Does citric acid "decay"?

sir_manning

Hi

I'm preparing about 80 mL of citric acid from anhydrous crystals, using 1g of crystals to 1 mL of de-ionized water. It is for the purposes of etching GaAs with a 3:1 citric acid to H2O2 (by volume) mixture. I wait at least a day after making the acid from the crystals before use.

My question is, does citric acid "decay" (become weaker) over time? I might be imagining things, but it seems that when I use acid that has been sitting around for more than 4 days, my etching rate is slower and/or inconsistent. The acid is stored in a clear glass container in an acids cabinet. Thanks!

alxm

That's a kind of funny question, since it's often used as a preservative.

Anyway, at those concentrations we can rule out bacteria eating it, and at room temperature citric acid is chemically stable. So offhand it doesn't sound likely.

Dadface

That's how I preserve my vinegar,by pickling it.

Orion1

It is not citric acid decomposition, but rather hydrogen peroxide decomposition.

Hydrogen peroxide decomposes (disproportionates) exothermically into water and oxygen gas spontaneously:
[tex]2 \text{ H}_2 \text{O}_2 \rightarrow 2 \text{ H}_2 \text{O} + \text{O}_2[/tex]

The rate of decomposition is dependent on the temperature and concentration of the peroxide, as well as the pH and the presence of impurities and stabilizers.

I recommend mixing your solution in an ice bath, which is just cold water and ice cubes and using your aqueous solution immediately after production.

Decomposition may be reduced by refrigerating your solution and avoiding exposure to radiation such as heat, light and sunlight.

It may be possible to restore your solution by digitally measuring the pH of a freshly mixed solution and restoring back to that pH by adding more hydrogen peroxide until the pH matches the original mixture formula.

It is also not necessary to wait to mix your acid formula.

Quote by Dadface That's how I preserve my vinegar,by pickling it.

Affirmative, adding something like 5 milliliters of Lemon juice (5% to 6% aqueous citric acid solution), acts as a preservative for the example of canning fruits and vegetables and juices.

However, prevention of contamination and elimination of all forms of bacteria from preserved food should be the greatest concern.

Reference:
Hydrogen peroxide - decomposition - Wikipedia
Citric acid - Wikipedia
Lemon - Wikipedia

minerva

You can keep the aqueous citric acid solution mixed in advance of when you use it, but you should add the appropriate amount of fresh 30% hydrogen peroxide (or whatever the stock concentration you're using is) to the citric acid solution immediately before you use it.

sir_manning

Thanks for the replies. The citric acid is kept at room temperature and the H2O2 is kept in a fridge. I usually mix the two about 20 minutes before using them. I'll try keeping the citric in the fridge too.

lightarrow

Also, hydrogen peroxide could slowly oxidize citric acid, I think.