline solution
from buret with continuous swirling. (Add first 3–4 mL dropwise
and remainder in steady stream.) Or:
(b) Add 50 mL quimociac reagent, cover with watch glass, place
on hot plate in well ventilated hood, and boil 1 min.
After treatment by (a) or (b), cool to room temperature, swirl carefully 3–4 times during cooling, filter into Gooch with glass fiber filter paper previously dried at 250°C and weighed, and wash with
five 25 mL portions of H2O. Dry crucible and contents 30 min at
250°C, cool in desiccator to room temperature, and weigh as
(C9H7N)3H3[PO4⋅12MoO3]. Subtract weight reagent blank. Multiply by 0.03207 to obtain weight P2O5 (or by 0.01400 for P). Report
as percent P2O5 (or percent P).
References: Z. Anal. Chem. 189, 243(1962).
JAOAC 45, 40, 999(1962); 46, 579(1963);
47, 420(1964).
CAS-7723-14-0 (phosphorus)
CAS-1314-56-3 (phosphorus pentoxide)