Darton´s test for gold ?

[orvi]

I found this interesting text on one website, issuing field test for gold in the ore. It just state this:

"A test known as Darton's is believed to be a valuable means of detecting minute quantities of gold in rocks, ore tailings, etc.

"Small parts are chipped from all the sides of a mass of rock, amounting in all to about | ounce. This is powdered in a steel mortar and well mixed. About half is placed in a capacious test tube, and then the tube is partly filled with a solution made by dissolving 20 gr. of iodine and 30 gr. of iodide of potassium, in about 10 ounces water. The mixture thus formed is shaken and warmed. After all particles have subsided, dip a piece of fine White filter paper in it; allow it to remain for a moment ; then let it drain, and dry it over the spirit lamp. It is next placed upon a piece of platinum foil held in a pincers, and heated to redness over the flame. The paper is speedily consumed; and after again heating to burn off all carbon, it is allowed to cool and is then examined. If at all purple, gold is present in the ore, and the relative amount may be approximately deduced. This method takes little time, and is trustworthy."
"

My understanding is that iodine/iodide should dissolve gold into the solution from crushed ore. Then, potassium tetraiodoaurate is in solution. This is soaked into the filter paper, which is then heated - I assume iodine will evaporate, leaving just non-volatile residue.
Then, the paper is essentially burned on platinum foil (platinum just for it´s corrosion resistance, or it has any other deeper meaning ?) and violet residue should indicate gold...
I can imagine that working, but at the same time, I have doubts about actual sensitivity.

Is this of any use in real life ? Do anyone has experience with it ?

Link to the page where I found this:
http://nevada-outback-gems.com/Prospecting_Basics/gold_field_test.htm

 

[Ultrax]

I recently wrote about this here: standard geological iodine test for gold

I use this test in the field. It works great, but I recommend replacing the water in solution with ethanol, it will increase the rate of dissolution of gold and silver in the ore by two orders (100 times).

If the perimeter of crystalline gold is clearly visible after evaporation, then you have more than 2 grams of gold per ton of ore.
If the golden border is thin and see poorly, then you have less than 2 grams per ton. Approximately.

P. S. You can even evaporate the solution on a hot piece of thin white ceramic (for example, from a cup or plate). The results are visible much better than on stainless steel.

 

[orvi]

Thanks, good to know it is used and it works. It make sense to me, but I had doubts about sensitivity.
Now, I can see that it is usable. Iodine is nice as it can complex silver and thus dissolve small particles of AuAg alloy. I will test this on one type of ore when I would have the sample homogenous, representative and ready.

 

[Ultrax]

For electrum, iodine-iodide (I₂-KI-C₂H₅OH) dissolution is generally an indispensable express method, since both gold and silver can be dissolved simultaneously in only two other ways: (a) cyanidation - which is obviously not suitable for field conditions, and (b) thiourea in an acidic environment with hydrogen peroxide - in general, this method is also good, but calcining thiourea is very unpleasant

 

[4metals]

I think this method could be a valuable tool for our endless stream of gold in my rocks members. But the method posted by Orvi and the experience posted by Ultrax have some differences.

Possibly if we clear them up we can come to a useable method to guide members to.

First of all Iodine and Potassium iodide are easy to acquire, so that is good. And using ethanol instead of water makes sense as well.

But some inconsistencies need clarification. The original post by Orvi calls for 10 ounces of water (280 ml) and a 1 ounce sample and Ultrax suggests 100 ml of ethanol and a 100 gram sample.

The Darton test calls for 20 grams iodine and 30 grams of potassium iodide and the method detailed by Ultrax does not specify the quantities of the chemicals. That needs to be cleared up.

The darton method suggests the leach be warmed and a white piece of filter paper be used to absorb some of the solution. The Ultrax method suggests evaporate all of the leach. (After a warm overnight water bath to leach) Suggestions of a platinum foil were mentioned in Darton but stainless was suggested by Ultrax as well as porcelain.

The Darton test uses a flame to heat the sample to ash the paper leaving a residue, very doable in the field, the ultrax method suggests drying at 300ºC in a porcelain dish? (not a field friendly method)

I appreciate the fact that Orvi posted the method he found, and I also appreciate the fact that Ultrax has hands on experience with this method, albeit slightly modified. What I am hoping to get out of this is a discussion that yields an easy to follow recipe to use either in a lab or in the field. And anyone with experience using this method should chime in with their opinion.

Hopefully we can get an understandable easy to follow recipe for the gold in my rocks folks!

 

[Ultrax]

The amount of reagents depends on the mass of solution you need, the sample of ore, and the desired accuracy of the result. Usually, 100 grams of solution per 100 grams of crushed ore is sufficient.

Therefore, I gave only the ratios for 100 grams of solution (this is one part of iodine (about 6 g) and four parts of potassium iodide - 24 g). Heating and dissolution for 8 hours, and then evaporation of the alcohol solution is necessary for an approximate quantitative(!) determination of gold and silver together. This result usually can be weighted.

A quick test can also be done in the field, without even overnight heating to 50C, 70% of ore's gold and silver dissolves within 2 hours at 20C.

I didn’t offer stainless foil, I offered a stainless spoon heated over a wood fire or just a piece of broken porcelain. To make a qualitative(!) conclusion about the presence of gold and silver, you can evaporate several dozen drops of the solution at one point on a spoon or a fragment of a porcelain cup.

There will be a dried puddle of salt residues, and along the perimeter of this dried puddle, there will be crystals of gold and silver (if there were any in your ore).

With the rapid evaporation of gold iodide, there is no violet residue - gold and silver just crystallize at the point of greatest concentration (along the perimeter of the salt puddle, which dries last in the flame).

If there is a lot of gold in the ore, you may even end up with a small gold mirror on the porcelain where the drops have evaporated. But such ores are great luck. And the general rule is that if the perimeter of the gold border is clearly visible - in ore there are 2 grams per ton and more, if the border is thin, then less than 2 grams per ton. If you see a mirror(!), it means your ore contains more than 10 grams per ton.

I would like to clarify separately - this is an express method for determining ore gold (!), that is, not visible to the naked eye.
For native gold, it gives correct qualitative but false quantitative results due to incomplete dissolution of the particles.