The results for ascorbic acid standard in the Ascorbic Acid Assay Kit (L-Ascorbate) (K-ASCO) seem to fluctuate by as much as 15%. Is there an explanation for this?

Yes. Ascorbic acid is not very stable and can lose as much as ~ 25% within 3 hours when stored at room temperature in light. The ascorbic acid should be stored at 4˚C or on ice in the dark and used as soon as possible after it is made fresh. If the ascorbic acid sample is not tested immediately then it may be stabilised by making the ascorbic acid in 3% metaphosphoric acid with 10 mM EDTA or in 1 mM tert-Butylhydroquinone (TBHQ, tertiary butylhydroquinone) and storing the sample in the dark at 4˚C or on ice. 

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