In case of Dichromate we use it to maintain the pH, otherwise

there is a mixture of Chromate and Dichromate. They donot

absorb at the same wavelength. So we cannot use Lambert-Beer's

Law.

For permanganate use of sulphuric acid is not for this reason

but to maintain the pH as in neutral or alkaline solution

permanganate get hydrolyzed to for manganese dioxide.