Preparation of Chromium (III) Salts

[MaximumTaco]

OK, so i have Cr(III) Oxide, Cr2O3, and i wish to dissolve this in conc. HCl to form Cr(III) Chloride - however, at room temperature, this reaction just doesn't seem to work readily - i would have thought it would. Would heating facilitate the desired reaction? Would the use of another acid, such as conc. Sulphuric acid, react more readily with the oxide without the need for heat?

 

[mrjeffy321]

Look here,
http://www.roguesci.org/theforum/chemistry-related/4725-review-chromium-compound-synthesis.html

A few quotes from the site,

Chromic chloride: May be prepared by dissolving hydrated chromium oxide in hydrochloric acid and evaporating off the liquid. It may also be prepared by adding, by weight, 1 part of either sodium or potassium dichromate to 6 parts of hydrochloric acid and gently heating for a few seconds to initiate the reaction. Chlorine gas will be evolved, so do this reaction is a well ventilated area.

Chromic trichloride classic method is to heat chromic oxide (Cr2O3) in a stream of sulphur chloride or carbon tet vapour in a tube furnace. Not very convenient. Dissolving Cr2O3 in concentrated hydrochloric acid and then evaporating to dryness produces a hydrate, if this is heated strongly it loses virtually all of the water but some decomposition to C2O3 occurs colouring the mass green. If the residue is heated to a high temperature, 700C seems in the right range, the chromic chloride sublimes leaving the chromic oxide behind.

I've had Cr2O3 sitting in a test tube with 30% hydrocloric acid for over 24 hours now, there seem to be nothing happening at all, the acid is still without any sign of turning Cr+3 - green. Is something wrong with the procedure Marvin mentioned or does it need high temperature or something? The difference between 30 and 37% can't be that great, can it?

I also can't see where anything I've quoted talks about adding Cr2O3 to hydrochloric acid. Nitro's method talks about chromium metal which I had previously thought not active enough to liberate hydrogen but I'm prepaired to suspend my disbelief on that count.

Pottery grade Cr2O3 is much much less reactive than the freshly precipitated hydrated oxide. Again I can't vouch for this, the closest experience I've had is with lapidary ceric oxide in my somewhat successful attempts to produce ceric sulphate, another potent oxidiser. I would be inclined to try sulphuric acid of different concentrations, starting with concentrated, and pounding the oxide into a very very find powder first. Reducing to chromium with aluminium is one possibility or its back to a furnace until you get enough in salt form to recycle.

Cr-metal from Cr2O3/Al thermite dissolves fast in 30% HCl.

You might try making some Chromium Oxide thermite to produce Chromium metal, then use this to prepare CrCl3.

 

[MaximumTaco]

I don't have Al to do a thermite reaction - i wonder if heating with charcoal will do it?

I'll try milling the powder very fine, and try conc. H2SO4 and see if it is any different.

I'm actually trying to synthesize Na2Cr2O7.

 

[mrjeffy321]

Here are instructions for the preparation of K2Cr2O7,
http://www.frogfot.com/synthesis/kdichromate.html
You easily substitute Sodium Salts into the reaction (as opposed to Potassium) to make it Na2Cr2O7.
Using this method, you could use your Cr2O3 as it is, without reacting it with acid first.

 

[GCT]

OK, so i have Cr(III) Oxide, Cr2O3, and i wish to dissolve this in conc. HCl to form Cr(III) Chloride - however, at room temperature, this reaction just doesn't seem to work readily - i would have thought it would. Would heating facilitate the desired reaction? Would the use of another acid, such as conc. Sulphuric acid, react more readily with the oxide without the need for heat?

What reagent grade of the chromium III oxide are you employing? You may need to recrystallize the oxide. Or at times, the sample may have aged to much and undergone chemical changes (redox reactions with oxygen for example). It may be the case that something is redoxing the Cr III in the sample.

Are you having trouble with the reaction overall or just the dissolution of the sample oxide?