50 C.cs. of aniline solution are measured out, treated with 50 c.cs. concentrated hydrochloric acid, and the mixture cooled by standing the measuring flask in ice water. 50 C.cs. N-nitrite solution are then added to it with gentle shaking, and the whole allowed to stand in ice water for 20 minutes. At the end of this time all the nitrous acid will have been used up except for the merest trace, and the solution is then made up to 500 c.cs. and is ready for use. Not less than 20 minutes must be allowed to elapse before using the solution as the diazotization under these circumstances takes some time. Such a solution will keep in the dark at o° for about 4 hours, and must always be freshly made for use.

Fig. 43. - Distillation of a liquid of high boiling-point.