Sciencemadness Discussion Board

Chromate and dichromate

TmNhRhMgBrSe - 22-8-2019 at 08:47

I have some questions about chromate and dichromate.
How to know when to stop adding hydroxide and test if I added too much? I think I added too much hydroxide so want to add some acids to neutralise the hydroxide. The chromate solution will be use to make insoluble chromate salts like calcium chromate.
I heated the chromate solution to boil off some water. When I heated the solution, it turned orange. Does the chromate dichromate equilibrium depend on temperature?

teodor - 22-8-2019 at 09:45

Did you see http://www.sciencemadness.org/talk/viewthread.php?tid=150474 ?

As far as I know there is always some sort of equilibrium. Usually I stop addition of hydroxide when no more changes in color. The dichromate forms the same way as a dibasic acid undergoes second ionisation. Well, there is a difference in pH range but it is still related to ionisation.

Also, as far as I know, there is not possible to make crystalls of dichromate with alkali earth cations, so there will be no such problem las dichromate contamination of CaCrO4. But it is my guess, I still like to see some studies about it.

So, I think based on the fact of the absence of calcium dichromate except in solution you can make CaCrO4 from K2Cr2O7 without hydroxide. Mix it with CaCl2 and distill the HCl over (well, it is just my speculation but I sure it will work).

Also I am sure that calcium chromate is quite soluble, like most of chromates. All chromates I saw except those of barium and possible silver make water coloured yellow.

There are some weird things about solubilities of chromates and I am making some small study about it. One of this thing is that solution of chromate in water can also produce dichromate ions.

Another problem is a strong absorption properties of less soluble chromates, you have to invent some tricks if you want to get a pure compound.



[Edited on 22-8-2019 by teodor]

Bedlasky - 22-8-2019 at 20:12

Calcium chromate is solube.

https://en.wikipedia.org/wiki/Calcium_chromate
Quote: Originally posted by teodor  

Also I am sure that calcium chromate is quite soluble, like most of chromates. All chromates I saw except those of barium and possible silver make water coloured yellow.


There some other insoluble chromates - Sr2+,Cu2+, Cd2+, Hg2+, Hg2(2+), Tl+, Pb2+, Bi3+ and maybe some others. CuCrO4 and Ag2CrO4 are dark red-brown, others are yellow or yellow-orange.

TmNhRhMgBrSe - 23-8-2019 at 12:08

Quote: Originally posted by teodor  
Did you see http://www.sciencemadness.org/talk/viewthread.php?tid=150474 ?

As far as I know there is always some sort of equilibrium. Usually I stop addition of hydroxide when no more changes in color. The dichromate forms the same way as a dibasic acid undergoes second ionisation. Well, there is a difference in pH range but it is still related to ionisation.

Also, as far as I know, there is not possible to make crystalls of dichromate with alkali earth cations, so there will be no such problem las dichromate contamination of CaCrO4. But it is my guess, I still like to see some studies about it.

So, I think based on the fact of the absence of calcium dichromate except in solution you can make CaCrO4 from K2Cr2O7 without hydroxide. Mix it with CaCl2 and distill the HCl over (well, it is just my speculation but I sure it will work).

Also I am sure that calcium chromate is quite soluble, like most of chromates. All chromates I saw except those of barium and possible silver make water coloured yellow.

There are some weird things about solubilities of chromates and I am making some small study about it. One of this thing is that solution of chromate in water can also produce dichromate ions.

Another problem is a strong absorption properties of less soluble chromates, you have to invent some tricks if you want to get a pure compound.



[Edited on 22-8-2019 by teodor]
But I didn't use any ammonia?

teodor - 23-8-2019 at 22:17

Oh, I see. You mean the experiment with heating in that thread. Ammonium buffer was added to keep constant pH to proof that dichromate is not formed due to pH change but because of heating. That's explain your results also, it has no any dependencies on ammonia properties.

[Edited on 24-8-2019 by teodor]