Why do you use 0.005M h2so4 solution for preparation of k2cr2o7 solution for UV calibration?

Answer 1

Because Cr2O72- ion is only stable for hydrolysis by keeping pH < 2.0

(pH < 2.0 means [H+] > 0.01 M, which equals > 0.005 M H2SO4)

To prevent the formation of chromate CrO42- ions in the equilibrium -below- keeping it to the left side (<-) by an excess (overplus) of H+ ions:

Cr2O72- + H2O <--> 2CrO42- + 2H+

Answer 2

In part, the absorbances of this salt are highly reproducible. Please see the link.