Silver Assaying

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JDmead

Active member
Joined
Mar 27, 2012
Messages
29
Location
Allentown, PA USA
I've gone through the process to learn how to perform assays on gold bullion, mostly from scrap jewelry. Now I'm being asked to learn how to perform assays on silver, predominantly sterling silver. Some jewelry, some break out and some flatware. I'm curious if the process is identical to gold assays, or if there is a significant difference.
 
Don't know how it is usually done, but when I was in school we did an analyze of the amount of silver in a solution via titration with NaCl while measuring the conductivity of the solution.
The conductivity drops while silver is transformed to silver chloride and when all silver is gone, additional NaCl increases the conductivity. Find the point of lowest conductivity and calculate the amount of silver in your sample.

I also think that there is a way via fire assaying to measure the amount of silver in a sample.
I found this link at the first hit when I did a search.
http://archive.org/details/textbookoffireas00bugbrich

Göran
 
The process is about the same as cupellation for gold fire assay.

The basic difference is that I would start with a larger sample size, up to 1 gram. I would also include a proof correction sample that closely ( or as closely as possible) matches the unknowns.

You will notice that the correction factor is now greater than 1.0, opposite of what is normal in assaying gold where the correction factor is usually less than 1. This just means you've lost some silver to the cupel and the air. This is normal for silver. Depending on your equipment, temperature, etc, the loss can vary between near zero and up to 2%.

The more you practice and the better you get, the smaller your loss will be. Play with the temperature, you do not need the furnace as hot as for gold fire assays.

All that being said, I never liked fire assays for silver. The potentiometric titration method or the gravimetric silver chloride method is much better in my opinion.

If you decide to stick with fire assay, part your silver beads, I have seen many sterling samples that contain appreciable amounts of gold.
 
I have fire assayed a lot of silver samples using proofs but I prefer the Volhard titration method. It's quite accurate and very simple and fast.
 
I like the potentiometric titration method over the silver chloride gravimetric or the Volhard although I prefer Volhard to control a silver cell. However most settlements are done with a fire assay. I don't know why that is but it likely is because anybody doing classic fire assays for gold has what they need for silver fire assays.

There is some tricky terminology used in silver assays which should be clarified. A corrected silver assay does not mean the same as a corrected gold assay. In a gold assay the proof corrections are used to adjust for silver retention in a silver assay corrected means that the slag (if any) and the cupel are assayed for retained silver and the result is added back into the result. I cannot think of anyone doing this for bullion analysis, possibly for ore fusions and mining samples but not bullion.

If there is any type of proof run with silver it is run as a known proof sample which is run along with the unknown in the same furnace lot. The difference between the result on the known proof sample and the result of the known proof sample are added to the unknowns with the assumption that they were both exposed to very similar conditions and likely experienced the same losses due to cupel absorption and volatilization. This is reported as a controlled silver fire assay. As Westerngs said, the losses can go as high as 2%.

It is important to know which type of fire assay you will settle on, it is entirely possible neither correction or control is applied and the number is reported as it comes from the balance. Being informed which technique is used can make a difference on large lots.

It is also important to check your silver for gold. This is done by parting the bead and weighing any insolubles which is usually gold. Gold is usually in old sterling silver at about 0.3 PPT (pars per thousand) but depending on what was in your sterling it can go higher. Most refiners will not pay on gold below 0.5 PPT and even the bigger platers get cut off around 0.3 PPT. But always check because it may be payable. Generally speaking a refiner will not assay for gold in your silver unless you request it, just another case of knowing your material before sending it out. A larger sample makes the measurement of small quantities of gold easier to quantify, if the sample is too small, even 0.5 PPT may be reported as trace.

Another thing that is a good practice to check is the silver content of the lead you are cupelling with. You should always use low silver foil if possible but an impurity in the lead is silver. I like the preformed lead boats that look like plumbing end caps made out of lead with a press fit cover, if you cupel them you will see a tiny little bead in the cupel. That weight should come off your sample weights. Every time I order a box of 800 of the boats, I have a few of them cupelled to figure the number for the correction.
 
Sorry for posting on a very old topic. I suddenly got interested in silver assaying to compare the silver chloride method of refining vs the electrolytic refining and I'm also interested in buying silver as I have rejected most of the offers I had before. If the assays would return a very small fraction then I guess an electrolytic refining is not worth the time.

I am not a chemist but if anyone is willing to teach for a dummy like me I would really appreciate it.

I have a burette, flasks, fume hood, acids and .001g scale.

From what I understand silver is assayed thru the following steps:
1) Weigh the sample, preferably 1g.
2) Dissolve in 50/50 nitric.
3) Titrate w/ what? 32 percent HCL? 50 percent NaOH? What are the ratios? What ml to what gram of pure silver?
4) What are the indicators?

If there is a book you could recommend I would love to read it. I tried google with no luck understanding what they are talking about. Heh

Keita.
 
You might be interested in the Volhard titration method. The best explanation I've ever seen is in GSP's book. He sells it for $35.00, and it is money well spent.

Dave
 
autumnwillow said:
Sorry for posting on a very old topic. I suddenly got interested in silver assaying to compare the silver chloride method of refining vs the electrolytic refining and I'm also interested in buying silver as I have rejected most of the offers I had before. If the assays would return a very small fraction then I guess an electrolytic refining is not worth the time.

I am not a chemist but if anyone is willing to teach for a dummy like me I would really appreciate it.

I have a burette, flasks, fume hood, acids and .001g scale.

From what I understand silver is assayed thru the following steps:
1) Weigh the sample, preferably 1g.
2) Dissolve in 50/50 nitric.
3) Titrate w/ what? 32 percent HCL? 50 percent NaOH? What are the ratios? What ml to what gram of pure silver?
4) What are the indicators?

If there is a book you could recommend I would love to read it. I tried google with no luck understanding what they are talking about. Heh

Keita.
I once ran about 8,000 titrations, of maybe 100 total different metals or types, in a 9 month period. I also once ran about 6000 silver assays in a 4 year period, using the Volhard method, which I assume you want to use. And a lot more in between. I'm just saying I've done a lot of titrations.

Your final accuracy is never better than the least accurate piece of equipment you're using. In this case, this is the scale and the buret. Inexperience, especially in recognizing the end-point, and improper technique (like not reading the buret level or using the meniscus wrong) can severely affect the accuracy.

Your accuracy with a .001g scale. If your scale reads 1 .000g, it is actually saying it's somewhere between .995g and 1.005 (1.004999... really). This is not accurate enough to distinguish between .999 and .9999. At best, you would only know for sure that it was at least 99.5 percent pure.With a .0001g scale, the best you could guarantee would be 99.95 percent pure. However, when titrating, the buret accuracy would very likely be less than that of the ,0001g scale.

What I'm saying is that it's not that easy to determine whether or not it is 99.99 percent pure. If you properly operate your cell, you could very possibly produce 4 nines silver. Probably the most important thing is rinse the crystal very, very well. Spongy, fluffy crystal won't rinse well. Strive for a short stubby hard crystal. Keep the silver content of the cell at least 60g/l and the copper below about 80 g/l. Operate in a fairly clean room not close to a gas melting furnace or incinerator. Don't reuse muslin filters.
 
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