The type-II silicon clathrate, Na$_{\rm x}$Si136, having a residual sodium content as low as 37 ppm (x = 0.0062) has been prepared by thermal decomposition of NaSi under high vacuum in the temperature range 340-420 $^{\circ}$C followed by subsequent treatments under high vacuum, and completed by several treaments with iodine at 300-350 $^{\circ}$C. The final sample was characterized by XRD, chemical analysis and EPR spectroscopy. This latter technique proved to be particularly suitable to the characterization of highly diluted sodium atoms in the open host lattice of a type II clathrate of silicon and the quantitaive determination of the residual sodium content.
