Sulfuric acid, cryoscopy

This reaction was one of the earliest electrophilic substitutions to be developed in the history of aromatic chemistry and dates back to the mid-nineteenth century. The reagent used then (and now) was nitrating mixture, formed by mixing 1 mol of cone, nitric acid with 2 mol of cone, sulfuric acid. Cryoscopy (depression of freezing point, check back to your general chemistry text on colligative properties) shows that this results in the formation of four ions (Equation 12.2) ... 

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