View Full Version : Acidic RODI topoff??

Molar Mass
11/16/2014, 10:29 AM
I got a RODI unit on craigslist a few months ago, and it's been working with no problems so far. The in-line TDS meter on the 2-stage DI unit always ends up saying 1 ppm in 0 ppm out after about 5 minutes (and I'm using well water).

I use water leaving the DI unit for my salt mix and for top-off water. The pH of the saltwater ends up being 8.2 since there are buffers in the salt mix. The top-off water has been getting more and more acidic: 6.2 last week, 6.0 or lower today. The well water that the unit uses has a starting pH between 7.2-7.4, so I'm wondering if this is because the DI resin is almost used up. One of the 2 canisters is entirely orange and the other has only 1 inch of black left at the top.

Could nearly-depleted DI resin cause acidity issues? (If I have 2 canisters, is one acid and one base, or do both have acid and base?) If so, what should I use for top-off water? Should I try using just RO, which gets down to 1-2 ppm after a while? Or should I use salt to buffer the top-off (which might make it useless as top-off...)?

My FOWLR tank usually has a pH between 7.9 - 8.1. It might be low because of the acid topoff water. I'm using the api liquid test kits for pH and high-range pH.

11/16/2014, 10:36 AM
Well the RO/DI water has no buffer, so CO2 from the air is going to form carbonic acid in water which lower the pH [due to lack of buffers].

11/16/2014, 10:38 AM
the ph of RO/DI is extremely unstable.

Molar Mass
11/16/2014, 10:45 AM
Does this mean normal RODI water is that acidic and I just haven't been checking it up until now? The TDS of the output is still 0 if that inline meter is to be trusted.

11/16/2014, 11:01 AM
The pH of RO/DI water is not a parameter to worry about. It is simply a function of the CO2 in the atmosphere. No need to buffer or treat in any way. Anything worth worrying about would more likely raise the TDS.

Molar Mass
11/16/2014, 11:03 AM
So I should assume that any drop in tank pH after top-off is temporary, and I only need to replace the DI stages when TDI creeps up?

11/16/2014, 11:30 AM
yes; as mentioned above RO/DI has virtually no buffering capacity so CO2 will readily cause a significant drop in pH. In addition, many pH tests don't work well with DI water.

the carbonate in your tank should provide more than enough buffering capacity to compensate for the RO/DI. If you're having persistent pH issues, I would search elsewhere.

Molar Mass
11/16/2014, 11:37 AM

I just found some pages saying the same things:

I only need to worry about DI pH when the output increases suddenly from 0-1 ppm.
I'm going to mark this thread as solved :)
EDIT: I can't change the first post, so how can I mark this as solved?

11/16/2014, 07:40 PM
your last post pretty much did that!