O-Toluidinesulfonic acid

The filtrate is transferred to a 3-1. round-bottomed flask and concentrated under a pressure of about 50 mm. to a volume of 200 ml. (Note 5). After the mixture has been chilled overnight in a refrigerator, the crystalline acid (68-74 g.) is filtered by suction and dried (Note 6). An additional small amount (5-8 g.) can be obtained by evaporating the filtrate to dryness. This residue is allowed to remain in contact with 75 ml. of methanol for about an hour with occasional shaking. The solid is filtered, dried, and weighed. The total yield of o-toluidinesulfonic acid is 74-79 g. (79-83%) (Notes 7 and 8). 

---