Understanding Supersaturation and Its Impact on Gravimetric Ppt

[nautica]

Why would supersaturation be undesirable in gravimetric ppt?

What measures can be taken to decrease the relative supersaturation during the ppt?

 

[chem_tr]

Hello, I needed to find the definition of the term "supersaturation" by Googling, and find this:

"A supersaturated solution is treated with a seed crystal. When introduced into the solution, the entire mass of the solution solidifies. When poured over a seed crystal, a solid mass forms on top of that crystal." from http://129.93.84.115/Chemistry/DoChem/DoChem058.html.

This gives me the idea that supersaturation is dangerous for precipitation reactions as no precipitates occur before adding some crystals outside. The best measure is to avoid preparing the solution in a very small volume.

 

[nautica]

So is the problem that the solid mass occurs or is the problem that no ppt will form unless a seed is added??

 

[altered-gravity]

Hi nautica,

If I´m not wrong, in a solution of A in water, supersaturation is a non-stable state where the concentration [A] is higher than the solubility at that temperature. It can be created when the solubility of A increases with temperature, you prepare a saturated solution of A at certain temperature and then start cooling the sample very slowly.

If you don´t perturb the system (that´s impossible) it doesn´t ppt, in fact it pptes very slowly. If you take the sample without care (moving it quickly or cooling too fast) you´ll see a drastic precipitation of the excess. As chem_tr said it´s used to generate well-grown cystals of A introducing a little crystal seed of A (cos the seed perturbs the solution and gives a pattern of growing for the solid to add on, must be done very slowly). But the seed is not necessary for the precipitation.

The problem with gravimetry is that when you form the insoluble salt (nothing is completely insoluble), if the reaction is made at high temperature, important part of the salt can remain solved, even cooling the sample. If supersatoration occurs you can´t assure that all the salt is ppted.

You must study the curve of salt´s solubility vs temperature. If there´s no variation, no problem. If it increases with temperature, I would try to make it at low T, or perhaps to cool it quickly after adding the reactant.

 

[chem_tr]

Hello, I had an issue with supersaturation today, and wanted to share my findings with you, especially with Nautica, of course.

My experiment involves refluxing a dicarboxylic acid in acetic anhydride, and after I finished refluxing, I did not touch the flask and kept it unattended. When I saw that very little crystals appeared in the flask, I touched the flask and moved a little bit, by gentle shaking. Then a huge formation of crystals occurred, that's, I think, involved in supersaturation.

 

[altered-gravity]

That´s it! I think you´re right.

But i have a doubt, if you prepared the solution at normal temperature (25º) and then started heating, supposing that no solvent loose occurred with the reflux, how did concentration of dicarboxilic acid raise?

 

[chem_tr]

I think I'd better give one more detail; the dicarboxylic acid totally converts into the corresponding aldehyde since it reacts readily with acetic anhydride, thus converting it to acetic acid. This is not a solution, just a chemical reaction:

$$R-(COOH)_2 + (H_3C-C=O)_2O \xrightarrow {reflux} R-(CO)_2O + 2~CH_3COOH$$

As reflux continued, a clear solution developed in the flask, and upon cooling, crystals belonging to the anhydride began to separate, the rest of which precipitated when I disturbed the somewhat supercooled system.

 

[altered-gravity]

I´d better review my organic chemistry notes

Those meta-stable states are quite strange aren´t they? For example, under extremely controlled conditions, it´s possible to cool pure water to some degrees below 0ºC without freezing. The system behaviour is similar, if you perturb it slightly, it freezes drastically at all the volume.

There is much to learn about them.