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Topic: Why do Ammonium / Chlorides / Nitrates increase solubility of Ca(OH)2 in H2O  (Read 7344 times)

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Offline curiouscat

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I found the following extract in a patent I was reading (#):

"Solution of calcium hydroxide in water, uses an ammonium salt as a solvating aid. Anions, provided as chloride or the nitrate, are also used to increase the solubility of the calcium hydroxide in water. This method is effective in reducing the amount of water needed to dissolve calcium hydroxide."

Why would this strategy work?  ???

 Essentially, it seems to say co-addition of Ammonium Chloride or Ammonium Nitrate will boost solubility of Ca(OH)2 (normally very insoluble <0.1% w/w )? Is it because these compounds are Strong Acid + Weak Base? 

If so I could probably use that to predict quantitatively how much of a solubility boost I can get?

#Amended slightly for clarity.

Offline discodermolide

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I seem to remember something called the Common Ion effect. Where the addition of a common ion increases the solubility of a precipitate. Perhaps this is the reason.
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Offline curiouscat

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I seem to remember something called the Common Ion effect. Where the addition of a common ion increases the solubility of a precipitate. Perhaps this is the reason.

Thanks disco! The Common Ion Effect indeed exists but:

(a) AFAIK it decreases solubility not increases
(b) No ion is common between Ca(OH)2 and our solvating-aid here.

I could be wrong though.

Offline Raphael

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I am guessing it has to do with ionic strength and "ionic atmosphere". I will try to post more tomorrow night if no one else posts

Offline curiouscat

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I am guessing it has to do with ionic strength and "ionic atmosphere". I will try to post more tomorrow night if no one else posts

Thanks! Would be interesting to hear your ideas. I'm stumped.

Offline AWK

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This is not  an idea. This is reality. Ionic strength without common ion effect always increase solubility of any ionic compound. Moreover, calcium cation show tendency to weak complexation of oxygen from anions, eg you can strongly increase solubility of calcium sulfate using ammonium sulfate. In this case ionic strenght effect and complexation hardly prevail common ion effect which always decrease solubility.
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Offline curiouscat

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This is not  an idea. This is reality.

My bad.

"Would be interesting to hear your ideas explanations. "

Offline curiouscat

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This is not  an idea. This is reality. Ionic strength without common ion effect always increase solubility of any ionic compound. Moreover, calcium cation show tendency to weak complexation of oxygen from anions, eg you can strongly increase solubility of calcium sulfate using ammonium sulfate. In this case ionic strenght effect and complexation hardly prevail common ion effect which always decrease solubility.

Interesting! Thanks, I did not realize.

Is there a quantitative basis for this? I mean using solubility products I can estimate how much solubility will decrease by adding a common ion.

Is there a way to estimate how much more Ca(OH)2 I can dissolve via (a) ionic strength (b) complexation mechanisms?

Baseline solubility of Ca(OH)2 is very low: 0.173 g/100 mL (20 °C)

Say, I wanted to increase this to 2% w/w is that feasible? How much of the solvating aid would I expect to need.

Offline Borek

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Revisit Debye-Huckel theory. For not too high ionic strengths activity coefficients are always lower than 1, so concentrations must be higher -> solubilities must be higher.
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Offline curiouscat

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Revisit Debye-Huckel theory. For not too high ionic strengths activity coefficients are always lower than 1, so concentrations must be higher -> solubilities must be higher.

Ah! It's a non-ideality effect then.

Thanks! I was puzzled how solution-aids get around the solublity product limit but activities would allow that.

Offline ajkoer

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Look at the reaction:

 Ca(OH)2 + 2 NH4Cl <--> 2 NH3 (g) + 2 H2O + CaCl2

It is known that the solubility of ammonia decreases when NaCl is added to a solution. So as the Ca(OH)2 dissolves/reacts more ionic CaCl2 is created that decreases the solubility of the NH3 driving the reaction to the right. This implies that NH3 is expelled and Ca(OH)2 is dissolved.

Another way of describing what is happening is the increase in ionic interactions from the "non-common ion effect". As a result, "a sparingly-soluble salt will be more soluble in a solution that contains non-participating ions." See http://www.chem1.com/acad/webtext/solut/solut-6b.html . This is also referred to as the "Salt Effect" to quote from Wikipedia "
The salt effect[2] refers to the fact that the presence of a salt which has no ion in common with the solute, has an effect on the ionic strength of the solution and hence on activity coefficients, so that the equilibrium constant, expressed as a concentration quotient, changes. (see http://en.wikipedia.org/wiki/Solubility_equilibrium ). For more advanced details see http://www.jim.or.jp/journal/e/pdf3/45/04/1317.pdf . To quote from the abstract: "We developed a chemical model to analyze ionic equilibria in a cobalt chloride solution at 298K. The chemical model consisted of
chemical equilibria, mass and charge balance equations. The activity coefficients of solutes and water activity were calculated with Bromley
equation. Values of the equilibrium constants for the formation of cobalt chloride complexes at zero ionic strength and of the interaction
parameters were estimated by applying Bromley equation to the reported equilibrium constants at different ionic strength". Now, in the current context, note that NH4Cl/NO3 are salts of a weak base and strong acid, which are highly ionic and have correspondingly low pHs.
« Last Edit: February 28, 2013, 08:57:58 PM by ajkoer »

Offline billnotgatez

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@ajkoer
Quote
Ca(OH)2 + 2 NH4Cl <--> 2 NH3 (g) + 2 H2O + CaCl2
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