RO PH does change for a period of time after released to the atmosphere, and I cannot find the article where I learned that. After reading that article I tested my RO water fresh out of the pot and after sitting 24 hours in a plastic bucket in the dark at room temp.

Initial PH was 5.5. 24 hours later it was 6.0, 48 hours later it was 6.0.

It is my understanding that the loss of certain salts effects the osmolarity of the water and makes it unstable and it wants a carbon atom somewhere, which it gets from the atmosphere.

Also, Rusty is right, if you can use tap water, do it so you don't have to add s$it back in. I have to RO because my water has such high iron content it stains porcelain. We get acid rain at times, so that is out. I have to add back the cal/mag and then balance my PH back up to 6.5 or so. I only use the drop method of PH test. Do not test the effluent PH.

Everyone has different water. You will have to tailor to your supply.