Water PH dropping.... sun exposure?

I am using 2 liter bottles to water my plants and the water I have here is way to high in PH. When I treat the PH and get it to the appropriate level, I close the bottle and then on the next watering day I am ready to go and don’t have to do any more PH level changes…I thought. When I test the water and get it to the 6.0-6.5 range I cap the bottle and let it sit for a couple of days. I do this purely for convenience purposes. It does get sun where I have to store it. When I test the water before watering it typically drops down to 4.0 or in that general range and I have no idea why. I am going out of town and am going to have to leave pre-mixed PH treated water for my friend to water for me. My fear is that when she waters the water will have dropped in PH. Any suggestions or ideas why this may be happening??

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The oxygen in the water might be getting used up causing it to turn acidic over time because as it sits the minerals slowly but surely evaporate and separate from water

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I’m sure I read somewhere that water temperature affects PH.

Found something but not what I read before.

“The pH at room temperature is ~7 meaning there are about 10-7 moles of H+ per liter of water at room temperature. As the temperature increases, the ability of water to ionize in this way increases and so the concentration of H+ in solution will increase (and hence the pH will drop).”

How on earth did I end up in an 11 month old post? LOL

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I dont know but i ended up here too lol. The older posts have some interesting reading for when im bored and I never know what its going to be about but the answers are what i like to read the best and i figure if i read things enough i will start remembering