Determination of copper in magnesia cupel

[robertova89]

I have a problem to determine copper in a magnesia cupel, at the end of the cupellation the cupel blackens, it has a dark colour so I can’t determine if the cupel has copper in it. Could it be that the problem is the cupel or the flux that I'm using? Please help If anyone has ever had this problem before.

Thank you

 

[fireguy]

I would suggest that you analyze the material you are assaying, not the cupel. Typically copper in the ore or sample will stain the cupel green. The more copper present, the darker the color. Very high levels will look almost black. One workaround would be to drop a larger lead button and perform the cupellation with a larger cupel.
Example, if you typically drop a 30 gram button, add extra litharge and flour (reducer) and drop a 60 gram button and then use a #9 cupel:
http://www.lmine.com/mabor-magnesia-cupels-c-1_164_268/9-mabor-cupel-2-case-192-p-4854.html

It could also be that you have a different metal present that is causing this problem (tin, iron, etc.) An ICP scan for a variety of elements might identify the problem element.