Does citric acid (lemon juice?) evaporate quickly?

So, I had about 1 liter of water, and I left it out for 24 hours. The pH after 24 hours was around 8.0. Using lemon juice, I ph-ed it to 6.0. I used some of this to germ seeds. In another 24 hours, that same water had a pH again quite high (over 7.0).

I used a typical popcorn-sized bowl to hold the water.

Are the acidic components of the lemon juice evaporating, or is even more chlorine evaporating after the initial 24 hour period of sitting out?

Thanks guys.

PS. Is a store bought "ph-up/down" somehow better than baking soda / lemon juice ?

I have run into the same problem with my water when using a citric acid-based pH adjuster. According to some of the posts here and organic chemistry, citric acid is not an ideal choice to buffer an alkaline solution since it is such a weak acid. This means that it readily dissociates (or dissolves) but does not have a strong affinity for donating protons.
All in all, since citric acid is a weak acid, it is unable to offer "buffering capacity" and the pH of the recently adjusted solution tends to climb back into the alkaline range.
Another contributing factor could be the type of nitrogen available to your plants. If the nutrients that you use contain an imbalance of ammonia to nitrate, this can cause your pH to drift either up or down.
Someone should correct me if I am wrong, but I believe that when nitrate(NO3-) is absorbed by roots, the plant pumps a proton (H+ ion) into the roots to displace the negative charge of the nitrate. This would explain why your pH is drifting down IF your nutrient solution is higher in nitrates.
This tends to be the case in almost all hydroponic systems, since nitrifying bacteria that live in the system (no, you can't really get rid of them) selectively convert ammonia into nitrate. I'm not sure what effect this conversion has on pH, but the end result is that ammonia is removed and replaced by nitrate.

I hope this rambling made sense, I'm quite high. If you have ???... post 'em.

good answer.

another way to look at it is that there is probably very little alkalinity since it is tap water. In the pool business we just call it "ph bounce".With some hardness in the water it won't bounce so much.=BUFFERS = TDS.