The Vespiary

The Hive => Newbee Forum => Topic started by: TheBlindGenius on February 18, 2003, 02:14:00 PM

Title: Melting points
Post by: TheBlindGenius on February 18, 2003, 02:14:00 PM

What's up everybody.  I was wondering about a question I have had for quite some time.  I know that for an impure substance, the melting point is always lower that that of the same substance in a pure matrix, even if the contaminant has a higher melting point.  I have a couple of questions.

First, does the identity of the contaminant matter in determining the melting point depression?  For example, let's say a NIST standard of MDMA is acquired (far-fetched scenario) will 99.00% MDMA with 1.00% sugar melt at the same temperature as 99.00% MDMA with 1.00% reaction byproducts from a Pd/C CTH?

My second question is, does the melting point depression follow any sort of linear or other mathematical function when compared to the percent purity?  Perhaps a curve could be made that related percent purity to melting point by using triple recrystallized sample and making 100-50% standards, and taking their melting points.  For example for cocaine or heroin, when buying big amounts, the difference between 80% and 95% is very important.

Title: check your physchem book
Post by: hypo on February 18, 2003, 02:26:00 PM

on eutectic mixtures

(or google)

(no they are not linear)

(not all mixtures are eutectic)

(you'll only reach the eutectic point, when you have a mixed chrystal)

(you'll never really reach the eutectic point)

(melting point also depends on the rextallisation solvent)

(what really matters is how sharp your substance melts)

Title: Damn
Post by: TheBlindGenius on February 18, 2003, 02:30:00 PM

Not taking P. Chem till next semester.  I figured it would be in there somewhere.  In analytical and organic chem melting points are mentioned and discussed a bit but never heard that word eutectic.  I will check it out though.

Title: well...
Post by: hypo on February 18, 2003, 02:31:00 PM

you don't need to take physchem to read the book...
(or to check google)

Title: Whatever
Post by: TheBlindGenius on February 18, 2003, 02:45:00 PM

you say

Title: I don't think there are adequate theories to...
Post by: foxy2 on February 18, 2003, 09:59:00 PM

I don't think there are adequate theories to predict the effect without first running experiments with the substance and contaminant.  This is my intuitive guess.

You may bee able to get fairly good results with relatively pure compounds(less than 5% impurity) but beyond that I don't think you can trust a general theory.  Plus such things would bee dependant on many characteristics of each component in the mixture.

Title: Function of molecular weight.
Post by: terbium on February 18, 2003, 11:43:00 PM

I don't think there are adequate theories to predict the effect without first running experiments with the substance and contaminant. 
If you have a pure substance (A) with a small amount of another substance (B) dissolved in it then the freezing point depression is a linear function solely dependent on the number of moles of B. In the old days this was used to determine the molecular weight of a pure unknown substance. Camphor was used as substance A because of its large freezing point depression constant. A known weight of the unknown substance B was added, the freezing point depression measured and by this the number of moles of B and hence its molecular weight were determined. I once had an old thermometer about a meter long which had been made for determining differential temperatures to 1/100°C for this purpose.

Title: yeah,
Post by: hypo on February 18, 2003, 11:52:00 PM

i bet that is very easily feasible with MDMA  :P  :P  :P