How to test for Chromium(III) Oxide

I got some chromium(III) oxide ($\ce{Cr2O3}$) sold as pigment, called "Chrome Green". I've been having a hard time trying to react it with $\ce{HCl}$ to make $\ce{CrCl3}$. I thought it was just hard to break the $\ce{Cr-O}$ bond.


Now I'm just afraid that the pigment that I bought was actually cut with something else, like a silicate-type of pigment. How do I test for chrome(III) oxide, if I can't get it to become a salt in the first place?




The test for $\ce{Cr(III)}$ can be performed in the solid state using an [*oxidative melt*](https://de.wikipedia.org/wiki/Aufschluss_(Chemie)#Oxidationsschmelze). You will need


* chromium(III) oxide (obviously)
* sodium carbonate
* potassium nitrate
* a [*Bunsen*](https://en.wikipedia.org/wiki/Bunsen_burner) or [*Teclu*](https://en.wikipedia.org/wiki/Teclu_burner) burner
* a [magnesia trough](https://de.wikipedia.org/wiki/Magnesiarinne) or a crucible
* suitable [pliers](https://de.wikipedia.org/wiki/Bauchzange) to hold the trough


Mix the green pigment with up to the threefold amount of sodium carbonate and potassium nitrate, place the mixture on the magnesia trough and melt it.


A yellow melt indicates the formation of sodium chromate according to 


$\ce{Cr2O3 + 3KNO3 + 2Na2CO3 -> 2Na2CrO4 + 3KNO2 + 2CO2 ^}$ 


 2

Chromium(III) oxide contains $\ce{Cr^3+}$ which can be detected through salt analysis. Only specific test for $\ce{Cr^3+}$ is mentioned.


1. Source 1\*



> 
> $$\ce{Cr^3+(aq) + 3OH^{–}(aq) -> Cr(OH)3(s)}$$
> 
> 
> Chromium(III) hydroxide is formed when $\ce{Cr^3+}$ is treated with
>  ammonia or sodium hydroxide, which is soluble in excess alkaline
>  solution (amphoteric, dissolves in acids too) but not soluble in
>  excess ammonia . With sodium hydroxide, a dark green soluble
>  hexahydroxo–complex ion is formed.
> 
> 
> $$\ce{Cr(OH)3(s) + 3NaOH(aq) -> [Cr(OH)6]^{3–}(aq)}$$
> 
> 
> 


2. Source 2\*\*



> 
> Chromium can be taken through a series of colored tests which leaves
>  no doubt as to its identity. Chromium(III) forms a steel green
>  hydroxide which dissolves in excess strong base to give a deeply green
>  colored solution of the hydroxy complex. Treating this complex with 3%
>  hydrogen peroxide gives the yellow solution of the chromate ion, which
>  upon acidification with dilute nitric acid gives the orange color of
>  dichromate. Treatment of the cold solution of dichromate with 3%
>  hydrogen peroxide gives the intense blue color of a peroxide of
>  chromium. (The actual composition of this peroxide is not known, but
>  it is believed to have the empirical formula $\ce{CrO5}$). This
>  peroxide readily decomposes to the pale violet color of the original
>  hydrated chromium(III) ion. In low concentrations of dichromate the
>  blue color is fleeting, and attention must be focused on the test tube
>  during the addition of the hydrogen peroxide to avoid missing the
>  color change.
> 
> 
> $$\ce{Cr(OH)^{4-} (green) + H2O2 + OH^{-}→ CrO4^{2-}(yellow)}$$
>  $$\ce{CrO4^{2-} + H+→ Cr2O7^{2-} (orange)}$$ $$\ce{Cr2O7^{2-} + H2O2 +
> HNO3→ CrO5 (blue)→ [Cr(H2O)6]^3+ (violet)}$$
> 
> 
> The following color changes are all indicative of $\ce{Cr^3+}$. Add an
>  excess of 6 M $\ce{NaOH}$ to about one mL of test solution. To this
>  green solution add 10 drops of 3% $\ce{H2O2}$. Heat the test tube in
>  the water bath until the excess $\ce{H2O2}$ is destroyed as indicated
>  by the cessation of bubbles. Acidify the yellow solution with 3 M
>  $\ce{HNO3}$. Cool the resulting orange solution in an ice bath. To the
>  cooled solution add a drop or two of 3% $\ce{H2O2}$ and observe the
>  immediate fleeting blue color.
> 
> 
> 


**Reference**


\*<http://www.docbrown.info/page13/ChemicalTests/ChemicalTestsc.htm>


\*\*<http://www.wiredchemist.com/chemistry/instructional/laboratory-tutorials/qualitative-analysis>


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