danrox - 4-12-2008 at 11:03
Does anyone know a way to precipitate pure silver from a solution of silver nitrite?
Thanks
UnintentionalChaos - 4-12-2008 at 13:19
Hydrazine or ascorbic acid are probably your best bets. You could also plate it out of solution. A cheap but annoying procedure might be to treat with
stannous sulfate and the wash the precipitate with strong acid (a haloacid is probably your best bet) to dissolve out the hydrous SnO2 that probably
would co-precipitate.
Taoiseach - 4-12-2008 at 13:24
Boil with NaOH and cane sugar. That will precipate Ag as a fine powder. Wash with water and dry in warm air.
bfesser - 4-12-2008 at 14:30
I can verify that aqueous L-ascorbic acid works, at least on aqueous silver(I) acetate. Upon combining the solutions (L-ascorbic acid solution
freshly prepared), a cloudy light grey precip formed immediately. After centrifugation and removal of the supernatant liquid, the solid was pressed
onto a white sheet of paper with the end of a glass stir rod, forming small flakes with a metallic silver luster--obviously the metal, and not the
oxide.
You probably won't need a centrifuge to settle the silver out--I was just in a hurry. Just shake the container a bit to agitate it into forming small
clumps and wait for it to settle before decanting and filtering.
[edit]
I forgot to mention that the L-ascorbic acid was AR grade from Mallinkrodt. You probably don't want to use cheapo drug store stuff, as any chloride,
iodide, bromide, etc. will also precipitate out the silver as their respective salts. Not saying you need analytical reagent grade, but perhaps at
least lab reagent.
[Edited on 12/4/08 by bfesser]
Fleaker - 4-12-2008 at 14:52
Add a stoichiometric quantity of NaOH. Rinse the formed Ag2O with cold distilled water. Then heat to 400 C and you're left with pure silver.
Jor - 4-12-2008 at 15:02
I currently have about 23 grams of pure silver chloride sitting on my bench, and I'm still not sure what would be the best way converting it to
elemental silver.
Converting to oxide and heat is no option, I do not have a hot burner, and money doesn't allow me to buy it.
So I want to reduce it. Fleaker you told me heating with NaOH (not in excess, why?), to obtain Ag2O. Ok. And then? Would adding 'cane sugar' described
here many times be suitable? When I put cane sugar in google, I get the brown sugar. Is the white, normal stuff, sucrose, also suitable? I have
glucose as well...
Wouldnt it be fast to just first complex it to the ammine. That seems much faster, need to leave a little Ag2O though as a seed to formation of the
Ag-crystals, or I will be lest with a mirror.
I also have other common reducing agents, say sulphites, but you said they wouldn't work.
So what's your thoughts fleaker?
Fleaker - 4-12-2008 at 15:28
Jor, avoid using excess base because the silver will form a complex, Ag(OH)2- which is soluble. This is also true of adding too much chloride to
silver chloride which forms an analogous complex.They (oxide and chloride of Ag) have small Ksp values but when you add excess precipitation reagent
you run the risk of losing it in the filtrate!
Keep sulfites away from silver. Also, keep formaldehyde away from silver (a rule of mine, as I use HCl as a precipitant for silver nitrate when I
refine it) for the obvious reason of carcinogenic chloromethyl ether formation.
With respect to sucrose (refined white cane sugar), you will recall that saccharose is a disaccharide made from glucose and fructose, both of which
are reducing sugars and are ring cyclized. Also, look at the proper name alpha-D-glucopyranosyl-beta-D-fructofuranoside and the structure. Adding the
saccharose to an excess base solution will immediately cleave the bond between the glucose and fructose components, and the base will also open up the
ring taking the hemiacetal back into the aldehyde which will be oxidized by the Ag+ to the carboxylic acid which forms soluble carboxylate. So the
base converts the sucrose, a non reducing sugar, into its two reducing sugar components, glucose and fructose, either of which are happy to reduce the
sugar. This is my understanding of it, but I am also not an organic chemist.
I wonder if the Tollens test is effective on hemiacetals as they are? Anyone know off hand?
Edit: Jor, you aqueously react the silver chloride with hydroxide and filter off the formed Ag2O. In some cases, it's just easier to use sodium
carbonate and get Ag2CO3 from a silver nitrate solution and heat that to 250C, whereupon it becomes completely silver oxide, and heat that further to
320C where it gives up oxygen to yield silver metal. As you can imagine, it's a bother to have to use a standard NaOH solution to do it right! So if
this fellow wanted, he could use excess Na2CO3 and react that with his silver nitrite. Boil it down and then heat, then leach away the excess.
[Edited on 4-12-2008 by Fleaker]
Tacho - 5-12-2008 at 02:44
I have produced fine gray silver powder by mixing insoluble silver chloride with a solution of sodium hidroxide and adding table sugar wile swirling
and gently warming. The reduction was sudden and exotermic (careful with boiling). The resulting silver powder is very fine as talc, but not fine
enough to become black and untreatable.
I'm pretty sure silver nitrite will behave the same way as silver chloride.
Sorry but, if I took any notes on quantities, I lost them long time ago.
Jor - 5-12-2008 at 14:48
Very strange. To 23 grams of pure AgCl was added a solution of 6,2g NaOH in 100mL H2O. I heated. The white solid quickly turned dark. I heated for
about 10 minutes with stirring, and next portions of sugar were added. I do not know how much i added, I didn't weigh it out. But I believe it was a
good excess, as it was about 10-15mL of total volume of sugar crystals. I also added 3 more big spatulas of glucose. All this because when sugar
addition started the liquid went so dark I could'nt see the silver being formed. After about 10 more minutes of heating, I decanted the lquid. This
contained a small amount (no more than 100mg) of very fine dark powder. I disposed of this. I heated the grey power/lumps until it was completely dry.
I weighed out around 19,5g. I added a sample to a mini-testtube, and added a few drops of conc. Nitric acid. It reacts immediatly. I assumed I had
relatively pure silver metal.
I added this all to a flask and started adding a few mL nitric acid 65% at a time, with heating. A lot of NO2 is evolved. After about 10mL-15mLs of
HNO3, I stopped, as there wasn't a vigorous reaction anymore (although it still reacted). I had a yellow solution with quite a lot of yellowish
precitipate on the bottom. I don't know what went wrong. Any ideas?
The only explanation I could think of that AgCl precitipted, wich seems to be yellow because of the yellow solution above it. But I wonder where the
huge amounts of chloride come from. After heated with the sugar, and I stopped, I decanted the liquid above it, and quickly rinsed the grey
powder/lumps twice with water.
[Edited on 5-12-2008 by Jor]
Taoiseach - 6-12-2008 at 03:22
@Jor
I precipated silver powder from AgCl some time ago and run into similar problems. AgCl tends to form small crumbs which hardly react with the sugar.
They might even be coated with a layer of Ag2O which protectes them from reduction. So you need to boil & stir the AgCl+NaOH+sugar solution for
quite a long time. I took the solution from the burner from time to time, allowing the precipate to settle and then crushed the small crumbs with a
spoon. I repeated this until no white spots of AgCl would appear any longer. A lot of mechanical action is required to assure everything reacts
thoroughly.
btw make sure you wash the silver powder carefully before adding the HNO3! Wash at least 5 times with boiling water, then 2x with destilled water.
I also observed a slight yellow coloration of the AgNO3 solution, altough the silver powder was very pure and reagent grade HNO3 was used. I figure
this is due to NOx dissolved in the solution, as the coloration disappeared when solid AgNO3 was crystallized. When dry AgNO3 is heated carefully it
becomes yellow too, because of small amounts of NO2 being formed. When it is left to cool while being agitated (so that the NO2 can escape from the
crystals!) it becomes a slightly off-white color but the yellow tint completely goes away!
Thus your yellow precipate is most probably AgCl left over from incomplete reduction, the yellow color coming from small amounts of NOx in the
solution.
unionised - 6-12-2008 at 14:00
Just a thought, is it worth keeping the sodium nitrite by-product?