Sciencemadness Discussion Board

cobalt carbonate impurities

jamit - 20-9-2022 at 21:23

Hello, I just did some synthesis of cobalt chloride using pottery grade and pyro grade cobalt carbonate as my cobalt source. I added hardware grade hydrochloric acid until fizzing stopped and then filter to remove any insoluble impurities. After crystallization, I got the following product, that is more reddish than pinkish crystals. I compared the color after drying and it looks nothing like the color of reagent grade cobalt chloride hexahydrate. I believe I have impurities in my cobalt source, but what could they be? Manganese, iron, copper, etc immpurities.

I added water to my original cobalt carbonate and it forms a sluug and it's not pink but more like dark pinkish, so obviously it's not pure cobalt carbonate. Any help would be appreciated if you can help me with what my initiate cobalt impurities might be and how to purity it?


woelen - 20-9-2022 at 23:31

Pottery grade chemicals can contain many impurities, but given your description of the effect on color, I expect that your main problem is the presence of iron. Iron gives a deep yellow color with HCl, and this certainly can shift the color of your product towards red/orange. Manganese(II) is (nearly) colorless, copper(II) is green/cyan under your conditions, so these will not produce a shift towards red/orange.

Checking for iron can be done with some thiocyanate salt. If you have that, then you can detect even tiny amounts of iron by adding a drop of your Co-solution to a solution of the thiocyanate salt. Any iron present will give a deep red color to the solution. Cobalt also gives a color (bright blue), but when iron is present, then that color certainly will be quite different, due to the presence of iron.

Is your HCl colorless? Sometimes, hardware store HCl is yellow. This most likely also is due to presence of iron.

Purification can be done by recrystallization. Dissolve all of your cobalt chloride in as little as possible of warm water and then let it cool down slowly. You could let it stand for a few days to get some water evaporated. You'll get new crystals, which are much purer. Do not allow all of the liquid to evaporate, once you have 80% or so of your original amount of solid, remove them from the liquid and press-dry them between filter paper (coffee filters are suitable), wrapped in paper tissue. Then let them dry in air. You could rinse them with a little acetone or diethyl ether, but do not rinse them with water. Cobalt chloride is too soluble for that.

[Edited on 21-9-22 by woelen]

teodor - 20-9-2022 at 23:40

The most probably impurity in cobalt salt could be nickel. There is a special procedures of separation these two elements.
The colours you observed is quite normal. It depends on a water contents and crystal forms and is not a sign of impurities on my experience.
When you start with a technical grade material you get technical grade product. What do you plan to do with the carbonate?
I never heard about usage of cobalt in pyro compositions. What is its effect on flame colour?

[Edited on 21-9-2022 by teodor]

jamit - 21-9-2022 at 05:53

Thank you woelen and teodor. I will check for iron contaminant using potassium thiocyanate and get back to you guys. thanks.