You can synthesize graphene oxide via Staudenmairs method .This oxidation method typically use concentrated sulfuric acid in combination with fuming nitric acid
Sodium nitrate reacts with sulfuric acid according to following equation :
2 NaNO3 + H2SO4 = 2 HNO3 + Na2SO4
Existence of sodium nitrate is necessary to produce nitric acid .Existence of both sulphuric acid and nitric acid is important to oxidation of graphite and intercalation of graphite oxide to produce graphene oxide