MadSci Network: Chemistry

Re: what is the chemical formula of the complex ion in benedicts solution?

Date: Tue Jul 6 17:22:06 1999
Posted By: Dr. Michael Gallagher, Senior Research Chemist
Area of science: Chemistry
ID: 929557523.Ch

Dear Jill,

As you know, Benedictís solution is made from Copper (II) Sulfate, 
Sodium Carbonate, and Sodium Citrate heated with a reducing sugar.
The reaction does not form a complex ion.  
HOWEVER, the Copper Sulfate dissolving in water does form a 
complex ion  (Cu(H2O)4) (+2) a nice light blue with a SO4(-2) ion for
electrical neutrality.
The reddish brown color formed is from the Copper (I) Oxide precipitate.
The reaction is:

2 Cu(+2)  +  2 OH(-1) + 2 electrons (from a reducing agent i.e. GLUCOSE)

                       with heating  ====>  Cu2O (precipitates)  +  H2O

An insightful question!!

Try this experiment for fun  
Instead of Sodium Citrate to provide the source of the hydroxide ions, 
try using dilute ammonia.  

Why does the color change??  
Does the reaction with glucose still work???
What potential problems could happen with this combination.

Dr. Mike Gallagher
Senior Research Chemist
J.R. Simplot Co.

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