Chemistry Stack Exchange is a question and answer site for scientists, academics, teachers, and students in the field of chemistry. It only takes a minute to sign up.
Sign up to join this community
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. You will need
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
$\ce{Cr2O3 + 3KNO3 + 2Na2CO3 -> 2Na2CrO4 + 3KNO2 + 2CO2 ^}$
Chromium(III) oxide contains $\ce{Cr^3+}$ which can be detected through salt analysis. Only specific test for $\ce{Cr^3+}$ is mentioned.
$$\ce{Cr^3+(aq) + 3OH^{–}(aq) -> Cr(OH)3(s)}$$
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.
$$\ce{Cr(OH)3(s) + 3NaOH(aq) -> [Cr(OH)6]^{3–}(aq)}$$
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
