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Author Topic: Why did my silver nitrate solution go dark in sunlight?  (Read 15812 times)

LukyTom

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Recently for a biology project, we had to investigate a local ecosystem. In this investigation, i had acquired some water from the aquatic environment (pond) which contains various living things. I did a salt test on it, H2O + AgNO3 and at first it became cloudly like it was expected to. However, when i put it in the sun for a few minutes, obvious supply of Energy, it had turned brown. Is that normal? I presume it isnt, since i had a another controlled test tube with NaCl+AgNO3 + E with no change to brown colour.

I am wondering, for all brilliant chemists out there, what compound could be contained in the water to cause such a reaction.

The pond is probably polutated and has a green and blue algae bloom within it.

Thxs
« Last Edit: 27/04/2009 21:06:58 by chris »

Chemistry4me

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #1 on: 15/04/2009 08:28:59 »
Quote
Silver nitrate can burn and causes VERY permanent brown stains on almost everything it touches. Stains won't be evident until exposed to UV light.

http://en.wikipedia.org/wiki/Van_dyke_brown

That's why they develop their photos in a dark room. :)

Chemistry4me

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #2 on: 15/04/2009 08:33:56 »
Sodium chloride and silver nitrate you should have seen a white precipitate of silver chloride at the bottom. No brown colour as the silver ions have reacted with chloride ions.

Raghavendra

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #3 on: 15/04/2009 11:03:47 »
Tollen's reagent?

Chemistry4me

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #4 on: 15/04/2009 11:06:54 »
He's not looking for aldehydes.

Raghavendra

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #5 on: 15/04/2009 11:08:57 »
ok

lightarrow

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #6 on: 15/04/2009 18:06:24 »
Recently for a biology project, we had to investigate a local ecosystem. In this investigation, i had acquired some water from the aquatic environment (pond) which contains various living things. I did a salt test on it, H2O + AgNO3 and at first it became cloudly like it was expected to. However, when i put it in the sun for a few minutes, obvious supply of Energy, it had turned brown. Is that normal?
Have you ever heard about photographic films? They are made of silver salts (chloride, bromide, iodide, ecc); in general, all silver salts photodecompose = become brown under the light. Example:

AgCl --(light)--> Ag + Cl.

The metallic silver generated is responsible of the brown colour.

If you noticed, the silver nitrate you have used is stored in dark bottles/containers, to avoid light entering it.

http://en.wikipedia.org/wiki/Photographic_film
« Last Edit: 15/04/2009 18:19:57 by lightarrow »

LukyTom

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #7 on: 21/04/2009 10:03:33 »
So you are saying that the colour turning brown was normal then? Because that test was done to find the amount of salt in the solution, by the amount of precipate formed. It was extremely milky, and I assume from the information youve provided me, that the AgCl was the cause of the brown colour?

IF so, then how didnt the control soloution of pure NaCl react and turn brown?

Thxs again for all the help.

Chemistry4me

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #8 on: 21/04/2009 10:24:41 »
My guess is that the pond water contained a mixture of chloride as well as hydroxide ions, when you put it into the silver nitrate, a white precipitate of AgCl formed, which you observed as being cloudy. The hydroxide ions did not react as silver hydroxide is unstable, however when you put it into the sunlight, the reaction occured which created a brown colour, which is silver oxide. But that's only a guess.

Raghavendra

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #9 on: 21/04/2009 10:56:18 »
Tollens reagent give the best results for intermolecular weak attraction

Chemistry4me

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #10 on: 21/04/2009 11:56:30 »
Do you have any idea what you are saying because I sure don't. :)

LukyTom

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #11 on: 21/04/2009 12:29:04 »
What sort of common continamites could give the result of Hydroxide Ions?

Chemistry4me

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #12 on: 21/04/2009 12:36:28 »
Now that I think about it, my guess probably isn't very accurate. :(
So just to make sure, you also put the NaCl + AgNO3 solution into the sunlight right? For how long?

lightarrow

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #13 on: 21/04/2009 12:58:56 »
So you are saying that the colour turning brown was normal then? Because that test was done to find the amount of salt in the solution, by the amount of precipate formed. It was extremely milky, and I assume from the information youve provided me, that the AgCl was the cause of the brown colour?
Yes.

Quote
IF so, then how didnt the control soloution of pure NaCl react and turn brown?
You mean that a solution of NaCl didn't react with a solution of AgNO3? Or you are saying that it did react and made the milky AgCl but this didn't turn brown?
In this last case it could be that the first solution also contained something that catalyzed the photodecomposition of AgCl, or iodides/bromides and maybe AgI and AgBr photodecompose faster than AgCl.

Bored chemist

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #14 on: 21/04/2009 18:26:56 »
Silver chloride turns dark when exposed to sunlight, generally it goes purplish then grey then black.
This same reaction (or one like it) is used as the basis of photography.
However most modern photography relies on the light producing a so called latent image then developing this latent image to get a visible one.
Lots of different chemicals can be used as developers and I suspect that some of the "general stuff" present in pond water might well have done two things. First it made the photodecomposition of the silver chloride more rapid and second the "general stuff" in the pond water was oxidised to "brown stuff".

On an unrelated note, Raghavendra is now serving word salad.

lightarrow

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Re: Why did my silver nitrate solution go dark in sunlight?
« Reply #15 on: 21/04/2009 18:37:03 »
Silver chloride turns dark when exposed to sunlight, generally it goes purplish then grey then black.
This same reaction (or one like it) is used as the basis of photography.
However most modern photography relies on the light producing a so called latent image then developing this latent image to get a visible one.
Lots of different chemicals can be used as developers and I suspect that some of the "general stuff" present in pond water might well have done two things. First it made the photodecomposition of the silver chloride more rapid and second the "general stuff" in the pond water was oxidised to "brown stuff".
Do you mean oxidised by the Ag+? Then the brown stuff is also Ag (again :)).

LukyTom

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Why did my silver nitrate solution go dark in sunlight?
« Reply #16 on: 02/05/2009 03:33:42 »
So from reading all the posts, I assume that the milky solution turning brown was normal. And that the other control soltuion didnt due to containmenants.

If thats the case then thank you!

 

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