Estimate the phosphorus content of the given food sample by the Fiske-Subbarow method.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Aim. To estimate the phosphorus content of the given food (ragi) from its ash solution by the Fiske-Subbarow method.
2 · Principle. Phosphate + ammonium molybdate in acid → phosphomolybdic acid; ANSA reduces it to molybdenum blue. Absorbance at about 660 nm is proportional to phosphate (Beer-Lambert law) and is compared with KH₂PO₄ standards.
3 · Requirements.
Ash solution of the food, diluted 1 in 10
Ammonium molybdate in sulphuric acid
ANSA reducing reagent
Standard KH₂PO₄ (10 µg P/mL)
Colorimeter with red filter, test tubes, pipettes
4 · Procedure.
Blank, standards 10-50 µg P, and test (1 mL diluted ash solution).
Make each to 8 mL; add 1 mL molybdate and 0.4 mL ANSA; make to 10 mL.
Stand 10 minutes; read at about 660 nm against the blank.
5 · Observation. [Table: tube, mL of standard, µg P, absorbance; graph of absorbance against µg P with the test marked]
6 · Calculation. P (mg/100 g) = µg from graph × dilution factors × (100 ÷ sample weight) ÷ 1000. Example: 28.33 × 50 × 20 × 10 ÷ 1000 = 283.3, about 283 mg per 100 g.
7 · Result. The phosphorus content of the given food (ragi) is 283 mg per 100 g.
8 · Precautions.
Gloves and goggles; never pipette the acid molybdate by mouth.
Phosphate-free, detergent-free glassware.
ANSA kept in a brown bottle.
Same standing time for all tubes; test within the standard range.
Scoring tip: Draw the standard curve with labelled axes and show each dilution factor step by step.
