I have doubts about what do will say with "crystallize sodium hydroxide... If you want to purify sodium hydroxide for electrochemical studies, probably the best way is to prepare a saturated solution (over 20 mol/L) to precipitate carbonate (the solubility of sodium carbonate is much smaller than the corresponding hydroxide) and the metals (with exception of Pb, Zn and Al, which will form the corresponding complexes).
Inside the solid, which will remain in the bottom of this solution, the metal hydroxides will be precipitated and will not be present in the sodium hidroxide solution. This solution must bedilluted with water of good quality and so you will be free of interferences.