Sciencemadness Discussion Board - Test for Ag3PO4

Test for Ag3PO4

Pyro - 26-10-2013 at 11:42

Hello,

I tried to make AgNO3 a while ago by reacting 31,1000g of Ag with HNO3.
I then tried to dry it in what I thought was an airtight container with CaCl2. Then came the fire.

When it was finally dry (I still assumed the container was airtight) I started transferring it to a container.

With the residue in the beaker I intended to do an Ag experiment, but the stuff didn't dissolve, then it dawned on me that the H3PO4 vapor must have gotten into the dessicator.

The problem is that this is nice white powder that doesn't seem to be light sensitive.
The sources I found list it as being yellow.

Ideas to test it?

That is a piece of white copy paper it's on

[Edited on 26-10-2013 by Pyro]

woelen - 26-10-2013 at 11:48

Add a tiny amount of the powder to dilute HClO4. If you don't have HClO4, then try with very dilute H2SO4. If it dissolves, then this is an indication of being a phosphate. You can also try a test with molybdate in dilute H2SO4. If this on heating gives a yellow solution, then there is phosphate in the compound.

bismuthate - 26-10-2013 at 11:52

Mixing an excess of the material in copper acetate and the blue should dissapear and ther should be a percipitate.

Pyro - 26-10-2013 at 12:00

Thanks for the quick replies, I don't have CuAc so i'll use Woelens method. How dilute are we talking? 5% 10%? I still have that sodium molybdate you gave me, Ill use that.
Is it even possible that its Ag3PO4 since I found no source saying it's white?

I am looking to sell it, I have almost 120g. but need to be sure that it's pure.

woelen - 26-10-2013 at 12:05

If you want to assess its purity, then you have a much harder job. The test with molybdenum is a qualitative test. If there also is nitrate or anything else in the compound, then you still see the yellow color.

I would use a solution of 5% H2SO4 and only add a very small quantity of your supposed Ag3PO4.

So, if you want to assess its purity, then you also need to test for nitrate (I assume you dissolved silver in HNO3). Another thing is that you may not have Ag3PO4, but Ag2HPO4 or even AgH2PO4, or a mix of these.

bismuthate - 26-10-2013 at 12:05

Wiki says it's yellow and photosensitive. Shine a really bright light on it.
P.S. where did you get that much silver?

[Edited on 26-10-2013 by bismuthate]

Pyro - 26-10-2013 at 12:17

oops, I fucked up!
I estimated the amount based on molar mass. It's 40'ish grams.
And I bought it at the jewelry store.

Woelen: If it tests positive I will flush it with large amounts of dH2O to remove all soluble silver salts. It could only have come in contact with CaCl2 and H3PO4.

blogfast25 - 26-10-2013 at 12:48

Quote: Originally posted by Pyro

And I bought it at the jewelry store.

Jewellery silver is often quite contaminated.

I also don't really get where the H3PO4 pops up from all of a sudden? Phosphoric acid solutions have really low vapour pressures...

I also doubt very much that silver phosphate would dissolve in very dilute H2SO4. You really need quite strong HClO4 for that, otherwise it will take an age, if it happens at all...

[Edited on 26-10-2013 by blogfast25]

Pyro - 26-10-2013 at 13:01

I bought bullion at the store, 99.9% (or in the silver grading scale 999.0)

remember the fire? 900g of high purity RP burnt up, the P4O10 vapors then reacted with atmospheric moisture to form H3PO4

woelen - 26-10-2013 at 13:41

@blogfast25: Phosphoric acid only is a weak acid and in an acid like H2SO4 or HClO4 phosphates are protonated to form H3PO4. Quite a few insoluble metal phosphates dissolve in dilute mineral acids. The complication with silver phosphate is that HCl does not work, because AgCl is insoluble and H2SO4 also is troublesome, because Ag2SO4 is very sparingly soluble. Hence my suggestion to use dilute H2SO4 and only a small amount of Ag3PO4, which with water only gives some opalescence, which should disappear when the H2SO4 is added.

blogfast25 - 27-10-2013 at 05:27

Quote: Originally posted by woelen

@blogfast25: Phosphoric acid only is a weak acid and in an acid like H2SO4 or HClO4 phosphates are protonated to form H3PO4. Quite a few insoluble metal phosphates dissolve in dilute mineral acids. The complication with silver phosphate is that HCl does not work, because AgCl is insoluble and H2SO4 also is troublesome, because Ag2SO4 is very sparingly soluble. Hence my suggestion to use dilute H2SO4 and only a small amount of Ag3PO4, which with water only gives some opalescence, which should disappear when the H2SO4 is added.

I know all that but because Ag2SO4 is also poorly soluble I think it's a weak test.

It might in effect be easier to convert the Ag3PO4 to Ag2O by treating it with strong NaOH: at least there should be a colour change and the obtained product (after filtering off) should redissolve in HNO3.

@Pyro: no, I didn't know/remember your P fire, sorry...

[Edited on 27-10-2013 by blogfast25]

bismuthate - 27-10-2013 at 05:56

I just realized that you coud mix it with phosphoric acid and then pour it on copper to see if it reacts. If it does then you have AgNO3 (mostly).
P.S. were you and your dad OK?

blogfast25 - 27-10-2013 at 09:51

Quote: Originally posted by bismuthate

I just realized that you coud mix it with phosphoric acid and then pour it on copper to see if it reacts. If it does then you have AgNO3 (mostly).

Schemes like that would require extraordinary control of quantities used. Sure if it was pure AgNO3 and you could displace the nitrate with phosphoric acid (based on the phosphate's poor solubility) you'd, in theory, have some nitric acid in there, which would attack copper. In reality this is a very far fetched way of proving you have AgNO3. The simply fact that the material seems mostly insoluble in water already tells us it's probably mostly NOT AgNO3.

bismuthate - 27-10-2013 at 09:54

True.
Pyro did you happen to expose the silver to heat? I find it hard to believe that there is still enogh H3PO4 in the air to react with it.

[Edited on 27-10-2013 by bismuthate]

Pyro - 27-10-2013 at 10:08

My AgX has been on the paper all day, it hasn't darkened at all.
I haven't been able to access my lab yet to test it, But what else can it be? It's white, not yellow. is it possible that it's just extremely pure Ag3PO4?

@blogfast:I don't fancy working with molten NaOH, seems too risky for somebody like me

@bismuthate:we were fine, after a while... all the details are in a thread somewhere

Pyro - 27-10-2013 at 10:11

I made AgNO3, that was in a ''dessicator'' which mustn't have been airtight. then came the fire, I assume th H3PO4 vapor reacted with the AgNO3, this is also plausible because the solid smells strongly of HNO3

woelen - 27-10-2013 at 10:16

Well, if it smells of HNO3, then it certainly is not a useful compound, which you can sell.

I would try to recover all the silver from this and retry the oxidation with HNO3. Maybe addition of a large excess amount of NaOH and a suitable reductor (ascorbic acid or hydroquinone) reduces all silver to the metal. I hope that it is not bound so strongly to the phosphate that the reductor cannot reduce it.
The reduced silver can be heated to separate it from crap, the metal melts and any impurities float on top of the metal as scum. Heating the metal to melting is hard though. Do you have the equipment to do that?

bismuthate - 27-10-2013 at 10:20

Ahh. I see now.
Also I don't believe that the NaOH would have to be molten. Using an excess of NaOH solution on silver compounds will leave you with insoluble silver oxide.
Edit whoops woelen posted while I was typing sorry bout that.

[Edited on 27-10-2013 by bismuthate]

Pyro - 27-10-2013 at 10:21

Woelen, what do you mean? I havn't washed it so it is probably just free HNO3 in the solid.
the basic mercury nitrate you gave me also smellt strongly of HNO3/NO2 just like this AgX

Well, not really. It would also not really be worth it for 40EUR worth of silver. can really it not be Ag3PO4 that is contaminated with HNO3/NO2?

blogfast25 - 27-10-2013 at 12:48

Pyro:

Of course the NaOH wouldn't have to be molten. Even a 20 - 30 % solution would do it.

The method relies on converting a poorly water soluble compound (like Ag3PO4) into a completely insoluble one like Ag2O. That is the result of the equilibria that exist in the solution:

Basically Ag3PO4(s) + 3 NaOH(aq) === > 3 AgOH(s) + Na3PO4(aq)

It works because the AgOH (which then decomposes to Ag2O) is much more insoluble than the Ag3PO4. No heat is required although gentle heat will speed things up a bit.

This 'trick' works in countless similar situations.

[Edited on 27-10-2013 by blogfast25]

woelen - 27-10-2013 at 13:06

If my previous post was unclear than I want to apologize for that. As blogfast25 writes, a solution of NaOH will do the job.

I am not sure though whether Ag3PO4 is less soluble than Ag2O. If you have a strong reductor added to the mix, then you get metallic Ag and that certainly separates from the rest. A well-known method of retrieving silver from photography waste solutions is adding a solution of NaOH with hydroquinone (which photographers have/had anyway for developer-purposes) to the waste solutions. This precipitates metallic silver. Ascorbic acid is a more modern environmentally more benign reductor.

Pyro - 27-10-2013 at 13:21

Woelen: What a coincidence, I just bought 100g hydroquinone.

but I just as well figure to make sure it's pure Ag3PO4 and sell it for a profit. could the smell not just be free HNO3?

blogfast25 - 27-10-2013 at 14:19

Quote: Originally posted by woelen

Ascorbic acid is a more modern environmentally more benign reductor.

Glucose + ammonia will reduce Ag2O, in the classic silver mirror chemistry:

http://www.rsc.org/Education/EiC/issues/2007Jan/ExhibitionCh...

Don't waste any hydroquinone on that...

[Edited on 27-10-2013 by blogfast25]

Pyro - 27-10-2013 at 14:28

I'd rather make sure it's pure Ag3PO4 and sell it like that, it's worth more sold like that

woelen - 28-10-2013 at 00:07

If it smells of HNO3, then it definitely is not pure Ag3PO4 and then you cannot sell it like that.

You mentioned the mercury nitrate you have, which also smells of HNO3. This also means that it has (some) impurity in it, but having a nitrate of a very easily hydrolysed metal ion smell of HNO3 is acceptable (just like acetates smelling of acetic acid), but having a phosphate smell of HNO3 is not acceptable, at least not if it is sold to you as pure phosphate.

blogfast25 - 28-10-2013 at 04:23

And I'm fairly sure silver nitrate fetches a better price than silver phosphate, silver for silver...

[Edited on 28-10-2013 by blogfast25]

Pyro - 28-10-2013 at 05:23

all I want is about 50eur for the lot, that will cover my initial silver cost.

Woelen, If it is Ag3PO4 that has been made with the H3PO4 mist, it will contain HNO3. If I were to sell it I would purify it first.
3AgNO3+H3PO4->3HNO3+Ag3PO4
I will wash it and let you know how it smells then

woelen - 28-10-2013 at 07:14

Reality unfortunately is less clean than your reaction equation. I'm quite sure you will have stuff like Ag2HPO4, AgH2PO4 and coprecipitated AgNO3 in the mix as well. I do not know the solubilities of these acidic phosphates.

The only usefulness of your lot is its silver content. For the rest it is an ill-defined mix and if someone purchased it, then that person needs to do a lot of processing to release the silver from it. I personally would prefer either pure silver, or a soluble silver salt like AgNO3.

[Edited on 28-10-13 by woelen]

Pyro - 28-10-2013 at 07:24

Ah, damn it. I guess the only way is to make it into silver again.
Oh well, looks like I have to do some actual work

thanks for the help

blogfast25 - 28-10-2013 at 10:00

Quote: Originally posted by Pyro

Ah, damn it. I guess the only way is to make it into silver again.
Oh well, looks like I have to do some actual work

thanks for the help

Woelen's 100 % right Pyro: you can't expect to sell an 'accidental' product of uncertain composition; it's not fair on the buyer either.

Have some fun making a silver mirror, then redissolve as AgNO3. Making good quality AgNO3 I've been told is not as easy as it looks (even without P fires to boot!)

Pyro - 28-10-2013 at 15:11

It's easy with a good dessicator

AJKOER - 2-12-2013 at 09:17

I suspect that the acid fumes from the fire reacted with CaCl2 releasing HCl. This reacted with your AgNO3 forming AgCl and releasing HNO3 (which you still smell).

However, AgCl is very photo sensitive (see Wikipedia http://en.wikipedia.org/wiki/AgCl ) as is the phosphate (http://en.wikipedia.org/wiki/Silver_phosphate ). So, I do not believe you still have AgCl or any Ag3PO4 or other silver salt. In a very hot fire, it is gone (Silver vaporized, gases like Cl2 formed).

What I suspect you now have is CaO with traces of Ca(NO3)2 (see http://en.wikipedia.org/wiki/Ca(NO3)2 ) containing HNO3 fumes as the latter calcium salt is very hygroscopic.

I hope I am wrong.

Pyro - 2-12-2013 at 10:50

The Ag wasn't exposed directly to the fire. I boiled it with NaOH and hydroquinone for a good hour last week, all I got was a black sludge that didn't react with HNO3. so I disposed of it.
but thanks for the effort!