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I want to do rust removal by electrolysis (sodium carbonate, water solution) Can I use a stainless steel basket as a cathode or will it produce hexavalent chromium ?
I know if I use stainless steel as the anode it does produce hexavalent chromium, but what if it is a stainless cathode ?
I have a solution of sodium carbonate, water and hexavalent chromium. What can I do to eliminate the chromium or make it safe, preferably safe enough to dump in the drain if that is possible.
In anode, things get oxidized. That is why the chromium in anode would become hexavalent chromium (hexavalent chromium is the oxidized form of chromium).
In cathode, things get reduced (opposite of oxidized). That is why you could remove rust, because rust is the oxidized form of iron, so if you reduce rust, you would get iron.
You will not obtain hexavalent chromium, because that is not possible through reduction of chromium; that is only possible through oxidation of chromium, which is impossible at a cathode.
You can reduce hexavalent chromium to trivalent chromium with sulphite in acidic environment:
$$\ce{Cr2O7^2- + 8H+ + 3SO3^2- -> 2Cr^3+ + 3SO4^2- + 4H2O}$$
Then you can precipitate out the trivalent chromium with sodium hydroxide:
$$\ce{Cr^3+ + 3NaOH -> Cr(OH)3(s) + 3Na+}$$
There's no reason a ss basket couldn't work as a cathode, although you need to control current (density) and voltage. If I were to post a question here asking about a solution I made of tri-nitro-toluene (TNT) and how I should "dispose" of it, or of plutonium, I would hope no one would give me the correct technical answer. The practical answer is: if you have to ask, you shouldn't be handling it!! Chromium is an element. It is forever. There are all sorts of LAWS concerning how you must handle waste containing it. Reducing Cr(VI) to Cr(III) does reduce the hazard, but does not eliminate it. You can not legally dump caustic solutions down the drain in the USA, which means using NaOH is a really bad idea. It is against the law. When I was involved in small scale (hundred of gallons) use of Cr(VI), we used ferrous sulfate to reduce it and to precipitate out the Cr(III). There are generally two methods: one uses sulfuric acid to get to a low pH and sodium metabisulfate to reduce it, the other ferrous sulfate and alkaline pH. In either case, you'd have to then neutralize the wastewater back to neutral. Cr is fairly easy to trace through a sewer system; that means if you dump it, be prepared to spend some time in jail.
