Sciencemadness Discussion Board

Educated / Experienced Estimation of Expected Recrystalization Exclusion Rate Towards Metal Compounds

VeritasC&E - 6-7-2021 at 13:35


Does anyone have enough experience to give me data / examples to estimate the exclusion rate towards (heavy) metals per recrystallization round of (A) amino acids or (B) metal salts?

Maybe we can compile a list of factors which most influence the exclusion rate.

I guess that it depends on the solvent being used (maybe water yields greater exclusion rates than ethanol because in general it seems to better dissolve metal compounds), on the exact method used, and on the compound's solubility characteristics (I'd expect a greater exclusion rate on greater solubility swings / lower solubility compounds), for instance.

Examples:

I want to recrystallize Zinc Chloride in Ethanol in the 25-12C range. It contains 30ppm of metals other than zinc.

I want to recrystallize Glycine in water in the 100-0C range. It contains 20ppm of various metals.

What range of exclusion can I expect in each case for those other metals? Am I gonna get a 80-90% reduction in other metals per round or more like a 5-10%?

[Edited on 6-7-2021 by VeritasC&E]

Tsjerk - 6-7-2021 at 21:39

Very important is the rate of crystallization and the nature of the compound to exclude. With a slower rate, the compound to exclude will have more time to be "pushed" away from the crystals that are forming, and a compound that doesn't "fit" in the crystals will be easier to exclude. When both are good you can easily get into the 95%+ range.

VeritasC&E - 7-7-2021 at 01:39

Quote: Originally posted by Tsjerk  
Very important is the rate of crystallization and the nature of the compound to exclude. With a slower rate, the compound to exclude will have more time to be "pushed" away from the crystals that are forming, and a compound that doesn't "fit" in the crystals will be easier to exclude. When both are good you can easily get into the 95%+ range.


Hi Tsjerk!

Thank you for your contribution! I usually crystalize over 24 hours or so, with fairly slow stirring. The rate is higher at the beginning, but gets slow after a few hours, and then very slow as it approaches the final temperature.

I clearly see the concept of "a compound that doesn't "fit" in the crystals will be easier to exclude" but what does it really mean in practice? How do you estimate that in a practice?

In the glycine example for instance I'd expect much of the metal impurities to be present as metal glycinates. Should
I consider these metal glycinates to be fitting or not fitting well into the crystal matrix? What range of exclusion (fork) should I expect against them per crystalization cycle?

[Edited on 7-7-2021 by VeritasC&E]