Aesthetic Iodine/Iodide Precipitation

[solar_plasma]

Since it was only a small amount of dark cemented material, I had heated it in a test tube, catched most of the iodine vapours in some potassium iodide solution in two erlenmeyer flasks. The decomposed, maybe glodcontaining, black powders I added to some incinerated filters and other small black gold recoverings. Removed basemetals,dissolved in AC.


But copper I iodide should be white. I had not seen any white insoluble material. It may have been dirty in the grey materialde scribed above,which came from "cementation" on copper. BUT there is no need for cementation, if we only go after the hydroxides.

In water copper I iodide is quite insoluable, so maybe it is more soluble in other conditions or it is left in the leached material which should not make any headache, as long as gold I iodide is soluble in the leach.

Next time I would treat the leach, when it is still brown. Then adding NaOH or KOH and seperating the formed hydroxides, noting their respective precipitation pH and their colour. I just need to know the range for gold III hydroxide precipitation, which first have to try on a gold standard solution.

Btw measuring ORP and pH under the leaching seems critical after what I read.

 

[solar_plasma]

I have tried iodine/iodide on samples of pure copper, pure zinc (both at pH over 7(made me wonder)) and a heavily plated sugar spoon, which contains 1,6g silver (plating mark "100" = 100g/2400cm² (pH by accident 1,5, adjusted with HCl, - there are different informations, but it should be higher)

copper: leach discolorized, white pricipitate of CuI, the excess copper is not tarnished
zinc: leach discolorized, white pricipitate of ZnI₂, the excess zinc is still greyish
silver: the leach was in excess, still brown after 2h, no pricipitate after 2 h, spoon got white tarnish, - when I rubbed some white tarnish off with glass wool, the wool got yellow

combined from english and german wiki (some of the solvents are very dangerous and nothing for the hobbyist):

CuI:
white
solubility: 80mg/L 18C H₂O
soluble in ammonia and potassium solutions

soluble (complex): in NH₄OH, in KI, in KCN
unsoluble in dill. acids or alcohol

ZnI₂:
white
solubility: 4500g/L 20°C H₂o
soluble in diethylether
soluble in alcohol

AgI
yellow
solubility: 30 microgramm/L H₂O
soluble in acid
very slightly sol.in NH4OH
soluble in KCN

AuI
Yellowish to greenish-yellow
soluble in liquid NH₃
soluble in alkali iodides
soluble in alkali cyanides

edited 25.10.2013, 21:27, source: Römpp Chemielexikon, 10th edition

I am very excited to see the spoon next monday.

 

[solar_plasma]

Next experiments will take care of ORP and pH and elaborate the effect on the selectivity of the leach.

They say ORP should be kept at 450-475mV
https://www.google.com/url?q=http:/...ds-cse&usg=AFQjCNHjQwaoUH2-N4W1cKMN0SLRfnlKWQ

or at 400-900mV
https://www.google.com/url?q=http:/...ds-cse&usg=AFQjCNEDBGV6Ns5XUj3Jf4JlY-VSnGE7aQ

the pH at 2
https://www.google.com/url?q=http:/...ds-cse&usg=AFQjCNF9sEuQelhZSr3oPiVfK0NrJNHPDA

pH at 4,5
https://www.google.com/url?q=http:/...ds-cse&usg=AFQjCNHBo6DNzjO5PAfqs9f3foil6uiZTA

there is also mentioned pH 1-6,5
https://www.google.com/url?q=http:/...ds-cse&usg=AFQjCNH8V6dstxuiv14sCtfdVqc479CPpg

the leaching will stop when rising over pH7
https://www.google.com/url?q=http:/...ds-cse&usg=AFQjCNE_YcHhZUELDzzW9R2vTZYIIYX_MQ
If there are differences in the pH&ORP setting regarding the type of leach (only iodine or iodine/iodide) and regarding the leached material, is another point which has to be clarified.

 

[solar_plasma]

Left:
The silver plated spoon in iodine/iodide/HCl/pH1,5 (at the beginning) developed a thick,unsoluble, yellow crust of silver iodide, which comes mostly easily down. The basemetals beneath do not look attacked.

Next challenge will be to find the best method to get the silver out of this compound without losing the iodine, which has to be reused, if this method should make sense.

I hope I can oxidize 2I^- to I₂⁰ and at the same time form a soluble compound of the Ag⁺. So, I have to find out and reread all documents, if HNO₃/H₂O₂ can be used as the oxidizer.

Middle:
Zinc iodide is soluble and leaves a colourless clear liquid. pH over 7,5 at the beginning

Right:
Copper iodide is also somewhat soluble, but here we got a white precipitate of copper iodide,too. pH over 7,5 at the beginning

 

[solar_plasma]

Next challenge will be to find the best method to get the silver out of this compound without losing the iodine, which has to be reused, if this method should make sense.

And here is the answer, - I knew I already had read about it 8) . I'm sorry it's in german, but the equations are international:
http://www.chf.de/eduthek/wuchu/1996_3.pdf

 

[jonn]

Hi Solar, I'd be interested to know if you ever had any success with this process.

 

[solar_plasma]

I have got 1,5g silver from this spoon the iodine way. But is has no advantages. From all methods I tried on silver plating the H2O cell had most advantages.

Regarding gold, I would also say, no advantages, but a lot of disadvantages, especially since you never really know, where your gold is and you never get all iodine back.