Does acetic acid evaporate?

[Ink]

I use a solution with 8,5% acetic acid, prepared by myself. That lasts for about 6 months.
During this time will acetic acid evaporate and reduce its concentration in the solution?
Solution is inside a 1,5 liter plastic bottle and is automatically dosed by a pump. The bottle cap has a small hole to allow air in.
It seems to me that solution smells more intensely when is new and inorganics doesn't reduce so much when the solution gets older... I've just prepared a new bottle, so I'll see at least if there is some difference in the effect on inorganics.

Thanks.

Luca

 

[disc1]

Yes, some of the acetic acid will evaporate away over time.

 

[Ink]

Enough to significantly reduce its concentration in the solution over 6 months?
I mean something like from 8,5% passes to 6%?

Thanks.

Luca

 

[disc1]

That depends on how much is escaping through the little hole. Left completely open there would be very precious little left after 6 months.

 

[bertoni]

Maybe. We would need to know the difference in the evaporation rate between the water and the acetic acid to say. I have no idea as to what might happen. If you covered the hole with a bit of cotton gauze or something like that, it might reduce evaporation but allow enough air in to prevent any sort of vacuum problem.

 

[Ink]

I think water evaporation is not significant at all.

I think I will shift to manual dosing... I don't like that, but if there is significant reduction in acetic concentration, it makes auto dosing totally unuseful...

 

[dkeller_nc]

The vapor pressure of acetic acid is little lower than the vapor pressure of water (10 mm Hg for acetic acid vs 13.5 mm Hg for water) at room temperature. Raoult's Law states that the vapor pressure of a component of a mixture is proportional to the vapor pressure of the pure component at that temperature and the mole fraction of that component in the liquid phase.

Since your mixture is mostly water with a little acetic acid, and the vapor pressure of acetic acid and water at room temperature are close, most of what evaporates will be water. One could calculate the change in the liquid percentage of acetic acid over time, but I would not think it would vary all that much.

However, on a much more practical note, it's a really bad idea to put many weeks or months of a dosing solution into a reservoir. All it takes is a malfunction of the dosing pump, controller, or perhaps even a plumbing mistake that allows siphoning of the liquid to completely trash your aquarium.

As a general rule of thumb I don't put more than a couple of week's worth of solution into any of my dosing /ATO containers for that reason.

 

[el_pinguino]

Why not just test this empirically? Measure the pH weekly and see if it decreases, increases or remains the same.

 

[Ink]

Dkeller, that's a good suggestion. By the way I've never heard anybody report problems due to malfunction of a dosing pump. I think it could brake and stop dosing, rather than blocking in continuos function. It's also impossible to siphon all the solution to the tank, because it's against gravity, at least in my set-up.

Thanks.

 

[Randy Holmes-Farley]

I agree that vinegar becomes more concentrated as it evaporates, but that the evaporation is slow. I pump directly from a 1 gallon container, and I cut its lid to allow the tubing to enter, but there's no need to get crazy about sealing it. I pump up hill, so there is no chance for siphoning.

pH is not the best way to measure vinegar concentration because it is not that sensitive to acetic acid concentration. Doubling the concentration only drops pH by about 0.15 pH units. :

 

[dkeller_nc]

By the way I've never heard anybody report problems due to malfunction of a dosing pump.

Hmm - I sure have. Typically it's with 2-part solutions, and usually with a DIY setup - 2 "dumb" peristaltic pumps and a controller.

 

[Ink]

Thanks Randy, I was even afraid acetic evaporates leaving a less concentrated solution, no the opposite.

Dkeller, I'll keep in mind to avoid future problems.

Thanks to everybody.