The Amateur Chemist

Synthesis of Potassium Dichromate from Stainless Steel


This is still an ongoing experiment, but I think I finally figured out how to do this. I started with a stainless steel spoon. I hooked this up to a power supply, and electrolyzed it in some water. This will corrode the spoon away quite fast, depending on what kind of power you are applying to it. I waited till the water was thick with the sludge that is produced by this. I then filtered, vacuum was much faster, the sludge. This electrolysis produced the hydroxides of the metals, iron + nickel + chromium. Then we have to separate the chromium from the iron and nickel. 

The only way I could do this was through the fusion of the hydroxides with sodium or potassium hydroxide. Many people suggest to make the metal chlorides, then add an excess of sodium hydroxide to this solution. The chromium hydroxide will originally precipitate out, then go back into solution, leaving the iron and nickel hydroxide as precipitates. This, however, does not work. There is so much iron contamination in the mix, that it sort of "locks" up the chromium into it, not allowing it to go back into solution. I remember someone saying once that they thought that it might actually be making 'Chromite', or FeCr2O4, which is the ore that chromium is extracted from. 


So, to re-extract the chromium, we fuse the sludge with sodium or potassium hydroxide. This means, we take an equal amount of the now dry sludge and the hydroxide and heat them up in a stainless steel container till the hydroxide melts. You can help this process along by adding a little potassium nitrate or chlorate to the mix. Keep heating this for a while. I heated mine for 15 to 20 minutes. Let it cool, then put some water over it. Let it soak for a day or too, and the sodium/potassium chromate will leak out into the water. It should be yellow. You can then acidify to the dichromate and let evaporate to the crystal form.

From all the methods I tried, this was by far the easiest. It was only a couple of steps, and doesn't require many chemicals.  You can look at my notes from my other tries below. None of them worked very well.







The original plan was this.  Dissolve/react as much stainless steel as I could in concentrated (31%) HCl.  This would make iron chloride, nickel chloride, and chromium chloride.  Add a major excess of sodium hydroxide to this green solution. This will react to form iron, nickel, and chromium hydroxide. At first, all three will precipitate out, but then as the solution contains an excess of sodium hydroxide and reaches a ph of greater than 13-14, the chromium hydroxide will redissolve. Then filter this mixture, and I have a solution of green chromium hydroxide. I have succesfully removed the iron and nickel.  Then oxidize the chromium hydroxide to (sodium?) chromate with hydrogen peroxide. As it turns into chromate, the solution will go from dark green to a bright yellow.  Then to convert this to dichromate, we need to acidify the solution.  You can use HCl, but sulfuric acid works better.  However, the newly formed dichromate will react with the excess hydrogen peroxide to form a dark blue compound that is unstable and will eventually give the original green +3 chromium ions again.  To counter this, we boil the solution to destroy  the hydrogen peroxide.  Boil until no more bubbles of oxygen form.  Then add the sulfuric acid. The solution will go dark orange, maybe even red as the dichromate is formed.  The sodium dichromate is HIGHLY soluble, while the potassium dichromate is almost insoluble.  We add potassium chloride to this solution. This forms potassium dichromate and sodium chloride. Cool this solution slowly, down to around 0 C.  The lower the better.  The orange/red potassium dichromate is formed. Filter, dry, and now you have potassium dichromate from stainless steel.  



However, upon actually performing this we run into a major problem. When we add the excess sodium hydroxide, the chromium does not redissolve as expected.  Upon investigating this, and doing some research, it is determined that the problem is because of the large concentration of iron.  The website above is still true, because they use a pure source of +3 chromium ions. No contamination of iron or nickel. In our solution, the large concentration of iron makes the chromium co precipitate with it. The chromium is sort of locked into a large molecule with the iron, and can't redissolve. Upon filtering, the liquid was clear, not yellow as expected.  This was a major problem.

I did some more research, and I might have found a solution.  If you take the precipitate you get and add concentrated (30%) H2O2 to it, it will sort of split the iron and chromium back up again. It oxidizes the iron hydroxide into iron oxide or rust, and the chromium into soluble chromate. The iron oxide stays precipitated, and the chromate goes into solution.  The reaction was boiled for several minutes to get complete oxidation.  It was then filtered, and a bright yellow solution was obtained, with the iron (and nickel hopefully) separated out.  This took a long time, and was quite a waste of very valuable concentrated hydrogen peroxide.  I needed a new oxidizer. I figured out that bleach, or sodium hypochlorite will also oxidize the chromium and iron. I tried a number of experiments, and to my surprise it was discovered that you don't even need to add the sodium hydroxide to the solution first. The bleach can be added directly to the dissolved chlorides.  This way you don't have to waste NaOH, only cheap bleach. I added bleach until it stopped fizzing (it fizzed a lot), then added a little bit extra.  The reaction produces choking, noxious gas, so it must be done outdoors or in a fume hood.  I added a little bit, then ran away, came back several minutes later, and repeated.  I then filtered the solution. I got a nice bright yellow, free from any impurities (hopefully). The stuff in the filter, iron oxide, had a nice brownish look to it just like it should have. I then (stupidly) decided to add the sulfuric acid and make the dichromate right away.  It started fuming, white smoke, and turned to a dark green.  I was very confused at first, but then I remembered that I had added extra sodium hypochlorite (bleach), and that sulfuric acid + bleach yields chlorine gas. Stupid me.  Also, I think the bleach would have reacted with the formed dichromate just like h2o2 and converted it back into dark green +3 chromium ions.  Luckily, there is a easy way of destroying extra hypochlorite. Boiling.  

This is where I am at right now. I will boil the mixture (outside, as it forms chlorine) until I think that there is no more hypochlorite left.  Sodium Hypochlorite upon boiling will disproportionate into sodium chloride and sodium chlorate, both of which are highly soluble, so we can easily wash them out later.  We'll see what happens.