Stability studies on ascorbic acid extraction at Megazyme showed that metaphosphoric acid plus EDTA was very effective at stabilising ascorbic acid. The preparation of metaphosphoric acid/EDTA buffer is provided in the K-ASCO kit booklet:

To prepare 3% (w/v) metaphosphoric acid plus 10 mM EDTA dissolve 3.0 g of metaphopshoric acid in 80 mL of distilled water and add 10 mL of 100 mM EDTA solution. Make to 100 mL with distilled water and store at 4°C between use.  To make 100 mM EDTA, weigh 2.92 g of EDTA and add to approx. 80 mL of distilled water adjust pH to approx. 7 to dissolve the EDTA using 4 M NaOH and make to 100 mL.

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