Vapour Diffusion Experiment

In a vapour diffusion experiment where equal volumes of precipitant and protein are added in the drop, both the precipitant and protein concentration will double.

Phase Diagram for vapour diffusion experiment, no crystals

However, if crystals begin to grow, the concentration of protein in solution will decrease.

Phase Diagram for vapour diffusion experiment, crystals growing

In batch crystallization the precipitant and protein concentration stay the same.

Point A: Protein stays undersaturated
Point B: Protein crystallizes and the concentration of protein in solution drops to saturation
Point C: Protein precipitates, but crystals may still grow

Phase Diagram for batch experiments

In a dialysis crystallization experiment, the concentration of the protein is constant (if one assumes the stretching of the membrane is negligible and that the initial solution fills the chamber completely).

In a salting-out experiment, the precipitant concentration increases.

Phase Diagram for a salting-out dialysis experiment

Dialysis has the advantage the the precipitant concentration can be altered during the course of the experiment.
You can also increase the concentration of one precipitating agent while decreasing the concentration of another.

Phase Diagram for a dialysis experiment, changing buffers

Dialysis can also be used to exploit the salting-in region of the phase diagram by forcing the protein out of solution by lowering the precipitant concentration.

Phase Diagram for desalting