11.4 Microwave digestions can be completed in less than 45 minutes from start to finish. Allow the samples to digest for the same length of time. All nine results should be comparable if digestion is complete for the largest sample weights. If the results are different, the digestion period must be extended or sample size reduced. At the end of the normal digestion time, add a few drops of fuming nitric acid down the inside wall of the digestion vessel.
Digestion is incomplete if charring occurs upon contact of the two liquids.
Allow a narrow beam of white light to pass through the sample digest at right angles to the axis of the container. If the light beam is dispersed when viewed from the top of the container, digestion is probably incomplete. If the light beam goes through the solution with more or less no dispersion, digestion is probably complete. Drops of water hitting a hot light bulb could cause it to shatter.