[curiouscat]

NH₄Cl has been reported in several patents to increase the solubility of Ca(OH)₂ in water by an  order of magnitude.

Would there be any chance of a chemical reaction (Temp ~ 60 C)? I was hoping not, since dissolved Ca(OH)₂ is the reagent of interest to me.

Possibilities I worried about:

(1) NH₄Cl  NH₃  + HCl


(2a) NH₄Cl + H₂O  NH₄OH + HCl

(1b & 2b) Ca(OH)₂ + 2 HCl  CaCl₂ + 2 H₂O

(3) 2 NH₄Cl + Ca(OH)₂  2 NH₄OH + CaCl₂
 

Are any of these reactions likely?

Previously I thought the solubility enhancing effects are from activity coefficents alone (at high ionic conc.) but the fact that NH4Cl is the only salt that has such a great effect on solubility of Ca(OH)2 causes  me to wonder.

[Borek]

Technically adding NH₄Cl you are not increasing solubility of Ca(OH)₂, you are dissolving it in a weak acid.

Ca(OH)₂ + 2NH₄⁺ Ca²⁺ + 2NH₃ + 2H₂O

[curiouscat]

Technically adding NH₄Cl you are not increasing solubility of Ca(OH)₂, you are dissolving it in a weak acid.

Ca(OH)₂ + 2NH₄⁺ Ca²⁺ + 2NH₃ + 2H₂O

Thanks! No simple way of knowing if or not this affects the availability of the lime for a reaction, right?

e.g. Say I'd a reaction needing x parts of saturated lime solution before. If apparent solubility has been enhanced 10 times due to NH4Cl can I bank on (roughly) using 1/10th the quantity of lime solution?

[Borek]

I would say it depends.

If what you need is just Ca²⁺ situation is quite different than if you need just high pH. And the situation is even more different if you need both.

[curiouscat]

I would say it depends.

If what you need is just Ca²⁺ situation is quite different than if you need just high pH. And the situation is even more different if you need both.

This reaction is what I need it for. I'd say I need the Ca²⁺ ions but not sure how I decide if I need the pH or not.



More specifically, this reaction:


Somehow the first trial (with NH4Cl as a solubility aid) gave a very low conversion  and I'm trying to debug why.....Equivalent amounts of Lime (with no NH4Cl added)  provided as a slurry gave almost quantitative conversion. Which was counter-intuitive to me. Dissolution ought to have helped but it actually hurt?

[AWK]

This is quite poor method and great result cannot be expected

http://link.springer.com/article/10.1007%2Fs00706-008-0083-5?LI=true#page-1
http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv1p0494
http://www.freepatentsonline.com/2582114.pdf

[curiouscat]

This is quite poor method and great result cannot be expected

http://link.springer.com/article/10.1007%2Fs00706-008-0083-5?LI=true#page-1
http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv1p0494
http://www.freepatentsonline.com/2582114.pdf

What's your basis for concluding that it is a poor method? None of those references have any Ca(OH)₂ and definitely no  NH4Cl?  (That last patent does use Ca-Acetate! Interesting find! Thanks. Looking it up)

Note that this isn't for a small scale lab prep. So not using I₂ / KOH / peroxy compounds has other cost / logistical / safety / effluent advantages. Ca(OH)₂ seems a good choice; the search now is about ways to optimize the scheme.

What am I missing?

Sure those are alternatives. Maybe I wasn't clear: Ca(OH)2 by itself is proving not a bad alternative. I'd have expected Ca(OH)2 + NH4Cl to be better but somehow it is not. And that is the surprise.

The comparison  isn't between Ca(OH)2 and other routes but by Ca(OH)2 but with and without NH4Cl.