Question

In LUMINOL SYNTHESIS, why is hot water added after mixture is cooled to 100 degrees C.? "Allow the solution to reach a temperature of ~190 o C and start a timer. The temperature of the reaction must be held between 190-200 o C for two minutes to obtain optimum yield. (Put one person in charge of carefully monitoring the time and temperature.) The easiest way to achieve this is to partially remove the test tube from the sand-bath when the temperature reaches 190 o C (by lowering the labjack that the sandbath is sitting on) and let it sit with the bottom of the test tube just barely covered with sand. Lift and lower the labjack as needed to maintain the temperature within the optimum range. After 2 minutes at 190-200°C, remove the test tube from the heat and start to boil about 15 mL of water on a hot plate. Once the reaction mixture has cooled to less than 100 °C (very important to wait until it is below 100 °C) pour 10 mL of the boiling water into the mixture."

Answer 1

The main pupose of adding hot water after mixture is cooled to 100 ^oC is to accelerate the coagulation of the light yellow 3-nitrophthalhydrazide as a yellow precipitate. Further reduction of 3-Nitrophthalhydrazide to 3-Amino phthalhydrazide can be accomplished by treating the 3-nitrophthalhydrazide with 10% solution of sodium hydroxide (now reaction mixture turns to red color) and adding sodium dithionite, Na₂S₂O₄ (also called sodium hydrosulfite), as a reducing agent. The red color of the solution quickly gives way to the yellow color of the 3-aminophthalhydrazide.

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