Which technique among the following is most appropriate in separation of a mixture of 100 mg of p- nitrophenol and…

Step 1: Understand the Problem

We need to separate a mixture of p-nitrophenol and picric acid (2,4,6-trinitrophenol). These are both aromatic phenols with similar chemical structures, making separation by common methods difficult.

Step 2: Evaluate Each Method

(a) Steam distillation ❌ Both p-nitrophenol and picric acid are solids with high melting points and are not steam-volatile in the traditional sense. Picric acid is also explosive when heated strongly. Steam distillation is not appropriate.

(b) 2–5 ft long column of silica gel ❌ Column chromatography would work in principle, but for only 100 mg of material, a 2–5 ft column is excessive. Also, the similar polarities of these compounds (both are nitrophenols) make column separation less efficient.

(c) Sublimation ✅ p-Nitrophenol can sublime (it sublimes at about 45°C under reduced pressure). Picric acid does NOT sublime readily under these conditions. This difference in sublimation behaviour allows selective purification of p-nitrophenol.

(d) Preparative TLC ❌ Prep TLC can separate small amounts of compounds but for 100 mg this is less efficient and practical. Silica gel TLC has moderate capacity and separation of two structurally similar compounds may be poor.

Step 3: Conclusion

Answer: (c) Sublimation

Key Points to Remember:

• Sublimation is the method of choice when one compound sublimes and the other does not
• p-Nitrophenol sublimes; picric acid does not (picric acid is also explosive — avoid strong heating)
• For small quantities (mg scale), sublimation is fast, efficient, and gives high-purity product
• Picric acid = 2,4,6-trinitrophenol (three nitro groups make it explosive and less volatile)