Sciencemadness Discussion Board

Making Chromium salts fron Chromates

RU_KLO - 22-12-2022 at 10:16

Hi,

I want to make different Chromium salts (Chlorides, Carbonates, Sulphates), starting from Potassium Chromate/Dichromate.

Read some post here (Der Alte long post) and wiki.

Currenly having a yellow Potassium Chromate solution + H2O2 (not removed yet).(from steel whool/KOH/H2O2)

This is what I found:

1) to make dichromate: remove H2O2, add acid (HCL/H2SO4) until it turns orange. Cristallize.
a. are HCL and H2SO4 interchangable (or one acid is prefered over the other?)

2) to make Na chromate/dichromate:

from a paper (only abstract:
Through investigating the solubility of potassium nitrate and sodium chromate, the cooling crystallization and evaporating crystallization method of sodium chromate preparation was designed. The terminal temperature of cooling crystallization and material ratio were studied, and the sodium chromate crystals getting by stepwise evaporating crystallization were analyzed. The optimal operating conditions of crystallization process were determined, with the mass ratio of potassium chromate and sodium nitrate from 1:0.9 to 1:1.2, and the terminal temperature of cooling crystallization of 4°C. The integrated technology of the Na 2CrO 4 preparation from K 2CrO 4 through crystallization was proposed. The circulation process for Na 2CrO 4 preparation was studied. The good purity of sodium chromate products is obtained by recrystallization, which is increased from 81.4% to 92.2%. The final sodium chromate crystal is uniform with larger size.
(Preparation of sodium chromate from potassium chromate via crystallization process - January 2012Xiandai Huagong/Modern Chemical Industry)

also patent: CN101844810B

a. Is there a simplier way, like adding Na2CO3? (or NaHCO3) to the solution? if yes, should be H2O2 removed first?


2) to make Cr(III) salts (this is where it gets difficult to get information).
What I did "understand", I need to get the chromium hydroxide: "The Cr can be isolated as hydroxide if so desired and used to produce any Cr+++ salt with a suitable acid."


Now how to get the hydroxide from Potassium chromate?

1) turn into dichromate, mix with ethanol, heat to remove acetaldehyde
or Isopropyl alcohol without heating

from: "For example, with ethanol (a primary alcohol), you can get either ethanal (an aldehyde) or ethanoic acid (a carboxylic acid) depending on the conditions.

If the alcohol is in excess, and you distil off the aldehyde as soon as it is formed, you get ethanal as the main product.

If the oxidising agent is in excess, and you don't allow the product to escape - for example, by heating the mixture under reflux (heating the flask with a condenser placed vertically in the neck) - you get ethanoic acid.
"

Will Cr(OH)3 precipitate? (so it could be retrieved by simple filtration?)

Could this be done from Chromate directly (or needs to be dichromate?) if yes, H2O2 should be prior removed?

2) reduce it with Zinc power in diluted acid? (adding zinc powder to chromate/dichromate solution + diluted acid?

a. Should H2O2 be removed first?
b. Works with chromate/dichromate, or should be dichromate only?

From here:
"The reduction of dichromate(VI) ions with zinc and an acid

Dichromate(VI) ions (for example, in potassium dichromate(VI) solution) can be reduced to chromium(III) ions and then to chromium(II) ions using zinc and either dilute sulphuric acid or hydrochloric acid.

Hydrogen is produced from a side reaction between the zinc and acid. This must be allowed to escape, but you need to keep air out of the reaction. Oxygen in the air rapidly re-oxidises chromium(II) to chromium(III).

An easy way of doing this is to put a bit of cotton wool in the top of the flask (or test-tube) that you are using. This allows the hydrogen to escape, but stops most of the air getting in against the flow of the hydrogen."
https://www.chemguide.co.uk/inorganic/transition/chromium.ht...


I will get Cr(II) / Cr (III) ions, how will I get the hidroxide? (or what will I get? if HCL : CrCl2/CrCl3, if H2SO4 -> CrSO4 / Cr2(SO4)3
(color of the solution will indicatre if Cr(II)-blue or Cr (III)-green

3) other better way?

NOTE: I undestand that Chromium is toxic.(very toxic)


Thanks,

blogfast25 - 22-12-2022 at 11:38

'Sacrificing' the beautiful and increasingly banned chromates and dichromates to make some Cr +3 salts seems utter folly to me.

Start from Cr +3 alum instead:

https://www.google.com/search?q=chromium+alum&rlz=1C1CHB...

Or be really 'adventurous' and extract Cr +3 from SS, which contains about 11 w% of it (as alloyed chromium, of course).
Quote: Originally posted by RU_KLO  


1) to make dichromate: remove H2O2, add acid (HCL/H2SO4) until it turns orange. Cristallize.
a. are HCL and H2SO4 interchangable (or one acid is prefered over the other?)



Please. We're chemists here: it's HCl, NOT HCL.

Welcome to SM! :)

[Edited on 22-12-2022 by blogfast25]

sceptic - 23-12-2022 at 00:23

Quote: Originally posted by RU_KLO  


Currenly having a yellow Potassium Chromate solution + H2O2 (not removed yet).(from steel whool/KOH/H2O2)


Are you sure that you actually do have potassium chromate? As far as I know, steel wool is always made from low-carbon steel, which does not contain chromium. If you want to make chromates, you would need to start with stainless steel. It sounds like you have a solution of iron (III) compound, which would also be yellow.

j_sum1 - 23-12-2022 at 00:26

Nope. You can get stainless steel wool. It is a real pain if you get it by accident.

RU_KLO - 23-12-2022 at 04:15

Quote: Originally posted by sceptic  
Quote: Originally posted by RU_KLO  


Currenly having a yellow Potassium Chromate solution + H2O2 (not removed yet).(from steel whool/KOH/H2O2)


Are you sure that you actually do have potassium chromate? As far as I know, steel wool is always made from low-carbon steel, which does not contain chromium. If you want to make chromates, you would need to start with stainless steel. It sounds like you have a solution of iron (III) compound, which would also be yellow.


Yes, it was "stainless steel 10Grs wool".

Amos - 23-12-2022 at 07:29

First and foremost: Do not use hydrochloric acid to produce dichromate from chromate solutions; if you heat the solution at all after doing so, dichromate will begin to oxidize HCl to chlorine and your solution will quickly go a deep brown. Additionally, in the presence of a lot of HCl, potassium chlorochromate will crystallize instead(chem player has a great video on intentionally making this on bitchute.com). You probably want to boil your chromate solution way down and then add 50-75% sulfuric acid(to avoid superheating) slowly with stirring until a red-orange color emerges. Potassium dichromate happily crystallizes in nice big chunks upon cooling, if you don't add too much acid or have too much water left. Too much sulfuric acid or too high a dichromate concentration results in rapid, haphazard formation of crystals that are translucent due to inclusions of sulfuric acid.

I've done a decentbit of chromium(III) chemistry; Chrome Alum (potassium chromium sulfate) is the natural byproduct whenever you conduct oxidations in organic chemistry using potassium dichromate and sulfuric acid(Jone's reagent). That's a pretty good bet for getting a usable, weighable precursor, as it's water soluble and not hygroscopic. Patiently evaporate the solution, as boiling solutions of Cr(III) ions will generate frustratingly hygroscopic aquo complexes that are green in color and nearly impossible to remove from solution. When it is basified with carbonate or hydroxide solutions, however, what I think forms is actually a basic sulfate salt, occurring as a granular blue-grey powder. It looks nothing like the fine green gelatinous suspension described in literature as Cr(OH)3, but I'm not actually sure that the composition of that is well-understood either. Because of that and the difficulty of crystallizing some of the classic water-soluble salts(sulfate, nitrate, chloride), it's not the most attractive candidate for building a relaxed, easy collection of compounds like copper. It does, however, have a rich library of complexes with various ligands. You can also easily make a couple of compounds directly from dichromates by their violent reaction with oxalic acid.

[Edited on 12-23-2022 by Amos]

blogfast25 - 23-12-2022 at 07:31

Quote: Originally posted by sceptic  


Are you sure that you actually do have potassium chromate? As far as I know, steel wool is always made from low-carbon steel, which does not contain chromium. If you want to make chromates, you would need to start with stainless steel. It sounds like you have a solution of iron (III) compound, which would also be yellow.


Assuming he's used a non-oxidising acid like hydrochloric or sulphuric the Fe would be present almost 100 % as Fe +II, not the ferric form. And in acid conditions it will not oxidise to Fe +III quickly.

sceptic - 23-12-2022 at 11:41

Quote: Originally posted by blogfast25  
Quote: Originally posted by sceptic  


Are you sure that you actually do have potassium chromate? As far as I know, steel wool is always made from low-carbon steel, which does not contain chromium. If you want to make chromates, you would need to start with stainless steel. It sounds like you have a solution of iron (III) compound, which would also be yellow.


Assuming he's used a non-oxidising acid like hydrochloric or sulphuric the Fe would be present almost 100 % as Fe +II, not the ferric form. And in acid conditions it will not oxidise to Fe +III quickly.


He describes the mixture containing hydrogen peroxide, so it would be a pretty strongly oxidizing solution.

If you do have a chromate solution and want to get chromium hydroxide, I would think you could electrolyse the solution with a zinc or iron anode, with the electrodes separated far apart. The anode would be oxidized to insoluble oxides/hydroxides, and insoluble chromium (III) hydroxide would form at the cathode. With some divider between the electrodes, the substances could be easily separated from the potassium hydroxide solution.

blogfast25 - 23-12-2022 at 12:30

Quote: Originally posted by sceptic  


He describes the mixture containing hydrogen peroxide, so it would be a pretty strongly oxidizing solution.



Ooopsie. Didn't see the peroxide there. My bad.

RU_KLO - 25-12-2022 at 11:14

Quote: Originally posted by sceptic  
Quote: Originally posted by blogfast25  
Quote: Originally posted by sceptic  


Are you sure that you actually do have potassium chromate? As far as I know, steel wool is always made from low-carbon steel, which does not contain chromium. If you want to make chromates, you would need to start with stainless steel. It sounds like you have a solution of iron (III) compound, which would also be yellow.


Assuming he's used a non-oxidising acid like hydrochloric or sulphuric the Fe would be present almost 100 % as Fe +II, not the ferric form. And in acid conditions it will not oxidise to Fe +III quickly.


He describes the mixture containing hydrogen peroxide, so it would be a pretty strongly oxidizing solution.

If you do have a chromate solution and want to get chromium hydroxide, I would think you could electrolyse the solution with a zinc or iron anode, with the electrodes separated far apart. The anode would be oxidized to insoluble oxides/hydroxides, and insoluble chromium (III) hydroxide would form at the cathode. With some divider between the electrodes, the substances could be easily separated from the potassium hydroxide solution.


The process was: stainles steel wool (10gr) disolved in HCl (green solution). recipitation with sodium carbonate.
KOH solution + H2O2 (30%) added. Filtered. Yellow solution.

(from wiki: The chromium(III) hydroxide can be easily made by dissolving stainless steel in hydrochloric acid, followed by precipitation with sodium carbonate.[2]

As I need some Potassium chromate (for qualitative analisis), the idea was:
1) remove H2O2 by heating
2) cristallize.

but I also want to perform some Cr chemistry. (learning process) so the idea is starting from potassium chromate to get other salts of Chromium.
But information founded, they all start from dichromate instead of chromate.

Its cheaper and easy for me to get HCl + wool + Sodium carbonate, than Chrome alum (Chromium(III) potassium sulfate)