Chromatography Forum

Bruce, how do you know that supersaturation is extremely unlikely . . . . .
I know only that many recrystallizations which I performed for purification purposes took days or even weeks with seed crystals present. So unless one waits, much longer than the 2h suggested by marina how does one know?
Or is the suggestion here that if supersaturation is prevented in maybe the majority of cases it is allright to assume that there is no supersaturation in all cases?

Did you publish any of your solubility determinations? Where?

I see a few problems with the methods Marina and others suggested:
- The problem of supersaturation has already been mentioned, so it might not be a good idea to use elevated temperatures. The same problem with ultrasonication.
- How do know, that you have reached equilibrium? You can shake for a few hours or even days - you still can not be sure.

I recommend, since you are working with pesticides, that you try the offical method CIPAC MT157. Actually these are two methods, column elution method and flask method. The column elution method is used for solubilities below 10^-2 g/l; the flask method above 10^-2 g/l.

In short, the flask method uses magnetic stirring of an excess of substance in water (solvent), temperated to 20 °C. Samples are taken in a geometric time series, i.e. after 0.5, 1, 2, 4, 8 ... hours. Samples can be filtrated by an immersion filter or centrifuged. Determination of concentrations can be by any method. If concentrations do not significantly change between two consecutive measurements you're done.

This method should be reproducible and avoids the problems mentioned above. At least it's official...

Hans,

Results were confidential company reports, however I also performed controls using published data to confirm my technique.

All of my comments apply to a material that is pure. Recrystallisation to obtain pure products is significantly different to solubility determinations.

In my experience with the issues of seed crystals in solubility determinations, if they don't induce crystallisation in a supersaturated solution within minutes, they are probably not the correct form ( allotrope/polymorph ) or purity.

I've been in team that spent days trying to crystallise a chemical from nC7-EtOAc using client-supplied seed crystals. In the end we prep HPLCed some of the product, allowed that to crystallise and used that.
Crystals immediately ( < 1 hour ) appeared

With regard to use of ultrasonics/heating, when I compared them I found no significant difference, other than great shortening of time, but users should always be wary. I found that temperature control was by far the most critical parameter, but maybe pesticides are different.

Actually I think the correct CIPAC method for solvents may be another number ( MT181? ). From recollection, I think 167 is only for low solubility water testing, but may be wrong... There were also some ASTM solubility methods that I looked at.

I agree with H.Thomas that the original poster should use an industry method if possible.

Bruce Hamilton

Bruce, you're right - MT181 is the method for solubility in organic solvents. But only >10 g/L. For lower solubilities the adaption of MT157 is recommended.

Place high amount of your analyte in all the solutions you prepare....keep them in shaker for 24 hour filter the solutions and quantitate them against a cal curve using the sovent mixture in which your analyte has high solubility...

Dear H.Thomas or Bruce Hamilton,

Could you please tell me where i can find this method ( MT181 or an other i am confused !) , is there is a book or web?

thanks for you all ,all.

The CIPAC-Methods are published in handbooks, that are sold by CIPAC for around 50 GBP each, a collection of all handbooks on CD is 250 GBP (http://www.cipac.org/cipacpub.htm)

The CIPAC-Methods are published in handbooks, that are sold by CIPAC for around 50 GBP each, a collection of all handbooks on CD is 250 GBP (http://www.cipac.org/cipacpub.htm)

Thank you very much my dear