Is it possible to pass saturated solutions of compounds with very high solubility through filters with small pores (e.g. passing a saturated aqueous ZnCl2 solution at 90C through a 2mcm glassfiber filter with the help of partial vacuum)?

Will it work well? Are there preferred methods for this? If not, is there any (reasonable) solution to filter out microparticles from such solutions?


Filtration is a mechanical operation. Thus, if you have a solution of $\ce{ZnCl2}$, which (assuming you use water as solvent) dissociated into ions of $\ce{Zn^{2+}}$ and $\ce{Cl^-}$, a typical filter will not retain these ions. Instead of using some vacuum (potential danger to evaporate some solvent, thus altering the concentration of he solution), you may consider filtration under pressure (e.g., across a Schlenk fritt for larger scale, or a syringe filter at smaller scale [e.g., testing a spin coating]).

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If the solvent capacity for $\ce{ZnCl2}$ is not (this much) temperature dependent, you equally could perform the filtration near room temperature (e.g., $\pu{432.0 g}$ $\ce{ZnCl2}$ per $\pu{100 g}$ of water at $\pu{25 °C}$ listed here).

  • $\begingroup$ Hi Buttonwoods! Thank you for your answer. You made me discover the Schrenk Fritt! $\endgroup$ – Hans May 26 at 12:18
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    $\begingroup$ If the intend is to purify $\ce{ZnCl2}$, then -- beside the anticipated scale (g, kg, t) -- it must be clear in advance what the nagging impurities (and their concentration) currently are, and what (after purification) would be a level good enough. Otherwise the work will not find an end. It could be that the undefined impurities are equally (or better) soluble in the solvent, too. Equally, filtration would proceed from coarse to fine filters just to prevent blockage, too. A suitable pressure depends on the material, yet in addition e.g., shape and dimension of the container, too. $\endgroup$ – Buttonwood May 26 at 13:24
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    $\begingroup$ Small scale preparative column chromatography uses a gentle pressure (perhaps 1 bar) top push the eluent across the column, but column's preferred diameter is below 10 cm. What is the purpose of the ZnCl2 after purification? What did you find in Amareggo / Perrin «Purification of Laboratory Chemicals»? Given this entry I would not wonder if it is some kind of sublimation instead of a (directed) crystallization. Of course one would start with a coarse filter. $\endgroup$ – Buttonwood May 26 at 13:33
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    $\begingroup$ If sublimation, or high vacuum distillation (example, seems easier to perform than for magnesium) is not an option (physics group in an university?), then this demanding specifications of purity are better delegated to chemical supplier selling the material with an certificate of analysis. E.g., sigma sells this with ≥99.995% purity (#429430, max. 50.0 ppm trace metal), or at 99.999% purity (#229997, max 15 ppm trace metals). Crystallization from solution typically is not directed enough. $\endgroup$ – Buttonwood May 26 at 20:55
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    $\begingroup$ "Directed" as in "zone melting" with a discrete layer of molten material passing the solid sample, e.g., to purify semiconducting metals in microelectronics (open access example). While a melting point of $\pu{290 °C}$ could be attained with some resistor wiring around an horizontal glass tube, I do not know if after hours of contact normal lab glass eventually would be eaten away from a melt of $\ce{ZnCl2}$ ... $\endgroup$ – Buttonwood May 27 at 12:00

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