Estimate the iron content of the given food sample by the thiocyanate (Wong's) method.

1. 1 · Aim. To estimate the iron content of the given food (bajra) from its ash solution by Wong's thiocyanate method.
2. 2 · Principle. Potassium persulphate keeps the iron ferric. In acid, Fe³⁺ + thiocyanate forms blood-red ferric thiocyanate. Its absorbance at about 480 nm is proportional to iron (Beer-Lambert law) and is compared with ferrous ammonium sulphate standards.
3. 3 · Requirements.
   • Ash solution of the food
   • Dilute sulphuric acid
   • Saturated potassium persulphate
   • Potassium thiocyanate
   • Standard iron (10 µg Fe/mL)
   • Colorimeter, acid-washed tubes and pipettes
4. 4 · Procedure.
   • Blank, standards 10-50 µg Fe, and test (5 mL ash solution); each made to 6.5 mL.
   • Add 1 mL acid, 1 mL persulphate, then 1.5 mL thiocyanate.
   • Read at about 480 nm against the blank within 15 minutes.
5. 5 · Observation. [Table: tube, mL of standard, µg Fe, absorbance; graph of absorbance against µg Fe with the test marked]
6. 6 · Calculation. Fe (mg/100 g) = µg from graph × (100 ÷ 5) × (100 ÷ sample weight) ÷ 1000. Example: 35 × 20 × 10 ÷ 1000 = 7.0 mg per 100 g.
7. 7 · Result. The iron content of the given food (bajra) is 7.0 mg per 100 g.
8. 8 · Precautions.
   • Gloves and goggles; acids in a fume hood; never pipette by mouth.
   • Acid-washed glassware and iron-free water.
   • Persulphate before thiocyanate.
   • Read within 15 minutes, before the colour fades.

Scoring tip: A neat standard curve with the test value marked on it, plus the role of persulphate, carries most of the marks.