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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.

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    $\begingroup$ "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. " Just curious...why do you do this? $\endgroup$ – paracetamol May 30 '17 at 7:41
  • $\begingroup$ I take ascorbic acid as a vitamin C supplement. Its acidity tends to upset my stomach, so I add sodium bicarbonate to neutralize it. $\endgroup$ – BrunoFacca May 30 '17 at 14:19
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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.

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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.

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  • $\begingroup$ Thank you for your detailed answer. I did not know the reaction also released water. I read that mixing ascorbic acid with water causes it to oxidize in a few hours. Wouldn't the water releases by the reaction oxidize the ascorbic acid (or sodium ascorbate)? Sorry for my ignorance, I'm a complete dummy when it comes to chemistry. $\endgroup$ – BrunoFacca May 29 '17 at 18:11
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    $\begingroup$ Water isn't (typically) an oxidant; however, the sodium salt of ascorbic acid is likely to be more easily easily oxidized than the free acid. Both ascorbic acid and sodium ascorbate are often used as an antioxidant additive in foodstuffs, where oxygen typically acts as the oxidant. $\endgroup$ – JSK May 29 '17 at 18:29
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The answer by Mr. Rice is incorrect: ascorbic acid and sodium bicarbonate mixed in water yield sodium ascorbate, which is actually more soluble in water than ascorbic acid. It is considered a salt of ascorbic acid, but it is not table salt. The main problem with taking sodium ascorbate is that you are increasing your sodium intake. Liposomal vitamin C is somewhat better in terms of bioavailability. However, it is not made with powder. The common brand on the market that comes in little packets is made with sodium ascorbate in solution that is encapsulated in liposomes.

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  • $\begingroup$ Thanks for your answer and welcome to StackExchange. $\endgroup$ – BrunoFacca Nov 7 '18 at 19:46
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Hydrating the ascorbate and bicarbonate increases surface area and allows for increases reaction when mixed with water. The resultant produces salt, CO2 and water. CO2 will escape quickly (fizzing). Water will evaporate slower. It leaves behind the salt. It will likely also have a little of either ascorbate OR bicarbonate. It will eventually be solid but could be powderized again. However, it will be non-reactive to water alone. If bicarbonate remains, you could add water and ascorbate to release a small amount of CO2 and water again.

For the comment on talking ascorbic acid and buffering it with bicarbonate, you're wasting money if you're hoping to get the health benefits of vitamin C. Ascorbic acid is very delicate and will be neutralized by the bicarbonate. You're essentially getting salt, water, and carbon dioxide. If ascorbic acid upsets your stomach, try switching to liposomal vitamin c. It's powderized and encapsulated in fat. It maintains chemical integrity and allows for increases bioavailable absorption. It's considerably more expensive but you can get a TRUE 1000 mg per day for about 43 cents per day. I prefer the capsular version versus the packets that you mix with water. The packets are more expensive, messy, and tend to cause waiste and thus dosage variation.

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  • $\begingroup$ Thanks for your answer and welcome to StackExchange. $\endgroup$ – BrunoFacca Nov 7 '18 at 19:46

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