Re: How do I keep the oxidation of Vit C powder to a minimum?

Ascorbic acid is volatile in solution. Its half-life may be on the order of minutes to an hour. Oxidation is the usual fate, and is accelerated by presence of O2, metal catalysts (Fe, Cu, Mn, etc.), heat and light.

To keep ascorbic acid as stable as possible, keep it undissociated (low pH ~ 3), remove all metal ions (e.g., demineralize or add EDTA), flush O2 with N2 and add a more sensitive antioxidant (e.g., ferulic acid) or stabilizer (e.g., bovine serum albumin or gum), and store in a dark refrigerated container.

To use on the skin, it is best to use a microemulsion of the water-in-oil type.

There is a large literature on ascorbic acid creams in cosmetic use. See, e.g., http://www.freepatentsonline.com/7282597.html

J Pharm Biomed Anal. 1997 Mar;15(6):795-801.
Stability of vitamin C derivatives in solution and topical formulations.
Austria R, Semenzato A, Bettero A.
Università di Padova, Dipartimento di Scienze Farmaceutiche, Italy.

The stability of ascorbic acid, ascorbyl palmitate and magnesium ascorbyl phosphate (VC-PMG) in both standard solutions and topical formulations was investigated by direct RP-HPLC analysis after sample dilution with a suitable aqueous-organic solvent mixture. The results showed that, whereas the two vitamin C derivatives were more stable than ascorbic acid, the ascorbyl esters showed significant differences. Esterification with palmitic acid in 6 position did not prevent hydrolysis of the molecule, either in solution or in emulsion; only the special preparation of products with high viscoelastic properties was able to reduce the typical behaviour of this compound. Conversely, the introduction of the phosphoric group in 2 position protected the molecule from break-up of the enediol system, confirming VC-PMG as a very stable derivative of vitamin C that may be easily used in various types of cosmetic products.