ZnCl2  is already a weakly acidic salt. The preparation of a saturated solution of it  just  needs adding the salt slowly to water in a beaker with continuous stirring. Proceed until you reach saturation where the added salt does not disappear. To use it for the Refractive index measurement you must be aware of the temperature not to increase and the salt crystals will then formed. 

Zinc(II) chloride (ZnCl2) is highly soluble in water. Conc. aq. sol. of this salt are strongly acidic, owing to HCl generated by its partial hydrolysis.