Put a watch glass over the top of the beakers and take them to your hood. At the appropriate point in the procedure, add each reagent to your synthesis beaker, in your hood, not at the repipet. Do not remeasure the volume of any solution delivered by repipet, since greater accuracy is not necessary. Rinse the ammonia beaker with water as soon as you add it to your reaction beaker. Keep your reaction beaker in the hood at all times, to minimize fumes.
Addition can be more rapid at first than towards the end.
After addition is complete, allow the solution to sit with occasional stirring for 10 minutes or until bubbling stops, whichever comes first. The slow addition of H 2 O 2 minimizes the extent of a non-productive side reaction that hydrogen peroxide undergoes by not allowing large concentrations of peroxide to build up. M HNO 3 . Let the solution cool for 10 minutes. The precipitate should become visible at this stage.