ph drops in prepared water

[paats]

OK this one has me stumped.When I premix my water for storage after a few days the ph starts falling off.I have a r/o filter hooked up to clean up my own tap water.I checked the ph of the water coming out of the r/o machine and it`s right at 8.HOWEVER....when I store the water(by the way it`s not just salt water,I store plain r/o water for make up and it happens to this too) the ph starts to head south.It takes a few days but the change becomes more dramatic the longer the water sits and eventually I have to use a buffer to get the mixed water back in line.I`m tempted to blame the charcoal filter on my r/o,maybe making a carbonic acid?Any ideas?

 

[ChrisRD]

It's very difficult to get an accurate pH reading on RO/DI water as there are very few ions left in the water (which is what you are measuring).

How are you storing the saltwater? What type of container? Is it tightly sealed? Do you have a powerhead or something that keeps it mixing?

 

[Anonymous]

Don't be too concern about pH of freshwater until the salt is added. The CO2 in the air will lower the pH of RO water by existing in the aqueous solution as carbonic acid (as you mentioned), but not from the carbon filter.

CO2 will also affects saltwater (as you noticed too). But when using the water for water change, it is usually not important as it will go up with proper gas exchange (protein skimmer, surface agitation, etc.)

There are proper ways to check the pH of ultrapure water, but typically reefers don't do it the right way to get a meaningful measurement. It is not as simple as dipping a cheap pH meter into the solution like high school chem experiment, but entails degasing and inert environment that it is just not worthwhile to do.