[quimico]

Hello everyone.
Can somebody help me on explaining me how does it work (Chemically or physically )the separation of a residue of water containing PVAc (Polyvinyl Acetate) by putting it into a tank with  salt brine. Why does the brine must be saturated? and I have a critical step in the SOP that says this:
Salt brine container must always contain rock salt to maintain salt saturation with a PH level of approx. 4.2.
It's something I don't understand, how the pH of the salt brine can be 7?. we are using ice salt (NaCl) and I tried an experiment and it remained around 7.. do they refer the pH 4,2 as the blend of brine salt and the PVAc residue? why should this be the pH.
What should be the disposal of the solid PVAc after its separation?