[yso191]

I recall reading somewhere that water left exposed to the atmosphere will pick up CO2 (I think), and thereby change the pH.

I have had 9 gallons of RO water sitting in my garage uncovered for about 6 weeks.  I am wondering about hitting my mash pH target.  Is this effect negligible or should I not use the water due to an unknown pH?

[kylekohlmorgen]

A volume of water left 'open' will come to equilibrium with the environment its left open to.

In your case, you'll probably get some oxygen and CO2 pickup, but nothing to be worried about.

RO permeate can be quite corrosive, so I wouldn't use it if it was stored in a metal container.

If not, then you might taste it to make sure you didn't pick up any off flavors from the garage

[yso191]

Interesting.  It was stored in a metal container - a keg.  What is RO permeate?

[mabrungard]

RO permeate is a fancy term for the RO water that we use. It has permeated through the membrane and left most of those ions we don't want, behind.

There is also the RO reject water. That is the water that couldn't make it through the membrane that has all those excess (now concentrated) ions.

[yso191]

Well the water looks good, smells good and tastes good, so I'm going to use it.  If the beer doesn't turn out fantastic I'll blame it on old water.

[kylekohlmorgen]

RO permeate is a fancy term for the RO water that we use. It has permeated through the membrane and left most of those ions we don't want, behind.

There is also the RO reject water. That is the water that couldn't make it through the membrane that has all those excess (now concentrated) ions.

Steve - sorry for not clarifying. Don't confuse the fancy words with vast knowledge/experience on RO; my limited experience comes from work, and its mostly with industrial units.

Martin - am I off base with the corrosive comment? Is storing RO permeate in a stainless vessel (like a keg) okay for extended periods?

[mabrungard]

RO can leach ions out of the stainless steel, but it is at a very low rate. RO is not officially corrosive to good grades of stainless steel. The biggest concern I see is for those wanting ultra-pure water...if you keep it in stainless steel, there will be a very low concentration of the steel components in the water.