Have you used Sodium Metabisulphite as antioxidant? Try it at concentrations of 0.01–1.0% w/v, or perform a calculation based in the amount of ascorbic acid you have in your formulation to try a molar relationship 1:1 (perhaps a big amount can result, so try other proportions like 1:10 or 1:100). I have beeen successful in stabilizing effervescent tablets with a combination of metabisulfite and EDTA. Greetings.

Thanks Mr. Luis Estrada...

I've been adding any antioxidants such as Sodium Metabisulphite and also Sodium Formaldehyde Sulfoxylate (@ 1.0% w/v) and no different appearances... 

For 2-first days, the color were light orange, but after that it'll darken to brownish...

Maybe I will calculate the molar relationship, just like your suggestion...

Thank you very much, Mr. Luis Estrada... :)

Hello Yulia

I believe that you vitamin c may be contaminated, try to determinate its purity first. A question, Is this a composition?, Is this a solid or liquid mixture?, may be other components affect the stability of Vit C, vitamin C is very stable in solid fase (dry).

Regards

Mauricio

Ahhhh the stability of Vit C depends on pH. Acid or mild alkaline medium are prefered.

Regards

Mauricio

I agree with Mr Mauricio Osorio : traces of other components (as impurities ) will affect easely the antioxidant activity of Vit.C.

Best regards

All the above contributions are very true. Another contributing factor to the pitfall you are facing now, is that these reactions must be carried out in dark systems (meaning no exposure to sun rays or even bright lights in the laboratory). Any little amount of such light will cause those colors formation.  Also, remember to always transfer immediately the prepared Vit C into an amber bottle to prevent  degradation (which is the browning of the color you are currently observing).

I hope it helps.

Stay blessed.

DAA.

We have experiments using the System oxidizing beta-carotene / linoleic acid, DPPH free radical and voltametric differential pulse.

There are two mechanisms of antioxidants either by preventing free radicals formation by interacting with and then neutralising or by replacing the drug. Vitamin C or Ascorbic acid is water soluble antioxidant. Its mechanism of action by replacing the drug in oxidation process, thereby a decrease in its assay is obviously during stability. The stability of Vitamin C in aqueous solution is low, especially in the presence of metal ions which accelerated the process of oxidation. Using EDTA as a metal scavenger is not preferable as It chelates with di/tri-valent cations and forms water soluble chelates but at the same time it initiates free radical reactions, thus making it a potentially dangerous pro-oxidative compound. Also, on exposure to oxygen, ascorbic acid will undergo further oxidative decomposition to various products including diketogulonic acid. So, you can overcome this problems by tipping Nitrogen gas during the formulation process. Also, using Coated Ascorbic Acid is preferable and better than Uncoated in stability.

I think you have got an idea from above suggestions to solve your problem.

Good luck.

Ascorbic acid may undergo photochemical oxidation due to which the colour is darken. Always use freshly prepared solution.

Thanks for all your suggestion... :D I would like to examine it... :D