you need to add acetic acid. I afraid that the calculations were not correct for acid , As it should have pH about3.9-4.0

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Depends on your chitosan Medium, low, or high molecular weight, the way I do it is to weigh the chitosan and mix it in the distilled water and put the beaker on a stirrer and let the stirrer do the job, after an hour add the 0.1 molar acetic acid dropwise. also you can try, ascorbic acid, lactic and maleic acid as well.