Buffering RODI water and trying to emulate nature--an advanced question...

[babyblues71]

I’ve been reading some pros and cons of buffering rodi top-off water before it enters a reef tank to replace evaporated water within the main tank. Some people say that buffering top-off water helps to keep more steady pH and kH values once the water enters the tank water and mixes with the salt water. Others, on the other hand, state that when water evaporates from the main tank, all of the buffers are still in there and all that really left the tank was the “wet” part. So there’s no “need” to buffer rodi water according to the second crowd. So, now I've been analyzing two big sources of freshwater that enter the actual ocean. First, I instantly think of rain. Rainwater has an acidic pH of around 5.6 on average, but that’s due to the water making contact with carbon dioxide in the atmosphere as it falls to the ground. And the Amazon river, which is the freshwater source that seems to empty the “most” into our oceans (If I’m wrong about this, please let me know), has a pH between 5.1 to 7.8, but is generally on the neutral 7 scale. Therefore, if the majority of the water entering the ocean via rivers or rain water is more to the neutral side or slightly acidic, then first this leads me to believe that the ocean is primarily a pH of 8.14 (Used to be 8.2, but rising C02 levels mixing with the top layer of ocean water have made this drop) mainly because of carbonate rocks, organisms, etc. within ocean water. The entering freshwater, in retrospect, at least from what I see, only plays a very miniscule part in changing the chemistry of the ocean because, if it did, the ocean would have been a pH of around 7 a long time ago. It also leads me to believe that the pH of the entering top-off freshwater into a marine system would be better suited to be a neutral 7 just like in nature, which is what rodi initially “is” once it’s made. However, even though rodi is already at a neutral 7 range, one must remember that there “are” tds’s within water in the Amazon, which then get into the ocean. So, if one wanted to buffer rodi water prior to it entering a marine tank to add those same solids “but” wanted to make sure that its pH stayed at 7, in the top-off resovoir, what would “you” put in the rodi water that is natural? No chemicals---perhaps some kind of natural material that’s generally used for a discus tank substrate? Curious what your take is on all of this….

Brent

 

[sherm71tank]

My ro di water doesn't have much of a ph value at all. It's empty. The air (with co2), or baking soda will change it easily. I maintain my alk and calcium with 2 part. You could certainly go to the trouble of buffering your top off water with the needed parts but it sure seems like too much work to me! What was the question?

 

[babyblues71]

So, if one wanted to buffer rodi water prior to it entering a marine tank to add those same solids “but” wanted to make sure that its pH stayed at 7, in the top-off resovoir, what would “you” put in the rodi water that is natural? No chemicals---perhaps some kind of natural material that’s generally used for an Amazon fish based freshwater tank? Some kind of river gravel substrate or something? PS--I could be wrong about this, but I've heard a lot of pH meters and tests won't test rodi properly and will always give false readings. I could be wrong about that, though.

 

[babyblues71]

Ah--just found this...interesting.

"Water is transported in different forms within the hydrological cycle or 'water cycle'. It is estimated that each year about 502 800 km3 of water evaporates over the oceans and seas, 90% of which (458 000 km3) returns directly to the oceans through precipitation, while the remainder (44 800 km3) falls over land"

from http://www.gipsymoth.org/WaterFresh.asp

So, if the pH of rainwater is an average of 5.6, then even if our RODI water does absorb some co2 and becomes a little acidic, perhaps we should just top-it-off like that and NOT buffer it because this is how it works in nature and the buffers in the marine water is what adjusts the incoming precipitated rainwater. My guess is, is that the people who were having trouble with pH drops in their main tank by adding straight RODI water had low alkalinity due to their salt mix's buffer system not being up to par, or perhaps not enough crushed coral substrate in their main tank to properly buffer the incoming water. Just a thought...

Brent

 

[seattlerob]

I think your question is whether to use straight RO/DI water for your topoff, or to use 'buffered' RO/DI.

I would answer you that either one will work. Either way, as far as your reef is concerned, you are still going to need to measure pH, Alk, Ca, Mg, etc. In all likelyhood, you'll be supplementing your reef with some type of additive those to keep them within natural ranges. Even when using 'buffered' water, many times additional supplements are needed to keep those things at the right levels.

I believe the most common method of 'buffered' topoff is kalkwasser (which is what I use). Kalkwasser provides the benefits of being a balanced Ca and Alk supplement, as well as having a pH raising effect (although you have to be careful not to dose too much too quickly, otherwise pH will rise too much).

If using just baking soda to buffer RO/DI, IMO you would make it harder to maintain steady Alk & Ca.

Anyways, to answer your question directly, I would put kalk (aka pickling lime) in the rodi water. I believe it's natural.

hth,
rob

 

[seattlerob]

Brent, your guess about people having trouble with pH drops due to low alk is spot on. Alk is a buffer that helps prevent pH swing. So, they have a direct relationship. At least, that's my understanding.

With that being said, also know that Alk will drop over time, at a rate that will vary depending on what you have in your reef. So, it's not just what salt you use, or what kind & how much substrate. You really need to be supplementing what's getting consumed by your tank's inhabitents.

The supplementing process can happen via topoff, or other methods. Many people use topoff and other methods combined to keep Ca, Alk, Mg, pH all stable.

hth,
rob

 

[Billybeau1]

You guys got some reading to do.

The pH of ro/di is insignificant and has little to no effect on your tanks actual pH.

While alkalinity has some affect on pH, is not the important item in the scheme of things.

The relationship between alkalinity and pH


http://www.advancedaquarist.com/issues/may2002/chem.htm


One should use straight ro/di for top off.

 

[pledosophy]

I mix my Seachem Reef Buffer in my Ro to bring up the pH of the top off to 8.3 and it keeps my tank there for me too.

 

[Fredfish]

The answer is... it depends... Which 'nature' are you trying to emulate? I would expect that you will see both salinity and pH swings in oceanic waters near the amazon depending on season and which tide it is.

Even in an enclosed lagoon you will probably see swings.

On the other hand, reefs that are further from shore will not be exposed to the same levels of fresh water or changes in salinity, temperature or pH.

Now, from everything that I have read, many organisms can withstand such swings, but will have an optimal range that they do best in.

To me, it can't hurt to err on the side of caution with buffering.

Fred

 

[drummereef]

I agree with Billybeau 110% on this issue. Buffering your top off water is not necessary and in some cases can cause an overdose (precipitation) upon addition. There are too many factors in regards to evaporation to properly measure the amount of suppliment that would be necessary to accomplish this. The pH of the top off water, in this case RO/DI, is a totally seperate issue imo. The small addition of top off added to the tank over time, even if the pH is lower, won't negatively effect the pH of the system. To me, buffering top off is asking for it. Using kalk in top off or using a 2-part suppliment (along with straight RO/DI top off) as your main suppliment program is an appropriate means for keeping your alk/ca/mag in check.

 

[Icefire]

Buffering RODI is only good top raise alk.
RODI ph is basicy 7 (without co2) so once in the tank it's get like tank water

 

[MrZ]

Billions of years ago the oceans were freshwater. They are continuously getting more salty as freshwater laden with salts and minerals empties into the worlds oceans and the water evaporates. This happens so incredibly slowly because the oceans are so large. If you buffered your top off water, your alkalinity would quickly come out of wack with your calcium because your tank is so small in relation to the oceans.

If you want to do something more productive with your topoff water then kalkwasser is a great addition because it is balanced calcium and alkalinity in the same ratio in which corals use it for calcification.

If you want to think of it as a more natural analogy, think about evaporation versus precipitation. Pure water evaporates from the surface of the ocean and enters the atmosphere which cools, condenses, and rains back down into the ocean as pure water (with a little polution for good measure). I'm not sure that the fact that the water that falls over the earth's landmass picks up quite a bit more dissolved salts will help us out much trying to keep a reeftank - but I've been wrong before.

 

[MCsaxmaster]

I think you're perhaps overthinking...or underthinking, I haven't decided which

The pH of pure water, (and I mean pure, no dissolved gases, nothing) will be about 7.0. If you allow gases to dissolve in from the atmosphere the CO2 will cause the pH of the water to drop to something closer to 5.5 (generally a bit higher than rainwater since rain also uses nitrogen and sulpher aerosols as nucleation sites and hence produces acids). If you add this to some sea water you are adding water and some dissolved gases. This dilutes the sea water slightly (diluting that alkalinity and everything else dissolved in the sea water) and adds the dissolved gases. If we assume that the tank is at or close to equilibrium with respect to dissolved CO2 and that the DI water is at or close to equilibrium we aren't increasing the concentration of dissolved CO2 (more of the same is still the same). We ARE reducing the concentration of alkalinity ever so slightly. Reducing alkalinity reduces pH, however this change is so small it is essentially negligable under normal circumstances.

Let's use an example.

Let's say that sea water has an alkalinity of 2.5 meq/kg, a salinity of 37 psu and it is in equilibrium with the atmosphere which has 380 uatm CO2. The pH will be 8.22 (NIST scale). If we dilute the water with DI (380 uatm CO2) to a salinity of 34 psu the alkalinity will drop to 2.297 meq/kg. This will cause the pH of the water to drop from 8.22 to 8.21 (NIST scale). That is, in terms of running an aquarium, meaningless.

Now, if the DI water is not very pure and contains some anions in excess of cations (i.e., it has acidity, the opposite of alkalinity) which is very, very unlikely then you could see a larger pH drop. For the most part, nobody in the US has acidic groundwater though (most of us have alkaline groundwater) hence, if anything, the opposite effect is much, much more likely.

Let's say that the RO water is supersatrated with CO2. Let's say that instead of 380 uatm of CO2 it has more like 1000 uatm CO2. Here we have a dilution effect and we are rasing the CO2 partial pressure of the water, both causing a drop in pH.

So, the new alkalinity will still be 2.297 meq/kg due to dilution but the new pCO2 will be about 425 uatm. These together will cause the pH to drop from 8.22 to 8.17. Again, this is no big deal and essentially meaningless to the animals in the tank. The relative change in salinity is a much bigger environmental change and the creatures cope just fine.

There is no reason that the alkalinity, pH or anything else about DI needs to be adjusted before addition if you are maintaining alkalinity through some other means.

Chris

 

[Henry Bowman]

MCsaxmaster, thanks for explaining in detail why it doesnt make sense to add anything to your RO/DI unless you are supplementing an already existing demand in your tank.

Since the tank is buffered if your alk is in line and RODI is not (buffered) the RODI has a negligable and possibly short term effect on the tanks chemistry that makes little if any difference to the tank's inhabitants.

BillyBeau suggested more reading (good recomendation) and Drummerreef said exactly what I was thinking. Keep adding stuff to your RODI and you are ultimately asking for disaster since evap is not consistent over time.

 

[babyblues71]

Thanks for the replies, everyone. I have decided not to buffer my rodi water with anything. At one time I was actually storing some Aragonite in the RODI container, but it was releasing phosphates from the material as it SLOWLY dissolved, so I stopped doing that. However, my main inquiry for this thread was due to the fact that I kept getting so many different opinions when doing google searches that I really didn't know who to believe....

Brent

 

[drummereef]

I think you made the right choice.