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Author: Subject: Sensitive test for gold
Bedlasky
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[*] posted on 18-9-2019 at 13:11
Sensitive test for gold


Hi.

I read in the book about testing for gold with TiCl3. So I decided to test it and it works really well!

1. I prepared TiCl3 solution by dissolving Ti powder in conc. HCl on hot water bath for half an hour.

2. I pipeted small amount of 0,01M AuCl3 solution in to the test tube and diluted it with some water.

3. I pipeted small amount of TiCl3 in to the test tube containing AuCl3. After one second solution turned in to intense dark blue. This colour was good apparent against light. In some angle of view it looked like a mix blue and gold colours.

Colour of solution is caused by gold adsorbed on hydrous TiO2. This colloid reflects light and give solution dark blue colour. Overall equation for this reaction is:

Au3+ + 3Ti3+ + 18H2O --> Au + 3TiO2 + 12H3O+

This test is very sensitive. It indicates gold since concentration 10-4mg Au/2ml.

I tried this test also with copper sulfate today and it works too (colour of solution was redish brown). I suppose this test works for all noble metals. I maybe in the future try it with Ag and Bi salts (and if I will be lucky maybe with Pt and Pd). But I haven't any of these salts in the present.

Here are some pictures of Au test:

[Edited on 18-9-2019 by Bedlasky]

Au na TiO2 (2).jpg - 2MBAu na TiO2 (1).jpg - 1.5MBAuCl3.jpg - 1.6MB
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[*] posted on 18-9-2019 at 13:49


Nice.
Is this a valid substitute for the stannous chloride test?
I have always found my TiCl3 solution goes cloudy as it hydrolyses to a TiO2 suspension. Shelf life is not long. Still, it is robably an easier prep than SnCl2.




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Tsjerk
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[*] posted on 18-9-2019 at 14:29


Nice test, but isn't it a disadvantage it also reacts to copper?

What is hard about preparing the stannous chloride? Just put some tin in HCl and wait or add peroxide. Remove your tin and weigh, now you also know the concentration of your solution. Just make sure you always have an excess of tin and you don't decrease the total volume.

[Edited on 18-9-2019 by Tsjerk]
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Bedlasky
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[*] posted on 18-9-2019 at 14:56


Quote: Originally posted by Tsjerk  
Nice test, but isn't it a disadvantage it also reacts to copper?


No, because colours are different. And if you have mix copper/gold, then gold have much stronger coloration than copper.

Quote: Originally posted by j_sum1  
Is this a valid substitute for the stannous chloride test?


Probably yes. I never read about SnCl2 test until your post.

[Edited on 18-9-2019 by Bedlasky]
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[*] posted on 18-9-2019 at 16:20


AFAIK, stannous chloride is a pretty standard test for gold and PGMs in solution with different colorations for different elements.



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[*] posted on 18-9-2019 at 23:29


Making SnCl2 by adding peroxide is not an option. If you add H2O2, then the tin is oxidized to its +4 oxidation state and you easily get hydrous SnO2 in that case.
Dissolving tin in HCl can be done, but the reaction is very slow and must be carried out with exclusion of air.




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