RO water with high pH...why?

[khemo]

Hey guys

I bought a new RO unit recently and I just got a chance to test it today. The RO water is comming out at pH= 7.8 which is higher than my tap water pH which is 7.2. Any reason for this?

The unit has a valve on it so you can adjust it to the TDS you want. I got the RO water comming out at kH = 3, GH = 4 dH. I'm only using API test kits but I still don't understand why the pH will come out higher.....

Anyone care to explain?

much appreciated

 

[Peixetos]

Because some CO2 gassed off and the water became thus more alkaline (with less carbonic acid).

 

[khemo]

So if I stored the RO water for a week i would expect the pH to reduce back to close to neutral as the water regains CO2 from the atmosphere?

The water has been stored for 1.5 weeks now and still no change in pH :\

Has anybody tried adding Hydrochloric acid?

 

[ordloh]

You can get it neutral if you turn your kH down to zero. Adding HCl bring your kH down too, you might as well just do it through your RO unit. I don't think such small changes in pH matter, my tapwater has a pH of 7.8, I put it straight into my aquarium and my fish don't seem to mind. pH goes down over the week due to the high fishload and heavy feeding.

 

[Philosophos]

What's happening (possibly) is that your tap water pours out at 7.2 because of atmospheric CO2, where as the RO has had time to sit and gas off, so your tests are coming back higher pH despite lower KH. If you've got high TDS in your area, you may want to consider adding on a 4th stage to remove the larger solids, and a flush kit to prolong membrane life. There are some places (here for instance) where the TDS levels are so high that even 4 stage RO units blow their membranes within 6 months of regular use.

HCL is going to offer inconsistent pH reduction unless you use an auto doser or add it at least semi-regularly. You may be able to piggyback it in on your micros after an initial pre-dosing during WC's. HCL is already something used to prevent CSM+B in liquid mix from spoiling. Just theory here; I've only used HCL to sterilize and prevent mold in micros.

-Philosophos