StealthMode
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Accidental Reduction of Cr6+ to Cr3+ with HCl
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.
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woelen
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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.
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StealthMode
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So, the solution here is to boil first, to remove most of the water and disproportionate the hypochlorite, then acidify after cooling, right?
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blogfast25
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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>,
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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]
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