Trying to understand alkalinity

[delphinus]

Most times I see alkalinity explained, I see it explained as (let me paraphrase a little): "alkalinity is the measure of how well a solution can resist changes in pH." Ok, so this much I kind of remember from chemistry in school, so ok, I buy that. But I guess my question is, why would a depressed alkalinity, in and of itself, lead to depressed calcification?

I'm guessing that if pH drops (because of depressed alkalinity), that this could have an impact on calcification rates. Ok. But let's assume for a moment that if pH is within a reasonable range anyways, why should the alkalinity then have any effect? There must be more to the story?

Let me try an example: Say if you had a reverse photoperiod on a refugium or sump that aids in pH stability. Assuming a good calcium number (say 440), but with still slow SPS growth, then the "suspect" is that the alkalinity is depressed. But if it's alkalinity and we just saw that pH is more or less stable and calcium is good, then there has to be more to the picture than simply "alkalinity is the resistance to fluctuations in pH."

...

I have only recently been trying to pay close scrutiny to my Alk numbers. One thing I've seen lately I'm at odds to explain and I'm hoping someone can explain this one.

Last week I got the following numbers from my tank:
Calcium: 485
pH = 8.4
total alk = 2.4 meq/L
borate/hydroxide alk = 0.5 meq/L
carbonate = 2.4 - 0.5 = 1.9 meq/L

I did two 25% water changes 4 days apart, and then tested the tank again.

The makeup water tested as follows:
Ca: 750
pH = 8.0
total alk = 6.1
borate = 1.5
carbonate = 6.1 - 1.5 = 4.6

After the two water changes, the tank tested as follows:
Ca: 400
pH = 8.4
total alk = 2.6
borate = 1.2
carbonate = 1.4

Look at those alkalinity numbers. Huh? Does any of this make any sense to you SPS gurus?

The total alk seems reasonably steady although it did go up a little. But why would the carbonate alkalinity go DOWN after two water changes where the carbonate was higher in the makeup water? I would have guessed that I would have seen a slight "bump" towards the makeup water values in ALL parameters, not this sort of see-sawing like that. I know that as Alk goes up I should see calcium go down so I guess that part makes sense to me but not the carbonate/borate thing. Where did my carbonate go? :confused:

[Mutagen]

Hi Delphinus,

some comments and some questions.

First, how are you distinguishing between your hydroxide alkalinity and the carbonate alkalinity. Are you titrating to a 7.0 pH endpoint or perhaps a phenolphthalein endpoint? In either case, this is going to give you erroneous results, there is way too much interference from CO2 for this procedure to work. I think you should just work on the total alkalinity number which is the same number most test kits give as carbonate alkalinity.

Second, alkalinity is not a measure of the ability to resist a pH change. A buffered solution will resist changes in pH. Alkalinity is a measure of the water's ability to neutralize a strong acid down to a pH of about 4.3. This is the pH where the carbonate ions in solution are all converted to CO2. All test kits measure alkalinity in this manner. The difference between the two terms can be confusing as people often use the terms interchangably, and the alkalinity in our systems is responsible for the buffering effect. This happens since we introduce alkalinity into our system with the addition of carbonates and bicarbonates which are also buffering agents. In the case of dripping calcium hydroxide we introduce the carbonate indirectly by combining CO2 from the air or biological processes with the hydroxide to form bicarbonate and some carbonate.

So to summarize, buffered solutions resist changes in pH. Buffered solutions are created by adding buffering agents to the water. For salt water systems, this will be either directly or indirectly from the addition of carboante and bicarbonate.

Alkalinity is a measure of the ability of the water to neutralize a strong acid, but is not a measure of buffering capacity. It is possible to have a highly alkaline water system that is not buffered. It is also possible to have a buffered solution that is not alkaline at all.

The calcification question is much more difficult. I'm not sure its fully understood even by the experts, but then we need an expert to answer that question. So here is my simplified way of looking at the issue. Calcification will require calcium and carbonate or bicarbonate. Higher alkalinity, in salt water systems, implies higher carbonate and bicarbonate concentrations so there will be more raw material available for calcification. Also, higher concentrations of calcium and carbonate move the system closer to where calcium carbonate would naturally precipitate so it may require less energy for the animals to carry out calcification as well.

As for the numbers you gave I can only guess. Calcium went down after adding calcium and alkalinity and alkalinity didn't go up as much as would have been expected. It looks like some calcium carbonate precipitated taking more calcium out of solution than was added. It is my experience that this can happen slowly and not be obvious like the often described snow storms some people create. By the way, that is a very weird salt mix at 750 calcium and 6.1 meq/l. The calcium would NOT be stably in solution under those conditions.

Hope this is of some assistance, Mutagen

[MiNdErAsR]

Moving this to the Reef Chemistry Forum....

[Randy Holmes-Farley]

Delphinus:

Those are some very good questions, and I think that Mutagen gave a very good set of answers. I am just in the finishing stage of writing an article that goes into great detail about akalinity, so you might look for that when it is posted.

Now, I do have some problems with your alkalinity breakdown.

You cannot distnguish carbonate alkalinity from borate and hydroxide alkalinity with any normal test. All of these happen between tank pH and about pH 7-7.5.

What you can distinguish is all three of those from bicarbonate alkalinity, which happens between about pH 7 and pH 4 or so.

What were you doing to get these numbers? It's hard to interpret these changes without knowing what you did to get them. However, I will point out that the true carbonate-only alkalinity (not bicarbonate) is very pH sensitive. It is a far greater portion of the total alkalinity at pH 8.5 than it is at pH 8.

<< But let's assume for a moment that if pH is within a reasonable range anyways, why should the alkalinity then have any effect? There must be more to the story? >>

Yes, as Mutagen indicated, it is the bicarbonate (or maybe the carbonate) and not the pH that is needed by a coral to form calcium carbonate. THIS IS EXACTLY WHY ALKALINITY IS IMPORTANT: IT IS A SURROGATE MEASURE OF BICARBONATE THAT IS VERY DIFFICULT TO ATTAIN ANY OTHER WAY [I capped this sentence to make it stand out for anyone just skimming the post]

I'm more than happy to have a followup discussion as this is a topic that confuses many people, and is oversimplified by others.

[delphinus]

Thanks for the replies Mutagen and Randy.

When I first started off writing those questions I had written it as "why are my SPS slow to grow." But in course of editing I had sorted of whittled that bit out because I am fairly certain that the answer to that question is that the alkalinity is depressed ... but I was sort of confused as to why -- I guess that part of the problem is that I was thinking "alkalinity" meant "buffering."

It sort makes sense now thanks to your explanations.

The test kit I am using is "Seachem." It gives me one titration for "total alkalinity" and another titration for "borate/hydroxide." It tells me the "carbonate" is the "total alkalinity" minus the "borate/hydroxide." As far as interpreting the numbers it doesn't go into any great detail it more or less just says that "carbonate alkalinity should not be at the expense of borate/hydroxide alkalinity" (um ... or vice-versa. Don't remember offhand and the instructions are at home and I'm at work right now so I can't check). I don't think it ever refers to carbonate as bicarbonate. So as to what exactly I am testing here, I'm not sure.

If it is true "carbonate" and not "bicarbonate" then it sounds like maybe I should just be paying more attention to the "total" number and the pH at which I'm testing this value, and just using this "carbonate" value as a measure of confidence in tested pH value? If it's low or close to zero then I assume my pH is closer to 8.0, and more nonzero it is then the higher the pH must be?

I am completely wet here. Thanks for your help. I look forward to your article, Randy.

Thanks..

[Randy Holmes-Farley]

Delphinus:

<< The test kit I am using is "Seachem." It gives me one titration for "total alkalinity" and another titration for "borate/hydroxide." It tells me the "carbonate" is the "total alkalinity" minus the "borate/hydroxide." >>

I don't doubt that they say it, but they are wrong. Unless you have a very specific reason for measuring anything else, total alkalinity will suffice for nearly all reefkeeping needs. I assume that you are not using Seachem salt, which has a load of its own baggage with respect to alkalinity because it overloads on borate in the salt mix.

Actually, now that I think of it, was that salt mix Seachem? Maybe that's why the alkalinity is so high, because it is screwed up with excess borate.

Assuming that most of the tank is not Seachem salt, then the total alkalinity is marginal but OK.

[delphinus]

The brand of salt is Instant Ocean, I don't know if it's completely different than Seachem salt or if it's just repackaged or what ....

The calcium tests very high on the makeup water because I have exceptionally calcium-rich city water, and I thought IO also had its own calcium in it... I mix the makeup water with a powerhead for at least 2-3 days before using it. I don't know if it was just the last bag of IO I was using but it seems after testing the Ca value that I now have an explanation for the white powder over my heater and powerhead after mixing the stuff my last few water changes.

Basically what I'm seeing is that nothing in the tank appears to be declining, but I am not seeing any calcification. No new coraline growth, plus no appreciable growth of SPS. I know of some people boasting up to 1" growth of some SPS per month .... so I thought I'd explore this alkalinity as a possible cause of the difference in growth rates.

Thanks VERY much for your explanations, this has been of a great help in my attempt to understand what's going on. I look forward to that article, Randy! :D

[Randy Holmes-Farley]

Tony:

<< The calcium tests very high on the makeup water because I have exceptionally calcium-rich city water, and I thought IO also had its own calcium in it... >>

Instant Ocean is a good choice, and the calcium makes sense. It's the alkalinity at 6.1 meq/L that seems out of whack for Instant Ocean. FWIW, Seachem and IO are very different salt mixes.

There are two other chemical things that seem to inhibit coralline and calcification: high phosphate and high organics. I can't judge if you actually have a problem or not, but these are other avenues to think about. Have you ever measured phosphate, and do you skim?

[delphinus]

Hmmm, I haven't tested phosphate in a while, good catch, thanks. I've generally only tested for phosphates when I've had a hair algae or cyano outbreak that I couldn't attribute to anything else. Nothing like that is happening at the moment. I'll test PO4 tonight when I get home from work and see what kind of number is there. (It generally has never been a problem in this tank, but that's not to say it couldn't start to be!)

The tank is indeed skimmed. It's a smallish tank so I'm using a smallish skimmer. Usually there is very little skimmate pulled out of this tank (unless I mis-adjust the skimmer and get too wet of a skimmate). There are no fish in this tank so pretty much the only inhabitant that gets regularly fed is a carpet anemone, so I don't think that there would be particularly a lot of DOC in the water column.

What is it about the 6.1 alkalinity in the makeup water that raises the red flag? Is it too high or too low, or just seems inconsistent with the Ca value? Maybe I should make up a new batch and test it again and see if I get a consistent value.

This batch of makeup that I had tested to give that 6.1 value had a LOT of precipitate in the bucket. The one thing that I did differently in mixing this batch over others is that I added too much salt at first, and then diluted it down with fresh water until I got the SG lowered down to where I wanted. (Then I let it sit with a powerhead and a heater for a couple of days, hoping that I would be mixing it up enough that way.) Normally I first add too little salt and then slowly add salt until I get the SG raised to where I want it (and then let it sit with a powerhead... etc.). I guess the latter method would be better to avoid the risk of a supersaturated solution that precipitates stuff out that normally shouldn't be precipitated out?

[Randy Holmes-Farley]

<< What is it about the 6.1 alkalinity in the makeup water that raises the red flag? Is it too high or too low, or just seems inconsistent with the Ca value? Maybe I should make up a new batch and test it again and see if I get a consistent value. >>

Ar 6.1 meq/L it seems way too high to me. IO usually tests out in the 2's, so 6.1 meq/L would seem beyond normal batch to batch variation. Making and testing a new batch would be a good idea. It might be a test error, a bad batch, or some other problem.

<< Normally I first add too little salt and then slowly add salt until I get the SG raised to where I want it (and then let it sit with a powerhead... etc.). I guess the latter method would be better to avoid the risk of a supersaturated solution that precipitates stuff out that normally shouldn't be precipitated out?>>

Yes, that is a good idea. The other way wouldn't explain a high alkalinity, however, but it would explain a low one. The 6.1 value is in meq/L and not dKH, right?

I don't worry too much about mixing, but it would not be good for a hypersaline solution to sit around for very long as it does risk precipitation as you suggested.

[delphinus]

<< Ar 6.1 meq/L it seems way too high to me. IO usually tests out in the 2's, so 6.1 meq/L would seem beyond normal batch to batch variation. Making and testing a new batch would be a good idea. It might be a test error, a bad batch, or some other problem. [...] The 6.1 value is in meq/L and not dKH, right? >>

Yes, meq/L not dKh. Wow, that is quite a variation from "normal" IO. I'm not sure what the alkalinity is of the water out of my tap (can I use my same test kit on that, or does it have to be saltwater?). I'm not using RO or DI water so that might be throwing things off. Of course... test error, "tester error" and bad batch are all certainly possibilities that should not be ruled out either, though.... and after all, since it is only one test value, instead of a real "sample", so perhaps some skepticism should be in order regardless.

Thanks once again. This has been an invaluable help for me.

[Randy Holmes-Farley]

Yes, total alkalinity tests work fine on fresh water.

FWIW, if you are using tap water you might consider testing it for copper at some point. A number of people have seen elevated copper in their tap water, and that's a toxicity concern for many invertebrates.