Estimate the amount of ascorbic acid in the given fruit by the 2,6-dichlorophenol indophenol method.

1. 1 · Aim. To estimate the vitamin C (ascorbic acid) content of the given fruit (guava) by titration with 2,6-dichlorophenol indophenol.
2. 2 · Principle. Ascorbic acid reduces the dye 2,6-dichlorophenol indophenol to a colourless form and is oxidised to dehydroascorbic acid. When all of it is used, the excess dye shows faint pink in the acid medium. The dye is standardised with pure ascorbic acid (dye factor).
3. 3 · Requirements.
   • 4% oxalic acid
   • Standard ascorbic acid (0.1 mg/mL)
   • DCPIP dye with sodium bicarbonate
   • Fresh fruit
   • Mortar, 100 mL flask, burette, pipettes, conical flasks
4. 4 · Procedure.
   • Titrate 5 mL standard + 10 mL oxalic acid with dye to faint pink (V₁).
   • Grind 5 g fruit in 4% oxalic acid, make up to 100 mL, filter.
   • Titrate 5 mL extract + 10 mL oxalic acid with dye (V₂).
5. 5 · Observation. [Two burette tables: dye standardisation (V₁) and sample (V₂), with concordant values]
6. 6 · Calculation. Dye factor = 0.5 ÷ V₁. Vitamin C (mg/100 g) = dye factor × V₂ × (100 ÷ 5) × (100 ÷ sample weight). Example: 0.125 × 4.2 × 20 × 20 = 210 mg per 100 g.
7. 7 · Result. The vitamin C content of the given fruit (guava) is 210 mg per 100 g.
8. 8 · Precautions.
   • Gloves and goggles; never pipette oxalic acid by mouth.
   • Work fast, away from light, heat and copper.
   • Standardise the dye fresh each day.
   • Take the first faint pink lasting about 15 seconds.

Scoring tip: Show the dye factor as a separate line before the final formula; examiners check that step.