kyro8008
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Fehlings and Cu2O...
I had a load of spare rochelle salt and made up a fair amount of fehlings solution in an attempt to obtain some Cu2O - (using glucose). It worked
fine; I had to use about 5 layers of filter paper to get the Cu2O out though. Unfortunately the Cu2O proved hard to get form the filter paper and it
was a total failure. However I made some interesting observations and just wondered if anyone knew why:
1. I did a few different runs one after the other; the precipitate in one of the experiments was much darker - brown even compared with the other runs
which were orange. Is this due to Cu2O being oxidised to CuO? - (it might have been left in longer).
2. After filtering, the filtrate was a deep orange colour and red in the run with the brown precipitate. Is this due to very fine particles of Cu2O
giving it that colour? It looked like a solution to me not a suspension...
Thanks in advance.
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12AX7
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I would expect colloidial Cu2O to be yellow to orange, not deep orange to red, but YMMV. You might have better luck adding the reducing agent slower,
so as to make larger crystals of Cu2O (it'll probably look brick red).
Let it settle, if it doesn't form a sludge on the bottom, it must be something really weird!
Tim
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neutrino
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Cu<sub>2</sub>O indeed has many colors. See woelen's post in this thread.
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kyro8008
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thanks for the information; when I get some time ill try a few more variables and see how it goes..
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akre
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I was doing Cu2O but with Benedict's solution. I later discovered that I could use fructose as the complexing agent leaving out the sodium citrate.
As for the colours, I get all colors from green to dark red depending on the ratio of Cu(II) and Cu(I). Heating slightly usually brings the reaction
to dark red.
[Edited on 23-1-2006 by akre]
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CherrieBaby
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If one is using it on a preparative scale the dilutions used in normal Bendict's or Fehling's 'recipes' aren't needed are they? CuSO4, tartrate and
NaOH are all very soluble (if not miscible) in water. Those high dilutions are used because Bendict's and Fehling's are nearly always used an
analytical reagents.
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Magpie
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Recently vacationing in Montana I learned about the smelting of copper sulfides that formerly took place in Anaconda. This inspired me to make some
cuprous oxide from copper sulfate using Fehling's solutions and dextrose as reductant. I know this isn't likely the most efficient way to do it but
it works nicely. My yield was only 75% as I was working on a fairly small scale. Here's a picture of my 1 gram of product:
[Edited on 26-8-2008 by Magpie]
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chemkid
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Perhaps try centrifuging to seperate the Cu2O from the liquid. I know most don't have access to a centifuge (myself included), but a simple two
cylinder one could be made with a dremel, a bolt a small piece of metal sheet, a drill and some centrifuge tubes.
Chemkid
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Magpie
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I separated the Cu2O from the mother liquor using a Buchner funnel and filter paper, dryed this in an oven, then scraped off the powder, without any
difficulties.
I titrated the hot stirred Fehling's solution with dextrose (glucose) until the blue color was gone. The liquor color was a faint green then. When I
turned the stirrer off the Cu2O quickly settled to the bottom of the beaker. The amount of dextrose required was just a little more than the
theoretical 0.005g/ml of Fehling's solution.
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woelen
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One way to get a more coarse precipitate is to have very slow reduction. I also made some Cu2O from glucose and Fehling's solution, but I did this in
the cold, standing at a cold place overnight. The precipitate you get in that case is brick red (like Magpie's) and fairly heavy, it quickly settles.
I separated the precipitate simply by repeated decanting and shaking with distilled water. After 4 rinses, it is clean and then you can let it dry on
a petri dish. After drying you can scrape off the solid. It also is remarkably compact. Five grams of solid is only a very small amount in terms of
volume, the powder is fluffy, but compact.
I also made bright yellow Cu2O, this can be made easily, but hardly can be separated, as the precipitate is slimy and very voluminous. It hardly
settles at the bottom. Quick formation of Cu2O can be achieved with a strong reductor, like ascorbic acid. Too strong a reductor (e.g. hydrazine) does
not work, then you get metallic copper.
[Edited on 27-8-08 by woelen]
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daragh8008
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Sorry to bring up such an old thread, but I have been trying the last few days to make a known concentration of cucl in HCl. Since I found that the
cucl tended to decompose before it had even been dried (made from cu ii salt and nacl sodium metabisulfite) I though it would be easier to weigh out
cu2o and dissolve up in HCl. The cu2o I made from copper nitrate and ascorbic acid in a 1:1 molar ratio. A few goes gave me a brown mud impossible to
filter. When one batch did work giving a deep red, I tried dissolving about 100mg in 36% HCl. Only a tiny fraction dissolved giving a deep yellow
soln, the rest refused to dissolve. I gave up after adding 30ml of HCl and boiling for 20 mins. Am I missing something here. I though cu2o was fairly
soluble in HCl. Perhaps the cu2o was mostly cu? Any suggestions. I could't find any details for the reduction of copper 2 salts with ascorbic acid so
I just went 1:1. Has anyone tried this.
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blogfast25
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kyro8008: perhaps peptisation occurred?
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