Very precise gold content analysis ?

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Noxx

Noxx

Well-known member
Joined
Feb 24, 2007
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Location
Quebec, Canada
Hello guys,
I'm curious to know how rafineries do to evaluate the exact gold purity of a sample. Let say I know my gold is 99,5%+ pure but I want to know exact purity... How will I do ? I have access to analytical equipment here at my school so I could do the test myself.

Thanks in advance.
 
In general, except for maybe the huge refineries, they don't attempt to find out the exact purity. Most go by the appearance of the bar, just like we do.

Fire assay is the most accurate method on the planet, but is still only accurate to .02%. Therefore, when you're between 999 and 9999, there is no direct method available to give you truly accurate numbers. This is why I tell people not to waste their money on assays of high purity gold.

The most accurate method is to run a spectro for all of the impurities and then subtract the total percentages of these metals from 100. An indirect method.

You could get a ballpark number with fire assay. It can accurately distinguish between 995 and 996, but not between 9995 and 9996.
 
If your school is large and well funded, it will have and ICP system in place. A properly done ICP by a good chemist will tell you to more than 5 decimal places. It's the method of choice for trace metals analysis.


Louis


ICP is this: http://en.wikipedia.org/wiki/ICP-MS
 
While it's far from scientific, I found you could fairly well judge the quality of gold in how it melted and performed once melted. When it will melt without oxides forming, stays brilliantly bright when melted and allowed to cool without a flux covering, and forms broad crystals on the surface, plus pulls a generous pipe upon final solidification, the gold is of relatively high quality---something that would require no apologies. The slightest contamination eliminates no less than one of those features, and in some cases, all of them.

It's not scientific, as I said, but it allowed me to gauge my quality such that I could produce what you see below.

Harold
 

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Yes exactly Scott,
It's called gold shot and it's made from pouring liquid gold, from a certain height, to a metal tank full of cold water.
 
Only 0,1%?

I have trouble believing the website, despite the source just on the basis of personal experience with it, and that many ultra high purity metals compounds come with a certificate of analysis by ICP technologies and those are routinely 99,999%+ (gold included). I doubt that gold would be the exception to a technology that can give ppt analysis of certain metals.


Good reading for you:

http://www.ivstandards.com/tech/reliability/part09.asp


Found the problem:

"A more detailed technical review of gold assaying techniques has been published in the WGC technical journal, Gold Technology, issue no. 22, July 1997. A more recent overview is to be found in Gold Technology, issue no. 32, Summer 2001." Off GSP's link. Antiquated information. At best it's 7 years old--most likely nigh on 15. Much has improved since then.
 
http://www.ivstandards.com/tech/icp-ops/
Noxx, this should tell you more than you want to know:

ICP Operations
A Guide for New ICP Users

By Paul Gaines, Ph.D.
Edited and prepared for the Web by Brian Brolin


This guide is intended for anyone operating and preparing samples and standards for measurement using ICP (ICP hereafter refers to either ICP-MS or ICP-OES). Our last guide, Reliable Measurements: A Guide for Trace Analysts, focused upon the task of achieving reliable trace measurements by ICP. This series will not focus on any single topic but rather upon a multitude of day-to-day tasks required by all ICP operators. The topics will be fundamental in nature and are intended as an aid for the analyst who is completely new or somewhat new to the technique of ICP.
 
This looks like a good place to tie this in for a study link also. :wink:

XRF :arrow: http://www.learnxrf.com/Principles_of_XRF.htm
 
Scott2357 said:
Just how much gold is that? Why does it look like cereal? Is that what they call shot?
Click to expand...
There was roughly 20 ounces in the bowl, as I recall.

As Noxx suggested, the shot is prepared by pouring molten gold into water, but it must come from a metered hole in the bottom of the device. Pouring from the lip of a vessel will yield what I commonly refer to as corn flakes, flat, irregular and thin bits of gold, not round pieces like you see. I can't explain why, but I know it works.

I learned to make shot from a refiner in Sparks, Nevada. it's likely the only bit of information that was ever offered to me by a refiner. They're very tight with any processes they use.

Harold
 
Harold,

I may know why the difference in shot vs. corn flakes. The molten gold even though a heavy metal is also a liquid and should behave like most other liquids. I've done some work with high speed photography and water. I'm not saying it's exactly the same but probably behaves similarly. In the case of water falling from a hole, all edges of the drop break away from the container at the same time. Also, the edge area is on the back and small since gravity has already stretched the drop. Only the small mass that breaks away last will be pulled in as it falls creating minimal oscillation and turbulence giving you a relatively stable round drop as it falls. The gold drop contacts the water much like an Olympic diver, breaking the surface with minimal disruption and little back splash.

On the other hand, from the vessel lip the drop rolls as it's released because the lip is holding it from underneath on one side and gravity pulling it over the edge causes it to begin to roll. Worse the last release mass gives it a little kick in same direction as the roll in progress. So you wind up with an end over end, oscillating, and turbulent gold drop as it enters the water. Back to the diving analogy, it's more like a kid doing a cannon ball off the edge of the pool. The drop spreads out quickly as it encounters high surface resistance from the water. Of course, this assuming no wind and a short fall distance so wind resistance is not a factor. HTH
 
Thanks, Scott. What you said makes sense, and is bolstered by the fact that on rare occasions, I'd end up with a long piece of gold, the same size as the port in my melting dish. I simply melted the gold in one dish, then poured it to a second one with a 1/8" hole drilled in the center. That dish had a torch playing on it at all times while I was melting the pure stuf. That insured that the gold would be molten until it was totally discharged. If the gold had any turbulence, I'd think there would be no way it could form the string that I described.

Way cool! After all these years, a plausible explanation.

Harold
 
Lou,

As I understand it, to do ICP, the gold must be put into solution. It, therefore, must be weighed and then diluted. I just can't visualize doing all these steps without introducing errors. And, as you know, these errors are additive. Can you tell me, step by step, how this could be done, in the case of gold bullion, to give a 5 decimal place accuracy. I'm not toying with you. I really would like to know. I guess we could start with a Cahn microbalance.

Chris
 
I'm no expert but I don't think you need to weight your gold. You dissolve it in Aqua Regia then you run it through your ICP and depending on the flame wavelengths, the ICP is able to tell you what metals are present quantitatively.

Or maybe I'm wrong haha...

If you dissolve your gold in very high grade reagents, it shouldn't add any contaminants.
 
I'll keep it short. Toying with me or not here is how its done in a nutshell:

1.) Mass with microgram balance of your choice, a Sartorius SE2 is a prime example of a scale good to 0.0000001 g. Take triplicate mass, avg. and deviation.
2.) dissolution in a PTFE vessel using spectral grade acids (as in trace metals, usually $100/L or more from Baker). The dissolution vessel has been leached repeatedly with acid and alkali and rinsed repeatedly with 18,2 MΩ water.
3.) Take it to a calibrated ICP-OES, remove a sample with a calibrated syringe (should also be massed) that has also been pre-leached with acid/alkali and rinsed. The syringe must have no metal components.

Much of it comes down to sample size.


And since these errors are additive and tend to propagate any mistake can quickly throw off the results.
You should also remember this for the fire assay technique as well--it has the same limitations with respect to error causation, if not more. Thinking of a few off the top of my head I would say:

  • vapour pressure of values (even solid Pt has a vapour pressure that must be accounted for in analytical work)

    mechanical losses during transfer from cupel to balance

    impurities in inquarts, litharge, cupels etc.

    impurities in reagents during any dissolution

    inaccuracies in mass measurement

    incomplete cupellation

    deposition due to capillary 'creep' (failed agglomeration in button)

    etc.

As you can see, there are many possible sources of error in fire assay and also in ICP.

I have nothing against fire assay--for what it costs and what it provides it is a good technique, but I would be highly hesitant to say it is better than ICP technologies or vice versa.

[/i]
 
ICP-MS (inductively coupled plasma- mass spectrometer)
sounds like the basic principle of a spectrographic analysis, but with a shortened acronym.
lets see if i can attempt to answer your question. ( i had to refer to my laucks lab guide)
a spectrographic analysis volatizes a specimen with a 23,000 volt arc source, enabling the scientist to determine minerals present through a light intensity of the emission spectra.
( certain minerals and metals give off various light spectrums when volatized.)
a sample of material is subjected to the intense heat of an electric arc and becomes a vapor. in the process it emits light which is divided by the spectrograph into a series of lines. a mechanism in the spectrograph takes a picture of these lines, and when the image is projected from the negative on the screen, the expert chemist can identify and measure the component parts of the specimen by the arrangement of the spectral lines.

the spectropgraph can detect and accurately measure traces of an element which constitute less than .001% of the sample.
hope this helps more than confuses.
 
Ever took a prism and held it in the light. If i am not mistaken that is how we first discovered the sun is made of hydrogen converted into helium .
It's all to do with electron orbits. That's your simplest spectrometer. Again a great man Niels Bohr.
 

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