ANS:C - alkalis

Raschig rings made of porcelain are not suitable for treating concentrated alkalis. Porcelain is generally resistant to acids but can be attacked by strong alkalis, such as concentrated solutions of sodium hydroxide (NaOH) or potassium hydroxide (KOH). When exposed to concentrated alkalis, porcelain Raschig rings can undergo chemical reactions that lead to degradation, dissolution, or damage to the structure of the rings. Therefore, they are not recommended for use in processes involving concentrated alkalis. For treating concentrated hydrochloric acid, nitric acid, or sulfuric acid, porcelain Raschig rings can be suitable as they are generally resistant to these acids. However, when selecting packing materials for acid treatments, it's important to consider factors such as the concentration, temperature, and specific chemical properties of the acids involved to ensure compatibility and effectiveness of the packing material.