I want to prepare glycol chitin according to the protocol but I don't know which solution should I use for dialysis? I have already thought about glacial acetic acid. 

I think that the reaction of acetic anhydride (excess) with aqueous solutions, will be too exogenous.

Ethylene glycol chitosan (EGC, 40 mg) was treated at 4 ºC in a mixture containing 400 mg of NaHCO3 and 4.5 ml of acetic anhydride and kept at 4 ºC for 24 h. After addition of 200 ll of acetic anhydride the mixture was allowed to stand for 24 h at 4 ºC. After dialysis, acetylated ethylene glycol chitosan (1 mg/ml) was used as a substrate for the assay of chitin deacetylase.