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Pyro
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Test for Ag3PO4
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]
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woelen
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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.
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bismuthate
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Mixing an excess of the material in copper acetate and the blue should dissapear and ther should be a percipitate.
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Pyro
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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.
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woelen
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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.
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bismuthate
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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]
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Pyro
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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.
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blogfast25
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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]
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Pyro
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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
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woelen
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@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.
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blogfast25
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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]
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bismuthate
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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?
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blogfast25
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| 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).
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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.
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bismuthate
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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]
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Pyro
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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
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Pyro
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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
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woelen
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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?
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bismuthate
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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]
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Pyro
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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?
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blogfast25
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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]
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woelen
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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.
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Pyro
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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?
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blogfast25
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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]
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Pyro
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I'd rather make sure it's pure Ag3PO4 and sell it like that, it's worth more sold like that
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woelen
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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.
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