Basic Chemistry Questions relating to buffering top-off water

[medicreefer]

So, I've been reading various forums and there seems to be a consensus from some experts and others more knowing than myself, that one should always aerate and buffer any RO/DI (and I'm assuming also distilled) water used for reefing. This apparently applies to both top-off water and water used to make saltwater mixes. The idea is that because RO/DI water is so depleted of any ions and buffering capacity, and because it is very low in pH, with high carbonic acid and nitric acid levels, if you don't aerate and buffer it, it taxes the buffering capacity of the tank (or salt mix) as the carbonic and nitric acids "eat up" the alkalinity, leading to low system alkalinity.

I understand the need to aerate to drive off CO2 and so always aerate my water for making up new saltwater. Starting today, I'll also drop an airstone into my top-off water bucket too. That problem is solved. My real question has to do with adding the buffer to top-off water.

Assuming that basically only pure water evaporates from my tank while the ions and other disolved components that make up saltwater stay behind, if I top off with pure distilled water I'm replacing just the water that evaporated right? Now by adding a buffer, which is usually sodium bicarbonate to the top-off, I'm adding more sodium ions to the system (which never evaporated to begin with right?). So, over the long run am I not adding too much sodium, causing it to rise in relation to the other ions that make up saltwater? This would shift the balance of ions in my tank away from natural seawater over the long run as my Na would rise, particularly in comparison to Cl based on my understanding of the chemistry involved. I know people have a similar problem with long term use of calcium-chloride to boost calcium levels. They end up with unnaturally high levels of chloride versus sodium and it messes up their params. Wouldn't the reverse happen adding buffer to top-off water? (PS I'm not worried about the carbonate part of the equation because I understand that part of the sodium bicarbonate is easily used up by my livestock, however, the Na ions are not.)

I love this hobby but the more into it I get, the more I think of this stuff. I hate doing things just because somebody says so, without understanding it for myself.

Thanks.

[Randy Holmes-Farley]

So, I've been reading various forums and there seems to be a consensus from some experts and others more knowing than myself, that one should always aerate and buffer any RO/DI (and I'm assuming also distilled) water used for reefing.



I would not assume they are more knowledgeable, Misinformed is more like it, although they probably think they are being smart. In short, they are wrong and you are right.

Top off water is a fine way to deliver something you already know the tank needs, but the top off pH, etc. itself is not a reason to buffer it.


I discuss it here:


Reverse Osmosis/Deionization Systems to Purify Tap Water for Reef Aquaria
http://reefkeeping.com/issues/2005-05/rhf/index.htm


from it:

http://www.reefkeeping.com/issues/20...hf/index.php#8

Final Effluent pH

Aside from the issues discussed above concerning the effluent’s pH when the DI resin becomes depleted, the final pH coming out of an RO/DI system should not significantly concern reef aquarists. Many aquarists with low pH problems have asked, for example, if their aquarium’s low pH may be caused by their replacing evaporated water with RO/DI water that they measure to have a pH below 7. In short, the answer is no, this is not a cause of low pH nor is it something to be generally concerned about, for the following reasons:

1. The pH of totally pure water is around 7 (with the exact value depending on temperature). As carbon dioxide from the atmosphere enters the water, the pH drops into the 6’s and even into the 5’s, depending on the amount of CO2. At saturation with the level of CO2 in normal (outside) air, the pH would be about 5.66. Indoor air often has even more CO2, and the pH can drop a bit lower, into the 5’s. Consequently, the pH of highly purified water coming from an RO/DI unit is expected to be in the pH 5-7 range.

2. The pH of highly purified water is not accurately measured by test kits, or by pH meters. There are several different reasons for this, including the fact that highly purified water has very little buffering capacity, so its pH is easily changed. Even the acidity or basicity of a pH test kit’s indicator dye is enough to alter pure water’s measured pH. As for pH meters, the probes themselves do not function well in the very low ionic strength of pure freshwater, and trace impurities on them can swing the pH around quite a bit.

3. The pH of the combination of two solutions does not necessarily reflect the average (not even a weighted average) of their two pH values. The final pH of a mixture may actually not even be between the pH’s of the two solutions when combined. Consequently, adding pH 7 pure water to pH 8.2 seawater may not even result in a pH below 8.2, but rather might be higher than 8.2 (for complex reasons relating to the acidity of bicarbonate in seawater vs. freshwater).

[medicreefer]

Thanks for the great, detailed reply and taking your time to explain thing to me. I've read many of your articles and have certainly learned a ton from them.

I'm glad to hear I wasn't crazy in doubting that advice. Your examples focused more on pH while what I had read focused on alkalinity but I get the comparison and the point. It's just interesting because a writer on another forum even wrote in a FAQ that using unbuffered water to make up new saltwater or top-off is "downright dangerous". So just to clarify, there's also nothing wrong with adding salt mix directly to distilled or RO/DI water to make up new saltwater, correct? Along those lines, if I make up a new batch of saltwater to SG of 1.025-1.026 but the alkalinity and/or calcium is lower than the manufacturer claims, am I to assume that this has nothing to do with me having added the salt to unbuffered water and that there is another cause? How important is aerating the water prior to making up new saltwater or adding to the tank?

Thanks again for your time.

[Randy Holmes-Farley]

So just to clarify, there's also nothing wrong with adding salt mix directly to distilled or RO/DI water to make up new saltwater, correct?

Correct. I know of no salt mix that needs any additional alkalinity buffering. Many folks complain they are too high.


Yes, the same applies to alkalinity in top off. None is removed by evaporation, so why should replacement have it. You can choose to add it to top off if your tank alkalinity is low, however.

How important is aerating the water prior to making up new saltwater or adding to the tank?

Aeration of top off Ro/DI is unimportant, IMO, since you add very little at a time, but if you want to add O2 by aeration, that’s fine. For salt water changes, it depends on how much you change at once. If you do big changes, aeration is important. I do 1% daily and very slowly, so aeration is unimportant.

Along those lines, if I make up a new batch of saltwater to SG of 1.025-1.026 but the alkalinity and/or calcium is lower than the manufacturer claims, am I to assume that this has nothing to do with me having added the salt to unbuffered water and that there is another cause?

Another cause, such as not meeting their specs, test kit errors, salinity errors, etc.

[gmatt]

I have a question about this issue for anyone. pH is not my concern, however alkalinity is.

I first started noticing some of my sps closer to the return in my tank acting like they were having issues with alk. So i started looking at several factors. I noticed after i moved recently that my newly made up salt water had a much lower alk after mixing. I keep my tank at 8.5 dKH. So i checked my makeup and it was 3 to 4 dKH. This would obviously be a problem with a larger water change. I never had this issue before always using tropic marin pro reef (yes normally a little lower than other brands. But never this low. So I checked my RODI and it was less than 1 dKH.

Im not sure why this is so low but i have to obviously have to add buffer to my newly mixed salt water. But this makes me believe that I should add buffer to my top off RODI as well.

Does this make sense? does anyone else have this issue or do you add buffer to your top off water to raise alkalinity?

Thanks in advance for any help.

[disc1]

RODI should be 0 dkH. If it is not, then it is not pure water. The alkalinity comes from the salt mix, not the water.

You would only need to add alkalinity to your top-off water if the tank needs more alk and you are using the top off to dose it. Alkalinity is just like the salt in the tank, it doesn't evaporate. It does get used up by the corals in the tank, so there may be a need to add some. But that is a completely different issue than top-off.

Either way, if you are not using the top-off as a vessel to dose the tank with, then there is no need whatsoever to add any alkalinity, pH buffer, or anything else to your top-off water. It should be just water. All those things are set by the salt that is in the tank.

[Randy Holmes-Farley]

Never heard of a salt mix with 3-4 dKH. Sounds like a testing error.

What salt are you using now?

[gmatt]

Originally Posted by Randy Holmes-Farley

Never heard of a salt mix with 3-4 dKH. Sounds like a testing error.

What salt are you using now?

I am using Tropic Marin Pro Reef. I am using the same test that i use on my tank (8.5 dKH) Elos alkalinity test.

double tested after i saw the low alk.

[bertoni]

Well, that's disappointing, one way or the other. I might try stirring up the dry salt mix, to see whether that helps. The tank measures 8.5 dKH with the same kit? Hmm, I might get a second opinion just in case the kit is reading very badly, but I admit that's a long shot.

[fishflorist]

I got the information which I was just about to start a thread and ask.
That is how to change water. I found Randy's artical http://reefkeeping.com/issues/2005-10/rhf/index.php#13 but it is too long to read ......

Randy at 4# said he do 1% daily, and I remenber that tmz is doing water change in similar way (from the "tank of the Month"), did I remember right ?

So I'm thinking about doing this, maybe 2% every other day, instead of 11% every 2 weeks. But it looks like I need to spend much more $$ on buying salt ~~

[cougareyes]

I know this is an ancient thread but I am dealing with this issue at the moment. First off test your fresh salt water mix you will be surprised, Brightwell, Seachem Reef, and Red Sea Coral all tested below 8 dKH. I have heavy LPS tanks and like to keep my tanks at 9.5 dKH. Not only does LPS corals consume alot of alk but your top off water will lower it as well. Before I caught my problem my alk was at 5, weekly water changes with alk of less than 8, top off water that was 0(ro/di) and the usage of the stock. So maintaining alk is bit more complicated that just thinking by doing weekly water changes you are fine. MY PROBLEM now is trying to dial in my buffering of my top off water. Does anyone have a formula that works for them or a way to test he top off water to get it dialed in.

[Randy Holmes-Farley]

To Reef Central

Not sure what you mean by top off water will reduce alkalinity, but unless it is acidic tap water with mineral acids in it, top off won't reduce alkalinity.

If you know the needs of the tank in alkalinity, it is easy to calculate how much alkalinity to put into the top off in the form of, say, baking soda.

[cougareyes]

Wow, most of use ro/di water in our reef; which has 0 dissolved solids and ph is around neutral, which is 7. You do not 100% of carbonates added back into the water from evaporation. I'm not sure how you can't understand how this will decrease your ph and alkalinity. Obviously I'm in the wrong place to find my answer.

[Randy Holmes-Farley]

Originally Posted by cougareyes

Wow, most of use ro/di water in our reef; which has 0 dissolved solids and ph is around neutral, which is 7. You do not 100% of carbonates added back into the water from evaporation. I'm not sure how you can't understand how this will decrease your ph and alkalinity. Obviously I'm in the wrong place to find my answer.

Truly you may be in the wrong place if you are unwilling to listen when a professional chemist with a PhD from Harvard and dozens of published reef chemistry articles explains something to you and that is your response.

Adding RO/DI never ever lowers the alkalinity of reef aquaria (except as it dilutes what was there).

Think of it this way.

You have 9 dKH.

You evaporate 1% of the volume, and the alkalinity becomes 9.09 dKH since all you removed was totally pure fresh water. Then you replace that 1% of evaporated water with totally pure fresh water, and the alk drops back to the original 9 dKH.

Hence, there is no change in the alkalinity.

The exact same argument can be made for pH, although it is a bit more complicated because it is not even clear whether adding pH 7 pure fresh water to seawater at pH 8.2 lowers, raises, or leaves the pH unchanged. The effect is likely vanishingly small, and when you replace the evaporated water with pure fresh water, it goes right back where it was.

[Breadman03]

Originally Posted by Randy Holmes-Farley

Truly you may be in the wrong place if you are unwilling to listen when a professional chemist with a PhD from Harvard and dozens of published reef chemistry articles explains something to you and that is your response.



Adding RO/DI never ever lowers the alkalinity of reef aquaria (except as it dilutes what was there).



Think of it this way.



You have 9 dKH.



You evaporate 1% of the volume, and the alkalinity becomes 9.09 dKH since all you removed was totally pure fresh water. Then you replace that 1% of evaporated water with totally pure fresh water, and the alk drops back to the original 9 dKH.



Hence, there is no change in the alkalinity.



The exact same argument can be made for pH, although it is a bit more complicated because it is not even clear whether adding pH 7 pure fresh water to seawater at pH 8.2 lowers, raises, or leaves the pH unchanged. The effect is likely vanishingly small, and when you replace the evaporated water with pure fresh water, it goes right back where it was.

RHF is no joke. He has proven credentials and is an invaluable asset to this forum.

[GainesvilleReef]

Randy,
I have been looking for the opportunity to thank you for all you have done for our hobby. I got into this about a year and half ago and would have been lost without your articles as a resource. Just the other day, I used one of your posts to lower the Alk of fresh saltwater to adjust it to tank levels for a large water change.

And, thanks to the other chemists like Bertoni that make this forum one of the best informational forums anywhere.

Clearly, I am in the right place to get answers.

[tmz]

Wow, most of use ro/di water in our reef; which has 0 dissolved solids and ph is around neutral, which is 7. You do not 100% of carbonates added back into the water from evaporation. I'm not sure how you can't understand how this will decrease your ph and alkalinity. Obviously I'm in the wrong place to find my answer.

Wow is an apt reaction to your post.

Listen and read and you will learn from Randy.
Great patience to take the time to explain it again ;very nice Randy.

Pure water evaporates;nothing else. Top off replaces pure water with pure water. There is no alkalinity involved. Simple. It won't change the alkainity in the tank nor the pH at least not fro very long.

Calcifying organisms in the tank consume alkainity and calcium which often requires dosing those beyond the additions from salt mix. A biotic preciptiation can also use some. None of that relates to top off water.

CO2 levels in the tank from equilibration with room air drive the pH. Again no relationship to replacing pure water with pure water.

You do not 100% of carbonates added back into the water from evaporation.

What is that supposed to mean? It makes no sense as written.

[tmz]

Fish florist,

I know this is an old question. Missed it when you first asked it .
Yes I do 1% water changes daily and have for many years.

[Randy Holmes-Farley]

Thank you, Tom, GainsvilleReef and Breadman03.

Happy Reefing!

[hilgert]

Originally Posted by Randy Holmes-Farley

Truly you may be in the wrong place if you are unwilling to listen when a professional chemist with a PhD from Harvard and dozens of published reef chemistry articles explains something to you...

Your PhD is from Harvard? Dang, I thought is was from Wichita Junior College.

EDIT: To be clear, RHF is the best...and I'm [fairly] sure Wichita Junior College is an okay school.

[Breadman03]

Originally Posted by Randy Holmes-Farley

Thank you, Tom, GainsvilleReef and Breadman03.

Happy Reefing!

What can I say? LinkedIn keeps suggesting I connect with you, so I did a little reading. The highlights of your accomplishments takes longer to read than War and Peace. I wouldn't want to clog up your meaningful connections with some reefer or I'd send the request.

[Randy Holmes-Farley]

Thanks for the comments.