1) After acid hydrolysis, why is there a need to neutralize the water sample with NaOH? What happens if the water sample wasn't neutralize prior to the addtion of the reagent?
2) Why can't you stand the sample for more than 30 minutes?
Please give us more information about your procedure.
It sounds like you are following one of the Standard Methods procedure.
Your digestion will end up with somewhat varying acid concentrations. To have a consistant composition of the digestate the sample is neutralized.
The 30 minute time you mention sounds like youj are using the ascorbic acid coloring procedure. Dince the color development is time dependent you need to be consisent (for this reagent) with time. Also, as I recall the color begins to fade if you go longer than 30 minutes. There are some coloring reagents that will develop their color and remain stable for a long period of time (vanadomolybdophosphoric acid) but the one I suspect you are working with is not one of them. For the ascorbic acid method,