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Chemistry question about chrome reduction

2006

I'm a general plater in a small job shop in Michigan. The more experience I get the more I'm interested in the chemical reactions on a molecular level.My question involves the very important hexavalent chrome reduction to trivalent chrome for proper disposal in our waste treatment system.We use sulfuric acid to lower the p.h. and sodium bisulphate as a reducing agent.When the reaction takes place are the chrome atoms gaining electrons from the oxygen that's in the complex ion with them or are they getting them from the sodium bisulphate.


Your response will be greatly appreciated.

Charles P. Schwartz
plating shop employee - Madison Heights, Michigan, USA

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First of two simultaneous responses -- 2006

I think you must mean sodium bisulfite. Sodium bisulphate has no reducing properties.

The electrons come from the SO3(2-) going to SO4(2-). Three moles of sulfite ion are oxidized to sulphate ion, giving up six electrons. These are picked up by two moles of hexavalent chromium, which are reduced to trivalent chromium.

Dave Wichern
Consultant - The Bronx, New York

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Second of two simultaneous responses -- 2006

The electrons needed come from oxidizing the bisulfite to sulphate.

One way of writing this reaction is

3CrO3 + 3Na2S2O5 + 3 H2SO4---->
3Na2SO4 + 2Cr2(SO4)3 + 3 H2O

There are may good references on this subject. You should have one in your shop.

Lyle Kirman
consultant - Cleveland Heights, Ohio

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2006

Hi Charles,
You are perhaps actually using sodium metabisulfite. The electrons for the reduction of Cr(+6) to Cr(+3) comes from the oxidation of sulfur, from S(+4) in the sulfite ion to S(+6) in sulphate:
2 Cr (+6) + 3 S(+4) = 2 Cr(+3) + 3 S(+6)
The various sulfites used as reducing agents all form the acid sulfite ion HSO3(-1) in acidic solution. Since hex. chrome can be dichromate or chromate depending upon the concentration and acidity, the chromium reduction reaction can be written as either
Cr2O7(-2) + 3 HSO3(-1) + 5 H(+) = 2 Cr(+3) + 3 SO4(-2) + 4 H2O
or
2 HCrO4(-1) + 3 HSO3(-1) + 5 H(+) = 2 Cr(+3) + 3 SO4(-2) + 5 H2O

More interesting is hex. chrome reduction by sodium hydrosulfite ⇦this on eBay or Amazon zincaffil links] , Na2S2O4, which has sulfur in the +3 valence. In acidic solution, S(+3) is oxidized to S(+6):

Cr2O7(-2) + S2O4(-2) + 6 H(+) = 2 Cr(+3) + 2 SO4(-2) + 3 H2O
Note the greater chemical efficiency of hydrosulfite cf. bisulfite.

In alkaline solution, S(+3) of hydrosulfite is quickly oxidized to S(+4):
2 CrO4(-2) + 3 S2O4(-2) + 2 H2O + 2 OH(-1) = 2 Cr(OH)3 + 6 SO3(-2)
The sulfite SO3(-2) produced can further oxidize to S(+6) as sulphate, thereby reducing more hex. chrome, but this reaction is exceedingly slow in alkaline solution.

See letters #26494 & 12215 for enlightening discussions. Lots of info on the Internet, too.
If seriously interested in the chemistry, get the Handbook of Effluent Treatment & Recycling for the Metal Finishing Industry ⇦[this on on Amazonaffil links], 2nd Edn., ISBN 0-904477-14-2.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

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2006

Many waste water treatment operators tell me that the reduction of Cr(VI) with sulphite at low pH seems to proceed less fast and is less complete if no iron is present in the waste water.

This makes me think that the reaction is actually happening in two steps:
- 1 sulphite is reducing 2 Fe(III) to Fe(II)
- 3 Fe(II) reduce 1 Cr(VI); in the same reaction Fe(II) is oxidized back to Fe(III)
So Fe(II)/Fe(III) takes the role of a catalyst.

Some chemical arguments support this:
- sulphite and chromate are both complex molecules, for which the exchange of electrons (while at the same time exchanging oxygen with free H+) is almost impossible at molecular level
- Fe(II) is able to reduce Cr(VI) at neutral pH, while sulphite can only do so at pH < 2,5 (in other words, at a pH when Fe(III) is fully dissolved and chemically present as Fe3+).

If Fe-level is very low, add some of the flocculant (e.g. FeCl3) that you would add in the next step of the waste water treatment cycle, already before the Cr(VI) reduction step.

Best regards,

Bert GIELEN
- Gent, BELGIUM

what is the procedure of analyze the sodium bisulphite in mixture of sodium bisulphite & sodium sulphite

Deepak Sharma
- Gujrat, India
July 14, 2009