March 04, 2021, 01:45:14 AM
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Topic: Mixture of strong and weak acids and a calcium-base. What will fall out?  (Read 394 times)

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Offline schnorch

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Hello,
again I am looking for some help. Last time I got it here.


At first something simple:
I have a solution from water and calcium hydroxide and add sulfuric acid. The most part will form calcium sulfate and fall out of the solution. This is I think because of the low solubility of calcium sulfate(2.4g/l).


Now a bit more complicated:
I have a solution from water and calcium salts from weak organic acids like butyric acid for example. Those salts have an even lower solubility than calcium sulphate.
If I now add sulphuric acid, what will happen?

1. Will the sulfate-ion grab Ca2+ from the weak-acid-salts and form calcium sulfate what mostly falls out? Then the weak acids are "free".
Or...

2. Will the sufuric acid have no chance to get the Ca2+, because of the very low solubility of the weak-acid-salts? Then the salts and the sulfuric acid wouldn't react together.

I think it's No.1. But I don't know why. Perhaps something because sulfuric acid is a strong acid. But this is not something like a complete explanation. Perhaps my thinking that a low solubility is related to a high stability is wrong in a way. This has lead me to No.2.
Perhaps someone has something to read or keywords to google for me, where I get a full explanation, what happens. Where can I read, why the sulfuric acid is able to grab the Ca2+ out of an almost unsoluble salt?


Thanks in advance.

Offline AWK

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #1 on: February 17, 2021, 06:12:23 AM »
Quote
Those salts have even lower solubility than calcium sulfate.

I doubt the truth of this statement.

Of course, the stronger acid displaces the weaker ones from the chemical compound.
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Offline Borek

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #2 on: February 17, 2021, 07:38:14 AM »
Oxalate has a very low solubility. Acetate is quite soluble. Stearate has a very low solubility again.

Then, a lot depends on the acid solubility. Butyric acid is miscible with water, but solubility of stearic acid is below mg per 100g of water, so it will crash out even without calcium present.

Basically there is no single answer, the outcome depends on the acid.
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Offline schnorch

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #3 on: February 17, 2021, 07:41:02 AM »
Quote
Those salts have even lower solubility than calcium sulfate.

I doubt the truth of this statement.
In this case the solubilities of the weak-acid-salts are really lower than the one of calcium sulfate. For example calcium butyrate has according to data I found online 0.7g/l, calcium decanoate 0.05g/l and calcium octanoate 0.01g/l.

Of course, the stronger acid displaces the weaker ones from the chemical compound.
Is this a general rule? Do you have something to read for me?


Offline schnorch

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #4 on: February 17, 2021, 07:50:34 AM »
Oxalate has a very low solubility. Acetate is quite soluble. Stearate has a very low solubility again.

Then, a lot depends on the acid solubility. Butyric acid is miscible with water, but solubility of stearic acid is below mg per 100g of water, so it will crash out even without calcium present.

Basically there is no single answer, the outcome depends on the acid.
In my case it's about carboxylic acids. Not the small ones like formic or acetic acid, but at least butyric (C4H8O2) or bigger (C5H10O2, C6H12O2, C7H14O2 and so on).
In general I think the bigger the less soluble are them. And weaker (higher pka) and more volatile.

The background is, I wonder if it is possible, to store such acids by adding calcium hydroxide and boiling the water off and then to release them again by adding water and sulphuric acid and filter out the calcium sulfate. There is a paper that claims it and for me it sounds well. But I want to understand the process more in detail.

Offline AWK

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #5 on: February 17, 2021, 09:05:02 AM »
Quote
Those salts have even lower solubility than calcium sulfate.

I doubt the truth of this statement.
In this case the solubilities of the weak-acid-salts are really lower than the one of calcium sulfate. For example calcium butyrate has according to data I found online 0.7g/l, calcium decanoate 0.05g/l and calcium octanoate 0.01g/l.
Your information sources are unreliable.
The solubility of calcium butyrate in the water at room temperature is on the order of 200 g / L.
It is only at about 10 carbon atoms in the acid residue that the solubilities of this acid salt and calcium sulfate are comparable and then lower.
Of course, the stronger acid displaces the weaker ones from the chemical compound.
Is this a general rule? Do you have something to read for me?
Have you ever read the section on acids in a chemistry textbook?
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Offline schnorch

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #6 on: February 17, 2021, 12:19:20 PM »
The 0.7g/l I have from here:
https://www.researchgate.net/post/Is-there-anyone-who-can-provide-the-salting-out-protocol-for-butyrate-extraction
The solubility of in water by 75 ºC ― a seemingly reasonable temperature that could be selected for the intended purpose ― would be 0.700 g/kgH2O for calcium butyrate...
It's of course possible that you know it better.
Calcium decanoate: https://foodb.ca/compounds/FDB011454 2.6e-05 g/L
Calcium octanoate: https://foodb.ca/compounds/FDB017360 0.00081 g/L

I have no chemical background. But now and then I stumble into something I want to know. Most often related to alcoholic fermentation. Then I try to find out and this way I collected some chemical and physical knowledge. But of course I know, it's not comparable to study chemistry from the basics.
I know what pH and acidity means and what the difference is. I know how pH buffers work and I am able to calculate them. And I know what the difference or the main difference between a strong and a weak acid is: The rate of dissociation.
Is this higher rate of dissociation the only reason, a strong acid would take the Ca2+ and the weak acid wouldn't?
If I have one Ca2+, one sulfate-ion and two butyrate-ions, what is the reason that the sulfate-ion binds to the Ca2+ and the butyrate-ions don't?

Offline AWK

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #7 on: February 17, 2021, 01:03:25 PM »
The solubility of calcium lactate was probably first reported by Michel Chevreul almost 200 years ago in "Recherches chimiques sur les corps gras d'origine animale" as 17.58 g per 100 g of water at 15 C.
Data from the discussion (not from the publication) only occasionally are reliable, and your chances of assessing the reliability of the data will increase significantly if you study chemistry thoroughly, even by yourself.
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Offline schnorch

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #8 on: February 17, 2021, 02:51:49 PM »
I don't understand why you now mention calcium lactate. You mean it is impossible that calcium butyrate has a such low solubity because calcium lactate is better soluble? The solubility of calcium lactate is according to wikipedia 58g/l at 20°C by the way.

And by the way:
I found the data of the international critical tables, Vol 4, page 230
The solubility is 0.84g/kg at 20°C

Offline schnorch

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #9 on: February 17, 2021, 04:01:33 PM »
But I am not sure with this value for calcium butyrate.
On page 216 it says:
M = Gram-formula-weight (corresponding to formula given in
the heading) per 1000 g H2O.

If M=0.84, does this mean the solubility is 0.84g per 1000g H2O?

Or is it gramm per mol, right?
Perhaps someone here knows the answer?

There is a formula on page 216:
x = Mole fraction = M / (M + 55.1)
With M = 0.84 x would be 0.015. Is x the soluble mole fraction?

Offline AWK

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #10 on: February 17, 2021, 04:38:21 PM »
Moles per liter (180 g/L at 20C) perfect fits to Chevreul data (175.8 g/L at 15 C)
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Offline schnorch

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #11 on: February 17, 2021, 05:25:54 PM »
And how did you calculate the 180 g/l ?

Ok, i got it. 0.84 x molar mass of calcium butyrate = 180

Offline schnorch

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #12 on: February 17, 2021, 06:00:50 PM »
Do you think such sources are reliable?

http://www.thegoodscentscompany.com/data/rw1594401.html
http://www.thegoodscentscompany.com/data/rw1594391.html

They write very low solubilities for calcium octanoate (1.2mg/l) and decanoate (48mg/l) at 25°C.


Offline AWK

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #13 on: February 17, 2021, 09:43:18 PM »
Nor reliable at all.
The solubility of C10 derivative should be much lower than C8 derivative.
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Offline schnorch

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Re: Mixture of strong and weak acids and a calcium-base. What will fall out?
« Reply #14 on: February 18, 2021, 08:25:59 AM »
So the solubility gets lower the larger the molecule is (within this class of molecules). The data of the international critical tables does support this not 100%. It claims calcium formate (the smallest molekule of this class) is less soluble than calcium acetate (the second smallest one). But the other derivates seem to follow this rule. C10 is way less soluble. But there is no data for molecules greater than C10 unfortunately.


All right,
based on what I have learned now, I rephrase my question:

If there are calcium salts of weak and large carboxylic acids with a lower water solubility than calcium sulfate, will sulfuric acid be able to take the Ca++ from them and fall out or not?
And if the answer is yes, is it, because sulfuric acid dissociates more than this weak acid, or is it because of something else? 

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