I've seen a "2.303" term multiplied to $R$ in Van't Hoff equation several times.
Here is one example:
http://en.wikipedia.org/wiki/Van_%27t_Hoff_equation#Error_propagation
$$\Delta H^\circ=2.303R\frac{\log K_1-\log K_2}{\frac1{T_2}-\frac1{T_1}}$$
What is this number and where does it come from?