[shorae_chen]

when i add drop-wise 2mL of very dilute NH3 solution to 1mL of AgNO3 solution. i got a dirty brown precipitate when i add one drop of dilute NH3 solution. but when i add the 2nd drop, the ppt dissolved. what is the compound formed ? which is the acid and base involved in this reaction?? thanks alot

[Alberto_Kravina]

when i add drop-wise 2mL of very dilute NH3 solution to 1mL of AgNO3 solution. i got a dirty brown precipitate when i add one drop of dilute NH3 solution. but when i add the 2nd drop, the ppt dissolved. what is the compound formed ? which is the acid and base involved in this reaction?? thanks alot

Since you did the reaction by yourself and as I see you made very good observations and described them perfectly I think that it is VERY easy to write the equation starting with the reagents.

[Albert]

which is the acid and base involved in this reaction?? thanks alot

I would not think about this reaction in terms of acids and bases.

[hmx9123]

Here's a hint: how else can you write NH₃ when it's in aqueous solution?

[woelen]

This reaction  is less trivial than one would expect.

At first, you get a precipitate. The NH3 forms NH4(+) and OH(-). The OH(-) reacts with the Ag(+).

When more NH3 is added, then a complex is formed. Ag(+) ions form a fairly stable complex with NH3, this complex is [Ag(NH3)2](+). So, any precipitate formed redissolves again, but now the Ag(+) is locked up in the complex.

When the solution is allowed to stand for a long time, and when the concentration is sufficiently high (or the pH is suffciently high), then the complex [Ag(NH3)2](+) decomposes, giving highly complicated macroscopic structures, with Ag(+), coordinated to the nitrogens of the ammonia, but with hydrogen atoms replaced by Ag. A complicated non-stoichiometirc compound is formed, which sometimes is called "silver nitride", but that name by no means precisely describes the nature of this compound. Anyways, the compound formed in this solution is VERY explosive and for this reason one should not store solutions with the [Ag(NH3)2](+) ion in it.

[Borek]

At first, you get a precipitate. The NH3 forms NH4(+) and OH(-). The OH(-) reacts with the Ag(+).

AFAIR to add to the system complexicity this AgOH precipitate is not stable and decomposes, so when you add only some NH₃ and let it stand for some time, adding excess NH₃ later may not dissolve the initial precipitate completely.

Someone check me on this.

[hmx9123]

Woelen, aren't you thinking about fulminating silver (i.e., like what you get from Tollen's Test)? 

That is:

"A black crystalline substance, Ag₂O.(NH₃)₂. When dry it explodes violently on the slightest percussion."

Not to be confused with:

"Silver fulminate, a white crystalline substance, Ag₂C₂N₂O₂; also called fulminate of silver. When dry it is violently explosive."

Both compounds are extremely unstable--the former can explode simply upon standing and is extremely dangerous.  The latter is also extremely dangerous although it is less likely to explode upon standing.

[woelen]

I indeed was thinking of fulminating silver. That formula, which you give, however, certainly is not the precise composition of this stuff. In fact, the precise composition of this compound cannot be easily written down in a single formula, it is a complex mess of silver(I) ions, oxide, hydroxide, ammonia, amide, nitride, and water groups.

I have also seen references like Ag3N.xH2O and Ag3N.yAgNH2.zH2O. One thing we at least agree upon, and that is the extremely explosive nature of this stuff  .

[hmx9123]

As far as I know, the exact formula is undetermined due to the extreme reactivity (i.e., explosive character) of the material.  I didn't write the stuff in quotes, I just did a quick google search because I was too lazy to type anything.  The best idea of what it's comprised of would include a synthesis for the material, but I don't really think that's responsible to post in this discussion.

[AWK]

see:
Jenkins, Ian D.
Tollens's test, fulminating silver, and silver fulminate
JCE, 1987, 164
   

[hmx9123]

That's a nice little article.  Seems that it's Ag₃N and Ag[sub2[/sub]NH.  Davis states in his 1943 text:

"Fulminating gold, silver, and platinum (Latin, fulmen, lightning flash, thunderbolt) are formed by [ ... ].  They are perhaps nitrides or hydrated nitrides, or perhaps contain hydrogen as well as nitrogen and water of composition, but they contain no carbon and must not be confused with the fulminates which are salts of fulminic acid, HONC.  They are dangerously sensitive, and are not suited to practical use."