Continue adding gel until the column is filled with packed beads so that the top of the column is visible. If you use up all the gel-NaCl soltuion before the column is filled, make some more. Attach the tubing to the top of the column so that the 0.15 M NaCl buffer will start to drip on to the top of the column. Open the stopcock at the bottom. If it does not leave fast enough, liquid will start to leak out where the top syringe is attached to the column.
Sodium azide is poisonous.
Wear gloves and goggles. Wash your hands well after using the sodium azide solution as well as the gel which is stored in a sodium azide solution. Sodium azide reacts with copper, lead, and acids to create dangerous products. Keep it away from these materials. Let buffer run through the column for about 15 minutes.