"Pszemol" wrote in message
...
"George" wrote in message
news:gC09e.516$r53.251@attbi_s21...
- it does NOT take a lot of CO2 to lower pH of purified water with no
buffers,
in fact, it takes VERY LITTLE CO2 to change pH, due to the lack of buffers.

No comment on above?? ;-)

- it does not *dilute* buffers, it rather brings them BACK to the level
before water evaporated leaving higher concentration of buffers there.

Please explain how water with no buffers can bring water with buffers "BACK
to the level before water evaporated leaving higher concentration of buffers
there". My sal****er was already depleted of buffer before I added the RO
water (as indicated by the low pH - 7.5 as opposed to 8.4). If the purified
water with no buffer has a low pH due to its CO2 content, adding this low pH
water to the salt water will in fact deplete the buffer in that sal****er
because it will use up however much buffer it takes to neutralize the CO2.

You are partially right, but not the ro water is the problem here...
Actually in terms of buffers you do not care about dilutions
- if your water has buffering capabilities to neutralize addition
of 1 mol of HCl before top off then after adding even additional 10%
of pure water it will STILL HAVE CAPABILITIES TO NEUTRALIZE same amount
of acid. When you have not enough buffer for your animals then you have
not enough buffer, period. Adding pure water does not decrease or increase
your total tank buffering capabilities.

When water evaporates it does not take with it your buffers,

No it does not. What takes the buffers is CO2, which is added to the
water by micro/macroalgae at night, and by the animals in the aquarium.

CO2 is only part of the problem here. And as you noticed yourself now,
the problem is in your tank water, not your top-off purified water...
Totaly opposite to what you were arguing before.

So the problem is NOT in your top-off purified water - that is for sure...
If you had alk level 10, then you let 10 gallons of water to evaporate and
then you add 10 gallons of purified water you will be back to ~10 alk
level.

Unless something else, such as the plants and animals in the tank are using
up the buffer, which we've established that they do.

So, once again, the problem is not related to your top off water with pH 5.

Small amounts don't. That is true. However, my wavemaker causes a lot of
evaporation, so I have to add water to the tank daily.

Mee too. It still does not make any difference!
CO2 freely dissolves in your tank water exactly as in your ro water.
The difference is in the buffers: you do not have them in the ro-water
so that is why you see pH drop.
See, this is exactly what takes you off track here...
High buffering capacity of water means it is hard to CHANGE pH of water
with addition of acid. Low buffering capacity of water means it is very
easy to change pH of water with even very small addition of acid (like CO2).

Point taken. I guess where I got this all mixed up was the fact that I never
had to worry about low pH in my take up water before, because I used DI
water.

And this is another strange thing in your description.
DI water SHOULD have the same issues as RO or distilled water.
Real DI water should be even cleaner than RO water, so it should
be even more easy to change its pH with CO2 introduction...
If you never noticed low pH of your DI water I would bet it was not very pure
water...

DI resins remove dissolved gases like CO2, so you shouldn't see the same level
of pH drop you see with RO. That was my point.

I also aerated the water, which should help drive off the CO2. Right?

Yes. But if you have a lot of CO2 in your indoor air (humans, pets, etc)
the equilibrium after aerating will be on the higher levels of CO2 in the
water, which translates to lower pH levels of the pure water.

Not much I could do about that, since my tank is in the basement.

By the way, I just made up a new batch of RO water today, and it
tested below 4.5 (the lowest range of my test kit).

And this does not surprise me. What I do not understand is why
do you even bother measuring pH level of purified water...

Sigh. Because I didn't realize that RO membranes allowed CO2 to pass through.
CO2 not only lowers pH, but promotes algae growth. When I saw the low pH levels
in my tank (which, in 14 years of running, I've seen it maybe twice), I decided
to test the make up water.

It is completely irrelevant to our situation.
The pH of mixed salt water before a water change is a totally
different story and you need to measure it for sure...
But RO water? Forget it. DI water? Forget it. Distilled ? The same.
All these water types will always have low pH readings when after
being in contact with air containing CO2. This is a fact of life.

After couple of hours of aerating, the level of CO2 in solution will reach
equilibrium with the ambient CO2 levels.

With aeration, yes.

Without aeration the same. But it just take a little longer.

Well, after four days with no aeration, the pH was 5.0. That bothers me, if for
no other reason than it makes me wonder what the CO2 levels are in my basement.

As long as you have water/air surface contact the gases will diffuse
and will exchange between water and air... Plain and simple physics.

Doesn't calcium react with CO2 in solution to form calcium carbonate (CaCO3)?
Yes, it sure does. Of course, any CO2 sequestered this way will only be
replaced by CO2 from the atmosphere. Correct?

You do not want this process to occur. This is the snowy precipitation
of calcium from the solution. When you see this something bad happened
to your water. You need to do everything to prevent this from happening.

I know that. I've never had that problem. I don't let my calicum levels get
that high.

Point taken. However, there is a direct relationship between alkalinity and
pH. And it makes sense to me to add the buffer to the make up water instead
of putting it directly into the tank.

It really does not matter. The way I put the buffer is I dissolve
a tablespoon of buffer powder in luke warm cup of purified water
and add the solution directly to the tank, in the high velocity spot, usually
into the outlet of the return water from the sump...
The point is that you not need to worry about pH of pure water.
If you like it better to add buffer to top off water - do it...
But don't you tell me you do it "because this water has scarry
pH level 5"... You do it because you do not have enough buffer
IN YOUR TANK WATER due to the living processes taking part in it.
Addition of CO2 to your system with top-off water is so small
that any pH fluctuations of pH after top-off would be worrying.
You need MUCH MOOOOORE buffering capacity in your tank than
it is needed to buffer small addition of CO2 from ambient air.
You will see details after you read the article by Mr. Randy Holmes.

And if you test the tank water, and find that the Calcium, alkalinity
and pH are off before you add unbuffered water, low pH water?

Again, it really does not matter.

So why do you worry about minuscule levels of CO2 in your RO water?

Because I saw a notable drop in the pH of my tank after I introduced the RO
water. That was the reason for my concern in the first place, and why I
investigated the pH of the RO water.

Very interesting. How much water you have added and how fast?

I added a couple of gallons over the course of a day during the weekend to a 55
gallon tank with an 18 gallon refugium.

Did you take measurements just before adding water and just after?
What is your normal error of pH measurement and what pH change
you call "notable drop"? I would guess you need to do a massive
top off and do it very fast to notice any influence of CO2 on pH.

The pH went from 8.2 to 7.5. I measured the pH before I added the water,then
after, well about two hours after.

Maybe in the last act of desperation I will suggest following reading:
http://www.advancedaquarist.com/issues/may2002/chem.htm
There are some charts in this article - try to match amount of CO2
which will cause pH drop to 5 in water having 0 buffering capacity.

Thanks for the link. I'll get back to you. I'm running some more tests.

Calcium level of 280 and pH 7.5 is worrying. What salt mix do you use?

Instant Ocean. I've always used this brand.

BTW - To really know your pH changes you need to replace your color pH
tests with electronic pH-meter and make sure it is correctly calibrated...

Yeah, I'll do that as soon as I pay off my car! I used to have access to a
really good probe when I worked at an Environmental Engineering firm here in
town. But since I've been working at another firm out of town, I don't have
that access (this firm doesn't have as much equiptment as the other one did).

I am using one and had been using color test - I am speaking from experience.
Of course, pH-meter is not good for directly testing pH of ro/di/distiled
water.