Preparation of indigo
beaker, dissolve 1 g 2-nitrobenzaldehyde in 5 ml of water and 5 ml acetone. To the solution under stirring přikapáváme 1M NaOH solution until alkaline to litmus. (1M NaOH solution prepare by dissolving 4 g NaOH in 10 ml of water consumption is approximately 5 ml.) Reaction mixture spontaneously heated and the color changes from green to brown. The solution after the addition of NaOH let stand 5 minutes, the precipitate filtered and rinsed with ethanol and finally with ether. Indigo is obtained in the form of dark blue crystals. Yield 0.2 g (23% of theory).