Ro/Di and pH questions

[pezenfuego]

Again, I put this the New Hobbyists section. I don't think it will harm anyone to read it and this is probably the most active forum.
I have a few questions about pH and am confused as to why ro/di water has an acidic pH...or at least tests as though it does.
So pH or per hydrogen (potential hydrogen/potenz hydrogen) is caused by OH- and H+ ions. If there is more of one than another, then the pH will read something other than 7. I can accept that reverse osmosis fails to filter these ions and therefore affects the pH. However, deionization on the other hand should effectively remove these ions, giving the pH a neutral balance.
Am I right? Is my DI resin just dead? I do have 1 or 2 tds...

 

[kpclown]

I've tested a few things as I found I was still importing excess nutrients despite using a RO/DI filter system. The first thing I noticed was that the first few cups of water had a high reading of TDS. After a few minutes the water would test better but it still was not pure. The next thing I noticed was that by slowing the flow into the filter the filter had more time to remove impurities. You can try to restrict the flow so that the filter can work a bit better.

 

[xcali1985]

Most filters require a certain amount of pressure to preform at its best. To fast or too slow will limit its ability.

 

[pezenfuego]

I don't think that MY filter is the problem seeing as the tap that is going into it has a pH of 7.4 and it is dropping to 6 (or maybe below).
I think there is a reason. Aside from that matter, when I mix saltwater (well I don't any more hehe), I get a very stable pH of 8.2
Bang has explained to me that the pH of this water doesn't matter and that dust will bring it to neutral levels. I believed him because he definitely knows what he's talking about. I just didn't understand why.

 

[bang guy]

Originally Posted by PEZenfuego
http:///forum/post/3229981
Again, I put this the New Hobbyists section. I don't think it will harm anyone to read it and this is probably the most active forum.
I have a few questions about pH and am confused as to why ro/di water has an acidic pH...or at least tests as though it does.
So pH or per hydrogen (potential hydrogen/potenz hydrogen) is caused by OH- and H+ ions. If there is more of one than another, then the pH will read something other than 7. I can accept that reverse osmosis fails to filter these ions and therefore affects the pH. However, deionization on the other hand should effectively remove these ions, giving the pH a neutral balance.
Am I right? Is my DI resin just dead? I do have 1 or 2 tds...
A TDS of 1 or 2 isn't a problem but the "2" reading can indicate it's almost time for new DI rosin. I wouldn't be too concerned about the acidity. It's possible that while breathing near the sample while taking the PH test the sample picks up CO2 and forms Carbonic Acid.

 

[pezenfuego]

Originally Posted by Bang Guy
http:///forum/post/3229995
A TDS of 1 or 2 isn't a problem but the "2" reading can indicate it's time for new DI rosin. I wouldn't be too concerned about the acidity. It's possible that while breathing near the sample while taking the PH test the sample picks up CO2 and forms Carbonic Acid.
Okay, that would make sense. I dismissed that thought because carbonic acid is very weakly acidic and shouldn't have produced such a low pH value. BUT in theory the water has no ions in it. If even a weak acid comes in contact with the sample and there are no bases in the sample to counteract it, the sample would show a very low pH.
As soon as something is added to the water, the pH takes a major swing due to its instability.
I am filling in blanks...I'm just not sure if I'm filling them in correctly.
Normally, this would be the time that I replace my di resin, but since I don't really need water with a tds of 0, I think I'll pass.

 

[bang guy]

I took my DI offline. The plants seem to appreciate the extra Phosphate and Nitrate.

 

[beaslbob]

Just wanted to hitch hike or expand on what Bang Guy is saying.
In our tanks carbonate is the dominate buffer affecting pH with the equation:
1. CO2 + H2O > H2CO3
2. H2CO3 <--> H+ + HCO3- <--> 2H+ + CO3--
Reference Dr. randy holmes-farley reefkeeping article "High pH: Causes and Cures"
By reducing co2 in 1) you lower the h+'s in 2 resulting in higher pH.
So you can have a low pH in the initial water (from ro/di) then the action of the tank raises (or lowers) the pH by how it handles the co2.
In my tanks both FW and marine I balance out the systems through the use of plant life. FW plants and salt macro algaes. All the tanks have a pH over 8.4 (api high range test kit) even the FW tank that have peat moss in the substrate. My Fw have no circulation but the marine do have sumps/refugiums with macro algaes.
From that I conclude that at least for my operation the vastly overriding consideration is how much carbon dioxide is being consumed by the plants. My systems become a net consumer of carbon dioxide and producer of oxygen in a 24 hour period. Even the lowest pH with plant life is much higher then the higest pH before I added macros.
my .02

 

[pezenfuego]

Originally Posted by beaslbob
http:///forum/post/3230225
Just wanted to hitch hike or expand on what Bang Guy is saying.
In our tanks carbonate is the dominate buffer affecting pH with the equation:
1. CO2 + H2O > H2CO3
2. H2CO3 <--> H+ + HCO3- <--> 2H+ + CO3--
Reference Dr. randy holmes-farley reefkeeping article "High pH: Causes and Cures"
By reducing co2 in 1) you lower the h+'s in 2 resulting in higher pH.
So you can have a low pH in the initial water (from ro/di) then the action of the tank raises (or lowers) the pH by how it handles the co2.
In my tanks both FW and marine I balance out the systems through the use of plant life. FW plants and salt macro algaes. All the tanks have a pH over 8.4 (api high range test kit) even the FW tank that have peat moss in the substrate. My Fw have no circulation but the marine do have sumps/refugiums with macro algaes.
From that I conclude that at least for my operation the vastly overriding consideration is how much carbon dioxide is being consumed by the plants. My systems become a net consumer of carbon dioxide and producer of oxygen in a 24 hour period. Even the lowest pH with plant life is much higher then the higest pH before I added macros.
my .02
This information is very helpful. I was wondering if you could read my post (#6) and tell me if my assumptions were correct. You seem to know what you're talking about.