A series of urea solutions in deionized, double-distilled
water were prepared using reagent-grade urea. Refractive
indices relative to air were measured using high accuracy
Abbe Digital Refractometer (China) at wavelength
589 lm. The refractometer was calibrated daily before
measurements against the known refractive index of water
and tested intermittently utilizing the reported refractive
index of acetone.