Thursday
November 20, 2014

Homework Help: TLC purity

Posted by Sheryl on Friday, October 13, 2006 at 11:29pm.

Question: A student spots an unknown sample on a TLC plate and develops it in dichloromethane solvent. Only one spot, for which the Rf value is 0.95, is observed. Does this indicate that the unknown material is a pure compound? What can be done to verify the purity of the sample using TLC?

My answer: A single substance gives a single spot no matter which solvent is used to develop the plate. However, it might be necessary to try a number of solvents to separate the components and determine how many there are in a mixture. When dealing with compounds of similar properties, such as isomers, multiple developments of the TLC plate may be necessary to determine purity.

Does this look okay?

Thanks from Sheryl


test


My original responses did not post so I had to try the test.
For you question, you don't say anything wrong but I think the second sentence is likely to be confusing the first. Also, you don't answer the question. I would start with "No." Then leave sentence 1 as is. Make the second one something like this. "However, in mixtures, it may be possible to have a single spot if the appropriate solvent to separate the components of the mixture is not used." Then go on with the rest of what you wrote editing here and there to make it fit with the above.


There are 2 questions asked. The first is: whether the single spot indicates the purity of the compound. Does this sound okay:

A single spot does not necessarily indicate the purity of the compound. A single substance gives a single spot no matter which solvent is used to develop the plate. However, in mixtures, it may be possible to have a single spot if the appropriate solvent to separate the components of the mixture is not used.

And for the second part:
When dealing with compounds of similar properties, such as isomers, multiple developments of the TLC plate with a fairly nonpolar solvent may be necessary to effectively separate compounds and so to determine purity.

Is this okay?

Sheryl

It sounds ok to me. One final suggestion is to separate question 1 and follow with your answer. Then do question 2 and follow with your answers. That will minimize (to essentially zero) the possibility of the teacher confusing the two answers.

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