Long-term storage of Ascorbic Acid mixed with Sodium Bicarbonate

Question

Every day I mix two parts of ascorbic acid powder with 1 part of sodium bicarbonate powder and place the resulting powder into an airtight container. I have noticed that if the resulting powder is mixed with water within 30-60 minutes, it fizzles indicating the release of carbon dioxide as a byproduct of the acid-base reaction. However, if I leave it untouched (within a dry and airtight container) for 6 to 8 hours, it hardens. If I crush it (so it turns into powder again) and mix it with water, it no longer fizzles.

My questions are:

1. Why does it harden and no longer fizzles if mixed with water after a few hours? Does the acid-base reaction take place even without adding any liquids to the mixed powder?

2. Most importantly, does the reaction that causes the hardening and absence of fizzling cause any kind of degradation (e.g., oxidation) to the ascorbic acid? In other words, can I mix ascorbic acid with sodium bicarbonate (both in powder form), store the resulting powder in an airtight container and consume over a few months? Would that diminish the efficacy of the Vitamin C in any way?

Thank you in advance.

Update:

I have bought sodium bicarbonate from a different manufacturer and now after mixing it with ascorbic acid, nothing happens until water is added. It can sit for 24+ hours, without hardening and it will still fizz when mixed with water. I suspect the sodium bicarbonate from the previous manufacturer was slightly moist and this one is completely dry.

Answer 1

The reason that your solid mixture is clumping and hardening inside your airtight container is that the reaction between the two compounds is producing water as follows:

$$\ce{C6H8O6 + NaHCO3 <=> NaC6H7O6 + CO2 + H2O}$$

where $\ce{C6H8O6}$ is ascorbic acid.

This reaction proceeds rapidly in the presence of excess water, which allows the reacting species to dissolve and thus have ready access to each other. This results in the observed fizzing from the release of carbon dioxide.

In the solid phase, the initial reaction is limited to the surfaces of the particles coming into contact with each other. As the reaction proceeds, the once purely solid material becomes wet via the acid-base reaction producing water, and the remaining materials can somewhat dissolve, facilitating migration of the reacting species so that on a timescale of hours all of your material has reacted.

Regarding the stability of storing the solid mixture vs. storing the solids separately, you are probably better off storing them separately until use. Both ascorbic acid and sodium ascorbate are susceptible to being oxidized by long-term exposure to air. Although water itself shouldn't act as an oxidant here, it could facilitate reaction with oxygen in the air in a similar manner in which it facilitates the acid-base reaction, as discussed above.

Answer 2

The sodium bicarbonate (a base) is reacting with the ascorbic acid to form sodium ascorbate and carbon dioxide, as well as water. The carbon dioxide is responsible for the fizzing you note when you dissolve the mixture in water. When you mix the two solids, the reaction is relatively slow, since the reaction must take place at the interface of the two solids; however, it appears that allowing it to sit overnight is ample time. The hardening is probably due to the release of water, which causes to solid to aggregate; the solid will no longer bubble in water as it has already reacted and released its carbon dioxide.

As for the fate of the ascorbic acid, the sodium salt is stable, and will be converted back to the free acid in the presence of another acid. Just don't expect it to fizz.