[Archdevil155]

Hey everyone, just created the account to ask something..
Why do we need to add NH3 to CaCl2 on the way to make Ca3(PO4)2?
1st year on University of Chemistry in Greece here and I gotta write a report on an experiment we did the other day.. Based on the directions from a book they had given us, we needed to make Ca3(PO4)2. The steps were these:
1.Get 2,50 g of pure, dry CaCl2 in a 100ml beaker.
2.Add 25ml H2O so they dissolve.
3.Add SOME dense NH3.(Sorry if the word dense isn't correct I'm from, and study in, Greece.. Anyway I mean high concentration.)
4.Filter if neccesary.
5.In a different beaker dissolve 5,00 g Na2HPO4*12H2O in H2O.
6.Heat both solutions and then combine them while stirring.
7.Keep the solution warm for a while so that the sediment becomes granular and drops.
8.Filter using the Buchner filtration system.
9.Wash with warm H2O (twice with 5ml each time).
10.Dry in a drying oven.

So on step 3 why do we need to add the NH3?? It doesn't say and the teacher wouldn't explain.. We need to calculate how much we need to add but we do not know why add it in the 1st place.. So please If anyone can help me I'd be quite grateful..

[Borek]

Think about pH.

[Archdevil155]

Umm can you be more specific..??

[Borek]

What anion do you need to precipitate Ca₃(PO₄)₂? How does its concentration depend on pH?

[Archdevil155]

(PO₄)^3- I think.. So we probably get the Ca²⁺ from the dissolution of CaCl₂ right..? So doesn't NH₃ react with the Cl^- to give NH₄Cl? A former chemistry teacher of mine told me that the NH₃ reacts witht he Na₂HPO₄, not the CaCl.. He told me that the Na₂HPO₄ dissolves and gives Na⁺ and HPO₄^2- and that then the HPO₄^2- reacts with NH₃ and gives (NH₄)₂PO₄ if I remember correctly.. Then that dissolves in the water giving NH₄⁺ and PO₄^2-.. Is that correct??

[Borek]

It is all happening in water, so there is no reaction followed by dissolution.

You are right about PO₄^3- being the anion that is necessary. HPO₄^2- is a weak acid and NH₃ is a (weak) base. What happens when you mix base and acid?

Note that some of your formulae are off, so even if your thinking is correct, it is hard to tell what is a typo and what is an error.

[Archdevil155]

Oh yea.. I haven't really gotten used to thinkin' like that yet.. In high school that's how we were taught but in University they told us that actually the salt isn't formed, just the ions are around... That's what happens right?  Umm they react and neutralize each other, forming a salt. If one of the terms I use (or more) is wrong, please correct me. The terms in Greek and English are probably a bit different.. Oh, I didn't write any typos, just errors then. I have to hurry a bit so I can't check for all of them in case I recognize them myself.. I think that (NH₄)₂PO₄ should be (NH₄)₃PO₄ but I can't find others right now..  Please correct my errors if you can and I'll check again as soon as possible..

[Borek]

not the CaCl..

CaCl₂

PO₄^2-

You mean PO₄^3-.

[Archdevil155]

Wow, I can't believe the 1st one.. THAT was because of me hurrying and not paying enough attention.. But the 2nd one was because I had written (NH₄)₂PO₄, so THAT would give PO₄^2- if it was correct.. I should have corrected the PO₄^3- along with the (NH₄)₃PO₄ before.. Soo I guess you helped me just as much as I needed  Thanks a lot and I have just two more small questions.. On step 6 of the experiment I described we heat the two solutions so that they react more easily and quickly I guess right?? And my 2nd question is on step 7, why is it the fact that we keep the mixture warm, that makes the sediment granular and lets it drop??

[Archdevil155]

Ok.. I'm having trouble putting everything together and I've gotten kind of frustrated. Could you tell me the entire course of the experiment?? What happens in every step each time you add something, along with the reactions..? I'd be veeery grateful!

[Borek]

I had an impression you mostly got it. Ammonia makes pH high, shifting dissociation of HPO₄ to the right and increasing concentration of PO₄^3- - that in turn shift the precipitation equilibrium to the right. Warm solution helps to age the precipitate (see http://en.wikipedia.org/wiki/Precipitation_(chemistry)#Digestion).

[Archdevil155]

Yea I thought so too.. So CaCl₂ doesn't react with NH₃?? By the way if instead of the word "react" I should use a different word tell me.. By react I mean that NH₃ with CaCl₂ form together NH₄Cl and CaO. I found this reaction equation:
CaCl₂ + 2NH₃ + H₂O    2NH₄Cl + CaO.

[Archdevil155]

Oh and also, why does high pH shift the dissociation of HPO₄ towards PO₄^3-?

[Borek]

Oh and also, why does high pH shift the dissociation of HPO₄ towards PO₄^3-?

Because it is this way. I mean - it is obvious when you know a little bit about acid-base equilibria, but it is not something that can be easily explained in a few words.

Basically it is not different from neutralizing an acid (HPO₄^2-) with a base (ammonia).

[Borek]

CaCl₂ + 2NH₃ + H₂O    2NH₄Cl + CaO

Perhaps it may happen if you react solid with vapors, but for sure not in a solution.