# Microfiltration for saturated solutions of highly hygroscopic compounds?

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).

• 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. May 26 '21 at 13:24
• "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 $\ce{ZnCl2}$ ... May 27 '21 at 12:00