Salinity vs. Alkalinity, pH and Ca
- Thread starter karimwassef
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karimwassef
Active member
My question too. I love your articles on pH, Mg, Ca and Alk. We need more... Na and Cl
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.
karimwassef
Active member
Awesome! Love it.
So is there a curve that shows these coefficients at s function of dissolved NaCl?
So is there a curve that shows these coefficients at s function of dissolved NaCl?
karimwassef
Active member
It sounds like Na interferes with Ca precipitation the way Mg does?
Not really the same way.
Sodium, being positively charged, can crowd around the carbonate and effectively shield it from positively charged calcium "seeing" the carbonate on the other side.
Magnesium can do that same thing, but magnesium also has a more profound effect that impacts the kinetics of calcium carbonate precipitation, and not just the tendency for precipitation to happen (which Ksp describes).
Magnesium gets onto growing calcium carbonate crystals in place of calcium, and poisons the growing crystal for further precipitation because it has disrupted the order. That greatly slows down the process, even if it "wants" to happen. Normal seawater is this way. Calcium carbonate wants to precipitate since the solubility product is greater than 1, but it doesn't because magnesium keeps it from happening. Sodium does not have that effect.
karimwassef
Active member
You gave us s couple of points on it already. Is it calculated or empirically determined?
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.
ln(γ) = D. H. + ΣBiimj + ΣCijkmjmk
I don't think it is even worth describing what the terms all are.
karimwassef
Active member
Sounds like we need a PhD to figure it out. Do we know anyone like that?
karimwassef
Active member
So on a simpler note. Would varying salinity from 35 to 25 significantly change solubility of Ca in the salt water?
Is even saltier water better at holding Ca? What's the sweet spot? Would adding Mg to add in interference help?
Also, can my conductivity / salinity probe tell the difference between CaCl salt or NaCl salt? Can a hydrometer?
Is even saltier water better at holding Ca? What's the sweet spot? Would adding Mg to add in interference help?
Also, can my conductivity / salinity probe tell the difference between CaCl salt or NaCl salt? Can a hydrometer?
wayne in norway
New member
You wet skimmed the salinty down 7,8 ppt in a week? How big a tank is this - you realise you'd need to remove at least 20% of the total system volume as skimmate, and that's not accounting for effects of the top up system.
I am dubious of this personally.
I am dubious of this personally.
wayne in norway
New member
Also those devices cannot tell the difference between different salts. The differing effects on differing devices could, but it will be an extremely complex exercise
Yes, it will. Whether that is important or not may depend on other factors, like how hard you push the supersaturation of calcium carbonate. High pH is usually the biggest driver. A 0.3 pH unit rise is equivalent to a doubling of either alkalinity or calcium in terms of supersaturation and likelihood of precipitation.
Yes, saltier water holds it better. Magnesium contributes significantly to the effect in seawater, as does sulfate. The only sweet spot is where corals thrive. Adding more and more sodium, chloride, and sulfate, will make calcium and alkalinity less and less likely to precipitate. Again, may not be important for a reefer.
All of them will get off in measurement as you deviate from pure seawater ratios.
With a refractometer, 36.5 ppt NaCl looks exactly like 35 ppt seawater.
karimwassef
Active member
Tank volume is 600g
Skimmer output is 0.5g per day = 15g a month
I can easily remove double that if I skim wetter.
I calibrate and test my probe and compare to my hydrometer enough that I trust the reading.
Skimmer output is 0.5g per day = 15g a month
I can easily remove double that if I skim wetter.
I calibrate and test my probe and compare to my hydrometer enough that I trust the reading.
karimwassef
Active member
I run 8.4 pH so I watch Mg even more than Ca... I keep it at 1500 as much as I can. When my salinity dropped, this was the biggest factor for me and I added significant Mg to get back on target.
karimwassef
Active member
Should I measure sulfate?
karimwassef
Active member
I added four new coral frags. A stylophora, a montipora, a hydnophora and a Hollywood chalice. Other than my own clumsiness in mounting them, they adapted well to the low salinity and extended their polyps within the hour. Again, I think it's maintaining the other parameters that counts.
