Sciencemadness Discussion Board

Accidental Reduction of Cr6+ to Cr3+ with HCl

StealthMode - 2-5-2016 at 11:53

An aqueous solution of NaClO, Na<sub>2</sub>CrO<sub>4</sub>, and NaCl in water, in unknown proportions, was treated with a quantity of 30% HCl that may have been up to a 60% molar excess based on the following expected reactions:

Na<sub>2</sub>CrO<sub>4</sub> + 2HCl -> H2O + 2NaCl +Na<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>

NaClO + 2HCl -> Cl<sub>2</sub> + H<sub>2</sub>O +NaCl

The solution was initially an amber / orange colour, and darkened over the course of 2 hours at room temperature to dark brown / orange, with a hint of green that I thought resulted from iron impurities in the technical grade HCl. The solution was then boiled to 10% its original volume to remove the HCl and most of the water, whereupon I was left with a solution that looked distinctly like CrCl<sub>3</sub>, and produced no precipitate when treated with concentrated KCl solution (if the solution contained Na<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub>, a K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> precipitate would be expected).

The initial solution was produced by an incomplete reaction between 11% NaClO bleach and anhydrous Cr<sub>2</sub>(CO<sub>3</sub>;)<sub>3</sub>, which seems to require a great excess of bleach (much more than 100% excess, at least). Can anyone shed some light on what happened here? I wouldn't have expected the Cr<sup>6+</sup> to be reduced under these conditions.

woelen - 2-5-2016 at 12:11

Yes, hexavalent chromium is reduced by chloride ions at low pH and high temperature. I have had a similar experience when I wanted to make KCrO3Cl from K2Cr2O7 and HCl. In one of my experiments, I got quite some Cl2 and my nice red/orange dichromate solution became dark brown/black.

StealthMode - 2-5-2016 at 12:57

So, the solution here is to boil first, to remove most of the water and disproportionate the hypochlorite, then acidify after cooling, right?

blogfast25 - 2-5-2016 at 13:12

Quote: Originally posted by StealthMode  

The initial solution was produced by an incomplete reaction between 11% NaClO bleach and anhydrous Cr<sub>2</sub>(CO<sub>3</sub>;)<sub>3</sub>,


There's no such thing as 'anhydrous Cr<sub>2</sub>(CO<sub>3</sub>;)<sub>3</sub>'. From Cr3+(aq), soluble carbonate and bicarbonates precipitate hydrated Cr(OH)3, not Cr(+3) carbonate.

This explains, in simple terms, why:

http://www.chemguide.co.uk/inorganic/complexions/aquaco3.htm...

[Edited on 2-5-2016 by blogfast25]