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I've been trying to make Sodium dichromate $\ce{Na2Cr2O7}$ by following these steps:

  • Dissolve an inox spoon in $\ce{H2SO4}$
  • Filter the solution
  • Dissolve the obtained sludge in $\ce{NaClO}$
  • Heat the solution until slurries appear
  • Filter the solution

The solution I obtained was yellowish (light yellow), I tried to add some acid to see the orange color of the (hypothetical) dichromate but nothing happened!

I added some ammonia before acidifying the solution, nothing as well.

1st: Is this method the right one to obtain chromate/dichromate? 2nd: If yes, why didn't the orange color appear?

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  • $\begingroup$ While inox spoon's may indeed contain chromium, this reference states "at least 10.5 mass%" and "up to 26% is used for harsh environments". So, it is still a small fraction of your spoon that constitutes chromium. So it is more likely your first solution became yellow in color because of an other major constituent, iron. Yet I doubt a moderate oxidant like bleach is able to oxidize metallic chromium to yield a significant amount of chromates, $\ce{CrO4^{2-}}$ and dichromates $\ce{Cr2O7^{2-}}$, the later a known strong oxidizer. $\endgroup$ – Buttonwood Feb 18 '14 at 13:18
  • $\begingroup$ bleach is able to oxidate chromium to chromate in strongly basic conditions, i.e. under excess of bleach in concentrated sodium hidroxide. However, the method itself is questionable, as inox contains too small amount of chromium. $\endgroup$ – permeakra Feb 18 '14 at 16:30
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    $\begingroup$ Back in the days when cancer wasn't invented yet, people used to clean glassware with hot solutions of $\ce{CrO3}$ or $\ce{Na2Cr2O7}$ in sulfuric acid. If you work with (di)chromates, protect yourself and the environment. Please wear gloves and don't dump the solutions in the sink. $\endgroup$ – Klaus-Dieter Warzecha Feb 18 '14 at 19:01
  • $\begingroup$ Does one of these steps isolate iron from chrome? If not is there any method to do it? Remark that I'm not chemist but have some basic notions of chemistry. I wore gloves throughout the handling, how do I get rid of these materials then? $\endgroup$ – Pedro Feb 19 '14 at 6:05
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Note that the chromate ion $\ce{CrO4^{2-}}$ has chromium in the +6 oxidation state like dichromate $\ce{Cr2O7^{2-}}$. Chromate is yellow, dichromate is orange.

From the Wikipedia article on sodium chromate: sodium chromate is formed by reaction of dichromate with hydroxide:

$$\ce{Na2Cr2O7 + 2NaOH -> 2Na2CrO4 + H2O}$$

You may be making sodium dichromate, but the strongly basic conditions are converting it to sodium chromate.

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  • $\begingroup$ When I add acid in excess, it normally (re-)convert to $\ce{Cr2O72-}$? $\endgroup$ – Pedro Feb 19 '14 at 6:07

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