In the original method for synthesizing graphene oxide (Hummers’ method) the reagents used are: (KMnO4, NaNO3, H2SO4. Whereas, in the improved method by Tour and Co-workers NaNO3 was excluded, the amount of KMnO4 was increased, and the reaction was performed in a 9:1mixture of H2SO4/H3PO4for improving the efficiency of the oxidation process.
For more details, see attached file which contains the paper entitled "Improved Synthesis of Graphene Oxide " by Tour's group in which they describe the details of the improved procedure.