pH of DI water

[bajabum]

I just hooked up a DI unit to my RO to make water for my Black Onyx Perc hatch coming.
It measured 9.4.
If a few of you using DI could check the pH, I would appreciate a consensus of what is normal.

[OwenInAZ]

I get back from vacation on the 11th, I'll check DI in my lab then :)

[chiahead]

i airate my DI water in a storage tank 24/7 and it sits at about 6.2 ph

[Philwd]

I had fresh ph of 9.2 couple weeks ago. Not sure what it would be overnight.

[rwessels]

I heard that you really can't accurately measure the PH of DI water as there is nothing for it to buffer to. Does that make any sense?

[Philwd]

That's what I heard too. THat's what I chalked up my reading to anyway.

[bajabum]

I thought pure water had a pH of 7. Didn't give it a second thought until I mixed up 10 gallons of saltwater with it and the high pH carried over into the newly mixed saltwater.
My RO I am making the DI with is 6.8 and TDS of 7.
DI TDS = 0
Have an airstone in it, will check it in the morning.

[buffalobunch]

My DI always measures between 9.2 and 9.4. The rubbermaid in the garage is reading 9.28 this morning and it's been sitting out there for 3 days since the water was made. I thought the same as well for ph of pure water and got all bent out of shape one night. Started reading what it takes to buffer pure water and started testing it over the next week. Read the same everyday and since then don't even bother to test it.

[OwenInAZ]

I bet you can measure it. Otherwise, how could I routinely measure unbuffered solutions? :)

[rwessels]

Measurement of pH is fine but to measure very pure solutions such as DI water the meter you will need to use will be at the top end of the market.

Deionized water will quickly acquire a pH when exposed to air. Carbon dioxide, present in the atmosphere, will dissolve in the water, introducing ions and giving an acidic pH of around 5.0. The limited buffering capacity of DI water will not inhibit the formation of carbonic acid H2CO3. Boiling the water will remove the carbon dioxide to restore the absence of a pH value. In practice, the indication from chemical indicators can give a value of usually between pH 5.0 and pH 9.0 depending on the indicator used.

Most people don't worry about the PH of the DI water. It is far more important once you mix the salt in.