Division of cations when salting out a sulfate solution
Following on my previous attempts at electrolysis using a terra cotta flowerpot as a membrane/divider, I took 12AX7's suggestion of using a plaster
divider to avoid iron contamination (thanks 12AX7!), and made a one-piece unit (all plaster) with two wells separated by a small plaster wall. After
initial tests I coated the barrier with improvised sodium silicate solution since it seemed a little too porous; I also decided to stop using sodium
solutions since the NaOH formed at the cathode can attack the CaSO4 material. But my question still relates to the first trial I did, using Na2SO4:
I electrolyzed a solution of sodium sulphate (approximately 1M) at 6V using a lead anode and stainless steel cathode, and drew off three separate
batches of acidic liquid from the anode well over a period of a day, totaling a little more than 1.5 liters.
I boiled this down to 250ml. A small amount of early precipitate was discarded (I think this may have been CaSO4) but the bulk of the material that
settled out was kept.
On reaching this lower volume of solution without seeing any increased resemblance to sulfuric acid, which was the goal of this experiment, I realized
that while I had some H+ in solution (pH tested around 1 using paper strips) there was still enough sodium in solution that I was simply producing
bisulfate, possibly mixed with the original Na2SO4 as well.
I then dried 250ml of 91% isopropyl alcohol using CaCl2. I mixed this thoroughly with the collected precipitates, hoping to split the bisulfate into
H2SO4 in solution and Na2SO4 still as solid. A milky white solution was formed (but most precipitate was not dissolved, which was as expected). I
filtered this into the aqueous solution I had boiled down. Some fine particles passed through my filter, leaving a turbid IPA solution sitting as a
layer on top of the aqueous solution. After shaking, the layers remained separate, but the particulates passed into the water portion of the system.
Anyway, to get to the question: in theory, how should the cations sort out in this system? My hope is that since Na2SO4 is not soluble in IPA, while
H2SO4 is, almost all of the H2SO4 will end up in the alcohol while the Na2SO4 is in the water. The water, after all, is saturated with sodium cations.
But is there a theoretical way to calculate the equilibrium?
[Edited on 20-9-2009 by bbartlog]
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