Reducing Silver nitrate to metallic silver

[Lobby]

...

Volumetric Silver Titration
“Volhard’s method”
The analysis of silver bullion by titration is a very accurate for high silver bullion and fine silver. Samples which are soluble in nitric acid are perfect for this method. The converted silver chlorides from the aqua regia refining process also work well as the gold can be calculated gravimetrically after the parting process and the silver can be calculated via titration. This method eliminates cupellation and proofs required to correct for furnace losses. It also is preferred for fine silver assays providing the titrant is calibrated against fine silver when it is made up.
The basis of the reaction is as follows:
AgNO3 + KSCN = AgSCN + KNO3 (1)
3KSCN + Fe(NO3)3 = Fe(SCN)3 + 3KNO3 (2)
Reaction (2) does not occur until (1) is completed, therefore the addition of a small amount of a soluble ferric salt serves as an indicator by the formation of red ferric thiocyanate in the presence of an excess of the thiocyanate. When the titrant enters the solution it will produce a red streak which will re-dissolve on mixing until all of the silver has been converted and the solution turns red signifying the end point. Chlorides interfere by reaction with silver so all reagents, glassware, and water must be chloride free.
Potassium thiocyanate Prepare and standardize a solution. (M=N) 5 g KCNS Diluted to 1 L. Standardize against 0.5g fine silver.

Ferric alum indicator Dissolve 100g of crystals in 1 liter of distilled water, Add 2M Nitric Acid to clear the solution.
...

I'm in the process of setting up to do some Volhard titrations and have a question.

Rxn 2 shows the use of Ferric nitrate as the source of the iron indicator, but later in the procedure you mention ferric alum indicator: iron (III) ammonium sulfate.

I suppose all that's needed is a source of Fe, so it shouldn't matter what iron chemical is used. But before I purchased anything, I thought I'd ask.

Thanks, folks!

 

[IgorGreece]

Sodium, potassium, and ammonium formate will all reduce platinum and palladium, with no discrimination. The fact that this does not reduce the metal down is because a.) pH is too low, b.) temperature is too low. I would be certain to keep the temperature down when doing this reaction and maintain careful pH control. Most of my reductions with formate for PGM group metals are done at about 80*C at pH >6.

and HAuCl4, what we offer here, we offer. I for one am grateful to 4metals for the formate reduction. Beats hydroquinone, and it's cheaper.
Lou

Looks like PGMs drop @ pH 4.3-4.5 and T 70-100C
US Patent 6290749

 

[Palladium]

Just looking through the old stuff. Does anyone know what the ratio of sodium formate is for each gram of silver dropped from the nitrate solution? I'm not good when it comes to those type calculations.

 

[butcher]

silver nitrate
AgNO3 = 169.8g/mol
Ag = 107g/mol
(About 63% silver)

Sodium formate
HCOONa = 68g/mol

Formula:
Reduction of silver metal from a solution of silver nitrate using sodium formate as the reducing agent.
2AgNO3 (aq) + HCOONa (aq) --> 2Ag (s)+ CO2 (g)+ NaNO3 (aq)+ HNO3 (aq)

2AgNO3 = 2 x 169.8g/mol = 339.6g for (2mol AgNO3)
(OR 2(Ag) X 107g silver = 214 grams of silver in solution)

We want about 10% excess sodium formate
68g X 1.10 = 74.8 grams

So it looks like for 2 moles of silver nitrate (or 214grams of silver) we need about 75grams of sodium formate.
If my math is right.

I notice in the reaction above we are left with the byproduct of HNO3 and NaNO3 left in solution, this would be useful, (some base metal may be involved). you could add some sulfuric and distill off some useful nitric acid from this stuff.

 

[butcher]

I haven't gone back and re-read this topic (I will), this may have already been discussed.

Kodak patent for making high purity of silver nitrate.
Interesting using pH to separate base metals and most of the valuable metals from the silver nitrate salts before re-crystallizing fairly pure silver nitrate salts.

http://www.google.co.in/patents/US5000928
http://www.freepatentsonline.com/6290749.html
http://www.google.com.tr/patents/WO1987005591A1?cl=en

 

[Palladium]

Thanks butcher. I'm wanting to try 4 metals method on large volumes of silver nitrate. sure would be the cats meow for side stepping the cell when using it with sterling. Anyone have first hand do's and don't's or pointers from what metals posted? I'm on my phone today so please excuse my typing.

 

[Lou]

I think the formate method is really only useful if your waste disposal costs are very cheap; otherwise you have quite a bit of residual formate that needs to be electrochemically oxidized to CO2 before it can be disposed of cost-efficiently.

PPOP is higher with formate than the cell; actually, pretty much anything is higher on a price-per-ounce-processed than electrochemical refining. Gold/silver both are cheapest to refine if one can afford to do the cell maintenance and inventory significant units in the electrolyte.

Of course you only want to use 99% Au for Wohlwill and 85%+ Ag for the Thum/Mobeius.

 

[lazersteve]

I'm getting 99.99+% fine silver (on table top XRF) from silver chloride using the sugar and lye method.

With careful filtration and hot water rinsing prior to precipitation of the chloride using muriatic acid I can even get away with leaving a little copper nitrate in the silver chloride slurry prior to conversion. My muriatic, sugar, and lye costs on 50#'s of final pure silver is less than $20 for the batch and can be achieved in less than 1 week (preparing the silver chloride takes longer). The actual conversion step is nearly instantaneous and takes very little precision to achieve consistent results.

I've got a silver cell and formic acid, but I see no reason for me to go through all of the headaches, tie up silver, and create the wastes when I can get equal purity silver with the AgCl method of lye and sugar. The only exception may be if you are doing huge quantities of silver that has known trace gold or PGMs. My silver is treated to remove these impurities prior to conversion to the chloride.

Here's an example of my silver using this method:

The bar is not very pretty, but it is very pure. A pin sample shot 100% on table top XRF. The bar was melted using an induction furnace and cast into graphite. The source silver was sterling silver and was converted to Ag from AgCl via table sugar and lye.

Steve

 

[butcher]

That bar of silver makes that dollar bill look almost worthless. :lol:

 

[Palladium]

An easy method of producing 4 nines silver even if base metals are in solution is to reduce the solution with sodium formate as follows
1 Filter solution to remove all ppt's
2 Raise pH to 1.5 using liquid caustic
raise the temp to 130 degrees F
3 Reduce the silver by adding a solution made of saturated sodium
formate which has been reduced to a pH of 4 using formic acid (filter
any precipitate)
4 The solution will turn brown from the reduction of the silver and will
turn a bright blue (if the silver nitrate was from sterling) when the
reaction is completed.
5 Test for completeness of the reaction by sampling the solution on a
glass rod and dripping a drop into chlorinated tap water. White ppt
means more silver is present. Add more formate solution until test
yields clear drops into chlorinated tap water.

Needless to say no chlorides in the make up chemistry water
Works extremely well on sterling silver scrap digested in 50% Nitric/Distilled water. The starting pH is critical as is the pH of the formate reducing solution.

Is that ph adjustment in the hydroxide step really 1.5 ? Not questing you, but all the patents i've read calls for a higher ph value most seem to say about the 4-5 range. Seems with a lower ph you would still have a lot of contaminates to carry over in the next step. I'm fixing to do the 2nd half of my video and i don't want to screw it up because i missed that one point. Can anybody clarify that before i embark on this escapade? The rest i understand.

 

[g_axelsson]

An easy method of producing 4 nines silver even if base metals are in solution is to reduce the solution with sodium formate as follows
1 Filter solution to remove all ppt's
2 Raise pH to 1.5 using liquid caustic
raise the temp to 130 degrees F
3 Reduce the silver by adding a solution made of saturated sodium
formate which has been reduced to a pH of 4 using formic acid (filter
any precipitate)
...

Is that ph adjustment in the hydroxide step really 1.5 ? Not questing you, but all the patents i've read calls for a higher ph value most seem to say about the 4-5 range. Seems with a lower ph you would still have a lot of contaminates to carry over in the next step. I'm fixing to do the 2nd half of my video and i don't want to screw it up because i missed that one point. Can anybody clarify that before i embark on this escapade? The rest i understand.

As I see it (from a purely theoretical standpoint as I have never tried this myself) you filter off any solids first, then adjust pH and precipitate the silver. If the pH is adjusted too far some dissolved metals could precipitate and contaminate the silver in the next step. Obviously the pH adjustment isn't for precipitating contaminations as it is after the filtering step.

... theoretically speaking. 8)

Göran

 

[Palladium]

Hummmm.... No answer on the ph? Are those crickets i hear? :|

Moving right along then. That's what learning is about.

Part 2 is in post production now. 8)

 

[Tub Buster]

I have yet to find a volumetric analysis for the copper levels in the cell to aid the more frugal refiner. Any suggestions?

I, also, don't know of any good methods of titrating copper in this matrix.

That quote was from January, 2010. I'm reactivating this thread because I may have a workable method to titrate the copper. It is an iodometric assay, based on these reactions:

(1) 2 Cu(+2) + 4I(-1) => Cu2I2 + I2

(2) 2 Na2S2O3 + I2 => Na2S4O6 + 2NaI

Ionic copper plus ionic iodide combine, forming elemental iodine (I2), and then the I2 is titrated with sodium thiosulfate. When the free iodine is used up, a soluble starch indicator loses color and the assayer knows the endpoint has been reached.

The original method is for ore and mill product, so it contains procedures for eliminating other elements. However when testing a silver cell most of these interfering elements will not be a problem, so the method can be simplified.

The attached file is a scan and OCR from an out-of-print book. It contains notes on working with iodine that may be useful for anyone doing iodine leaching.

I hope this is of some value.

 

[Lou]

This is the copper assay after silver has been removed, no?


As for Ralph's question--formate works nicely on silver if there is no PGM present, in which case it contaminates the silver. A lower pH helps in preventing some of that, but Pd will still greatly contaminate silver produced in that process.


Lou

 

[nh6886]

Tub buster,

Thanks for sharing that. I am not doing this presently but I'm adding this to my collection of information for later.

Lou,

In your earlier comments on the formate method you mentioned waste disposal cost being high unless you electro chemically oxidize the formate waste, would you be willing to expand on that or point me in the right direction to find this information.

All the Best,
John

 

[Tub Buster]

This is the copper assay after silver has been removed, no?

The author writes,

"Lead, bismuth and silver react with potassium iodide to form insoluble iodides, but moderate amounts of these elements do not interfere. Interference from large amounts can be minimized by
adding a large excess of potassium iodide (6)."

Bromine could be used to remove silver, but it interferes with the assay,
and would have to be removed somehow. I'm attaching a spreadsheet of solubilities and K_sp's.

I don't understand solubility products. Someone educate me...

PS. Haloacetic acids (HAAs) are said to be carcinogenic. Just a caution--copper acetate is used in this assay.

 

[Research135]

This is the copper assay after silver has been removed, no?


As for Ralph's question--formate works nicely on silver if there is no PGM present, in which case it contaminates the silver. A lower pH helps in preventing some of that, but Pd will still greatly contaminate silver produced in that process.


Lou

Tried this and it worked:
1-Removed Pd quantitatively from nitrate solution using DMG.
2-Adjusted Ph to 1.5 in the nitrate solution, and 3.5 in the sodium formate solution added. Temperature at 40 degC. It took a while but all silver dropped. It is at least 9995.
3-Dropped the Pt using same formate solution with pH at 6.0 and temperature at near 100 degC.
4-All Pt and Pd losses less 1/10000. Silver losses less than 1/1000.


Thanks to 4metals and everyone else for posting these procedures. Fast, clean, precise.

 

[Mudville9]

I had some free time and played a bit with the sodium formate reduction method. Each lab run was slightly different from 4metals method as I wanted to determine the effect of the various parameters. Starting with a fairly strong, but dirty solution (Ag = 250 g/L, Cu 40 g/L, many minor elements), the resulting silver powder ranged from a low of 99.959% to a high of 99.993%.

Contaminants were measured by ICP-OES and silver by difference.
Lead did not present a problem. Starting solutions contained 3500 ppm and the powders contained 0-36 ppm, depending on the procedure used. Palladium can precipitate, but platinum stays in solution to a much greater degree. The majority of the copper in the powder is likely the result of my washing procedures. Tellurium, iron, selenium, bismuth, zinc and arsenic were the only other contaminants in the silver powder that were ever higher than 4 ppm, and these can be reduced to near zero through process modifications, if required.

I’m afraid I can’t discuss the actual procedures I used, but can confirm the validity of 4metals process, as written.

 

[FrugalRefiner]

Thanks for sharing what you can. 8)

Dave

 

[Lou]

Best to conduct two precipitations. First for Pd. Second for silver.