I'm trying to figure out how to concentrate a protein solution using dialysis against a polyethylene glycol solution. Because I don't have a way of knowing what my protein concentration is other than by trial and error, I wanted to calculate how much water would shift.
For example, I have a mixed protein solution at 1.88% (w/v) that I want to shift to 20% (w/v). I calculated that to concentrate 50 ml of my solution, I would need to remove 31.2 ml of water. If I make 208 ml of a PEG 6000 solution at 23% (w/v), it should remove 31.2 ml of water to reach an equilibrium of 20% for both solutions (I have 3.5 kDa dialysis tubing), but I doubt this works on such a simple calculation. I'm guessing I may need to be calculating on moles or something (which I can't really do for the protein solution; it's a mix of unknown proteins). I've also experimented with some excess protein solution I have on hand, and instead of removing a small amount of water, it removed pretty much all the water.
Is there a way to precisely calculate how much water will be removed to achieve the final concentrations that I need?
I don't know how to make the calculation, but consider that water will continue to leave the protein solution in the dialysis bag until the osmolarity inside and outside are the same. Since PEG is too big to enter the bag (ideally, if there is no low-molecular-weight PEG), this condition will not be reached until the protein solution is very concentrated indeed. In other words, nearly all the water will be drawn out, as you observed. This process is slow, however, so you can keep checking the bag contents over time until the volume is what you need.
Thanks Adam! I'm not sure how I forgot about the influence of osmoles, so that was a bonehead move. I did some more looking into this after reading your response, but there isn't a lot of information out there about how to calculate diffusion and dialysis for something like this. I did come across osmometers though, but I don't know how well they work with a protein solution. I'll have to see if I can find one on my campus, and then try to do the math. I don't think the math should be too difficult if I know the osmolarity of the protein solution (PEG has only one osmole when dissolved). The one challenge now will be the influence of electroneutrality on this process, which I suspect will be considerable considering the processing additives in my protein solution.
I was hoping there would be an easier way to do this (other than the math part) so that I don't have to keep constantly checking and measuring my solution volumes and freeze-drying to confirm concentration, but it's not looking good for that.
Hi,
You may use the web tool to calculate
https://www.alfa-chemistry.com/solution-dilution-calculator.htm
https://www.alfa-chemistry.com/mass-molarity-calculator.htm
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