Let $A$ be an algebra over a commutative ring $R$. If $R$ is noetherian and $A^circ$ is pure in $R^A$, then the categories of rational left $A$-modules and right $A^circ$-comodules are isomorphic. In the Hopf algebra case, we can also strengthen the Blattner-Montgomery duality theorem. Finally, we give sufficient conditions to get the purity of $A^circ$ in $R^A$.
