titration of ammonia give ammonium phosphate?

[qwerty]

Hi, my questions are,

 

if you want to titrate ammonia ion NH4+ with H3PO4,

 

firstly, the h3po4 is triptopic.. meaning it yields 3 hydrogen ions per molecule. IS THIS TRUE?

 

secondly,

the original equation is NH3 + H2O --> NH4+ + OH-

 

Now, titrating this base with H3PO4,

 

i believe the equation of titration would be:

NH4+(aq) + 3OH-(aq) + H3PO4(aq) --> 3H2O(l) + (NH4)3PO4(aq)

 

is (NH4)3PO4 a likely product? or would it be (nh4)2HPO4?

[budullewraagh]

aqueous ammonia can be titrated with H3PO4 to yield ammonium phosphate. depending on concentrations and overall solution pH (possibly changed by presence of other acids/bases) the monobasic, dibasic and tribasic salt can be obtained

[qwerty]

is that equation i gave above,

 

NH4+(aq) + 3OH-(aq) + H3PO4(aq) --> 3H2O(l) + (NH4)3PO4(aq)

 

correct?

[Borek]

Only if you let the solution dry. As long as there is some water, you will have some mixture of NH4+ and HPO4(2-). Too weak base and too weak acid for anything else.

 

For example solution which is 0.01M phosphoric acid and 0.03M ammonia (so should be perfect salt) will have pH of 8.96. Only 66.1% of ammonia will be in NH4+form. As for phosphoric acid - 1.7% H2PO4- and 98.3% HPO4(2-) (H3PO4 and PO4(3-) also present, although below 0.1% levels).

 

BATE pH calculator rulez

 

Best,

Borek

--

Chemical calculators at www.chembuddy.com

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[Tetrahedrite]

is that equation i gave above' date='

 

NH4+(aq) + 3OH-(aq) + H3PO4(aq) --> 3H2O(l) + (NH4)3PO4(aq)

 

correct?[/quote']

I think a more appropriate (simple) equation would be:

 

3NH3 + H3PO4 --> 3NH4+ + PO4(3-)

[budullewraagh]

actually, the acid dissociation constant of HPO4-2 is 4.2 x 10^-13 whereas the acid dissociation of NH4+ is 5.7 x 10^-10. according to these figures, formation of tribasic ammonium phosphate cant be favored as seen above, because HPO4-2 is a stronger base than NH4+.

[Tetrahedrite]

actually, the acid dissociation constant of HPO4-2 is 4.2 x 10^-13 whereas the acid dissociation of NH4+ is 5.7 x 10^-10. according to these figures, formation of tribasic ammonium phosphate cant be favored as seen above, because HPO4-2 is a stronger base than NH4+.

This is only true if the concentration of both the ammonia and the phosphoric acid are the same. At any rate the simple equation above is more correct than the equation given by qwerty which contains two acids on the same side of the equation

[budullewraagh]

variable concentration will only affect kinetics.

 

this:

3NH3 + H3PO4 --> 3NH4+ + PO4(3-)

is not completely correct because there must be an excess of NH3 to form the tribasic salt

[qwerty]

h2so4 is an acid, what are the other acids on the same side of the equatoin?

 

OH- is a base

 

and isnt NH4 is a base because it breaks up the water molecule to give the OH- molecule? ehhh

[budullewraagh]

NH4+ cant be a base. where are its electrons to donate? certainly not there. the pKa is 9ish, so it has acidic character. this is why ammonium salts are generally slightly acidic.