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Dehydro-L-ascorbic acid is usually found in plant tissues in low concentrations and is always associated with ascorbic acid in much higher concentrations. One of the most reliable and direct methods for its estimation is that originally proposed by Roe & Kuether (1943) which is based on the formation of a 2:4-dinitrophenylosazone. Many plant tissues, however, contain so little dehydro-L-ascorbic acid that after extraction and dilution with the phenylhydrazine reagents the extracts may contain less than 0 5 ,ug. of dehydroL-ascorbic acid/ml. and even lower values may be encountered if the tissue has been exposed to anaerobic conditions. If more concentrated extracts are prepared interference is often observed on the addition of the 85 % sulphuric acid reagent, owing to formation of coloured products. A more serious disadvantage, and one which makes it impossible to use the method at all, is that with some plant extracts hydrazones of other compounds are formed which absorb strongly in the same region (530 mu) as the red 2:4-dinitrophenylosazone formed from dehydro-L-ascorbic acid. The method has been modified to permit of the more accurate estimation of dehydro-L-ascorbic acid when present in small amounts and to effect the separation of the osazone of dehydro-L-ascorbic acid from other hydrazones formed from other compounds in plant tissues.

A note on the estimation of dehydro-l-ascorbic acid in plant tissues by the Roe & Kuether procedure