Metal finishing Q&As since 1989

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Analysis for Trivalent and Hexavalent Chrome in wastewater

August 23, 2011

Hi. We are a tannery processing hides using chrome powder (as Cr2O3, trivalent) as the main tanning agent. So our wastewater would naturally contain chrome, I think in its trivalent form still. A brief background on the tanning process - we pickle the hides to a pH of 2.8 using formic and sulfuric acids, add the chrome powder in, the lastly basify or neutralise the bath to fix the chrome onto the collagen structure using alkali such as magnesium oxide, sodium bicarbonate or sodium carbonate / washing soda [affil links] to a pH of around 3.8-4.0.

Our government regulation for discharge of wastewater is for hexavalent chrome and not for trivalent chrome. But the laboratory analysis they use to determine the amount of chrome in the wastewater is for total chrome, which I think is not fair for us. Total chrome would mean discharge of both tri and hex chrome, am I correct? I would like to ask if there are separate procedures for analysing tri and hex chrome? Also, with the process I gave above, is there a chance that the tri chrome are converted to hex chrome? How are the tri chrome converted to hex chrome? Is it easily converted?

PERRIE CLEMENTE

Perlita Clemente
pollution control officer - Meycauayan, Bulacan, Philippines

Hi, Perrie .

Although the term "trivalent chromate" is widely used, it's an oxymoron. "Chromate" means CrO3, and there is no way that the chrome in Cr⁺⁶O⁺²₃ can be other than hexavalent, so we probably shouldn't call it chromate :-)

Hopefully you do not use any hexavalent chromium, but I'm from the metal finishing industry rather than the tanning industry, and I don't know. Here in the USA plating industry, we have discharge limits both for hexavalent chromium and total chromium. First, the hexavalent chromium is reduced to trivalent -- not only to reduce its toxicity, but because hexavalent chromium is soluble and not filterable. Then the total chromium is precipitated out, filtered and landfilled rather than remaining in the wastewater -- so it doesn't matter so much whether it could theoretically become hexavalent again under some weird oxidizing condition.

I'm not a lab technician, so I'll leave it to others to propose workable analyses for aliquots of hexavalent chromium and total chromium, but as an operational matter in dealing with high volumes of wastewater, hexavalent chromate is visible in clarified wastewater down to nearly 1 ppm because of its strong amber coloration. If the wastewater is faintly green to blue it is probably free of hexavalent chromium; if it is faintly yellow to amber, it is probably not. An instrument called an ORP meter is used to measure the oxidation potential of the wastewater during operation to determine if there is hexavalent chromium in it. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 23, 2011

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Use the 1,5 diphenylcarbohydrazide [affil links] method.

The sample, pH adjusted w/ sulfuric acid to about 1, is boiled with potassium permanganate [on eBay or Amazon] solution for two minutes. The excess permanganate is destroyed by addition of sodium azide; then, after cooling, a solution of the color reagent in acetone [on eBay or Amazon] is added. Read at 535 nm.

For hexchrome only, simply omit the boiling and permanganate addition.

Dave Wichern
Consultant - The Bronx, New York
August 29, 2011