Another RO pH question

[zeroday]

For those that use RO water, do you notice that just 2 or 3 small drops of PH down takes the PH down by 2 or so? I think I mentioned before, I switched to RO recently. In my tap water, I had to use just about a half of ml to get it to where I want, from the starting PH.

 

[bobrown14]

You shouldn't need to change the pH in RO water??

Remember what a pH test is. Its testing the Potential for Hydrogen.

Let your RO water sit out a few hours and test the pH again. It may change to where you like it.

Generally RO starts out 6.5 - 7.0pH and will drop down to 5-5.5pH after exposure to air and a little time.

There's a chemical process going on. It's pretty cool stuff.

 

[zeroday]

I think I have a problem then.... My RO water sits in a 20 gallon resevoir with air stones in it. Then when I need to water/feed, I pull out what I need. When I do, the PH is around 8, with a TDS of 0ppm. I thought maybe something was in the res, so I dumped it (twice) with the same experience.

Also tried 2 PH pens, and the dropper to test PH. All three agree.

 

[bobrown14]

pH should be dropping not going up for RO water. Maybe the filter needs service?


Is this well water?

 

[zeroday]

pH should be dropping not going up for RO water. Maybe the filter needs service?


Is this well water?

No, town water. Filters (the entire setup), has about 2 weeks of use (or about 60 gallons of water). I got it from Bulk Reef Supply. Its a 6 stage with the water saver.

 

[zeroday]

Clarification, the town uses a well. Unfiltered, it is about 7.4ph and 160ppm of shit.

 

[bobrown14]

Thats a good one. It's difficult to test "pure" water accurately for pH there's nothing in the water.

Here's a quote about this issue I found interesting. BTW I've been using RO water for almost 20 years. It's always 6.5pH but I never test it.

Quote:

"
Don't worry about the pH of RO water; it is very difficult to accurately measure the pH of high-purity water, particularly with inexpensive, consumer-grade pH meters. They simply don't work well without some ions in the water.

Furthermore, the low concentration of dissolved solids means the pH can swing wildly when anything is added to it: there's no buffering capacity. Once you add nutrients to the water, the pH probe will work better, and an accurate measurement can be made; the pH can then be adjusted to optimize growth.

High purity water *should* have a pH of 7.0 fresh out of the machine (be it an RO unit or distillation unit), but the presence of atmospheric concentrations of carbon dioxide (which forms carbonic acid in solution) will quickly cause the pH to drop to ~5.6 upon exposure to air. With a fresh, clean RO membrane, there may be insufficient buffering capacity and the pH will drop to this point after a couple of hours or so. As the membrane ages, the pH may not move quite so quickly as there will be a higher level of dissolved salts."

 

[Nunyabiz]

Yep you cant accurately test RO water until you put something in it.
And when you aerate water it removes Carbon dioxide which lowers the carbonic acid which raises the pH.

 

[zeroday]

Thanks for the info @bobrown14 . I'll throw a ml of calmag and see if that makes it balance with my two testers.

 

[zeroday]

I have an Apera PH pen. Its interesting, when I put some water in a glass then put the PH pen in and stir, it goes way up to 10, then takes forever to come down and finally settle at 8 or so. I put an ML of calmag in the gallon of water, shook it up, then rested. It quickly (as expected) went to 6.8ph. Stirring the pen made it go up a little bit and then back down to 6.8.

Thank you both for the help!

 

[bobrown14]

There ya go. Just needs to have something in the water to get an accurate test.

I thought the process is RO water absorbs CO2 from the air, making a solution of Carbonic acid which is of course more acidic.

Here's the geek in me coming out.

H20 + CO2 = H2CO3

 

[Nunyabiz]

There ya go. Just needs to have something in the water to get an accurate test.

I thought the process is RO water absorbs CO2 from the air, making a solution of Carbonic acid which is of course more acidic.

Here's the geek in me coming out.

H20 + CO2 = H2CO3

Until its aerated