Chromium Electroplating Solution

[olmpiad]

I have recently acquired about 1/4 of a pound of Cr2O3 from a rock shop. I have heard that it dissolves in H2SO4 to form a solution that gives Cr upon electrolysis. However, I cannot seem to dissolve it. Are there any chemicals other than H2SO4 and Cr2O3 that are needed? :confused:

[woelen]

Cr2O3 in fact cannot be dissolved in any solvent, other than molten NaOH, KOH or other molten alkali salt. This is a well-known property of many metal oxides, that, once they are calcined, they become remarkably inert. This is because of formation of a very compact and stable crystalline structure for the oxide. This effect is most notable on the amphoteric oxides (e.g. Cr2O3, Al2O3, TiO2, SnO2, Fe2O3, Co3O4), it also exists, however to a lesser extent with other oxides. E.g. crystalline MnO2 also is very hard to dissolve.

 

Even warm concentrated H2SO4 will not dissolve your Cr2O3. So, if you want something nice with it, then either you have to dissolve it in molten NaOH (which is very dangerous!!!!), or you have to make a thermite mix of it (mix with Al-powder). The resulting (impure) chromium metal then can be dissolved in hydrochloric acid and the impurities (mainly Al2O3) remain behind. Igniting such a thermite requires very high temperatures, but once the reaction is going it is unstoppable. You have to do that OUTSIDE, and when the mix ignites, you have to be sufficiently far away. There may be drops of molten chromium being sprayed around! A magnesium ribbon probably is sufficiently hot to ignite such a mix.

[raivo]

Cr2O3 indeed can be dissolved in molten NaOH but there is one more problem with it. When quite small quantity of Cr2O3 (and also products of reacting it with NaOH) are dissolved in molten NaOH then melting temp goes up and this mass becomes solid and reaction stops. In reality just small amount of Cr2O3 in quite big amount of NaOH can be dissolved this way.

 

Results are much better if oxidiser (usally KNO3 or NaNO3) is added to initial mixture.

 

Industrial process uses Na2CO3 instead of NaOH. This is mixed with small dolomite pieces and heated to 800C (or even more) with constant stirring and good aeration for some hours. Sodium chromate is made this way and this can easily be convertod to dichromate or chromic acid.

 

If i recall correctly Cr203 thermite needs 800 - 1000C preheating for reaction to run. Ignition temperature is even higher.

[encipher]

If I'm not mistaken, I think what you are looking for is not Cr2O3, but CrO3 which is the acid anhydride of chromic acid. CrO3 is soluble in water and sulfuric acid. I wouldn't recommend you try chrome plating at home though. Since Chromium compound are toxic and carcinogenic. Also, you will have to dispose of the 'leftovers' properly. You cannot simply flush it away or dump it down the sink. It is illegal to do so since it is classified as hazardous waste. There are simply too many risks involved.