KMnO4,K2Cr2O7/H+

Consider hydrochloric acid(HCl)
The chloride ion will undergo reduction to become chlorine gas
redox reaction occur when acidify KMnO4,K2Cr2O7 , it cannot be used

Consider nitric acid(HNO3)
The nitrate ion will reduce to nitrogen dioxide
The result is just like HCl ,it cannot be uesd

Finally , only sulphuric acid can use

ywh1472004的答案非常正確，相信他吧！

Acidification can increase the oxidation strength of the reagent (H+ as catalyst)

KMnO4 can be acidified by H2SO4, but not HCl, as Cl2 will be eliminated.
K2Cr2O7 can be acidified by H2SO4 and HCl though.

唔可以use dil HCl
because the Cl ion will act as Reducing agent
so use dil H2SO4

因為在它們的氧化還原過程中需要有H+才可以產生反應, acidified就是要提供所需的H+

可以用dil HCl