Food Science
Read our general notes on Risk Assessment
I have done a titration using the Nuffield recipe for quantitative Benedict's with different fruit juices. Why does the cupper sulphate go to white copper thiocyanate and not red copper oxide? What is the chemistry behind this?
110310
Igloo writes .........
Conventional Benedict's solution is made up with solutions of sodium carbonate, sodium citrate and copper(II) sulphate. When reduction occurs (using a 'reducing' sugar) a reddish-brown precipitate of copper(I) oxide is formed, and it is the appearance of this reddish-brown precipitate which confirms that a reducing sugar is present. This is fine when a qualitative test is being carried out, but if a quantitative analysis is required, this reddish brown precipitate would mask the end-point of the titration, where the solution no longer contains blue copper(II) ions.
If the Benedict's solution is made up as before, but with an extra ingredient, potassium thiocyanate, the copper(I) ions formed during the reduction produce a white precipitate of copper(I) thiocyanate rather than of red copper(I) oxide. The disappearance of the blue copper(II) ions is still just about possible in the presence of this white precipitate in a way that it certainly isn't when red copper(I) oxide is there. The titration is still a fairly tricky operation to carry out, as you will know if you have read through the instructions, but the inclusion of potassium thiocyanate (forming what is known as Quantitative Benedict's Solution) makes the process relatively successful, rather than virtually impossible.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
Rate this page or react
Share your views on this page, 0 ratings so far
updated: 12 March 2010
