Wednesday, February 25, 2009

How to dissolve a high molar Urea without getting precipitated?

I have been trying different methods to dissolve a high concentration of urea (9M-12M), but keep getting precipitation.

I think I have found a method to make the urea solution that will not precipitate for at least several hours or even couple of days.

1) Warm up H2O (microwave or hot plate) to substantial degree so that whatever amount of urea is added will be dissolved
2) Add urea and dissolve (make sure it is completely dissolved)
3) Let it cool down slowly and gradually to room temperature

I have also tried different combinations without success

1) Start with cold water
2) Add urea and dissolve it as the water gets warmed up on hot plate
3) Let it cool down slowly and gradually to room temperature

Comment: Probably the cold water is not a good idea.

1) Start with hot water (microwaved)
2) Add urea and stir
3) The water temperature is not hot enough so microwaved the whole thing (did not use hot plate so could not maintain the same water temperature)
4) Dissolve it completely this time and let it cool down slowly and gradually to room temperature

Comment: Probably some urea particles are not dissolved completely the first time and re-warming does not help. Even though it dissolves urea completely, the solution precipitate very fast with 1 hour after cool down.
In other words, you need to dissolve urea completely the first time. For whatever reason (most likely low water temperature), if it does not dissolve the first time it will not work.

1) Start with hot water (microwaved or hot plate)
2) Add urea and stir, completely dissolved
3) Cool it down with ice (monitor it so that it does not drop beyond room temperature) then leave it on the bench to cool down to room temperature.

Comment: cooling it down with ice is not a good idea. Even if you take it out from ice around 50 degree C and let it sit on a bench until it cools down to room temperature, it still precipitates pretty fast after complete cool down.

I wonder if someone out there have/had a same/similar problem making high concentration urea solution.


Sep 2012 Follow up: 
I have actually found a perfect solution to this problem.

1) Warm up water in microwave

2) Add urea (solid)

3) COVER THE BEAKER (or any other types of container you are using) so it won't get in contact with air then place the beaker on a heat stirrer. Keep the water hot but not boiling and keep stirring the solution until urea is dissolved.

4) Keep the solution in air tight container so the solution won't come in contact with air.

The way to cool down the solution was not the issue after all. The secret is to avoid contact with air.

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