Basic question: Ca and salinity

[karimwassef]

Hoping for a quick simple answer :)

At a given condition - say:
Temp = 80F
pH = 8.4
Alk = 8.0 dkH
Mg = 1600ppm

How much more calcium can be dissolved in 36ppm salinity than 34ppm salinity?
For example:
Max Ca = 500ppm at 34 salinity
Max Ca = 520ppm at 36 salinity


Is it linear over a range ?

30-40 salinity for example?

Thanks! :)

[disc1]

Less calcium would dissolve in more concentrated seawater. The maximum level really depends more on the alkalinity though.

[disc1]

If you're asking why the level is higher at a higher salinity it's because there's more salt mix there and that salt mix contains calcium so there's more calcium. But the maximum goes down as salinity increases.

[karimwassef]

It wasn't that there's just more calcium in the salt mix - I get that part.

I thought it was the availability of other agents that allows more Ca to dissolve. It was on a thread from a year ago or so...

Jonathan? Randy? Someone shared that with me at some point...

[karimwassef]

found it..
Yes. The ions Na+ and Cl- tend to stabilize other charged species that want to interact by "getting in the way".

One way to quantify this is with something called an activity coefficient. It is effectively a coefficient that you multiply by the concentration of an ion to get the effective concentration.

So when looking at the solubility product of something like calcium carbonate, you'd normally say the solubility product constant is the concentration of carbonate times the concentration of calcium:

ksp = [CO3--] x [Ca++]

If that measured value is above the ksp, calcium carbonate will precipitate, and below that value, solid CaCO3 will dissolve.

In reality, that simplistic equation only applies in dilute aqueous solution, such as fresh water.

In reality, the equation is modified by accounting for the activity coefficients, which effective make the concentrations look lower.

ksp = γco3 x [CO3--] x γca x [Ca++]

where γco3 is the activity coefficient of carbonate and γca is the activity coefficient of calcium.

So, back to the main point, Millero gives the activity coefficients of carbonate and calcium (and many other ions) in both NaCl and in seawater.

In dilute pure fresh water, they would be 1.0, indicating no special effect.

In NaCl solution equivalent to seawater salinity, they are 0.164 for carbonate and 0.259 for calcium.

So calcium carbonate is a lot more soluble in NaCl solution than in freshwater. By a factor of about 5 fold.

In seawater, the effect is even greater, with coefficients of 0.039 and 0.215, respectively, making calcium carbonate about 11 times more soluble in seawater than fresh. :)

Hope that made sense. :)

http://www.reefcentral.com/forums/showpost.php?p=23400808&postcount=43

[karimwassef]

Looks like I asked the same question back then - and forgot about it... still no answer then either.

Millero gives an equation that he uses to estimate them (page 158 of Chemical Oceanography, second edition), but I doubt you want to calculate it yourself,and I don't even want to have to write out the equation. The unexpanded form is:

ln(γ) = D. H. + ΣBiimj + ΣCijkmjmk

I don't think it is even worth describing what the terms all are. :D

[bertoni]

Magnesium helps keep calcium and alkalinity from precipitating by fouling crystal surfaces as they form. Maybe that's what you were having trouble remembering? I agree that computing exactly when precipitation will happen is beyond our skills and measurement capabilities.

[karimwassef]

No. I got that. See Randy's answer quoted above.

The question is how Na+ and Cl- interferes with the precipitation. Based on the answer above, more Ca can be dissolved in saltier water. How much Ca in how much saltier water?

Maybe I'll ask the question differently:

At the conditions above (temp, ph, alk, mg) - At what salinity can the maximum calcium be dissolved?

I'm not interested in optimum conditions for corals right now. Just starting from a theoretical view of what's possible.

[bertoni]

I suspect that the calcium and alkalinity would be soluble in a reasonably linear way over some short range of NaCl levels. That's a bit different from just adding more salt mix, which would decrease solubility overall. I'll have to see whether I can find my copy of Millero's book. It might help, although I'm not a chemist.

[karimwassef]

Thanks. My growth rate for SPS is very good with an Alk of 8 and Ca at 500, Mg at 1500 and salinity average ~34.

I let my pH wander from 8.3 to 8.4 (night, day) and I usually run closer to an average ~81F

I've noticed some growth spurts but I can't identify the drivers. I travel so knowing exactly what happens when isn't as easy.

The one variable that moves around the most on me is salinity because of my massive skimmer exporting salt and ATO adding fresh. I'll fix it with a brine refill, but haven't gotten around to it yet. It varies from 31 to 36. Mg moves around with it too, but I compensate by adding enough Mg to keep it at 1500. It's easier to add a gallon of Mg than to mix 200gal of salt mix when you're running to catch a plane. Not cheaper - but easier.

That creates an unusual mix where pH is 8.4, Ca is 500, Alk is 8, Mg is 1500, but salinity may be 31... Unnatural maybe but the corals are growing very fast - in spurts.

Temp also varies from 77 to 83. And I feed a lot.

So the salinity and temp are the volatile elements. Rest are basically in check.

Temp varies over 24hrs so I don't expect it to be the 'spurt' driver since that's more of a weekly or even monthly event. (yes- I know that temp stability is important.... But I snorkel and real reefs vary a lot too). The more shallow, the more extreme.

So, I think I've narrowed it down to salinity. My theory is that higher salinity allows more calcium to be dissolved and available. In some cases, I've measured close to 600 with high salinity (approaching 40). So I think there's a sweet zone for very rapid growth.

That's the background for reference only. I want to start with theoretical limits and work it back to a useful case.

[disc1]

With activity it becomes something that is not trivial to calculate. The bigger effect is with alkalinity. That's why I was saying it would be less because to me saltier water implies more alkalinity. I wasn't considering adding just NaCl like that.

[karimwassef]

Yes. Maybe I'll have to experiment. I can add kalk (let Alk rise too) or CaCl (adding more chloride ions) to different salinity mixes and see where I cause precipitation. I can have as many datapoint as mixes. As each precipitates, that would represent a datapoint. I can do this until they all precipitate.

[dkeller_nc]

That's not going to be as easy as it sounds. There are a good many other things in seawater that affect the solubility product (Ksp) of many compounds other than things that can be easily measured (like temperature, specific gravity, etc...). Just one of them is dissolved organics. Certain types of dissolved organics, particularly those that can act as chelators, can have a really radical effect on solubility products of ionic solids.

Here's (http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Equilibria/Solubilty/Solubility_Product_Constant,_Ksp) a pretty good explanation of the "common ion effect", which is what David's referring to, and the "uncommon ion effect" which is what Randy was discussing.

[dkeller_nc]

BTW - Here's (http://www.advancedaquarist.com/2002/3/chemistry) Randy's article discussing this subject in detail. Towards the end, he gives estimates for Ksp originally published by Millero for aragonite and calcite, as well as supersaturation constants for seawater at 25 deg C.

You could therefore calculate the relationship between changes in calcium concentration and carbonate/bicarbonate concentration, presuming that the supersaturation constant remains the same.

[karimwassef]

I love that article. I'm comfortable with the carbonate to calcium relationship. I'm not sure how to use that to map the Na and Cl to Ca relationship.

[disc1]

My theory is that higher salinity allows more calcium to be dissolved and available.

Unless you added more calcium then simply increasing the salinity doesn't cause calcium to magically come from nowhere. It may be possible to dissolve more in there, but you still have to actually put it in there for the level to go up.

[karimwassef]

I am adding more calcium.

Not sure where I gave the impression that adding Na and Cl magically creates calcium.

I want to know how much calcium can be absorbed by the saltier mix at the Alk and Mg I referenced above (8.0 & 1600)... The intent is to add calcium without initiating precipitation.

[karimwassef]

Ok. Let me know if you can get an analytical/theoretical answer.

Any recommendation on getting an empirical answer (see my proposal above)