I'm trying to find saturation point for dissolved zinc sulphate in water and ethylene glycol.

Based on my knowledge, water is more polar than ethylene glycol as reflected in its higher dielectric constant value. However, max solubity of this salt in water is roughly limited to 3M and for ethylene glycol is much higher (experiment in progress to determine the right number).

What causes this opposite behaviour ?

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    $\begingroup$ It has been a long time since I took a chemistry class, but I'm pretty sure that there is no general theory (over all salts and over all solvents) which allows for a good numeric solution for the question. $\endgroup$ – MaxW Feb 13 '19 at 18:35
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    $\begingroup$ If the observation is true, then I would assume that chelation effects are the root cause, but solubility is such a gross and complicated topic that I wouldn't bet a lot on that. $\endgroup$ – Zhe Feb 14 '19 at 15:20

Zinc is an amphoteric metal which means it can dissolve both in acidic and basic conditions. However, neutral pH is the transition point. Please see this article Recovery of Rare Earths from Waste Cathode Ray Tube (CRT) Phosphor Powder by Selective Sulfation Roasting and Water Leaching on its behavior in water and eg. I believe the reason is that in eg there is no mechanism for zinc to form insoluble hydroxide complexes which will decrease the solubility of zinc. But in ethylene glycol you simply cannot form hydroxides..

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